NORTHWESTERN LAKES OF THE UNITED STATES. 
59 
Measure 25 cc. of N/100 K 2 Cr 2 0 7 with a burette or a corrected pipette into 
the No. 4 casserole. Add 1 cc. of potassium hydroxide-potassium iodide solution 
and 2 cc. of concentrated hydrochloric acid. Titrate until a faint yellow remains, 
add starch, and continue until the blue just disappears. The number of cc. of 
thiosulphate required to titrate 25 cc. of N/100 K 2 Cr 2 0 7 is used directly in the 
formula for calculating the oxygen (p. 60) . 
5. Potassium bichromate N/100 . — Weigh 0.4903 g. of chemically pure K 2 Cr 2 0 7 , 
which has been dried at 130° C. for 30 minutes; dissolve it in distilled water and 
make up to 1 liter. One cc. of this solution is equivalent to 0.00008 g. or 0.055983 
cc. of oxygen at 0° and 760 mm. This solution is very stable if evaporation is 
prevented. No appreciable change could be detected in a solution that had been 
mixed for two years and shipped from Socorro, N. Mex., to Spokane, Wash., and 
return. 
6. Starch solution . — Mix 1 g. of potato starch with 25 cc. of cold water, then 
pour into 200 cc. of boiling water, boil a few minutes, let settle, and use clear solu- 
tion when cold. This solution will keep only a few days even if chloroform is added. 
During the last two summers Low’s starch was substituted for the above. A 
sufficient amo u nt was prepared before starting for the summer, and it did not 
change with age and shipping. The end point was as good or better than the 
regular starch solution, and a great deal of time was saved in the field. Low’s starch 
is prepared as follows (Low, 1919) : 
Make a cold saturated solution of commercial sodium chloride in distilled water and filter it. To 
500 g. of this solution add 100 cc. of 80 per cent acetic acid and 3 g. of starch. Mix cold. Boil until 
nearly clear, about two minutes. Add a little water to replace that lost by boiling, perhaps 25 cc. 
A true solution of all starch is thus obtained. No filtering or settling is required, and the solution may be 
cooled and used at once. It keeps indefinitely and gives sharper end points than ordinary starch liquor. 
METHOD OF PROCEDURE. 
The sample of water, about 250 cc., collected as described on page 57, is care- 
fully opened and 1 cc. of manganese chloride added to the lower part by means of a 
pipette. One cc. of the potassium hydroxide-potassium iodide solution is then 
added in a similar manner, using care not to stir the contents of the bottle. The 
stopper is then closed and the contents thoroughly mixed by shaking, after which 
they are allowed to settle. The bottle is then opened, and 2 cc. of concentrated 
hydrochloric acid are added to the center of the bottle by means of a pipette, after 
which the bottle is closed and shaken. The samples were treated this far as soon 
as possible after collecting, and usually the remainder of the process was com- 
pleted at once, but in a few cases it was necessary to keep them over night before 
titrating. When the precipitate is dissolved, or nearly so, the contents of the 
bottle are poured into a No. 4 casserole and titrated with standard sodium thiosul- 
phate, using starch, as the yellow color fades, to determine the end point. 
When potassium hydroxide is added to the sample containing manganese 
chloride, white manganous hydroxide, Mn (OH) 2 , is precipitated. If there is no 
dissolved oxygen present, it remains white. If the sample contains dissolved 
oxygen, it oxidizes the manganous hydroxide, forming higher oxides or hydroxides, 
which are brown. All of the oxygen is thus taken up by the manganous hydroxide. 
