Alkalinity
Introduction
Alkalinity is defined as a measure of the buffering capacity of water to neutralize strong acid. This
capacity is attributed to bases that are present in natural waters including OH-, HCO3-, and CO32-.
More alkalinity in your water sample means more buffering capacity of your water sample.
In order to determine Alkalinity of your samples, samples will be titrated with sulfuric acid or
hydrochloric acid to a certain pH end point (pH 8.3 for Phenolphthalein Alkalinity and pH 4.5 for
Total Alkalinity), and the volume of the acid used for the titration will be recorded.
The following reactions are occurring during titration;
pH range above 8.3
OH- + H+ → H2O
CO32- + H+ → HCO3pH range between 8.3 and 4.5
HCO3- + H+ → H2CO3
If carbonate (CO32-) is present in your sample, CO32- will consume one H+ when the solution is
titrated to pH 8.3, and it will consume another H+ during further titration from pH 8.3 to pH 4.5. If
the volume of acid required to get pH 8.3 is equal to the volume of acid used from pH 8.3 to pH 4.5,
your sample contained only CO32- (no OH-) as the major alkalinity component. If the pH of your
sample is below 8.3, and a certain amount of acid is required to reach pH 4.5, your sample contained
only HCO3- (no OH- and CO32-). If your sample requires a certain amount of acid to reach pH 8.3,
but no acid is required from pH 8.3 to pH 4.5, your sample contained only OH-.
Calculation
Phenolphthalein Alkalinity
= Amount of acid used to reach pH 8.3 (ml) * Normality of acid (eq/L) * 100,000 (mg CaCO3/eq) / sample volume (ml)
Where Normality = Molarity (moles/L) * the number of hydrogen exchanged in a reaction (eq/moles)
Ex. 2.0ml of 0.01M of sulfuric acid was required to titrate 50ml of your sample to pH 8.3. What is
the Phenolphthalein Alkalinity?
Phenolphthalein Alkalinity = 2.0 (ml) * 0.02 (eq/L) * 100,000 (mg CaCO3/eq) / 50 (ml) = 80 (mg/L)
Total Alkalinity
= Amount of acid used to reach pH 4.5 (ml) * Normality of acid (eq/L) * 50,000 (mg CaCO3/eq) / sample volume (ml)
Ex. 18.0ml of 0.01M of sulfuric acid was required to titrate 50ml of your sample to pH 4.5. What is
the Total Alkalinity?
Total Alkalinity = 18 (ml) * 0.02 (eq/L) * 50,000 (mg CaCO3/eq) / 50 (ml) = 360 (mg/L)
Procedure
1. Measure 50 ml or 100 ml of your sample into a 250 mL beaker or erlenmyer flask. Place your
sample onto a stir plate (make sure to put a bar magnet in the flask).
2. Measure initial pH of your sample. If the sample pH is below 8.3 (if above 8.3, do step 3 first),
add several drops of bromcresol green indicator. If the color of the solution turned blue, titrate your
sample with 0.02 N H2SO4 or HCl (you may need to dilute the acid provided in the lab) until the
color changes to yellow (pH 4.5). Record the total volume of acid used for the titration.
3. Measure initial pH of your sample. If the sample pH is above 8.3, add several drops of
phenolphthalein indicator. If the color of the solution turned pink, titrate your sample with 0.02 N
H2SO4 or HCl (you may need to dilute the acid provided in the lab) until color changes from pink to
clear (pH 8.3). Record the volume of acid used for the titration. Then, proceed with step 2.
4. Calculate both Phenolphthalein Alkalinity and Total Alkalinity using the formula provided above.
Reference
Snoeyink V. and Jenkins. D. 1980. Water Chemistry. JOHN WILEY & SONS, New York. ISBN
0-471-05196-9