_Reagents._—1. Dilute sulfuric acid. Dilute 1 part of concentrated
sulfuric acid with 3 parts of distilled water and free the solution from
oxidizable matter by adding potassium permanganate until a faint pink
color persists after the solution has stood several hours.
2. Standard ammonium oxalate. Dissolve 0.888 gram of the pure salt in 1
liter of distilled water. One cc. is equivalent to 0.1 mg. of oxygen. An
equivalent quantity of oxalic acid or sodium oxalate may be used.
3. Standard potassium permanganate. Dissolve 0.4 gram of the
crystallized salt in 1 liter of distilled water. Add 10 cc. of the
dilute sulfuric acid and 10 cc. of this solution of potassium
permanganate to 100 cc. of distilled water, and digest 30 minutes. Add
10 cc. of the ammonium oxalate solution, and then add potassium
permanganate till a pink coloration appears. This destroys the
oxygen-consuming capacity of the water used. Now add another 10 cc. of
ammonium oxalate solution and titrate with potassium permanganate.
Adjust the potassium permanganate solution so that 1 cc. is equivalent
to 1 cc. of ammonium oxalate solution or 0.1 mg. of available oxygen.
_Acid digestion._—Place in a flask 100 cc. of the water, or, if the
water is of high organic content, a smaller portion diluted to 100 cc.
Add 10 cc. of sulfuric acid solution and 10 cc. of standard potassium
permanganate and digest the liquid exactly 30 minutes in a bath of
boiling water the level of which is kept above the level of the contents
of the flask.[70][71a] If the quantity of permanganate is insufficient
for complete oxidation repeat the digestion with a larger quantity; at
least 5 cc. excess of the standard permanganate should be present when
the ammonium oxalate solution is added. Remove the flask, add 10 cc. of
the ammonium oxalate solution, and titrate with the standard
permanganate until a faint but distinct color is obtained. If 100 cc. of
water is used the number of cubic centimeters of potassium permanganate
solution in excess of the number of cubic centimeters of ammonium
oxalate solution is equal to parts per million of oxygen consumed.
If oxidizable mineral substances, such as ferrous iron, sulfide, or
nitrite, are present in the sample corrections should be applied as
accurately as possible by suitable procedures. Direct titration of the
acidified sample in the cold, using a three-minute period of digestion,
serves this purpose quite well for polluted surface waters and fairly
well for purified sewage effluents. Few raw sewages containing no trade
wastes need such a correction, but raw sewages containing “pickling”
liquors do need it. If the sample contains both oxidizable mineral
compounds and gaseous organic substances the latter should be driven off
by heat and the sample allowed to cool before applying this test for the
correction factor. If such corrections are made the fact should be
stated with the amount of correction.
_Period and temperature of digestion._—As the practice in regard to the
period and temperature of digestion has varied widely it is difficult to
compare the results obtained at one laboratory with those obtained at
another. None of the methods gives absolute results. They are all
relative[26][29][57] at best. Digesting 30 minutes at the boiling
temperature is herein designated the recommended method. If samples are
analyzed by any other method the method should be noted, and,
representative results by the standard method should be placed on record
for purposes of comparison.