ENGINEERING SCIENCE DEPARTMENT
Water and Environmental Technology Program
Name _________________
SPRING TERM
Aquatic Chemistry II, WQT-134
Laboratory #2
Alkalinity Modified from STM 2320
Applied Water and Spentwater Manual Chapter 13
Purpose: In this lab you will perform an alkalinity test on a water/waste water sample. “The
alkalinity of a water or wastewater sample is a measure of its capacity to neutralize acids.
The alkalinity of natural waters is due primarily to the salts of weak acids, although weak
or strong bases may also contribute. Bicarbonate ions (HCO3-) represent toe major form of
alkalinity (Operation of Wastewater Treatment Plants. Chapter 16. Laboratory procedures and chemistry).” It is expressed in
mg/L as equivalent CaCO3.
“Alkalinity contributes to the properties of wastewater, many of which also affect
biological processes (such as nitrification) and chemical reactions. Alkalinity is the
measure of a solution’s capacity to react with a strong acid (usually sulfuric acid H2SO4)
to a predetermined pH. The alkalinity of a solution is usually made up of carbonate,
bicarbonate, and hydroxides. Similar to acidity, the higher the alkalinity is, the more
neutralizing agent is needed to counteract it. In general, a treatment plant and its
collection system operates better with wastewater lower in acidity and higher in alkalinity
(http://www.lagoonsonline.com/laboratory-articles/acid.htm).”
The capability of a water to resist changes in pH is referred to its buffering capacity. It is the
sum of all the titratable bases. Alkalinity of a water is its acid-neutralizing capacity. The
measured value may vary significantly with the end-point pH used. Alkalinity is a measure of an
aggregate property of water and can be interpreted in terms of specific substances only when the
chemical composition of the sample is known. Alkalinity is significant in many uses and
treatments of natural waters and wastewaters. Because the alkalinity of many surface waters is
primarily a function of carbonate, bicarbonate, and hydroxide content, it is taken as an
indication of the concentration of these constituents. The measured values also may include
contributions from borates, phosphates, silicates, or other bases if these are present. Alkalinity in
excess of alkaline earth metal concentrations is significant in determining the suitability of a
water for irrigation. Alkalinity measurements are used in the interpretation and control of water
and wastewater treatment processes. Raw domestic wastewater has an alkalinity less than, or
only slightly greater than, that of the water supply. Properly operating anaerobic digesters
typically have supernatant alkalinities in the range of 2000 to 4000 mg calcium carbonate
(CaCO3)/L.
Factoids/Conversions
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Properly operating anaerobic digesters typically have supernatant alkalinities in the range
of 2000 to 4000 mg calcium carbonate (CaCO3)/L.
Commonly done to assess performance of a wastewater digestor: A ratio of 10:1 acidity
to alkalinity = good digestor, 5:1 ratio= bad digestor.
Important for coagulation/flocculation in plant. Rule of thumb: 34.2 mg/l of aluminum
sulfate requires about 17 mg/l of available alkalinity to form a good floc.
Advantages
 Very useful/common procedure in wastewater analysis and irrigation water analysis
Disadvantages/Interferences
 Variable sample volume that requires a procedural trial and error
 Analyze quickly within a few hours of sample collection
 Soaps, oily matter, suspended solids, or precipitates may coat the glass
electrode and cause a sluggish response.
 Allow additional time between titrant additions to let electrode come to equilibrium or
clean the electrodes occasionally.
 Do not filter, dilute, concentrate, or alter sample.
Sample collection: According to EPA sampling and preservation guidelines, samples for both
acidity and alkalinity measurements can be collected in a borosilicate glass or plastic bottles and
stored at 4 degrees Celsius for up to fourteen days. However, it’s always best to analyze fresh
samples collected with no headspace.
SUPPLIES/SAFTEY Please wear lab coats, safety goggles, and gloves at all times for this lab
procedure.
REAGENTS/STORAGE:
Sodium carbonate solution, approximately 0.02N: Dry 2 g primary standard Na2CO3
at 250°C for 4 h and cool in a desiccator. Weigh 1.060 ± 0.2 g (to the nearest mg), transfer to a 1L volumetric flask, fill flask to the mark with distilled water, dissolve and mix reagent. Do not
keep longer than 1 week.
Standard sulfuric acid or hydrochloric acid, 0.1N
Add 16.6 ml of 6 N sulfuric acid to ~700 ml of DI water in a 1000 ml volumetric flask. Bring to
volume with DI water.
Standard sulfuric acid, 0.02N: Dilute 200.00 mL 0.1000N standard acid to 1000 mL with
distilled or deionized water.
Dechlorinating reagents: A number of dechlorinating reagents may be used to remove residual chlorine
prior to distillation. Sodium thiosulfate (1/70 N): Dissolve 3.5 g Na2S2O3 5H2O in distilled water and
dilute to 1 liter. One mL of this solution will remove 1 mg/L of residual chlorine in 500 mL of sample.
Indicators
Bromcresol green
Mixed bromcresol green-methyl red indicator solution starts at blue green and goes to reddish
pink at pH 4.5:
Dissolve 100 mg bromcresol green sodium salt and 20 mg methyl red sodium salt in 100
mL distilled water.
*Phenolphthalein indicator solution, alcoholic, pH 8.3 indicator.
Lab Materials:
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Use any commercial pH meter or electrically operated titrator that uses a glass electrode
and can be read to 0.05 pH unit. Standardize and calibrate according to HACH
manufacturer’s instructions (note: use pH 7 first , then pH 4). Check calibration
efficiency by re-measuring pH 7 indicator solution.
Titration vessel: 250 ml beaker
Magnetic stirrer
Pipets, volumetric
Flasks, volumetric, 1000-, 200-, 100-mL
Burets, borosilicate glass, 50-, 25-, 10-mL
Polyolefin bottle, 1-L.
SUMMARY PROCEDURE
Modified from Alkalinity STM 2320 using (Applied Water and Spentwater
Manual)
Alkalinity Measurement: Hydroxyl ions present in a sample as a result of dissociation or
hydrolysis of solutes react with additions of standard acid. Alkalinity thus depends on the endpoint pH used. Determine sample alkalinity from volume of standard acid required to titrate a
portion to a designated pH. Look for a color change from pink to clear using the phenolphthalein
indicator at pH 8.3. Hydroxides and half of the carbonate alkalinity are neutralized
simultaneously using phenolphthalein or P-alkalinity. The procedure assumes:
1) hydrogen ions do not exist as a major component
2) carbonate is half titrated at the phenolphthalein endpoint
3) carbonic acid (H2CO3) is the major species remaining in solution at the methyl orange
endpoint (pH 4.3).
Titrate at room temperature using a properly calibrated pH meter, electrically operated titrator, or
color indicators (Standard Methods for the Examination of Water and Wastewater, 20th Edition,
section 2320).
Note: If no indicators are present, but one has a calibrated pH meter, it can be used to determine
the amount of mL used for the titration of the alkalinity-causing ions. The sample must be in
constant motion (using a stir bar) and a pH electrode must be fully immersed in the solution. As
one adds acid from a burette the pH meter is observed and when the pH of 8.3 is reached, the
amount of acid used is recorded and calculated as P alkalinity, when pH 4.5 is reached, the
amount of total acid utilized for the entire procedure is noted and the total or T alkalinity is
calculated.
Step-by-step Procedure
Modified from Alkalinity STM 2320 using (Applied water and Spentwater
Part 1 Standardization of Titrant (Sulfuric acid) (Applied Water and Spentwater
manual Ch 13)
Manual, CH 13)
This step is not needed, we bought 0.02 N Sulfuric acid from Hach!
1. Add 20 ml of 0.02 N sodium carbonate and add 5 drops (0.2 ml) of Mixed bromcresol
green-methyl red indicator solution to a clean 250 ml beaker (titration vessel) that
contains a magnetic stir bar on a stir plate.
2. Place 25 ml of 0.02 N Standard sulfuric acid in a 25 ml buret. Add slowly until solution
changes from blue-green color to a reddish pink color and then terminate titration.
3. Record the volume of acid consumed to change color of the solution ________ ml.
4. Calculate normality of Standard sulfuric acid where:
Normality = A X B = __________ N sulfuric acid (should be 0.02 N)
C
A = mL of Na2CO3 solution (should be 20 ml)
B = Normality of the Na2CO3 solution (should be 0.02N ml)
C = mL of Standard sulfuric acid acid read from the burette.
Part 2 Titration for Total Alkalinity (Applied Water and Spentwater
Manual)
1. Measure pH of your sample. record value __________________
2. Add 200 ml of sample and add 5 drops (0.2 ml) of Phenolphthalein indicator pH 8.3 to a
clean 250 ml beaker (titration vessel) that contains a magnetic stir bar on a stir plate.
Position titration vessel below a 25 ml buret filled with 10 ml of standardized sulfuric
acid, 0.02N.
3. If no colors develops, the pH of the sample is probably less then 8.3 and the P-alkalinity
is zero.
4. If a pink color exists , then the pH is above 8.3 and the sample should be titrated slowly
with 0.02 N sulfuric acid, in 1 drop increments, till its colorless (the original sample
endpoint).
5.
Record the ml of 0.02 N acid that it took to change from pink to colorless. ____ ml
(use this value to determine p alkalinity)
P Alkalinity mg/L as CaCO3/L=
(ml of acid to 8.3 or colorless )(Normality)(50 mg/meq as CaCO3)(1000 ml)
ml of original sample volume
6. add 5 drops (0.2 ml) of Mixed bromcresol green-methyl red indicator solution to your
titration vessel.
7. Record the starting point of 0.02 N on the buret _________ ml
8. titrate slowly with 0.02 N sulfuric acid, in 1 drop increments, till the solution changes
from blue-green color to clear to a reddish pink color (i.e. pH 4.5) and then terminate
titration.
9. Record the end point of 0.02 N on the buret _________ ml
10. Determine the volume of acid consumed to change color of the solution ________ ml
(7-9).
11. Add ml from step 5 and step 10 together ___________ ml (use this value for T
alkalinity)
T Alkalinity mg/L as CaCO3/L=
(ml of acid to 8.3=colorless+ ml to reach pH 4.5=pink)(Normality)(50mg/meq CaCO3)(1000 ml)
ml of original sample volume
Calculations
Results: The results obtained from the phenolphthalein and total alkalinity determinations offer a
means for stoichiometric classification of the three principal forms of alkalinity present in many
waters. The classification ascribes the entire alkalinity to bicarbonate, carbonate, and hydroxide,
and assumes the absence of other (weak) inorganic or organic acids, such as silicic, phosphoric,
and boric acids. It further presupposes the incompatibility of hydroxide and bicarbonate
alkalinities. Because the calculations are made on a stoichiometric basis, ion concentrations in
the strictest sense are not represented in the results, which may differ significantly from actual
concentrations especially at pH > 10. According to this scheme:
1) Carbonate (CO32–) alkalinity is present when phenolphthalein alkalinity is not zero
but is less than total alkalinity.
2) Hydroxide (OH–) alkalinity is present if phenolphthalein alkalinity is more than
half the total alkalinity.
3) Bicarbonate (HCO3–) alkalinity is present if phenolphthalein alkalinity is less
than half the total alkalinity.
Table 1. Volume conversions for drops
20 drops ~1ml
10 drops ~0.45 ml
5 drops ~0.2 ml
2 drops ~0.10 ml
Standardization Calculations (Applied Water and Spentwater
Manual)
Example: suppose 20 ml of 0.02 N sodium carbonate took 23 ml of the acid titrant to reach the
brom cresol green end point. What was the normality of the titrant?
Normality = A X B = __________ N sulfuric acid
C
A = mL of Na2CO3 solution (should be 20 ml)
B = Normality of the Na2CO3 solution (should be 0.02N ml)
C = mL of Standard sulfuric acid acid read from the burette.
Normality = 20 ml X 0.02 N = _0.017_ N sulfuric acid
23 ml
Alkalinity Titration Calculations(Applied Water and Spentwater
Manual)
P Alkalinity mg/L as CaCO3/L=
(ml of acid to 8.3 or colorless )(Normality)(50 mg/meq as CaCO3)(1000 ml)
ml of original sample volume
T Alkalinity mg/L as CaCO3/L=
(ml of acid to 8.3=colorless+ ml to reach pH 4.5=pink)(Normality)(50mg/meq CaCO3)(1000 ml)
ml of original sample volume
Example Problem (using STM 2320 equations)
from Operation of Wastewater Treatment Plants. Chapter 16. Laboratory procedures and chemistry)
Normality = A X B = __________ N sulfuric acid (should be 0.02 N)
C
A = mL of Na2CO3 solution
B = Normality of the Na2CO3 solution
C = mL of Standard sulfuric acid acid read from the burette.
The results for the titration and test of normality for H2SO4 are as follows:
1. Weighed 2.502 g NaOH = A,
2. Used 40 ml of Na2CO3 = B
3. Titrated using 18.9 ml H2SO4
4.Sample Size , ml = 50 ml
Normality Calculations
5. Normality H2SO4= (2.502 g X 40.0 ml) = 0.10 N H2SO4
(50 X 18.9 ml)
A sample of wastewater plant effluent was collected and tested for alkalinity. The results are as
follows:
6. Sample size , ml = 100 ml
7. ml of Titrant used A = 20 ml
8. Acid Normality, N = 0.02 N H2SO4
Total Alkalinity mg/L as CaCO3/L= A X N X 50,000= 20 mL X 0.02 N X 50,000= 200 mg/L
ml of sample
100 ml
Environmental Chemistry II, - WQT 134
p-ALKALINITY & t-ALKALINITY
Facility Name
Sample Date
Sampled by
Sample Time
Analyzed by
Sample Type
Date
Sample Location
Method
mls of sample
Water Temp__________________
** phenolphthalein P alkalinity **
ml of standard acid
"p" alk. =
______
** total T alkalinity **
, normality ____N
mg/L as calcium carbonate
Sample Location
total ml of standard acid
"t" alk. =
, normality____N
mg/L as calcium carbonate
mls of sample
Water Temp__________________
** phenolphthalein P alkalinity **
ml of standard acid
"p" alk. =
, normality ____N
mg/L as calcium carbonate
Calculations:
N1 x V1
N2 =
V2
N2 = normality of standard acid
V2 = ml of standard acid used
N1 = normality of Na2CO3
V1 = ml of Na2CO3 solution
** total T alkalinity **
total ml of standard acid
"t" alk. =
, normality____N
mg/L as calcium carbonate
STM:Alkalinity, mg/L as CaCO3 = A x N x 50,000
ml sample
A = ml of standard acid used
N = normality of standard acid
P Alkalinity mg/L as CaCO3/L=
(ml of acid to 8.3 or colorless )(Normality)(50 mg/meq as CaCO3)(1000 ml)
ml of sample titrated
T Alkalinity mg/L as CaCO3/L=
(ml of acid to 8.3=colorless+ ml to reach pH 4.5=pink)(Normality)(50mg/meq CaCO3)(1000 ml)
ml of sample titrated