Can someone please explain the Wrinkler method is simple terms? (1 Viewer)

[~Smurfette~]

I know the syllabus doesn't specify that we know this method of measuring dissolved oxygen content in a water sample, but I suppose it doesnt hurt to know it - better safe than sorry. I've gone through a few text book explanations and get most of it but they provide irritatingly complicated responses. Simplification anyone? Thanks!

 

[Dreamerish*~]

Trust me, there is no simple way.

Know the lot, or know nothing. As long as you're aware that the Wrinkler method is used for measuring the amount of dissolved oxygen in water, you'll be fine.

Anyhow, you asked for it:

Winkler Method: (This may look a little confusing, and I still can't write the equations from memory. Don't worry, we don't need it. As long as we know the oxygen-sensitive electrode method, who gives a toss about Winkler.) The Winkler method is a way of fixing the amount of dissolved oxygen in a sample and determining the DO by titration at a later time.

Manganese (II) ions and hydroxide ions are added to the water sample. The amount of insoluble brown manganese (IV) oxide produced depends on the amount of DO:

2Mn²⁺ + 4OH^- + O_2(aq) --> 2MnO_2(s) + 2H₂O_(l)

Acidified iodide solution reacts with the MnO₂, producing a yellow iodine solution.

2MnO_2(s) + 8H⁺ + 4I^- --> 2Mn²⁺ + 4H₂O_(l) + 2I_2(aq)

The iodine released is titrated against a standard sodium thiosulphate solution from a burette containing starch indicator. The starch indicator forms a blue colour with iodine, and the blue colour disappears at the end point.

2I_2(aq) + 4S₂O₃^2-_(aq) --> 4I^- + 4S₂O₃^2-_(aq)

It can be seen that each dissolved O₂ molecule gives 2MnO₂ which gives 2I₂ which reacts with 4S₂O₃^2-_(aq). Thus for each molecule of thiosulphate used at the end in the titration there was ¹/₄ mole of dissolved oxygen in the original sample.

 

[currysauce]

i think it has something to do with Manganese oxidising... i'll find info give me a sec

 

[Pointy Ears]

If its any easier, I have the method for the winkler test without the use of eqns.
However, because of this, I'm not sure if writing this answer is sufficient to gain full marks, but here it is for anyoone who wants it:

1. Mn(II) ions are added to the sample.
2. Dissolved oxygen oxidises Mn(II) to Mn(IV).
3. Sample treated with known excess of potassium iodide.
4. Solution titrated with standard sodium thiosulfate solution using a starch indicator.

Pls tell me if its worth using, or if i should just try and remember the goddamned eqns.

 

[~Smurfette~]

[QUOTE Pls tell me if its worth using, or if i should just try and remember the goddamned eqns.[/QUOTE]
I'd probably use equations depending on the marks allocated for the question. But I suppose if you're going to know the Wrinkler method, you should know at least one or two equations, or perhaps the overall equation just to validate your response. Thanks for your clarification Dreamerish, its definately helpful.

 

[paper cup]

does it say anything about having to know the chemistry of it?

 

[Dreamerish*~]

does it say anything about having to know the chemistry of it?

It's part of measuring dissolved oxygen in water, for which the oxygen-sensitive electrode can also be used.

The dot point is: Identify that water quality can be determined by considering ... dissolved oxygen.

It does not specifically state that we need to know how the electrode or Winkler's method works, but it implies we need to know about them.

I suggest know one of them in reasonable detail - choose the oxygen sensitive electrode.

And just mention that it can also be meaured by Winkler's method.