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
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
50°F, 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
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
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.
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 68°F (20°C). 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.
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
5. Compare with color standards as rapidly as
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
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
CALCULATION OF RESULTS.In calculat-
ing 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 differenceequals total
(OT) - (B-2) = total residual chlorine
Free available chlorine. From the value of A,
subtract the value of B-l. The difference equals free