LAB:
ACID – BASE TITRATION
AIM
Confirm a known concentration of a solution of sodium hydroxide by titration against a standard solution
of potassium hydrogenphthalate, C8H5O4K.
INTRODUCTION
You have already made a standard solution of potassium hydrogenphthalate, a primary standard and acid.
Potassium hydrogenphthalate has the formula C8H5O4K, but we can simplify the formula to HA. This simple
formula is often used to represent an acid with a complicated structure.
HA (aq) + NaOH (aq)  NaA (aq) +
H2O (l)
acid
base
salt
water
The acid reacts with the base/alkali sodium hydroxide to produce a salt and water. This type of chemical
reaction is called a neutralization reaction. To indicate when the reaction between the two colorless
solutions is complete (called the stoichiometric point or equivalence point) the indicator phenolphthalein is
used. Phenolphthalein is colorless in acid and pink in base/alkaline solution. The point at which the
addition of one drop (or even less) changes the solution from colorless to just faintly pink is called the endpoint and, in this case, shows that the reaction is just complete.
ASSESSMENT
Data Collection and Processing, Conclusion and Evaluation, and Manipulative Skills
APPARATUS AND MATERIALS
 Filter funnel, small
 Burette, 50 cm3, and stand
 Two beakers, 100 cm3
 Sodium hydroxide solution (0.10 M NaOH )
 Pipette, 25 cm3 and Pipette filler
 Standard potassium hydrogenphthalate solution
 Four conical flasks, 250 cm3
 Bottle of phenolphthalein indicator solution
 Wash bottle of distilled water
SAFETY
Sodium hydroxide solution is very corrosive. Even when dilute it can damage your eyes. Therefore you
must wear safety glasses and an apron throughout the experiment.
PROCEDURE
1. Using the funnel, rinse the burette with a little sodium hydroxide solution and then fill it with the
same solution. Record the initial burette reading.
2. Using a pipette filler, rinse the pipette with a little potassium hydrogenphthalate solution and
carefully transfer 25.0 cm3 of the solution to a clean dry 250 cm3 conical flask. Add two to three
drops of the phenolphthalein indicator solution.
3. Run sodium hydroxide solution from the burette into the flask, with swirling, until the solution
just turns pink. This first titration may be used as a trial run, because you will probably overshoot
the end-point. Record the final burette reading.
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Initial volume ±0.05 cm3
Final volume ±0.05 cm3
Volume of NaOH required ±0.1
cm3
Trial 1
Trial 2
Trial 3
0.00
2.70
1.00
42.25
42.70
41.50
42.20
40.05
40.50
4. Refill the burette with the sodium hydroxide solution, and again record the initial burette reading
to the nearest 0.05 cm3 (one drop).
5. Using the pipette, transfer 25.0 cm3 of the potassium hydrogenphthalate solution to another
clean conical flask. Add two to three drops of the phenolphthalein solution.
6. Carefully titrate this solution to the end-point, adding the sodium hydroxide drop-by-drop when
you think the color is about to change.
7. Repeat steps 5, 6 and 7 at least twice more or until consecutive results within 0.10 cm3
achieved.
8. Empty the burette and wash it carefully immediately after the titration, especially if it has a
ground glass tap. Rinse out all the other glassware with water and leave to dry. Finally wash your
hands.
9. Calculate the mean of the two (or preferably three) closest consecutive (concordant) volumes of
sodium hydroxide and quote this also to the nearest 0.05 cm3.
PRECISION
Precision refers to how closely a set of measurements agree with one another, in other words is a
measure of the reproducibility of a result. Random errors arise from imprecise measurements, leading to
the burette readings being above or below the “true” value. They can be minimized by increasing the
precision of the burette and doing repeated measurements (trials). Improve the precision of the burette,
by recording the readings to the nearest 0.05 cm3 (half of the 0.1 cm3 division). The reproducibility of the
volume measurements is increased through repeated measurements. Normally three consecutive
titrations should agree to within 0.10 cm3 and, strictly, you should repeat the titration until this is
achieved. However, you may not have the time to do this. With practice, your technique will improve so
that it is not necessary to do more than four titrations. The random error on the mean is  0.05 cm3. This
is more appropriate than adding the uncertainties, since that would be completely contrary to the purpose
of repeating measurements which is to reduce random error.
V(NaOH)mean = [23.50 ±0.05cm3 + 23.60 ±0.05cm3 + 23.70 ±0.05cm3]  3 = 23.60 ±0.05cm3
A more rigorous method for treating repeated measurements is to calculate standard deviations and
standard errors. These statistical techniques are more appropriate to large-scale studies with many
calculated results to average and not for a few trials.
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IB Internal Assessment Marking Rubric
Scoring rubric:
completely met = 2 ; partially met = 1 ; not met = 0
Designs a Method for Collecting Data
Sufficient data on the factor being investigated (independent variable) is collected. For example:
 Sufficient repeated measurements (trials) to improve the precision of raw data need to be
completed to calculate a mean.
 Trial runs are completed where necessary
 The need for consistent / concordant results in a titration
Data Collection & Processing
1. Recording Raw Data
Qualitative data contains sufficient detail on the chemically significant observations that enhance
the interpretation of the results. Look at the reactants and products in the chemical equation.
Describe the initial color and state of the reactants and the final color and state of the products.
Describe evidence of a chemical reaction occurring. These include: color change, change of state,
odor change, gas being produced (and the relative rate at which it is evolved), solid reactant
disappears, temperature change, solid is precipitated.
All units of measurement and absolute uncertainties are recorded. Column headings in tables are
clearly labeled and include units and uncertainties in measurement.
Raw quantitative data is presented in easily interpretable, organized and labeled table(s). A well
organized table will:
 borders/lines around text and numerical data
 not run over two pages
 columns to be compared placed next to one another
 be called a table and numbered consecutively
 have a concise descriptive title that relates the measured (dependent) and changed
(independent) variables on top of the table
 text and data centered
 rows and columns are evenly distributed
 11–12 point font size and a consistent font type for electronic tables
 correctly placed in document.
Consistent and correct use of decimal places and/or significant figures so that there is no
variation in the precision of the raw data. The level of precision should be consistent with that of
the raw data.
2. Processing Raw Data
Complete and correct quantitative analysis of the data is carried out. This could include
combining, manipulating raw data to determine the value of an answer, or taking the average of
several measurements and transforming the data into a form suitable for graphing.
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At least one sample per calculation showing the steps involved is shown. Identical calculations do
not need to be repeated.
For repeated trials, the final result for each trial is calculated. Average results are calculated
based on the final results of repeated trials. Exclusion of trials / collected data is stated and
justified.
3. Presenting Processed Data
Processed data and/or conclusions are summarized in an easily interpretable manner so that all
the stages to the final result can be followed.
Significant figures and units are used correctly in calculations and for presenting the numerical
results of processed data. One extra significant figure can be kept throughout the entire
calculation to reduce rounding errors. The final result is consistent with the number of significant
figures in the experimental measurements and any subsequent calculations based on them.
Uncertainties from the random errors are propagated through a calculation and expressed as
percentages and/or absolute uncertainty (the ± amount pertaining to the actual result).
Uncertainty should be estimated if only one measurement is done. The final total % uncertainty
should be cited to no more than one significant figure if it is greater than or equal to 2% and to no
more than two significant figures if it is less than 2%.
NOTE: Final average result of repeated trials is used for the calculation of experimental
uncertainty. The uncertainty on averages is the sum of the absolute uncertainties. Include any
other errors and uncertainties which may affect the validity of the final result. For functions such
as addition and subtraction absolute uncertainties can be added. For multiplication, division and
powers, percentage uncertainties can be added. If one uncertainty is much larger than others,
the approximate uncertainty in the calculated result can be taken as due to that quantity alone.
Conclusion and Evaluation
1. Conclusion
A statement is made that indicates whether the aim / purpose / research question were achieved.
This is stated explicitly and relates back to the details of the aim.
A valid interpretation of the results is made by describing observations and patterns revealed by
the data. The results and/or procedure are not restated but there a sound understanding of how
the procedure worked to give valid data.
The actual results are compared to the expected results (literature) by calculating the % Total
Error.
% Error = │(Experimental – Accepted) ÷ Accepted│ x 100
If appropriate the expected (from the hypothesis) and actual results are compared. It is stated
whether the results supported or disproved the hypothesis
Comment on the difference between the % Error and % Uncertainty.
If the % Error > % uncertainty (from random error), systematic errors are the main source of
error. If the % Error < % uncertainty, random errors are the main source of error. To overcome
this problem the experiment needs to be repeated more times.
Total systematic Error = % Error - % Uncertainty
A statement as to the level of confidence in the results is made.
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2. Evaluating the Procedure
Describe the sources of systematic error and/or random error encountered and difficulties with
the control of variables and how these affected the precision, accuracy and reproducibility of the
results. The sources of error should be obvious (not trivial) errors that can be eliminated. Include
any assumptions made which may affect the result. An assumption is a feature of the experiment
that you assume to be true but you do not or cannot test. Comment on the limitation of the
method chosen if appropriate.
Describe the effect of the systematic error on the magnitude and direction (result too high or too
low) on the final result. In other words did the systematic error cause the result to be lower or
higher than expected?
3. Improving the investigation
Suggest and explain feasible ways the investigation could be improved so the results will be closer
to what is expected. These modifications should address ways to reduce the systematic and
random errors described above, achieve better control of the variables, reduce approximations or
provide better procedures for measurement. In the graphical analysis many measurements of
the independent variable will help increase the precision of the data by ensuring that there are as
many points as possible are on or close to the line. Improvements should be specific (not vague),
realistic and lead to significant improvement in experimentation. They should not involve
unavailable equipment or materials. Completing more trials is only a feasible improvement if it
will improve the reliability of the results or reduce random errors. Obtaining better equipment,
more practice, avoiding spillages and spending more time on the experiment are not feasible
improvements. The best improvements are those that can be carried out with the current
schools resources.
Suggest a further question or an idea for an investigation you could explore that is
related to this concept.
General Directions
Ideas are expressed concisely. The reports is organized allowing for easy interpretation and
written using the passive voice. Instead of: We measured 50 ml of HCl (aq) acid in a 100 ml
cylinder, use: 50 ml of HCl (aq) was measured in a 100 ml cylinder. Pronouns [I, we, us, you] are
replaced with “the”. Chemical substances are named correctly and vague nouns like it, the
substance, they are avoided.
Author’s name, date practical work was completed in class and name of partner/s, IB SL/HL on
top right hand corner. Pages are numbered with 1.5 spacing between word processed text.
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