DETERMINATION OF COMPOSITION OF A CARBONATE –
BICARBONATE MIXTURE: METHOD OF ALIQUOT AND pH
TITRATION
BACKGROUND
Crude sodium carbonate, called soda ash, is commonly used as a commercial
neutralizing agent. The titration with standard acid to the bromocresol green end point gives
the total carbonate content. The results are usually expressed as the weight percent of
sodium carbonate in the crude sample. Since the samples are frequently non-homogeneous,
the method of aliquot portions is employed. In this experiment, the same basic principle is
used to find the percent composition of a mixture of sodium carbonate, sodium bicarbonate
in unknown sample. This is done in two ways: acid-base titration using two different
indicators and the pH titration using a pH meter. You are then asked to compare the two
methods.
PROCEDURE
PART I: Standardization of HCl
Follow the attached procedure for the standardization of HCl.
PART II: Analysis of a mixture of carbonate and bicarbonate
Follow attached procedures (from Harris web site).
OR,
PART II: Method of aliquots in the determination of percent sodium carbonate in an
unknown mixture
Weigh precisely into a clean 250-ml beaker a 5.0 g sample of the dried (110 oC for
one hour followed by cooling in a desiccator) unknown. Dissolve the sample in about 125
ml of distilled water. Place a clean funnel in a 250 ml volumetric flask and transfer the
solution from the beaker to the flask. Rinse the beaker, add the rinsate to the flask, and
finally dilute to the calibration mark with distilled water. Mix the contents of the volumetric
flask by inversion and shaking.
Pipet a 25 ml aliquot into a 125 mL Erlenmeyer flask and add 3-5 drops of
bromocresol green. Titrate with standard hydrochloride acid (prepared in part I) until the
indicator turns from blue to green. Then boil the solution to expel CO2. The solution should
return to blue color. Carefully add HCl from the buret until the solution turns green again.
Repeat the titration with at least two other 25 ml aliquots. At the end of the titration clean
the volumetric flak thoroughly. An alkaline solution should never be left in a volumetric
flask for a long period of time (why?). Transfer the solution into a plastic bottle for later use.
Report the mass percentage of sodium carbonate (Na2CO3) in the unknown. A
precision of 3 to 5 parts per thousand is not unusual for this analysis.
PART III. Analysis of a mixture of carbonate and bicarbonate by pH Titration.
The unknown soda ash is titrated with standard HCl using a potentiometric (pH) end
point measured with a pH meter and combination electrode. The indicator (bromocresol
green) end point results are compared with potentiometric end point results to observe the
clarity and variability of various techniques. The relevant reactions and end point changes are
as follows:
CO32- + H+  HCO3-
(First end point, pH = 8.3)
HCO3- + H+  H2O + CO2
(Second end point, pH = 4.5)
The purpose of the first titration is to locate approximately each of the two
equivalence points in the titration of carbonate ion with HCl. The second, or final, titration
is used to determine the second equivalence point in the titration.
Manual Titration Procedure
First, calibrate the pH meter using two standard pH buffers (pH 4.00 and pH 7.00). Transfer using a
volumetric pipet a 25.00 mL aliquot of the sample soda ash solution to a 250 mL beaker containing a
magnetic stirring bar. Add approximately 25 mL of distilled water (Of which the exact volume is not
important. Why?) to the beaker. Place the beaker on a stirrer, immerse the pH electrode, and start the
stirrer being careful not to touch the electrode to the stirring bar. Titrate with standard HCl, taking
both pH and mV readings every 2.00 mL. Plot a curve (using any spreadsheet software) of pH
versus volume of HCl added and locate the approximate end points (both of them) on the graph.
For the final titration, after transferring a 25.00 mL sample aliquot from the original sample solution
and diluting with 25 mL of deionized water, take pH readings every 5 mL to within 3 mL of the
second equivalence point (including both sides of equivalence point). Then take readings every 0.50mL interval. Near the equivalence point, take readings as quickly as possible because the pH will tend
to drift as carbon dioxide escapes from the solution. Enter your data into a spreadsheet and set up
data entry as mL HCl added in one-column and mV readings in the other.
Repeat the titration focusing at the second end point. From 0.9Ve2 to Ve2, add HCl in 0.5 mL
increments.
Automated Titration
Pipet 5.0 mL of the unknown solution and add 25.0 mL of water in the titration beaker. Prime the
pump with the standard HCl. Measure pump flow rate by delivering a known volume of HCl in a
10.0 mL measuring cylinder for a fixed time. Assemble the titration setup and ready for the program
run. Ask your instructor to help you in this step. The computer controlled pH measurement system
must be started the same time the pump is on. Record the pH until it drops to a stable acidic range.
The real time titration graph shows the two end points and the end of the titration. The titration
data- pH vs. time is recorded in XL format. You must convert the time to volume of HCl added by
the flow rate. A typical auto-titration plot is shown in the following figure.
pH vs. Volume
12.00
pH Data
First derivative
10.00
8.00
pH
6.00
4.00
2.00
0.00
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
-2.00
Volume (ml)
Gran Plot
A Gran plot will be used to graphically determine the equivalence points from the
titration data. This will be discussed in class. Other descriptions of plotting potentiometric
end points are provided in the textbook. Be sure and review these pages before you plot
your data according to the Gran function. Use the titration data at preceding and up to the
equivalence point (i.e., 0.9Ve to Ve) for the Gran plot. The acid dissociation equilibrium
 1
 -
pH
x A  ( V e2 - V H )
V H x 10 = 
 K a1  HA 
equation for the protonation of carbonate and bicarbonate ions can be used to derive the
Gran function below (read Harris web book!).
Where, VH = volume of HCL added; Ka1 = acid dissociation constant for first dissociation of
carbonic acid;  = activity coefficients; Ve2 = second equivalence volume; HA and A are the
conjugate acid base pairs, respectively. The above equation is given for the second end point
region. The fist equivalence point can be obtained similarly by using the data near the first
end point region. Which Ka value did you get from this region? Convince yourself by writing
appropriate equilibrium expression. For this experiment you can assume the activity ratio as
unity.
A plot of the Gran plot of VH x 10pH vs. VH for titration volumes near and up to the
equivalence point provides a straight line which is extrapolated to the x-axis to find Ve. You
should only consider the straight line portion to find the slope and the intercept. Calculate
Ka from the slope of the Gran plot. A typical Gran plot is shown below.
Gran Plot
1.00E+10
9.00E+09
8.00E+09
V. 10^pH
7.00E+09
6.00E+09
5.00E+09
4.00E+09
3.00E+09
2.00E+09
1.00E+09
0.00E+00
1.80
2.30
2.80
Volume HCl, mL
3.30
The Gran plot shown here was
done with the above titration data
in the region of first infection point.
This region should be selected such
that it covers data points from the
foot to the peak of the second
derivative (as your guide). Then
chose the linear portion to
extrapolate volume on the x-axis as
shown by the straight line.
Alternatively, you can use ‘trend
line’ to calculate the volume only on
the straight-line portions of the
data. You can find the Ka1 of
carbonic acid from the slope which
is = -1/Ka1.
DATA AND RESULTS
1. This is a composite lab. A complete lab report should be submitted with the following
information.
2. Present your data for the standardization of HCl. You also need to use the NaOH
previously standardized.
3. Present your data for the indicator based titration of the mixture. Use the attached
example calculation to calculate the percent composition. If a group is using the same
unknown, then you must present all the data of the group and calculate the average,
relative standard deviation and 95% . Make sure to have all the raw data from your group
members.
4. pH titration: Obtain the pH titration XL file. Plot the pH-volume, the first derivative,
and the Gran plots. Find the exact titration inflection points and the volume of HCl
from the Gran plots. You must do two Gran plots: one for the first inflection region and
the other for the second inflection region.
5. Also, obtain the Ka value from the Gran plot. Compare the Ka1 and Ka2 with those of the
literature.
6. Calculate the percent composition (and errors) of the mixture by using the inflection
volumes obtained from Gran plots. Example calculations are shown in the following
pages.
7. Finally, compare and contrast the two methods as an analytical tool for this analysis.
QUESTIONS
(Submit answers to the following questions with your lab report)
1. Explain why is it necessary to expel CO2 near the end of the titration?
2. The alkalinity of natural waters is usually controlled by OH-, CO32-, and HCO3-, which
may be present singularly or in combination. Titrating a 100.0 mL sample to a pH of 8.3
requires 18.70 mL of a 0.0281 M solution of HCl. A second 100.0 mL aliquot requires
48.20 mL of the same titrant to reach pH of 4.5. Calculate the concentrations of CO32and HCO3- in ppm. (Alkalinity is a measure of water's ability to neutralize acid)
NOTES


With appropriate planning of the work, the sample should be placed in the oven
overnight for proper drying.
Be extremely careful in the weighing and sample transfer operations because any error
here has a grave consequence. The results of the subsequent determination will be
incorrect even if the results of repeated titrations of aliquots of the sample solution agree
well.
Calculation Procedure
TITRATION OF A MIXTURE OF CARBONATE AND BICARBONATE
Part II. Titration using visual indicators
Find the volume of HCl for Phenolphthalein end point: Vp
Let MHCl = molarity of HCl and MOH = molarity of NaOH
Volume of NaOH added = Va
mols of bicarbonate = Va x MOH - Vp x MHCl
[HCO3-] = mols of HCO3- / Volume of aliquot
Calculate average, standard deviation and 95%
Find volume of HCl required for end point with Bromocresol Green (BG): V b
The BG end point indicates the complete conversion of carbonate and bicarbonate to
carbonic acid (pH about 4-5). We know
mols of CO32- / mols of HCl = ½
mols of HCl = 2 mols of CO32-
Total mols of HCl used = (Vb x MHCl) = mols HCl used for HCO3- + mols HCl used for CO32Vb x MHCl = mols of bicarbonate + 2 . mols of carbonate
Since mols HCO3- is known from the previous calculation, mols of CO 32- can be calculated.
[CO32- ] = mols of CO32- / Volume of aliquot
Calculate average, standard deviation and 95%
Calculate alkalinity, Alkalinity = 2 [CO32- ] + [HCO3-]
molarity
Calculate error in alkalinity based on standard deviations of [CO 32- ] and [HCO3-]
(Do error propagation)
Calculate wt% of CO32- and HCO3Find mols of CO32- and HCO3- in original sample from molarity
Find gms of CO32- and HCO3- in original sample from mols
Find wt% of CO32- and HCO3- in original sample,
= (g found x 100) / g sample taken
Example Calculation
You are given a mixture containing NaHCO3 and Na2CO3. You took 3.5000 g of the sample and
dissolved to 250.0 mL in a volumetric flask with deionized water. A 25.0 mL aliquot of the sample
required 36.0 mL of standard 0.100 M HCl to reach the Bromocresol green end point. Another
25.0 mL aliquot was mixed with 50.0 mL of standard 0.100 M NaOH. Then the solution is treated
with BaCl2 to precipitate carbonate as BaCO3(s) and immediately titrated with standard 0.100 M
HCl to phenolphthalein end point. The volume of HCl required was 40.0 mL. Calculate the wt% of
NaHCO3 and Na2CO3 in the mixture.
Vp = 40.0 mL
Va = 50.0 mL
mmols of bicarbonate = 50.0 x 0.100 - 40.0 x 0.100 = 1.00 mmols
mg of NaHCO3 = 1.00 mmol x 84 mg/mmol = 84.0 mg (in 25.0 mL aliquot)
[HCO3-] = 1.00/ 25.0 = 0.04M
Vb = 36.00 mL
36.00 x 0.1000 = 2 * mmols of carbonate + mmols of bicarbonate
mmols of carbonate = 0.5 (3.6 - 1.00) = 1.30
mg of Na2CO3 = mmols of carbonate x MW of Na2CO3
= 1.300 mmol x 106 mg/mmol = 137.8 mg (in 25.0 mL aliquot)
[CO32-] = 1.300 mmols / 25.00 mL = 0.052 M
Wt% Na2CO3 = 137.8*100/350.0 = 39.4%
Wt% NaHCO3 = 84.0*100/350 = 24.0 %
Part III: pH titration of CO32- and HCO3- mixture
Use the final pH titration data for the following calculation
 Plot mV vs. Volume of HCl added
 Take first derivative of mV-mL data and find the volume of HCl at two inflection points: V3 and
V4
 Do Gran plot: Find precise V4, Ka1 and their errors from linear regression. Compare Ka1 with
that of literature value ( Ka1 = 4.45 x 10-7 for H2CO3 == H+ + HCO3- )
 Use V3 from derivative and V4 from Gran plot to calculate CO32- and HCO3- as follows:




mols of CO32- = V3 x MHCl
mols of HCO3- = (V4 - 2V3) x MHCl
Calculate the molarity of CO32- and HCO3- in the original solution: mols/volume of aliquot
used. Also, calculate the alkalinity. Compare the results of the two methods.
Calculate the wt% of CO32- and HCO3- in the original mixture. Compare the results with that of
the indicator titration method.