25 May 2005
5. The P alkalinity is zero. The M alkalinity is due to bicarbonates only. No
hydroxyl or carbonate alkalinity are present.
Meaning of pH. The pH (value) is a measure of the acidity or alkalinity of
water. The test specifically measures the concentration of hydrogen ions in the water.
Neutral water will have a pH of 7. Water with a value of less than pH 7 is considered
acidic, while water with a value greater than pH 7 is considered alkaline. Examples are
Common pH values for materials on the acid side include: spinach with a
pH of approximately 5.5; orange juice and most soft drinks with a pH of
approximately 3.5; lemon juice with a pH of approximately 2.2; and 0.1
normality (N) sulfuric acid with a pH of 1.2.
Common pH values of materials on the alkaline side include: sodium
bicarbonate with a pH of approximately 8.4; milk of magnesia with a pH of
approximately 10.5; household ammonia with a pH of approximately 11.5;
and 0.1 N sodium hydroxide with a pH of 13.0.
The pH scale is logarithmic. A pH of 4 is 10 times as acidic as a pH of 5 and
100 times as acidic as a pH of 6. This is important to consider when adding a strong
acid to a system, because 0.1 N sulfuric acid will have more than 1 million times the
acidity of neutral water.
Relationship to Causticity. When the pH of water exceeds a value of 9.6
to 9.8, a measurable concentration of hydroxyl ions (OH-) is present in the water. As the
hydroxyl alkalinity (causticity) increases, the pH of the solution also increases. The
relationship between causticity and pH is shown in Table 6-5.