(OT) - (B-2) = total residual chlorine

Free available chlorine. From the value of A, subtract the value of B-1. The difference equals free available chlorine.

(A) - (B-1) = free available chlorine.

Combined available chlorine. From the value of total residual chlorine, subtract the value of free available chlorine. The difference represents combined available chlorine.

(Total residual chlorine) - (free available chlorine) = combined available chlorine.

pH TEST

The pH test measures the strength of acid or alkali in water. It is reported on a scale that ranges from 0 to 14. A pH reading of 7 is neutral (in a technical sense), values below 7 are acidic, and those above 7 are alkaline. Color comparators can be used to find pH by methods similar to those for determining chlorine residuals. The determination of pH in three simple operations is shown in figure 7-15.

Indicators

Many pH indicators are available, each with a limited range. The indicators used for treated water supplies are as follows:

The correct standards must be used with each indicator.

Procedures

In making the pH test, proceed as follows:

1. Fill the tubes to the mark with the sample.

2. Add the indicator to one tube in the amount specified by the manufacturer.

NOTE: Usually 0.5 ml (10 drops) for a 10-ml sample tube and proportionately more for larger tubes.

3. Mix and place the tube in the comparator.

Figure 7-15. - pH determinations in three steps: A. Add reagent; B. Remove tube; and C. Compare colors.

4. Match for color and read the pH directly.

5. If the color matches the standard at either the upper or the lower end of the range of the indicator, repeat the test with the next higher or lower indicator. For instance, if bromthymol blue is used and the sample matches the blue color of the 7.6 standard, the pH is 7.6 or higher. Therefore, use a phenol red indicator to check this value.