manufacturer of the comparator used. The general procedure consists of four steps as follows:

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

2. Place the compensating tube or tubes behind the color standard

3. Add 10 to 15 drops of orthotolidine to the test sample; mix and let stand for 5 minutes. Keep the sample in the dark during the 5-minute color development to reduce false color caused by manganese and nitrate compounds. If a blue color appears after 5 minutes, add more orthotolidine. If water is colder than 50F, warm it by holding the tube in your hand. Place the sample tube in the comparator and match the color, holding the comparator toward the light, preferably daylight.

4. Select the standard color nearest that of the sample and read the residual. If color appears to be halfway between two standards, report the residual as an average of two standards. Thus, if color appears halfway between 0.3 and 0.4 ppm, report the residual as 0.35 ppm.

Note that the orthotolidine test is not wholly accurate because the false color introduced by nitrates and manganese compounds cannot be entirely eliminated. However, the orthotolidine-arsenite test, described in the following section, does eliminate the false color error completely.

Orthotolidine-Arsenite Test

The orthotolidine-arsenite (OTA) test permits the measurement of relative amounts of total residual chlorine, free available chlorine, and combined available chlorine. This test has some limitations. Samples containing a high proportion of combined available chlorine may indicate more free available chlorine than is actually present, while samples containing a low proportion of combined available chlorine may indicate less free available chlorine than is actually present. Precise results depend on strict adherence to the conditions of the test. The conditions are the time intervals between the addition of reagents and the relative concentration of free available chlorine and combined available chlorine in the sample and the temperature of the water. The temperature of the sample under examination should never be above 68F (20C). The precision of the test increases with decreasing temperature.

EQUIPMENT. - You will need a color and turbidity-compensating residual chlorine comparator with commercial permanent standards.

REAGENTS. - The reagents used are orthotolidine (OT) and arsenite (A).

TEST PROCEDURES. - In testing, follow the procedures outlined below.

1. Label three comparator cells A, B, and OT.

2. Use 0.05 milliliter (ml) of OT reagent for each ml of the sample taken. For example, use 0.5 ml of OT reagent for a 10-ml sample and 0.75 ml for a 15-ml sample. Use the same volume of a reagent as is specified above for OT reagent.

3. To tube A, first add OT reagent, then add a measured volume of the water sample; mix quickly.

4. Within 5 seconds, add arsenite reagent; mix quickly.

5. Compare with color standards as rapidly as possible.

6. Record the result; the value obtained represents free available chlorine and interfering colors.

7. To tube B, first add arsenite reagent; then add a measured volume of water sample; mix quickly.

8. Immediately add OT reagent; mix quickly.

9. Compare with color standards as rapidly as possible.

10. Record the results as the B-1 value.

11. Compare the color standards again in exactly 5 minutes and record the results as the B-2 value; these values represent the interfering colors present in the immediate reading B-1 and in the 5-minute reading B-2.

12. To tube OT, containing orthotolidine reagent, add a measured volume of the water sample.

13. Mix quickly and compare with color standards in exactly 5 minutes.

14. Record the result; the value obtained represents the total residual chlorine present and total interfering colors.

CALCULATION OF RESULTS. - In calculating results of the orthotolidine-arsenite (OTA) test, follow the procedure below.

Total residual chlorine. From the value of OT, subtract the value of B-2. The difference’equals total residual chlorine.

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

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