Alkalinity and pH

Alkalinity and pH are related, but this relationship is not linear. We often focus on pH as the indicator of the tendency for metals to corrode, for example, but ignore the role alkalinity plays in the corrosion process. Hence, it’s a good idea to have a better understanding of the relationship between alkalinity and pH.

What is alkalinity?

Alkalinity is derived from the presence of bicarbonate (HCO3), carbonate (CO3) and/or hydroxide (OH) anions. In addition, dissolved carbon dioxide (CO2) plays a role in the carbonate alkalinity relationship. These substances are collectively referred to as total alkalinity.

Various chemicals are used to add alkalinity. Sodium bicarbonate (baking soda), sodium carbonate (washing soda) and sodium hydroxide (caustic soda) are readily available, low cost chemicals commonly used in many water treatment applications.

When one or more of these chemicals is added to water, they react to produce a pH that is determined by the equilibrium that exists between the various alkalinity components. These equilibrium reactions have been defined and can be measured in the lab. Rather than deal with the rigorous complex equilibrium relationships, however, water chemists have developed a simple method for measuring the amount and type of alkalinity present in any given water sample.

P and M alkalinity

Alkalinity is easily measured by titration with a standard solution of N/50 sulfuric acid using two color indicator solutions: Phenolphthalein and Methyl Purple. Phenolphthalein changes color at pH 8.3 and Methyl purple at pH 4.3. The titration test is named P alkalinity and M alkalinity by reference to the indicators used for each titration. (Other indicators are often used in place of Methyl purple such as Bromcresol green, but the M alkalinity designation remains)

During the titration, N/50 sulfuric acid is added slowly from a buret. This neutralizes the alkalinity and decreases the pH. At the endpoint of the P alkalinity titration, Phenolphthalein changes color from pink to clear, if P alkalinity is present. Methyl purple is added at the end of the P alkalinity test and the titration continued. As more N/50 is added, the pH decreases to pH 4.3 at which point the M alkalinity indicator color change takes place. (Some take the M endpoint at pH 4.5, but this is a slight variation from the norm and not significant for our discussion.)

Alkalinity does not exist below a pH of 4.3. pH values below this point are an indication of the presence of free mineral acidity (FMA).

It takes one part (one equivalent) of acid to react with one part of alkalinity. By measuring the amount of acid added during the titration, you can calculate the amount of alkalinity neutralized. And by using the P and M alkalinity determinations, you can also distinguish the type of alkalinity present as bicarbonate, carbonate and hydroxide.

P and M Alkalinity Relationships

Using the P and M alkalinity values determined by titration with N/50 sulfuric acid, you can determine the type of alkalinity present as hydroxide, carbonate or bicarbonate.

Condition

Hydroxide

Carbonate

Bicarbonate

P = 0

None

None

M

P = M

M

None

None

P = 1/2 M

None

M

None

P < 1/2 M

None

2P

M – 2P

P > 1/2 M

2P – M

2(M – P)

None

You will note that all three forms of alkalinity cannot exist at the same time. You can have one or two out of three, but not all three.

Generally, natural water supplies have bicarbonate alkalinity, but no carbonate or hydroxide. Boiler water, however, contains hydroxide and carbonate alkalinity, but no bicarbonate alkalinity.

A General Relationship Between Alkalinity and pH

Alkalinity and pH are best measured separately by titration and pH meter. However, if this is not possible, the following table shows a general relationship between alkalinity and pH. In this case, the P and M values are used to determine the percent P alkalinity of the total (M) alkalinity. For example, when P = 20 and M = 100, 20 divided by 100 yields 20% P alkalinity.

P as % of M alkalinity

pH (approx)

0

8.30

5

8.80

10

9.10

15

9.35

20

9.55

25

9.65

30

9.75

35

9.85

50

9.95

As you will note, total alkalinity alone (i.e. bicarbonate alkalinity) does not produce a pH above 8.3. P alkalinity (carbonate alkalinity) is required to increase the pH above 8.3. At pH values above 10, caustic alkalinity is present.

Summary

Water contains various forms of alkalinity such as bicarbonate, carbonate or hydroxide. The equilibrium relationship between the froms of alkalinity determines the pH. Generally, the higher the alkalinity content, the higher the pH.

The amount and type of alkalinity present is determined by direct titration with N/50 sulfuric acid, which gives the P and M alkalinity values. This, in turn, can be used to estimate the pH.

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