Lab course Master of Science (Semester 1) Applied Chemistry Analytical Chemistry Pre-­‐Lab: Basic Experiments for good laboratory practice Analytical Chemistry University of Siegen WS 2013/2014 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen Content 1 BASIC INFORMATION ABOUT LAB PREPARATION ..................................................................................... 2 1.1 LAB REPORTS ................................................................................................................................................ 2 1.2 ELECTRONIC BALANCES ................................................................................................................................... 5 1.3 VOLUMETRIC GLASSWARE ............................................................................................................................... 7 2 FORMULA FOR CALCULATION .................................................................................................................. 9 3 EXPERIMENTS ........................................................................................................................................ 11 3.1 3.2 3.3 3.4 E1: CALIBRATION OF THE GLASSWARE .............................................................................................................. 11 E2: GRAVIMETRIC DETERMINATION OF ALUMINUM ............................................................................................ 15 E3: VOLUMETRIC DETERMINATION OF ALUMINUM ............................................................................................. 17 E4: PHOTOMETRIC DETERMINATION OF IRON .................................................................................................... 19 4 APPENDIX .............................................................................................................................................. 22 5 REFERENCES .......................................................................................................................................... 23 I Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen Structure of the protocols 1.
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name and identification number of the student name and date of experiment, adviser date of protocol task of the experiment short introduction with theory carrying-­‐out of the experiment with reaction, what did you do with reagents, amounts, concentrations measured values detailed calculation with average, standard deviation SD, relative standard deviation in % RSD, formulas result in a final sentence critical evaluation of your result also in respect to possible errors evaluation of the whole experiment Before you start with the experiment you have to do an oral examination including execution and theoretical background of the experiment. The reports must be submitted to the adviser not later than one week after the experiment; later submissions will result in an F. 1 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 1 Basic Information about Lab Preparation Before each lab session, you should prepare by reading the lab manual and additional literature. We expect you to have a good understanding of the purpose, details of the procedure, the use of all chemicals and any significant hazards, and the underlying science of the experiment when you come to lab. You must pass an oral Pre-­‐lab Quiz at the start of each lab session to demonstrate your understanding of the material. Specific topics that you may expect to be quizzed on are listed at the end of each experiment. Each pre-­‐lab quiz is administered by a TA. TAs may choose to give quizzes individually, or in small groups. If you clearly have not read the manual, or have not attempted to understand the experiment, then you will be told that you will have to perform the experiment during a later session. As long as an honest attempt has been made to read and understand the experiment, you will be allowed to perform your experiment. As well as acting as an entry pass to performing your experiment, the pre-­‐lab quiz will be worth 20% of your lab report grade for that experiment. Although no pre-­‐lab report is required, drawing up tables in your lab notebook for your data and carrying out the preparation calculations (e.g., mass needed to make up solutions of given molarities) will help you finish quickly and efficiently. You are required to come in time. Once you pass the quiz, you will be allowed into the lab to start doing your experiment. 1.1 Lab Reports You will write a lab report on each of the experiments you perform during this course. Number all graphs, drawings, spreadsheets, etc. and refer to them in the appropriate part of the report. Don’t forget to answer all discussion questions asked in the manual for the experiment. 1.1.1 Report Format Scientific writing is not literary writing. You should aim to be brief, precise, and unambiguous. The reader should clearly understand what you are trying to say. Try to keep your verb tense consistent and appropriate. You may use passive voice. Avoid using jargon or slang and use full sentences. Do not refer to “I”, “we”, “our”, “us”, nor should you speak about yourself, e.g., “the student”. Twenty percent of each lab report grade is based on your presentation, formatting, and layout. Do not use pencil. There is no problem for turning in a completely handwritten report if your writing is neat and legible and your report is well organized and clearly laid out. Conversely, using a computer does not automatically guarantee you full presentation points. Use a legible (at least 12 pt and a 1.5 line spacing) font, lay things out well, label table columns, etc. 1.1.2 Purpose Brief summary of the principles demonstrated in this experiment (what you are going to do and why). 1.1.3 Data and Observations Report all data, including appropriate units, preferably in tabular form. Include any relevant observations (e.g., color changes, unintended spills, unexpected results, etc.) and note anything you did differently from the procedure given in this manual. 1.1.4 Data Analysis Follow the analysis steps in this manual, labeling them by number so we can follow what you are doing. Put a box around, underline, or in some other way highlight the important answers. Unless you are explicitly told otherwise in the data analysis instructions, if you studied multiple samples, 2 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen carry out all calculations for all replicates; i.e., do not average them together until the very end. You do not need to show all working for all replicate calculations. Show one complete sample calculation for the first replicate, writing out the each formula in words first, then plugging in the relevant data, including the appropriate units, clearly showing each step of your analysis. You may then simply summarize the calculation results for the other replicates in a table or spreadsheet, making sure you label the columns well with informative names and units. We recommend that you actually perform each step of the sample calculation with a calculator, rather than just writing in the results your excel spreadsheet gives you. This way your sample calculation acts as a check of your excel spreadsheet, so you can detect any errors you have made in your formulae. If the manual has told you to plot something, then that means you must include the plot in your report. 1.1.5 Results This section should report the results of the experiment separately from your calculations in a clear and concise format. Where multiple data sets are used, report each individual determination, the average with appropriate significant figures, the standard deviation, the percent relative standard deviation (%RSD), and the 90% confidence interval for your data. When using a line-­‐of-­‐best fit to data, report your correlation coefficient. See the Error and Statistical Analysis section of this manual for more information. If the Q-­‐test takes away one of your three replicates, then you cannot report a standard deviation. That’s ok, just talk about it in your conclusion. 1.1.6 Discussion questions Write numbered, coherent answers to all questions found in the lab manual. Use complete sentences and be sure your answer demonstrates understanding of the material. Like the rest of your report, the discussion section must be in your own words. Plagiarism will not be tolerated. 1.1.7 Conclusions The most scientifically important part of an experiment is your interpretation of your observations. For an analytical chemist, the accuracy and precision of your results is also important. These priorities are reflected in the allocation of some of your lab report grade to these topics. It may seem difficult for you to imagine what more there is to discuss once you have determined something like the % content of ascorbic acid in a Vitamin C tablet; however, here are some guidelines to help you: Summarize the point of the experiment. (What did you do? Why did you do it?) Discuss your results. (Can you compare to literature values or values given by a manufacturer? Do they seem reasonable? Why or why not?) In the cases where values are available for comparison, you should quantify the difference between your experimental values and the expected values. Rather than simply saying you were “way off”, a comment such as your value was 15% larger than the literature value would be more appropriate. List the possible sources of error in your experiment, being as specific as you can in your error descriptions; e.g., rather than saying “operator error”, describe exactly what aspect of the experiment you may have done incorrectly. Discuss any problems encountered and suggest ways around them if the experiment was repeated. Evaluate possible sources of error (operator, random, systematic, etc.) in the experiment. Discuss the most significant contributions to the error in this experiment and describe specifically how each of these significant errors would have affected your results (i.e., increased or decreased your answers). Give reasonable methods to eliminate or reduce these errors. Consider the experiment in terms of accuracy, precision, reproducibility, selectivity, and analysis time. Suggest ways to improve your procedure if you were to perform it again. 3 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 1.1.8 References Whenever you use material written by other people to help you create your lab report, you should reference your literature source; e.g., when you look up literature values, when you use reference information from the library, or when you read some information in a textbook to help you answer a discussion question. You should reference this lab manual and your textbook whenever appropriate. All facts, formulas, drawings, graphs, etc. used in the report that you did not create yourself require a citation of their source. Failure to reference this material has an implicit implication that they represent your original work. We grade your understanding of what you have read. The ability to find an appropriate quote is only half the battle, you must then frame it in your own words, applied to the particular experiment you have performed. Your references should be numbered in consecutive series in order of mention in your report. If a reference is repeated, do not give it a new number; use the original reference number. Reference numbers in the text should be superscripted outside the punctuation of the sentence or phrase to which the citations applies, or after the name of the author if referred to in the text; e.g., •
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Quantitation of chlordane in freshwater fish has been reported previously.1 When compared to previous reports of chlordane in fish,1 the results reported here... Work of Doe et al.1 has shown that … The complete list of literature citations should be placed at the end of your report, after the conclusions, but before the appendices. List references in numerical order and use the following formats. Journal references must include the author names, abbreviated journal title, year of publication, volume number (if applicable), and the range of pages of the cited article; e.g., (1) Smith, A. D.; Moini, M. Anal. Chem. 2001, 73, 240-­‐246. Book references must include the author or editor’s name, book title, publisher, city of publication, and year of publication; e.g., (2) Skoog, D. A.; Holler, F. J.; Nieman, T. A. Principles of Instrumental Analysis, 5th ed.; Saunders College: USA, 1998; Chapter 7. Website references must include the author (if known), title of site from browser window title bar, URL, and date accessed; e.g., (3) U.S. Environmental Protection Agency Home Page, <http://www.epa.gov/> (accessed July 1996). 1.1.9 Data rejection When you have a set of data with one data point that seems very different to all the rest, there must be a scientific basis for deciding whether to reject that point. You should not just throw it away without justification. If you have a series of repeated measurements of the same property (e.g., when you run three replicates of an experiment), then the Q-­‐test is a useful statistical way to determine whether one of your points in that series of measurements can be disregarded. The Q-­‐test tables are given in your text. (The value of Q for three observations is 0.94.) If you do not have a series of repeated measurements of the same property, then you cannot use the Q-­‐test. If you have 4 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen no basis for rejecting a data point, other than some level feeling that dropping a particular data point will make all your data look so much better, then this is what you should do: Do the analysis using all your data. Show that the results using all your data are very different than what you would expect. Point out how bad that one data point looks (e.g., the plot is linear except for this one point). Speculate about at least one specific experimental reason that could explain how that data point could have been in error. Redo the analysis dropping that data point. Make self-­‐congratulatory statements about how that fixed all your calculations. 1.1.10 Data reporting in your lab reports All experiments in this course involve running three replicates to reduce the uncertainty in your determined mean, and investigate the precision of your results. Use the mean of your replicates as your final answer; however, carry out all calculations for all replicates. Do not average them together until the very end. The standard deviation of the replicates is used as a measure of how widely distributed your individual measurements have been (and by extension, would be expected to be in the future), and should be reported as a ± range about your mean value. Also, we require you to list the percent relative standard deviation (%RSD); i.e., the ratio of the standard deviation to the mean, converted to a percentage. When quoting your results in the required format (mean ± standard deviation). Never truncate or round numbers during your calculations, even if some of the intermediate numbers were rounded to significant figures for reporting purposes along the way. Some experiments are treacherously susceptible to error if this rule is not followed. 1.2 Electronic Balances Unless accuracy is not critical to the procedural step being performed, all weighing in analytical lab is performed using a clean dry weighing bottle supplied at room temperature. Chemicals must not be weighed directly on the pan of any balance. Only weighing bottles, weigh boats and weighing paper should contact the pan. Any chemical that falls on the pan will be weighed along with the chemical in the container but will not be transferred to your experiment, resulting in errors. Never try to weigh an object heavier than the capacity of your balance. Temperature can affect the weight of an object. To be weighed accurately, all objects should be at room temperature. A warm weighing bottle will set up convection currents inside the balance case, which will buoy up the balance pan reducing the apparent weight. In addition to the weighing bottle and its contents, a balance also measures the weight of the column of air above the weighing bottle and this air is less dense above a hot object. Hot objects, such as crucibles, should always be cooled to room temperature in a desiccator (to prevent adsorption of water) prior to weighing. 1.2.1 Weighing by Difference This procedure explains how to create triplicate samples of the same solid using weighing by difference. 1. Keep your balance clean. Remove dust from the pan and floor of the balance with a brush before weighing your sample. 2. Weigh your weighing bottle with lid to get an approximate idea of its weight. Do not touch it with your hands — wear gloves, or hold it using a collar of paper around the neck of the bottle. Close all doors on the balance before making a weight determination. Any air movement can cause variation in weight in the mg range. 5 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 3. Many solids increase their weight by taking up water vapor or carbon dioxide from the atmosphere. We supply these solids pre-­‐dried in a desiccator. Remove the chemical from the desiccator. If you try and transfer chemical from the container while still in the desiccator you are likely to spill chemical into the desiccator. Keep the lid on the desiccator except when actually taking things in or out of it. Do not add chemical to the weighing bottle while it is still on the balance. Place the weighing bottle on the counter beside the balance, and use the spatula provided to transfer sample from its container to your weighing bottle. When you are finished return the chemical container to the desiccator. 4. The weighing bottle lid minimizes absorption of water vapor or carbon dioxide from the air during the weighing process. Use it. Weigh the weighing bottle along with its contents, including the lid, recording the weight using all significant figures. If the difference between this weight and the bottle weight recorded in Step 2 is not the quantity you require (roughly three times a single sample), remove the weighing bottle from the balance and add or remove chemical as appropriate. Do not return unused chemical back into the stock container. Put it in the solid waste. 5. Remove the weighing bottle from the balance and transfer approximately one-­‐third of its contents without loss directly into the first container to be used in your experiment. To transfer the solid, tip the weighing bottle sideways over the container, and use the lid of the weighing bottle to tap gently on the side of the weighing bottle, until an appropriate amount of solid is transferred. Transfer should be made into a container with a large enough mouth (e.g., Erlenmeyer flask) that there is no danger of losing solid during the transfer. 6. Weigh the weighing bottle and its remaining contents, including the lid, recording the weight using all significant figures. The weight of your first transferred sample is the difference between this weight and the weight you measured in Step 4. 7. Remove the weighing bottle from the balance and transfer approximately one-­‐half of the remaining contents without loss directly into the second container to be used in your experiment. 8. Weigh the weighing bottle and its remaining contents, including the lid, recording the weight using all significant figures. The weight of your second transferred sample is the difference between this weight and the weight you measured in Step 6. 9. Remove the weighing bottle from the balance and transfer the remaining contents without loss directly into the third container to be used in your experiment. 10. Weigh the weighing bottle and any residual solid, including the lid, recording the weight using all significant figures. The weight of your third transferred sample is the difference between this weight and the weight you measured in Step 8. 11. When you have finished using the balance, make sure that all weights are recorded and clearly labeled in your notebook, no objects are left on the balance pan, the balance and surrounding bench top is clean, excess solid is disposed of in the solid waste container, and stock chemicals are returned to their desiccators. Keep the balance room clean, tidy and minimize the paper you bring in there. If a balance does not behave properly, report it to the Stockroom. Do not try to fix it yourself. 6 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 1.3 Volumetric Glassware In analytical lab all volume measurements are made using volumetric glassware. Glass is inert to most chemicals, its coefficient of thermal expansion is small, and measurement is simplified by the transparency of this material. There are many types of glassware; some of which is designed for holding and transferring approximate volumes (e.g., beaker, graduated cylinder, Erlenmeyer flask, graduated pipettes). Volumetric glassware is designed for measuring and transferring accurately known volumes of liquids (e.g., volumetric flasks, volumetric pipettes, burettes). The volumetric markings on volumetric glassware are correct to within set tolerances given in below. Note that these tolerances are valid only if the glassware is scrupulously clean. 1.3.1 Burettes A burette is used for titrations and delivers a variable but accurately known volume. Your burette is composed of three major parts: The barrel is the graduated tube used to hold your solution. The stopcock controls the rate of flow of solution out of the burette. The tip is carefully machined to deliver regular size drops. 1.3.1.1 Cleaning your Burette The burette is a delicate and expensive piece of glassware and should be handled carefully. If the burette is broken in any way, it is best to replace it. In particular, each time before using your burette, check that there is no damage to the end of the burette tip. Any chips or cracks in a burette tip will have serious effects on the delivery of the titrant. Burettes must be carefully washed, to make sure that solutions uniformly drain down the inner surface of the barrel. A long brush may be used in the barrel. Some burettes allow you to remove the valve and burette tip to get easier access to both ends of the barrel. The burette is clean when there are no obvious deposits of material and if the burette drains cleanly without leaving droplets of solution on its inner walls. Clean the tip and stopcock by repeated rinsing. If the tip or stopcock becomes clogged, they may be unclogged by carefully pushing a piece of small diameter wire through the openings. If applicable, after cleaning, reassemble your burette. Care should be taken to assure that the stopcock is firmly seated or leakage will occur. Test for leaks by closing the stopcock, half filling the burette with distilled water, and watching for any liquid seepage. Stopcock grease is not required when using a Teflon stopcock. If you have a glass stopcock and feel you need grease, you may get some from the TA. 1.3.1.2 Rinsing and Filling a Burette Once your burette is clean and doesn't leak, it is ready for use. Solutions used in burettes are usually standardized (i.e., their concentrations are precisely known), so in order to avoid dilution, you must rinse your burette with your solution before filling; thus: 1. Lower the burette in its holder so the top of the barrel is within easy reach. Using a funnel, pour ~5 mL into the burette from a convenient vessel (a beaker is much better than a 1 liter bottle). 2. Remove your burette from its holder; tip and rotate the barrel so that the solution is brought into contact with the entire inner surface of the burette. 3. Allow the solution to drain completely out through the tip. 4. Repeat with a second 5 mL wash. 5. Close the stopcock and fill the burette to within 1 cm of the top. 6. Let the solution sit in the barrel with the stopcock closed. Make sure that there are no leaks. 7 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 7. Make sure that there are no air bubbles in either the barrel or the tip. Tap the barrel lightly to dislodge air bubbles (this is an indication of a dirty burette). To remove air from the tip, open and close the stopcock fully to force the air out. You may have to do this several times. 8. Allow the solution to drain until it is within the graduated portion of the barrel. 9. Label your burette with its contents, so you don't get your burettes confused. 10. Remove the funnel before taking any readings, or else it may drip additional solution into your barrel, destroying your volume measurement. Record the initial level of the solution in the burette. There is no reason to attempt to start each titration with the solution exactly at 0.00 mL; this practice wastes time and introduces operator bias. Once a burette has been rinsed and filled, it may be refilled as many times as necessary that same day with the same solution without repeating the above routine. Each day and with each change of solution, the burette should be thoroughly washed, rinsed with distilled water, rinsed with the standard solution and filled as above. Burettes are stored upside down so that material does not collect in their tips. 1.3.1.3 Reading the Burette Determinations using a burette involve an initial and final reading; the volume used is the difference between these two readings. Readings must be made correctly and in the same way each time. Remove any partial drops on the burette tip by touching the tip to the inner side of your titration flask. If you have been delivering solution rapidly, with the stopcock fully opened, wait a few seconds for all the liquid to run down the inside walls of the barrel before taking your reading. Readings are made from the bottom of the meniscus (curvature of the liquid in the barrel) for most solutions. With highly colored solutions, you may not see the meniscus and then any other consistent point of reference may be used. All readings with a burette or other volumetric glassware must be made at eye level. If you have to look up or down, there will be a serious parallax error involved. Raise or lower your burette in its holder to move the meniscus to your eye level. Your burette is graduated in 0.1 mL intervals. Read it to the nearest 0.1 mL. A plain white index card held behind the burette will make it easier to read. It is also a good idea to place a white piece of paper under your titration flask if the endpoint is being detected by an indicator color change. 1.3.1.4 Dispensing Solution Always hold the valve joint when turning the stopcock, so the burette will not break and so that you do not pull the valve and tip out of your burette by holding it too tensely. A technique that permits speed, accuracy, and control is as follows: Face your burette with the stopcock handle on the right side of the barrel. Operate the stopcock with your left hand from behind the burette, while swirling your solution with your right hand. Your left thumb and forefinger grasp the stopcock handle, while the other fingers of that hand hold the valve joint and tip steady. (Reverse for left-­‐handers.) Treat your first titration as a rough measurement to learn the approximate volume of titrant that will need to be added to reach the endpoint. For subsequent titrations, you can fully open the stopcock at first, allowing a fast stream of solution to enter the titration flask. As the endpoint is approached, the solution should be added more slowly, until it is added dropwise to reach a "perfect" endpoint (each extra drop added results in ~ 0.05 mL of error). Careful stopcock control allows dropwise addition. Be careful to ensure that each drop enters the sample solution. If you touch off a partial drop from the burette tip onto the inner wall of your sample flask, remember to rinse the partial drop down into your sample solution using a wash bottle. 8 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 2 Formula for Calculation Average !≡
Standard deviation (SD) !% =
Average volume !≡
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( 9 ) !!" ∙ !!" = !!" ∙ !!" = !"#$% ( 10 ) 9 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen Table 1: Table of symbol collection and their units. Abbr. Description Unit ω mass fraction 1% = 0.01 mg/kg = 10-­‐2 1 ppm = 1 mg/kg = 10-­‐6 M molar mass g/mol β mass concentration 1 g/L = 1 mg/mL Φ density of a solution 1 g/cm3 = 1 g/mL n amount of substance mol V volume 1 mL = 0,001 L z valency stage -­‐ V20 °C real volume of a vessel at 20 °C mL Gϑ weight of the contained water at ϑ [°C] g W20 weight of the water per mL volume at 20 °C W20 = 0,99717 g/mL g/mL fϑ correction factor -­‐ cϑ1;cϑ2 concentration of a solution at the temperature g/mL ϑ1 and ϑ2 fϑ1; fϑ2 correction factor for ϑ1 and ϑ2 -­‐ 10 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 3 Experiments 3.1 E1: Calibration of the glassware Quantification requires accurate weight and volume of sample and reagent. In the following experiments the accuracy of some volumetric glassware will be tested. Volumetric glassware should be handled with care and never be put on a hotplate. All volumetric glassware should be cleaned directly after using. 3.1.1 Calibration of 100 mL volumetric flask Take a dry volumetric flask. Weigh the empty flask without a stopper and give an exact weight precise to 1 mg. Fill the flask with water with known temperature up to the mark. Remove adhering drops of the neck of the flask with filter paper. Gross weight should be precise to 1 mg (without a stopper). The true volume of the water filling the flask up to the mark, depending on temperature and density of the water, can be calculated according to table (Table 2). Table 2: Weighted portion of water in the volumetric flask and calculation of the corresponding water volume at 20 °C. # Tara [g] (weight of empty flask; x.xxx g) 1 Gross [g] (weight of full flask; x.xxx g) Net [g] temperature V20 °C [mL] (weight of water; [°C] (calculated) x.xxx g) 2 3 average SD RSD R% Tasks to do in the protocol: Calculate the average, SD and RSD of the measurements and the deviation from the resulting value. Evaluate the deviation. 11 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 3.1.2 Calibration of a pipette Weigh an empty Erlenmeyer flask precise to 1 mg. First rinse, and then fill the pipette with water of known temperature to a short distance above the mark. Pour out the water until the meniscus is exactly on the mark and stop the outflow. The pipette is then allowed to deplete into the clean, weighed flask and held vertically so that the tip of the pipette is in contact with the vessel. Wait for 15 s after the depletion of the pipette has stopped. Remove any drop adhering to the outside of the pipette. Because of the capillary attraction there will be a residual volume in the tip of the pipette. The filled vessel is weighed and the temperature of the water noted. The gross weight must be precise to 1 mg. Repeat the measurements for at least five times! The capacity of the pipette is then calculated to a calibration temperature of 20 °C according to the table and the error is given in %R. Table 3: Weighted portion of water in the Erlenmeyer flask and calculation of the corresponding water volume at 20 °C. # Tara (weight of empty flask; x.xxx g) 1 Gross (weight of filled flask; x.xxx g) Net temperature V20 [mL] (weight of water; [°C] (calculated) x.xxx g) 2 3 4 5 average SD RSD R% Tasks to do in the protocol: Calculate the average, SD and RSD of the measurements and the deviation from the resulting value. Evaluate the deviation. 12 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 3.1.3 Drop error of a burette A titration cannot be more precise than one drop. To measure this error, fill the burette up with water of known temperature. The volume of a drop is determined by pouring out 20 drops and taking the weight. Usually 1 drop is the minimum error of a titration. Table 4: Weighted portion of water in the Erlenmeyer flask and calculation of the corresponding water volume at 20 °C. # Tara [g] (weight of empty flask; x.xxx g) 1 Gross [g] (weight of full flask; x.xxx g) Net [g] (weight of water; x.xxx g) temp. [°C] V20 °C [mL] V20 °C of one (calculated) drop [mL] 2 3 average SD Questions to answer in the protocol: 1
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How big is the drop error (in %) after a 10 mL titration? Compare this error to the error of 20 drops. How many mL have to be titrated to achieve a drop error as low as 0.1%? 13 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 3.1.4 Allowed tolerance of calibrated glassware Glassware with quality “A” has a blue labeling; glassware with quality “B” has a white or brown labeling. → Glassware with quality “A” is officially calibrated. → Glassware with quality “B” has an allowed tolerance factor of 1.5 to glassware with quality A. Waiting time before reading the scale: Pipettes: 15 s Burettes: 30 s Table 5: Absolute and relative error of calibrated Pipettes, volumetric flasks, measuring cylinders and burettes (quality A). Pipette Vol. 1 mL 5 mL 10 mL 25 mL 50 mL Error 0.9% 0.3% 0.2% 0.15% 0.1% Vol. 10 mL 25 mL 100 mL 250 mL 1000 mL Error (rel.) 0.1% 0.1% 0.07% 0.07% 0.03% Vol. 10 mL 25 mL 50 mL 100 mL Error (abs.) ±0.05 mL ±0.25 mL ±0.25 mL ±0.38 mL Error (rel.) 0.5% 1% 0.5% 0.38% of whole vol. volumetric flask measuring cylinder burettes Vol. 10 mL 25 mL 50 mL Error (abs.) ±0.02 mL ±0.03 mL ±0.04 mL Error (rel.) 0.40% 0.24% 0.16% of half vol. 0.20% 0.12% 0.08% of whole vol: 14 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 3.2 E2: Gravimetric determination of Aluminum 3.2.1 Keywords Requirements for gravimetric determination, solubility product 3.2.2 Basics Various metals (e.g. aluminum, iron, copper, zinc, cadmium, nickel, cobalt, manganese and magnesium) form under specified conditions of pH well-­‐defined crystalline precipitates with 8-­‐hydroxyquinoline (hereinafter referred to as oxine). These precipitates have the general formula M(C9H6ON)n, where n is the number of oxine-­‐ligands binding to the metal (M)-­‐ion. Bivalent metal ions react with 2 oxine-­‐ligands, trivalent metal ions react with 3 oxine-­‐ligands. 3.2.3
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Reagents 8-­‐Hydroxychinoline Acetic acid, 100% Ammonia, diluted Ammonium acetate 3.2.4 Drying of the sintered-­‐glass crucibles Dry the sintered-­‐glass crucibles pore-­‐size No. 4 at 130 °C in a drying-­‐cabinet, cool them down in a desiccator and weigh them (Tara). 3.2.5 Preparation of an oxine solution Dissolve 3 g oxine in a small amount of pure acetic acid (approx. 10 mL) and fill it up to 100 mL with water. Add diluted ammonia until the solution gets turbid. Dissolve the precipitation with diluted acetic acid. 3.2.6 Precipitation of Al(C9H6ON)3 Hand an empty and clean 100 mL volumetric flask to the adviser to get your unknown Aluminum sample. Fill it up to the mark with de-­‐ionized water and mix it. Transfer 25 mL of the solution for the analysis into a conical flask. Add 125 mL of water and heat the solution up to 50-­‐60 °C. Then add a quarter (approx. 25 mL) of the prepared oxine-­‐solution under stirring (1 mL will precipitate 0.001 g of Al). The complex Al(C9H6ON)3 will be formed. Complete the precipitation by adding a saturated solution of 4.0 g ammonium acetate in water. Stir the mixture while heating (50-­‐60 °C) to get a well formed precipitation. Filter the granular precipitate through a sintered-­‐glass crucible of porosity No. 4 with help of a Buchner flask under suction-­‐pressure. Wash with warm water to remove the excess of oxine. Dry the precipitation at 130 °C in the compartment dryer. Weigh the sintered-­‐glass crucibles after 1 h of drying and after that in steps of 30 minutes until the weight is constant. 15 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen Table 6: Weight portions and resulting Al-­‐concentrations for the three sintered-­‐glass crucible. # empty crucible [g] 2. weight with precipitate [g] 3. weight with precipitate [g] weight of weight Al(Ox)3 of Al [mg] [mg] concentration of Al [mg/L] 1. weight with precipitate [g] 1 2 3 average of Al [mg/L] SD RSD Additional Information: Molecular weight of Al: Molecular weight of Al(Ox)3: 26.9815 g/mol 459.4345 g/mol Questions and tasks for the protocol: 1
Evaluate the experiment. What are possible sources of errors? The gravimetric precipitation of aluminum oxinate is the starting point for the next experiment, the indirect titration of aluminum via substitution of the oxine with bromine. 16 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 3.3 E3: Volumetric determination of aluminum 3.3.1 Keywords Titration types, endpoint indication, equation of the bromometric titration of oxine 3.3.2 Basics The Al(Ox)3 precipitate gets dissolved in diluted hydrochloric acid. The bromine (Br2) is formed by adding bromate-­‐solution (KBrO3) to an excess of bromide (KBr). 5,7-­‐Di-­‐bromo-­‐8-­‐hydoxychinoline is formed by bromination of oxine. The substitution reaction of oxine proceeds slowly and no distinct endpoint can be observed, therefore an excess of bromate is used. The residual bromine is reduced with KI and the released iodine is titrated with thiosulfate-­‐solution and starch as indicator. 3.3.3 Reaction equation Al(C9H6ON)3 +3 HCl → 3 C9H7ON + AlCl3 BrO3-­‐ + 5 Br-­‐ + 6 H+ → 3 Br2 + 3 H2O C9H7ON + 2 Br2 → C9H5ONBr2 + 2 H+ + 2Br-­‐ 3.3.3.1 Back titration Br2 + 2 I-­‐ → I2 + 2 Br-­‐ I2 + 2 S2O32-­‐ 3.3.4
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→ 2 I-­‐ + S4O62-­‐ Reagents Precipitate from the gravimetric experiment HCl, conc. KBr 0,1 N-­‐KBrO3 standard solution, 0,1 N = 1/60 M KBrO3 Methyl red as indicator KI 0,1 N S2O32-­‐ standard solution starch as indicator 3.3.5 Procedure Use the precipitate from the second experiment to do the volumetric determination of aluminum. Dissolve the complex quantitatively in warm (~60°C) concentrated hydrochloric acid, use a Buchner flask to get the precipitate out of the pores completely. Collect the solution in a 250 mL reagent bottle, add a few drops of the indicator methyl red and 0.5-­‐1 g of pure potassium bromide. Titrate slowly with standard 0.1 N potassium bromate solution until the color becomes pure yellow. Check again with few drops of methyl red. (Allow the precipitation on the bottom of the flask to withdraw while checking the color). The exact end point is not easy to detect, and the best procedure is to add an excess of potassium bromate solution, i.e., a further 5 -­‐ 10 mL volume beyond the estimated end point, so that the solution now contains free bromine. 17 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen After 5 minutes reaction time and while stirring from time to time, add 1 g potassium iodide solution. The free iodine will be formed. Titrate the free iodine with a standard solution of 0.1 M sodium thiosulfate, using starch as indicator to determine the excess of bromine. Table 7: Inserted volumes of reagents for the determination of the Al concentration. # 1 Volume of 0.1 N KBrO3 [mL] Volume of 0.1 N S2O32-­‐ [mL] Corrected moxim per mAl per concentration volume 0.1 N crucible [mg] crucible [mg] of Al [mg/L] KBrO3 [mL] 2 3 average of Al [mg/L] Questions and tasks for the protocol: 1
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What is the stoichiometric factor of aluminum to bromine? Calculate the amount of aluminum (mg) per crucible and give the corresponding formula. Calculate the concentration of the Al-­‐solution. Calculate the average, SD and RSD of the experiment. Compare the volumetric result with the gravimetric result. Evaluate the experiment. What are possible sources of errors? 18 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 3.4 E4: Photometric determination of iron 3.4.1 Keywords Basic principle of photometry, measurement setup, calibration, determination of iron 3.4.2 Basics Iron(II)-­‐ions form an orange-­‐red complex with 1, 10-­‐phenanthrolin that is stable at pH 2.5-­‐9.0. The absorption is measured at the wavelength of 513 nm. With this method the sum of iron(II) and iron (III) can be determined in water and wastewater. Iron(III)-­‐ions are reduced to Iron(II) with hydroxylammoniumchloride. 2 Fe3+ + 2 NH2OH + 2 OH-­‐ → 2 Fe2+ + N2 + 4 H2O Then the reaction with 1,10-­‐phenanthroline is: Fe
2+
+
3
Fe(
N
N
2+
)3
N
N
2+
Figure 1: Scheme of the reaction of Fe and 1,10-­‐phenanthroline. 3.4.3 Cross interferences Cu, Cr, Co, Zn, phosphate > 1 mg/L Bi, Ag, Hg Concentration must be lower than the tenth of the iron concentration Cd > 50 mg/L Ni > 2 mg/L 3.4.4
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Required reagents Buffer solution: Dilute 40 g of ammonia-­‐acetate-­‐salt in 40 mL of acetic acid (100%, density 1,06 g/mL) and fill up to 100 mL with de-­‐ionized water in a 100 mL volumetric flask. 10% Hydroxylammoniumchloride solution: Dilute 10 g of hydroxylammoniumchloride in 100 mL de-­‐ionized water (already prepared). 0,5%-­‐Phenanthroline solution: Dilute 0,5 g of 1,10-­‐phenanthrolinehydrochloride in 100 mL de-­‐ionized water (already prepared). Diluted sulfuric acid: 1 volume of conc. sulfuric acid diluted with 3 volumes of de-­‐ionized water (already prepared). 19 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 3.4.5 Preparation of an iron stock solution (S1: 100 mg/L) Take 100 mg iron powder and dissolve it in 25% hydrochloric acid (10 mL). Heat the solution until the iron is completely dissolved. The solution is cooled down and then transferred quantitatively into a 1 L volumetric flask. Fill the flask with de-­‐ionized water up to the mark and mix it vigorously. 3.4.6 Preparation of additional stock solutions S2: Prepare a stock solution with the concentration of 10 mg/L by dissolving S1. S3: Prepare a stock solution with the concentration of 1 mg/L by dissolving S2. 3.4.7 Preparation of calibration solutions Prepare 6 stock solutions in a concentration range between 0 and 4 mg/L by dissolving the stock solutions S2 and S3. Use 50 mL volumetric flasks. Write down the used amount of stock solution 2 and 3 (Table 8). Table 8: Preparation of the serial dilution and absorption of the resulting iron-­‐solutions. Volume of the 2nd and 3rd stock solution Fe-­‐conc. Absorption 1 0 mL (Blank) 2 3 4 5 6 0 mg/mL 0.1 mg/mL 0.5 mg/mL 1.0 mg/mL 2.0 mg/mL 4.0 mg/mL After adding the stock solution to the volumetric flask, proceed as follows: 1
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Add 5 mL of buffer solution. Add 1 mL hydroxylammoniumchloride solution. Mix the solutions well and add 1 mL phenanthroline solution. The pH should be now between 3.4 and 5.5. Fill the volumetric flasks with de-­‐ionized water up to the mark and mix them. Wait for 15 minutes so the complex can assemble in the solution. Measure the absorbance with the photometer at 513 nm and write it into Table 8. 3.4.8 Sample preparation Add 5 mL of diluted sulfuric acid to the sample solution and fill up the 500 mL flask. Transfer 3 different volumes of the sample solution into three different 50 mL volumetric flasks. Add 5 mL buffer solution, 1 mL hydroxylammoniumchloride solution and 1 mL phenanthroline solution into each volumetric flask. Afterwards fill up the volumetric flasks with deionized water. The pH value should be between 3.4 and 5.5. 20 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 3.4.9 Measurement of the stock solutions and the sample solution Prior the measurements warm up the Philips photometer for at least 30 minutes. Measure the solutions with the according to the manual/ the instructions of the advisor. Use a wavelength of 513 nm. Before you measure the samples and serial dilution set the photometer to zero with the blank solution. Then measure the stock solutions with increasing concentration and note the absorption values. Measure the prepared samples, control that the sample values are in the calibration range. Table 9: List of measured absorption values for the standard and sample solutions. Standard/sampl
e-­‐ID conc. [mg/L] Abs. 1 Abs. 2 Abs. 3 average Questions and tasks for the protocol: 1
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Create the calibration curve using an excel chart, specify the linear equation and the correlation coefficient. Determine the unknown iron concentration of the sample using the calibration equation. Consider the dilution factor. Calculate the average, SD and RSD of the experiment. Evaluate the experiment, what was the best sample amount in your case? What are possible sources of errors? 21 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 4 Appendix In Table 10 the conversion factors f for the first experiment are shown. Table 10: Conversion of volume and concentration depend on the working temperature ϑ [°C] and the calibration [1-­‐3] temperature at 20°C.
Temperature [°C] Correction factor (f) Error in % (when no change of the temperature is observed) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 0,9989 0,9988 0,9987 0,9987 0,9986 0,9986 0,9986 0,9986 0,9986 0,9987 0,9987 0,9988 0,9989 0,9990 0,9991 0,9992 0,9994 0,9995 0,9997 0,9998 1,0000 1,0002 1,0004 1,0006 1,0008 1,0010 1,0013 1,0015 1,0018 1,0020 1,0023 1,0026 1,0029 1,0032 1,0035 1,0038 0,11 0,12 0,13 0,13 0,14 0,14 0,14 0,14 0,14 0,13 0,13 0,12 0,11 0,10 0,09 0,08 0,06 0,05 0,03 0,02 0,00 0,02 0,04 0,06 0,08 0,10 0,13 0,15 0,18 0,20 0,23 0,26 0,29 0,32 0,35 0,38 22 Applied Chemistry Analytical Chemistry pre-­‐lab WS 2013/14 University of Siegen 5 References [1] Home Page of Eurachem, <http://www.eurachem.ul.pt/> (accessed February 2014). [2] Ellison, S. L. R., Rosslein, M., Williams, A. Quantifying Uncertainty in Analytical Measurement, 2nd ed.; EMPA: Switzerland, 2000. [3] Bievre, P. de, Böttger, D., Eastwood, C. The Fitness for Purpose of Analytical Methods: A Laboratory Guide to Method Validation and Related Topics, 1st ed.; LGC: England, 1998. 23