MadSci Network: Chemistry

Re: Is there a type of analysis called a DO?

Date: Tue Dec 22 23:52:36 1998
Posted By: Charlie Crutchfield, , Retired, Retired
Area of science: Chemistry
ID: 914123876.Ch

Dear Miss Annette,

Yes there is, almost certainly what is referred to by "DO" is the analysis for Dissolved Oxygen. This measurement is used in water and sewage analysis.

There are two methods used for this, [1] chemical analysis by "Redox"using the "Winkler" method and [2] instrumentally by polarographic analysis.

In the chemical method:

|A]A solution of Manganous salt [usually the sulfate] is added to the sample., then the solution is made strongly basic by addition of NaOH or KOH. This forms a suspension of Mn[II] hydroxide.

[B]This Mn[II]OH reacts with the dissolved Oxygen to form brown Manganic[III]hydroxide.

[C] KI and sulfuric acid are next added. The solution is now acidic and the Mn [III] oxidizes the Iodide to Iodine.

[D] The Iodine is now titrated with standard [ 0.1000 N]Sodium Thiosulfate.

In brief, the dissolved Oxygen oxidizes the Mn[II] salt to Mnb [III]. Next, the Mn[III] oxidized the Iodide to free Iodine. This latter is then titrated [reduced]with Thiosulfate.

This analysis is very common and routine in water and sewage plants, but the procedure is not tolerant of sloppiness and a bit of practice and skill is needed. The sample must be collected and handled with considerable care, so that oxygen from the air does not enter and ruin the analysis. I regret I can not furnish you with more details on reagent concentrations, how much to add, and other procedural details, but my book giving details on this was borrowed and not returned.

For actual working details, contact your local water or sewage plant or any commercial laboratory that performs water or sewage analysis. Ask them about "DO" analysis. Or in your school library, look for "Standard Methods for the Analysis of Water and Sewage". I think the procedure is also given in A.S.T.M standard methods, in the volume on water, etc.

Charlie Crutchfield

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