To the Editor: To the Editor:

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 morc dissolved oxygen, illustrating the necessity for carcful sampling. Distilled water is sometimes found t o be supcrsaturated. 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.'

' CAMP,THDM.M H., "Water and Its Impurities," Reinhold, New York, 1963, p. GO, 201. RAYMOND A. SO.\IAIERS U N I V E N ~ IOTFYWIGCONSIN/STI