Water Chemistry Lab
014319
Lab #1: Characterization of acid base systems
Introduction
Characterization of acid-base systems in water which is governed by the carbonic
system requires the determination of two independent parameters, out of the following:
1. H2CO3* alkalinity.
2. Acidity or alkalinity with HCO3- as a reference species.
3. Total carbonate system species concentration, CT.
4. pH.
The most standard and common procedure is to measure pH and H 2CO3* alkalinity.
When pH measurement is unstable (e.g. CO2 is supersaturated, very low buffer
capacity etc.) the water can be characterized by two other parameters.
If another weak acid system is present in the water (such as the phosphate, acetic etc.)
its equilibrium coefficients need to be calculated and its total concentration has to be
analytically measured (PT for example) and it has to be considered when calculating
the alkalinity and/or pH. A common case is when characterizing wastewater.
Lab protocol
The lab is divided to two main sections:
1. In the first section you will need to produce a titration curve for phosphoric acid
(K2HPO4) by titration of HCl. You will be asked to extract the phosphate system
equivalent points and pK values (which can be extracted from the titration
curve). By knowing the equivalent point you can calculate the corresponding
alkalinity and total phosphoric acid system concentration (PT).
2. In the second section, two water samples will be characterized (synthetic tap
water and wastewater). The characterization will be conducted in different ways.
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In each case, two independent parameters will be measured and the other
parameters will be calculated based on the mathematical relations taught in the
course. In the case of wastewater, additional parameters will be given to you by
the lab TAs.
The types of water used in the second section:
1. Synthetic tap water, which is governed by the carbonic system only (HOCl / OClsystem is negligible). Synthetic tab water is tap water that was enriched with the
carbonate system, in order to elevate its alkalinity and acidity. It will be deoted
"tap water" in this lab protocol.
2. Filtered domestic wastewater with known total concentrations of phosphate
system and total concentrations of ammonia system.
Analyses performed on each water sample in the second section:
1. pH measurement and determination of alkalinity value (for example, in tap water
H2CO3* alkalinity is measured) by titrating to pH4.5~pH4.3.
2. pH measurement and determination of alkalinity value (for example, in tap water
H2CO3* alkalinity is measured) by Gran titration to three point below pH4 (do not
reduce the pH below pH3).
Analyses performed only on tap water sample in the second section:
1. Measuring acidity towards HCO3- by titrating to pH8.3 and H2CO3* alkalinity by
Gran titration.
Notice: when other weak acid systems are present in the water sample, on top of the
carbonic system, a suitable alkalinity equation should be used for the calculations.
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Lab protocol
Preparations:
Calibrate the pH meter with pH7 and pH4 buffer solutions.
First section

Every group will be given a 50 ml solution prepared by dosing K2HPO4 and
NaOH or HCl (at an unknown concentrations).

Each group will produce the titration curve. Titrate small amounts of HCl, wait
until the pH meter stabilizes and write the pH and the total volume of acid titrated
for each point (the titration curve is plotted based on from these values). You
should aim for pH steps of no more than 0.1 pH units.

By knowing the approximate values of the pK and equivalent points of the
phosphate system, you can guess when to titrate more or less HCl for a higher
resolution graph (based on the known buffer capacity).

The titration should end at pH3.

The titration curve should be presented on a graph where the X-axis is the acid
volume and the Y-axis is the pH.
Second section
1st method (repeat the analysis using tap water and wastewater)

Use a pipette to place 50 ml of sample in a beaker, add a stirring bar and set on
low stirring speed.

Write the initial pH. Stabilization of a well operating pH electrode takes about
half a minute. If this step takes too long, try changing the electrode or conclude
about the CO2 saturation value of the sample.
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Water Chemistry Lab
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
Start the titration while slowly stirring. When reaching pH5, start titrating of
smaller volumes of acid, until reaching pH4.3-4.5.

Write down the titrant volume needed and its concentration (usually 0.05 N)

Calculate the alkalinity using the following equation:
Alkalinity ( H CO * ) (mg / L as CaCO 3 ) 
2
3
Ve  C a
 50,000
Vs
(1)
Where: Ve is the HCl volume (ml); Ca is the HCl concentration (N); Vs is the sample
volume (ml).
This alkalinity definition is suitable for tap water, where the carbonic system is the only
weak acid system. For wastewater the calculation is similar but the alkalinity definition
is different.
2nd method (repeat the analysis using tap water and wastewater)

Use a pipette to place 50 ml of sample in a beaker, add a stirring bar and set on
low stirring speed.

Write the initial pH.

Start the titration, while slowly stirring, until reaching approximately pH4.

After the pH meter stabilizes, write down the values of pHx and Vx, and continue
for three to four more points below pH4.

Calculate Ve using the excel file for Gran titration, and calculate the alkalinity
using the previous equation (1).

You can combine both methods and analyses both alkalinities using the same
sample, i.e. continue lowering the pH to below pH4, after finishing the 1 st
method.
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3rd method (conduct the analysis using only tap water)
In this method the water is characterized by determining the Alkalinity (H2CO3*) and
acidity (HCO3-) without the need for knowing the pH.

Use a pipette to place 50 ml of sample in a beaker, add a stirring bar and set on
low stirring speed.

Start the titration to pH8.3 by adding strong base. If the initial pH is higher than
8.3 titrate strong acid (remember, Alkalinity (HCO3-) = -Acidity (HCO3-)).
Demonstration of CO2 effect on alkalinity and pH
CO2 will be added to a water sample, while the pH is measured.
Lab report
First section – plot the titration curve you got. Based on the curve, calculate:
a. The relevant equivalence point values.
b. The pK points
c. The alkalinity measured in the titration (towards the correct species).
d. PT concentration.
Mark the points on the graph.
Second section – answer the following questions in the lab report:
1. Write the alkalinity and acidity equations that you used.
2. Write all the combinations of two independent parameters that can be used for
characterizing tap water (carbonic system alone) in the lab.
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Water Chemistry Lab
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3. Develop the equations used for calculating alkalinity with Gran titration and
explain the difference between tap water and wastewater in that context.
Express all the reference species for the alkalinity of wastewater used with this
method.
4. Use the designated excel file for calculating the alkalinity with the Gran method.
Make sure you understand how this file is built.
5. For each of the methods and for both water samples, calculate the four
independent parameters and also the Acidity (CO32-) (i.e. complete the table in
the next page. Make sure you have the phosphate and ammonia total
concentrations given in the lab). Compare the different methods and analyze
the results. Explain why it is not customary to determine the value of acidity
(CO32-) by titration in the lab.
6. When bubbling CO2 into tap water and distilled water, what is expected to
happen to the pH? (Based on the demonstration). Evaluate approximately the
pH that can be reached in each case. Explain!
7. When bubbling CO2 to tap water, what will happen to the alkalinity and acidity
in its various forms (i.e. alkalinity and acidity with respect to different reference
species)?
8. When characterizing the wastewater sample, calculate the alkalinity of each of
the sub systems you measured. What is the percentage of each system to the
overall alkalinity? At what pH range does the ammonia system have any effect?
All calculations should be done without any neglections.
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Results table
Water
Method
sample
number
pH
Alkalinity(H2CO3*)
Acidity(HCO3-
Acidity(CO32-
CT
mg/l as CaCO3
Tap water
1
measured
Measured (direct to pH4.5)
calculated
calculated
calculated
Tap water
2
measured
Gran titration
calculated
calculated
calculated
Tap water
3
calculated
Gran titration
measured
calculated
calculated
wastewater
1
measured
*Measured (direct to pH4.5)
calculated
calculated
calculated
wastewater
2
measured
*Gran titration (measured)
calculated
calculated
calculated
* total alkalinity was measured, from which the carbonate alkalinity should be
calculated.
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