MS20 Laboratory: Physical and Biological Factors Affecting Oxygen ...

Standardizing the Thiosulfate Solution
If we know the oxygen content of a given water sample, we can use this as our standard and
calibrate the thiosulfate solution to it. This assumes, of course, that we know the exact condition of
our water sample, its salinity, temperature, and degree of oxygen saturation. If we use aerated
fresh water at room temperature, we can closely approximate the actual oxygen content of the
water by consulting the proper oceanographic tables.
Table One shows the theoretical saturations of oxygen at 0 o / oo salinity and at the temperature
ranges typically encountered in nature.
Table One. Theoretical Oxygen Saturation at Various Temperatures
TEMPERATURE 0 2 TEMPERATURE 0 2 TEMPERATURE 0 2
(ºC) mL/L (ºC) mL/L (ºC) mL/L
0 10.22 9 8.07 18 6.61
1 9.94 10 7.88 19 6.48
2 9.66 11 7.71 20 6.36
3 9.39 12 7.54 21 6.23
4 9.14 13 7.37 22 6.11
5 8.90 14 7.21 23 6.00
6 8.68 15 7.05 24 5.89
7 8.47 16 6.90 25 5.77
8 8.27 17 6.75
Sample 1, from the open aquarium at 0 o / oo salinity, will represent your standard. In order to
calibrate your thiosulfate solution, you will need to know how many milliliters of thiosulfate are
needed to titrate the standard.
Procedure
1. Transfer exactly 50 mLof the known sample into a 125 mLErlenmeyer flask, using a 50 mL
volumetric flask for the transfer (Figure Three, steps 6 and 7).
2. Fill a 50 mL buret with thiosulfate solution (step 8). Record the buret level in Table Two below.
3. Add approximately 5 mL of the starch solution (step 9) to your sample in the flask. The solution
in the flask becomes dark blue. Slowly titrate (add thiosulfate from the buret) until the solution
becomes colorless (step 10). You might hold a sheet of white paper behind the flask to help
determine when the solution becomes colorless. At this point, you may still observe a few blue
particles- don't worry about them. You have reached the end point! Record the final amount in
the buret in Table Two. After calculating the difference between the starting and ending points on
the buret, you will know the number of milliliters of thiosulfate required to render the solution
colorless.
MS20 Lab Oxygen rev.3/27/2005 Page 4 of 16