_Additional reagents._—1. Potassium iodide solution. Ten per cent
solution, free from iodate.
2. Standard sodium thiosulfate. Dissolve 1.0 gram of the pure
crystallized salt in 1 liter of distilled water. Standardize this
solution against the standard potassium permanganate. As the thiosulfate
solution does not keep well determine its actual strength at frequent
intervals.
3. Starch indicator. Prepare as directed in the section on dissolved
oxygen (pp. 65–66).
4. Sodium hydroxide solution. Dissolve 1 part of pure sodium hydroxide
in 2 parts of distilled water.
Certain widely practiced deviations from the standard procedure just
described are noted in the following paragraphs.
1. Heat the acidified sample to boiling, add the permanganate solution,
and digest for two minutes[16] at boiling temperature. This procedure is
facilitated by agitating the liquid constantly with a small current of
air to guard against bumping.
2. Same method as No. 1 except that the period of digestion is five
minutes.[121a]
3. Same method as No. 2 except that the permanganate solution is added
to the acidified sample when cold, and digestion is continued five
minutes after the sample reaches the boiling point. The advantage of
this method is that there is included the oxygen-consuming power of the
volatile matter present in some sewages and sewage effluents, which is
driven off by heat and thus escapes when the test is made in accordance
with procedures 1 and 2.
4. Same method as No. 3 except that the period of digestion is 10
minutes.[63][68c]
5. Digestion of the sample after the acid and permanganate solutions are
added is carried out abroad, especially in England, at approximately the
room temperature,[24a][69a][94f][100a] apparently to guard against
decomposition[17] of permanganate in the presence of high chloride, for
periods of three minutes, fifteen minutes, and four hours; many
observers record the oxygen consumed after all three periods, while some
record the result only for the four-hour period. At the end of the
period of digestion, add 0.5 cc. of potassium iodide solution to
discharge the pink color; mix; titrate the liberated iodine with
thiosulfate until the yellow color is nearly destroyed, then add a few
drops of starch solution and continue titration until the blue color is
just discharged. The number of cubic centimeters of potassium
permanganate solution in excess of the number of cubic centimeters of
sodium thiosulfate solution is equal to parts per million of oxygen
consumed.
6. Digestion in alkaline solution[104] is preferable to digestion in
acid solution for brines or waters high in chlorine. Place in a flask
100 cc. of the sample, or if it is of high organic content a smaller
portion diluted to 100 cc. Add 0.5 cc. of sodium hydroxide solution and
10 cc. of standard potassium permanganate and digest exactly 30 minutes.
Remove the flask, add 5 cc. of sulfuric acid and 10 cc. of the standard
ammonium oxalate, and titrate with the standard potassium permanganate
as in the acid digestion.