330 WATER HYGIENE 
3. Potassium bichromate solution. Dissolve 0.025 gm. of potassium 
bichromate and 0.1 c.c. of concentrated sulphuric acid in distilled water and 
dilute the solution to 200 c.c. 
Procedure.—Mix 1 c.c. of the tolidin reagent with 50 c.c. of the sample in 
a Nessler tube and allow the solution to stand at least five minutes. Small 
amounts of free chlorine give a yellow and larger amounts a yellow-green color. 
For quantitative determination, comparison of the color is made with that 
of standards in similar tubes prepared from the solutions of copper sulphate and 
potassium bichromate. The amounts of solution for various standards are 
indicated in Table XXVIII. 
Concentrations greater than 0.10 part per million of chlorine require use of a 
stronger bichromate solution, containing 0.25 gm. of potassium bichromate and 
1 c.c. of concentrated sulphuric acid dissolved in distilled water and diluted to 
200 c.c. The proper amounts of solution for stronger permanent standards are 
given in Table XXVIII. 
TaBLeE XXVIII 
PREPARATION OF PERMANENT CHLORINE STANDARDS FOR LOW 
CHLORINE CONTENT 
Potassium Bichrom- 
. . Copper Sulphate 
Value in Chlorine : ate Solution 
olution, 
Parts per millaon c 
C. | C.C 
0 OL 00 0.8 
.02 0 2.1 
.03 .Q 3.2 
04 .O 4.3 
.05 A 5.5 
06 8 6.6 
07 1.2 7.5 
.08 15 8.7 
.O9 1.7 9.0 
wl 1.8 10.0 
DissoLvVED OxYGEN 
Reagents.—1. Sulphuric acid. Concentrated (sp. gr., 1.83-1.84). 
2. Potassium permanganate. Dissolve 6.32 gms. of the salt in water and 
dilute the solution to 1 liter. 
3. Potassium oxalate. A 2 per cent solution. 
4. Manganous sulphate. Dissolve 480 gms. of the salt in water and dilute 
the solution to 1 liter. 
5. Alkaline potassium iodide. Dissolve 700 gms. of potassium hydroxide 
and 150 gms. of potassium iodide in water and dilute the solution to 1 liter. 
6. Hydrochloric acid. Concentrated (sp. gr. 1.18~1.19). - 
