7. Sodium thiosulfate. A N/40 solution. Dissolve 6.2 grams of chemically
pure recrystallized sodium thiosulfate in water and dilute the solution
to 1 liter with freshly boiled distilled water. Each cc. is equivalent
to 0.2 mg. of oxygen or to 0.1395 cc. of oxygen at 0°C. and 760 mm.
pressure. Inasmuch as this solution is not permanent it should be
standardized occasionally against a N/40 solution of potassium
bichromate. The keeping qualities of the thiosulfate solution are
improved by adding to each liter 5 cc. of chloroform and 1.5 grams of
ammonium carbonate before diluting to the prescribed volume.
8. Starch solution. Mix a small amount of clean starch with cold water
until it becomes a thin paste and stir this mass into 150 to 200 times
its weight of boiling water. Boil for a few minutes, then sterilize. It
may be preserved by adding a few drops of chloroform.
_Collection of sample._—Collect the sample in a narrow-necked
glass-stoppered bottle of 250 to 270 cc. capacity. The following
procedure should be followed in order to avoid entrainment or absorption
of atmospheric oxygen. In collecting from a tap fill the bottle through
a glass or rubber tube extending well into the tap and to the bottom of
the bottle. To avoid air bubbles allow the bottle to overflow for
several minutes, and then carefully replace the glass stopper so that no
air bubble is entrained. In collecting from the surface of a pond or
tank connect the sample bottle to a bottle of 1 liter capacity. Provide
each bottle with a two-hole rubber stopper having one glass tube
extending to the bottom and another glass tube entering but not
projecting into the bottle. Connect the short tube of the sample bottle
with the long tube of the liter bottle. Immerse the sample bottle in the
water and apply suction to the outlet of the liter bottle. To collect a
sample at any depth arrange the two bottles so that the outlet tube of
the liter bottle is at a higher elevation then the inlet tube of the
sample bottle. Lower the two bottles, in any convenient form of cage
properly weighted, to the desired depth. Water entering during the
descent will be flushed through into the liter bottle. When air bubbles
cease rising to the surface raise the bottles. Finally replace the
perforated stopper of the sample bottle with a glass stopper in such
manner as to avoid entraining bubbles of air.
_Procedure._—Remove the stopper from the bottle and add, first, 0.7 cc.
of the concentrated sulfuric acid, and then 1 cc. of the potassium
permanganate solution. These and all other reagents should be introduced
by pipette under the surface of the liquid. Insert the stopper and mix
by inverting the bottle several times. After 20 minutes have elapsed
destroy the excess of permanganate by adding 1 cc. of the potassium
oxalate solution, the bottle being at once restoppered and its contents
mixed. If a noticeable excess of potassium permanganate is not present
at the end of 20 minutes, again add 1 cc. of the potassium permanganate
solution. If this is still insufficient use a stronger potassium
permanganate solution. After the liquid has been decolorized by the
addition of potassium oxalate add 1 cc. of the manganous sulfate
solution and 3 cc. of the alkaline potassium iodide solution. Allow the
precipitate to settle. Add 2 cc. of the hydrochloric acid and mix by
shaking.
The procedure to this point must be carried out in the field, but after
the acid has been added and the stopper replaced there is no further
change, and the rest of the test may be performed within a few hours, as
convenient. Transfer 200 cc. of the contents of the bottle to a flask
and titrate with N/40 sodium thiosulfate, using a few cubic centimeters
of the starch solution as indicator toward the end of the titration. Do
not add the starch solution until the color has become faint yellow, and
titrate until the blue color disappears.
The use of potassium permanganate is made necessary by high nitrite or
organic matter. The procedure outlined must be followed in all work on
sewage and partly purified effluents or seriously polluted streams or
samples whose nitrite nitrogen exceeds 0.1 part per million. In testing
other samples the procedure may be shortened by beginning with the
addition of the manganous sulfate solution and proceeding from that
point as outlined, except that only 1 cc. of alkaline potassium iodide
need be added.
_Calculation of Results._—Oxygen shall be reported in parts per million
by weight. It is sometimes convenient to know the number of cubic
centimeters per liter of the gas at 0°C. temperature and 760 mm.
pressure and also to know the percentage which the amount of gas present
is of the maximum amount capable of being dissolved by distilled water
at the same temperature and pressure. If 200 cc. of the sample is taken
the number of cubic centimeters of N/40 thiosulfate used is equal to
parts per million of oxygen. Corrections for volume of reagents added
amount to less than 3 per cent and are not justified except in work of
unusual precision. To obtain the result in cubic centimeters per liter
multiply the number of cubic centimeters of thiosulfate used by 0.698.
To obtain the result in percentage of saturation divide the number of
cubic centimeters of thiosulfate by the figure in Table 14 opposite the
temperature of the water and under the proper chlorine figure. The last
column of Table 14 permits interpolation for intermediate chlorine
values. At elevations differing considerably from mean sea level and for
accurate work attention must be given to barometric pressure, the normal
pressure in the region being preferable to the specific pressure at the
time of sampling. The term “saturation” refers to a condition of
equilibrium between the solution and an oxygen pressure in the
atmosphere corresponding to 158.8 millimeters, or approximately
one-fifth atmosphere. The true saturation or equilibrium between the
solution and pure oxygen is nearly five times this value, and
consequently values in excess of 100 per cent saturation frequently
occur in the presence of oxygen-forming plants.
Table 14.—SOLUBILITY OF OXYGEN IN FRESH WATER AND IN SEA WATER OF
STATED DEGREES OF SALINITY AT VARIOUS TEMPERATURES WHEN EXPOSED TO AN
ATMOSPHERE CONTAINING 20.9 PER CENT OF OXYGEN UNDER A PRESSURE OF 760
MM.[F]
(Calculated by G. C. Whipple and M. C. Whipple from measurements of C.
J. Fox.)[27][119]
─────────────┬────────────────────────────────────────────┬───────────
│ │Difference
Temperature. │ Chloride in sea water (milligrams per │ per 100
│ liter). │ parts of
│ │ chloride.
─────────────┼────────┬────────┬────────┬────────┬────────┼───────────
│ 0. │ 5000. │ 10000. │ 15000. │ 20000. │
─────────────┼────────┴────────┴────────┴────────┴────────┼───────────
_°C._ │_Dissolved oxygen in milligrams per liter._ │_Parts per
│ │ million._
0│ 14.62│ 13.79│ 12.97│ 12.14│ 11.32│ 0.0165
1│ 14.23│ 13.41│ 12.61│ 11.82│ 11.03│ .0160
2│ 13.84│ 13.05│ 12.28│ 11.52│ 10.76│ .0154
3│ 13.48│ 12.72│ 11.98│ 11.24│ 10.50│ .0149
4│ 13.13│ 12.41│ 11.69│ 10.97│ 10.25│ .0144
5│ 12.80│ 12.09│ 11.39│ 10.70│ 10.01│ .0140
│ │ │ │ │ │
6│ 12.48│ 11.79│ 11.12│ 10.45│ 9.78│ .0135
7│ 12.17│ 11.51│ 10.85│ 10.21│ 9.57│ .0130
8│ 11.87│ 11.24│ 10.61│ 9.98│ 9.36│ .0125
9│ 11.59│ 10.97│ 10.36│ 9.76│ 9.17│ .0121
10│ 11.33│ 10.73│ 10.13│ 9.55│ 8.98│ .0118
│ │ │ │ │ │
11│ 11.08│ 10.49│ 9.92│ 9.35│ 8.80│ .0114
12│ 10.83│ 10.28│ 9.72│ 9.17│ 8.62│ .0110
13│ 10.60│ 10.05│ 9.52│ 8.98│ 8.46│ .0107
14│ 10.37│ 9.85│ 9.32│ 8.80│ 8.30│ .0104
15│ 10.15│ 9.65│ 9.14│ 8.63│ 8.14│ .0100
│ │ │ │ │ │
16│ 9.95│ 9.46│ 8.96│ 8.47│ 7.99│ .0098
17│ 9.74│ 9.26│ 8.78│ 8.30│ 7.84│ .0095
18│ 9.54│ 9.07│ 8.62│ 8.15│ 7.70│ .0092
19│ 9.35│ 8.89│ 8.45│ 8.00│ 7.56│ .0089
20│ 9.17│ 8.73│ 8.30│ 7.86│ 7.42│ .0088
│ │ │ │ │ │
21│ 8.99│ 8.57│ 8.14│ 7.71│ 7.28│ .0086
22│ 8.83│ 8.42│ 7.99│ 7.57│ 7.14│ .0085
23│ 8.68│ 8.27│ 7.85│ 7.43│ 7.00│ .0083
24│ 8.53│ 8.12│ 7.71│ 7.30│ 6.87│ .0083
25│ 8.38│ 7.96│ 7.56│ 7.15│ 6.74│ .0082
│ │ │ │ │ │
26│ 8.22│ 7.81│ 7.42│ 7.02│ 6.61│ .0080
27│ 8.07│ 7.67│ 7.28│ 6.88│ 6.49│ .0079
28│ 7.92│ 7.53│ 7.14│ 6.75│ 6.37│ .0078
29│ 7.77│ 7.39│ 7.00│ 6.62│ 6.25│ .0076
30│ 7.63│ 7.25│ 6.86│ 6.49│ 6.13│ .0075
─────────────┴────────┴────────┴────────┴────────┴────────┴───────────
Footnote F:
Under any other barometric pressure, B, the solubility can be obtained
from the corresponding value in the table by the formula:
S´ = S(B/760) = S(B´/29.92) in which S´ = Solubility at B or B´,
S = Solubility at 760 mm. or 29.92
inches,
B = Barometric pressure in mm.,
and B´ = Barometric pressure in
inches.