LAB # 2 MEASURING ACID PRECIPITATION
Written by Profs. Jonathan Gourley and David Henderson
Materials needed for this lab:
Lab notebook
Computer with Excel
Introduction
Please make sure you have read the background information on the course website (you should have
for the pre-lab). A supplementary handout with the same material will be made available in lab for
your reference.
Acid precipitation in the form of rain, snow, fog, and dry acidic deposition is a type of air pollution.
The series of exercises we will do this week include the following components:
1. Explore the differences between strong and weak acids using titration and pH measurements.
2. Measurement of pH and total acidity of rain and/or snow samples collected on campus and at the
Trinity College field station in Ashford, CT.
3. Analysis of the nitrate and sulfate composition of rain water.
Lab Procedures
There are three different procedures to work through for this lab. These can be done in any
order. You will work in pairs. Please rotate about the lab as needed to share equipment.
Organize all results in your lab notebooks. Everyone must wear safety glasses for this lab!
Safety glasses are very important for this lab.
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Experiment #1 – Measuring pH with digital probe
You will be provided with two unknown acids (A & B) and two rain samples (or one rain and one
snow). – Of the unknown acids, one is a strong acid and one is a weak acid. These acid standards are
of known concentration and are provided for practicing the techniques of titration and pH
measurement. The two rain samples were recently taken from here at Trinity from the top of the
Clement chemistry building.
The most accurate method of measuring pH is to use an electronic pH meter. The meter depends on
the availability of ions in the solution to carry charge in the electrochemical system.
Remember: contamination can ruin the results for everyone. Work cleanly and carefully.
Measure the pH of unknown acids
a. Test the pH of the samples using the graphical procedure for the IQ150 pH meter (see
attached). Make sure the pH readout is set for one decimal place. Carefully construct a clear
table in your lab notebook that compiles all of your readings. After calibration, take three trial
readings for each sample. The average of these trials will be used for later calculations.
NOTE!!! pH cannot be averaged directly. You must first convert to free hydronium ions see
below), average those values and then convert back to pH.
The probe reading stablizes when pH stops blinking. Between each trial re-check the buffer
solutions (pH 4 and 7) to make sure the probe is still calibrated. A good reading should fall
within + 0.2 pH. IMPORTANT: ALWAYS rinse probe in DDI water between testing of
each solution to avoid contanimation. Start with the pH measurements of the rain
samples and then move to the unknow acids. The rain samples have low ionic strength
and may be contanimated by the unknow acids. If your readings are more than .2 from
each other and/or the probe behaves erratically please let me know.
b. Recall that pH is defined as pH = -log [H3O+]. Use the pH measured for your samples to
calculate the molar concentration of free hydronium ions [H3O+].
c. You will record your results in an MS Excel data file. You will do this AFTER experiments #2
and #3. Do all calculations in Excel including the pH to hydronium ion conversion.
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Trinity College
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Experiment #2 – Measuring total H+ ion concentration using Titration
Unknown Acids Titration
1. Measure and transfer 25 ml of the sample to a 250ml Erlenmeyer titration flask.
2. Add one reagent pillow of indicator (phenolphthalein) to the sample. Phenolphthalein is
colorless in acid and changes to pink when all acids have been reacted with base. Be sure that
the indicator is completely dissolved.
3. Take the HACH Digital Titrator 1.600 N NaOH titrant tube and a dispenser tip out of the case.
Carefully remove the cap from the titrant and insert the dispenser tip.
4. Insert the titrant into the digital titrator. Depress the plunger into the titrant tube until it touches
the bottom of the plug in the titrant tube. Then turn the dispenser knob over a waste beaker
until a few drops of titrant emerge from the tip. BE VERY CAREFUL WHERE YOU
POINT THE TIP, IT MAY SPRAY TITRANT. SODIUM HYDROXIDE CAUSES
SEVERE BURNS. WASH THOROUGHLY WITH WATER IF YOU GET THIS
SOLUTION ON YOUR SKIN OR CLOTHES.
5. Rinse the tip with deionized water and zero the digital readout. Place the tip in the titration flask
and titrate the acid to the first very faint pink endpoint. Use a white background to observe the
titration so that you can see this pale color change. Read the digital readout at the endpoint and
record the number of digits in your lab notebook. Pour waste in waste container and rinse out
your flask with deionized water.
Analysis: complete the following calculations using Excel after you have finished with titrating
both the unknown acids and the rain samples (see below) and your station is clean.
6. Using the CLASS AVERAGE RESULTS from the titrations, determine the total moles of acid
in your sample using: HACH Digital Titrator 800 digits = 1 ml. Use the N number printed on
the titrant bottle (1.600 for this example). Use the following equation:
= total
digits 
1ml
1L
1.600moles


800digits 1000ml
1L
moles of acid in sample
7. Determine the concentration of your sample in moles/liter. Use the following equation to get the
concentration in terms of moles of acid/liter sample [H+].
moles  of  acid
x

25ml
1000ml
x = moles/L (solve by cross-multiplying)
Determine acid strength
Compare the concentration of free hydronium ions in solution that you calculated using pH
with the molar concentration of total acid you calculate from the titration data. Calculate
the ratio using the formula below. This comparison will give an idea of the strength of the
acids. Weak acids give ratios less than 0.04. Strong acids will give larger ratios.
H O 

3
moles / L  acid
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Titration of Rain Samples
Measuring total H+ ion concentration using Titration
a. Measure and transfer 50 ml of the sample to the Erlenmeyer titration flask.
b. Add one reagent pillow of indicator (phenolphthalein) to the sample. Phenolphthalein is colorless in acid
and changes to pink when all acids have been reacted with base. Be sure that the indicator is completely
dissolved.
c. Take the HACH Digital Titrator 0.16 M NaOH titrant tube. and a dispenser tip out of the case.
Carefully remove the cap from the titrant and insert the dispenser tip. NOTE THAT THIS TUBE IS A
DIFFERENT CONCENTRATION OF BASE THAN WAS USED FOR THE UNKNOWN
ACIDS.
d. Insert the titrant into the digital titrator as described above.
e. Rinse the tip with deionized water and zero the digital readout. Place the tip in the titration flask and
titrate the acid to the first very faint pink endpoint. Use a white background to observe the titration so
that you can see this pale color change. Read the digital readout at the endpoint and record the number
of digits in your lab notebook. Rinse out your flask with deionized water.
f. Repeat the titration 2 more times for each sample (use new sample each time), and take an average of the
3 readings (digits). Rinse the tip between each measurement with deionized water into the waste
beaker.
g. Cleanup titration station. Remove the tip from the titrant tube and replace the cap. Discard the titration
tip. Put the HACH titrator and reagents back in the case. Make sure your station is clean and dry before
taking out a laptop.
Analysis: all results and calculations should be done MS Excel after cleanup.
h. Using results from the titration, determine the total moles of acid in your sample using: HACH Digital
Titrator 800 digits = 1 ml. Use the N number printed on the titrant tube (0.16 for this example). Use the
following equation:
digits 
1ml
1L
0.16moles


800digits 1000ml
1L
= total moles of acid in the sample
i. Determine the concentration of your sample in moles/liter. Use the following equation to get the
concentration in terms of moles of acid/liter sample [H+].
moles  of  acid
x

50ml
1000ml
x = moles/L
Determine acid strength
Calculate the ratio of the concentration of H3O+ (as determined by pH measurement) divided by the
concentration of acid (as determined from titration). Determine whether your rain sample suggests
a strong acid or a weak acid.
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Experiment #3 - Determination of Nitrate and Sulfate Ion Content in Rain by Ion
Chromatography
As noted above, nitrate and sulfate ions can contribute to the acidity of rain. The EPA specified method for
analysis of anions like nitrate and sulfate is Ion Chromatography (IC). The technique was invented by chemists
at Dow Chemical and is now widely used for anion and cation analysis.
What is Chromatography?
Chromatography is a method for separating ions or molecules. In chromatography, a sample is placed on a
column packed with a porous solid support material. A liquid or gas is forced through the column and moves
the sample with it. Some means is provided to detect the sample molecules as they reach the end of the column
(the technical term is elute). If the solid support was completely inert, all sample molecules should elute at the
same time. However, if the support has some specific attraction for one type of sample molecule or ion, that
type will elute more slowly and will be separated from those molecules or ions that are less attracted to the
stationary support material. The more time a specific type of molecule or ion spends attached to the stationary
phase, the longer it will take to elute from the column. In some cases, chromatography allows separation of
several hundred different types of molecules with varying affinities for the stationary phase in the column.
In this experiment, the sulfate and nitrate ions are injected into a special column that attracts negative ions. The
most common method in IC is to use changes in the electrical conductivity of the eluting solution. The more
ions present, the greater the conductivity.
Ion Chromatography: Analysis of Rain (these steps may already be have done for you)
1.
Rinse a plastic syringe with 2 portions of your rain sample (1-2 ml each) into a waste beaker
2.
Draw 2 ml of rain into the syringe.
3.
Place a syringe filter on the syringe (filters are in the large plastic jar)
4.
Press the plunger to expel 1 ml of sample into the waste beaker.
5.
Press the plunger to place 1 ml of sample in a clean autosampler vial.
6.
Place a cap on the vial and seal it with the crimping tool.
7.
Label the vial with the date and location of the rain sample.
During lab, all the rain samples will be analyzed using the IC equipment. Prof. Gourley will run the EPA
certified standard solutions and then the rain/snow samples. He will provide the data for you to construct the
calibration curve and you will be provided with printouts of the data for the standards and your sample. Peaks
will be labeled and the table will provide the concentration of each ion in your samples. Use Excel to prepare a
calibration curve for chloride, nitrate and sulfate using concentration and peak area for all of the certified
standards. Obtain the equation for each calibration line using the best fit line function in Excel. Determine the
concentration of the chloride, nitrate and sulfate ions in the rain sample by substituting the peak area
(conductivity) of the rain sample into the equation and calculating the concentration.
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What is due for the Acid Rain Lab?
1. Introduction – A short (1-2 pages) summary of the experiments. Include who you worked with.
2. results and analyses of pH and titrations (unknown acids and rain samples) in Excel spreadsheet –
please organize your calculations and results in a clearly labeled table. Make sure to clearly show a
comparison between the two unknown acids and the two rain samples.
3. Excel file containing IC data and graphs showing calibration curves, one each for Cl- , NO3- and
SO4-2.
a. Using the calibration data provided graph the calibration curve for the three anions. Graph ppm
vs. conductivity for each anion (ppm should be on the x axis and conductivity should be on the
y axis). The conductivity values represent the area under the appropriate peak for each
standard.
b. Obtain the equations for each line. To do this, click on each data series, then under the Layout
tab of Chart tools click on “trendline” and make sure the box “Display equation on chart” under
“options” is checked.
c. Determine the concentrations of the anions in your rain sample by plugging in the conductivity
values given for each anion and solving for concentration (ppm). Recall that that your line
equations are in the form Y= mx+b and you will need to solve for x in each case.
4. Discussion (1/2 to 1 page)
a. Which is the strong acid and which is the weak acid, of unknown acids A and B.
b. Briefly discuss the comparative acidity and major anions of the two rain samples (or snow and
rain). How do you explain the difference in acidity of rain at the two sites? If you only
complete one rain sample, just briefly discuss the major anions found in your sample and
explain their sources.
All lab reports will be due in HARDCOPY form at the start of next week’s lab period. Email files WILL NOT
be accepted.
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