Notes on Determination of Dissolved Oxygen
To the Editor: Determination of dissolved oxygen, DO [J. CHEAI.
EDUC.,49, 427 (1972)l has also been one of thc more popular experiments in my course in analytical chemist r y for the past year. With the solutions prepared beforehand and with the water samples collected from the taps in the laboratory, the whole experiment is readily performed in a 3-hr lab period. William Stagg did not mention it, but when using 0.025 A[ sodium thiosulfate, the milliliters used correspond to the dissolved oxygen content in ppm. Our samples are collected mith Tygon tubing by running the water through the BOD bottle for several minutes. City water collected from a tap with an aspirator was found to have considerably more dissolved oxygen, illustrating the necessity for carcful sampling. Distilled water is sometimes found t o be supersaturated. Water which has been boiled in a n open beaker and cooled still has a measurable DO, showing that this simple procedure is not completely effective in removing DO. Finally, Henry's Law Constant is calculated from the DO of water which has been saturated by bubbling air through it. Calculations involve using the current barometric pressure and correcting for the vapor pressure of water. Results agree very closely mith those found in Camp.'
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CAMP,THDM.M H., "Water and I t s Impurities," Reinhold, New York, 1963, p. GO, 201.
RAYMOND A. SO.\IA~ERS U N I V E N ~ IOTFYWIGCONSIN/STRVENS POINT STKVI:NE POINT, WISCONSIN ,54481
Dissolved Oxygen Determination
To the Editor: I noted with interest Professor William R. Stagg's article in the June 1972 issue of the JOURNAL OF CHEMICAL EDUCATION concerning the determination of dissolved oxygen concent,ration in the introductory laboratory. This procedure has been part of our cnvironmental science laboratory curriculum for some time. As a chemist, I was pleased t o see the discussion of Henry's law as it applics t o this important environmental problem. Other readers of the article may be interested t o hear of simplifications in the procedurc that make it more suitable for the introductory laboratory. The staff of the National Environmental Training C ~ n t e in r Cincinnati recommend the use of the "whole bottle tcchniquc" described in the booklet "FWQA llrthods for Chrmical Analysis of Water and Waste," 1970, published by thc Analytical Quality Control Laboratory of the FWQA. This procedure calls for the titration of the cntirr samplc, after it has been trcatcd with manganous sulfatc and alkalino-iodide-aeide solutions follo>vcd by COIIcentrated sulfuric acid. I n order t o achieve 1: 1 chcmical equivalence, a 0.0375 N sodium thiosulfatc solution (Continued on page 1621 160
/ Journal of Chemical Education
