In waters that have been treated with calcium hypochlorite or liquid
chlorine it is frequently advisable to ascertain the presence or absence
of chlorine. As the reagents which have been proposed for its detection
are not specific for chlorine but give similar or identical reactions
with oxidizing agents or reducible substances care must be exercised in
interpreting the results of such tests: nitrites and ferric salts are of
common occurrence, and chlorates also may lead to misinterpretation in
waters treated with calcium hypochlorite.
_Reagents._—1. Tolidin solution. One gram of o-tolidin, purified by
being recrystallized from alcohol, is dissolved in 1 liter of 10 per
cent hydrochloric acid.
2. Copper sulfate solution. Dissolve 1.5 grams of copper sulfate and 1
cc. of concentrated sulfuric acid in distilled water and dilute the
solution to 100 cc.
3. Potassium bichromate solution. Dissolve 0.025 gram of potassium
bichromate and 0.1 cc. of concentrated sulfuric acid in distilled water
and dilute the solution to 100 cc.
_Procedure._—Mix 1 cc. of the tolidin reagent with 100 cc. of the sample
in a Nessler tube and allow the solution to stand at least 5 minutes.
Small amounts of free chlorine give a yellow and larger amounts an
orange color.
For quantitative determination compare the color with that of standards
in similar tubes prepared from the solutions of copper sulfate and
potassium bichromate. The amounts of solution for various standards are
indicated in Table 13.
Table 13.—PREPARATION OF PERMANENT STANDARDS FOR CONTENT OF CHLORINE.
───────────────────────┬───────────────────────┬───────────────────────
Chlorine. │ Solution of copper │ Solution of potassium
│ sulfate. │ bichromate.
───────────────────────┼───────────────────────┼───────────────────────
_Parts per million._ │ _cc._ │ _cc._
│ │
0.01│ 0.0│ 0.8
.02│ .0│ 2.1
.03│ .0│ 3.2
.04│ .0│ 4.3
.05│ .4│ 5.5
.06│ .8│ 6.6
.07│ 1.2│ 7.5
.08│ 1.5│ 8.7
.09│ 1.7│ 9.0
.10│ 1.8│ 10.0
.20│ 1.9│ 20.0
.30│ 1.9│ 30.0
.40│ 2.0│ 38.0
.50│ 2.0│ 45.0
───────────────────────┴───────────────────────┴───────────────────────