Chemistry - Winkler Method

[sarahtorrington]

Hi,
What is the Winkler Method to test for D.O.? I have some notes, but I'm pretty sure they're wrong.. (they talk about a colorimeter)
Thank you!

[RuiAce]

Hi,
What is the Winkler Method to test for D.O.? I have some notes, but I'm pretty sure they're wrong.. (they talk about a colorimeter)
Thank you!

Pretty sure they're wrong as well because the Winkler Method is a method of titration but from memory you don't actually need to know how it works.

[jakesilove]

Hi,
What is the Winkler Method to test for D.O.? I have some notes, but I'm pretty sure they're wrong.. (they talk about a colorimeter)
Thank you!

Hey!

I definitely DID know the Winkler method for my HSC. It is a titration, like Rui said, and can be used to find dissolved oxygen content and BOD etc. It isn't exactly simple, but technically the Board of Studies COULD ask you to specifically explain how to measure BOD. In fact, you hopefully did this experiment in class! Here's what I learnt; you don't need to understand the method, just be able to cite it.

- Titration used called the Winkler Method
- Dissolved oxygen oxidises Manganese 2+ and Manganese 4+ in alkaline solution
- Acidified with iodine in solution. Iodine ions become iodine gas.
- Iodine is titrated against Sodium Thiosulfate in the presence of a starch indicator
- This will tell you how much dissolved oxygen is in the liquid

Note that you could also just use an electronic oxygen sensor! This would be a good answer to a 1-2 mark question.

Hope this helps!

Jake

[sarahtorrington]

Thank you so much!!

[Happy Physics Land]

Hi,
What is the Winkler Method to test for D.O.? I have some notes, but I'm pretty sure they're wrong.. (they talk about a colorimeter)
Thank you!

Hey sarah!

Will supplement Jake's answer here with some formulae:

1. Dissolved oxygen oxidises Mn²⁺: 2Mn²⁺_(aq) + O_2(aq) + 4OH^-_(aq) --> 2MnO(OH)_2(s)
2. Acidification of resulting solution + addition of iodide ions: MnO(OH)_2(s) + 2I^-_(aq) + 4H⁺_(aq) --> I_2(aq) + Mn²⁺_(aq) + 3H₂O_(l)
3. Concentration of iodine determined through titration against sodium thiosulfate: I_2(aq) + 2S[/sub]2[/sub]O₃^2-_(aq) --> 2I^-_(aq) + S₄O₆^2-_(aq)
4. We have a net equation: 4S₂O₃^2-_(aq) + O_2(aq) + 4OH^-_(aq) + 8H⁺_(aq) --> 2H₂O_(l) + 2S₄O₆^2-_(aq)

If you dont remember the whole process please just remember the net equation for the titration. Keep in mind that, as you can see from the stoichiometric ratio in the net equation, thiosulfate to oxygen is 4:1. This is a very important ratio to remember if you want to do winkler's titration, because you would know your concentration and quantity of sodium thiosulfate, so to work out the amount of oxygen, just divide the moles by 4. Must not forget this ratio! The only risk with talking about oxygen meters, is that the teacher may think its a bit too simplistic, but it is fine if you can provide chemical equations that explain (or sort of explain) how an oxygen meter works.

Best Regards
Happy Physics Land

[RuiAce]

I have never seen any of those equations in my life before tbh

[jakesilove]

I have never seen any of those equations in my life before tbh

I remember learning one of them, but yeah it all depends on how much the HSC actually wants. Don't overlearn, but it can sometimes be useful to have a lot of equations to work with.