Potassium Chromate Indicator

[Whspa]

I'm setting up the 'Titrimetric Determination of Chloride in Water' prac but I'm not sure what to use for the Potassium Chromate Indicator. Can I just use 0.1M K₂CrO₄ or does it have to have AgN0₃ added, rested and filtered out?

Carol

[ellice]

Hi Carol,
We've just done this prac. The students added the chromate as an indicator, and then titrated with silver nitrate. I'm not sure what you mean with the resting and filtering out. (actually that sounds like something I'd like to do!!)
Cheers, Ellice.

[Whspa]

Hi, Ellice
When I Googled this test I found :
"Dissolve 50g of potassium chromate (K₂Cr0₄) in 100mL of DI water and add silver nitrate (AgN0₃) until a slightly red precipitate is produced. Allow the solution to stand, protected from light, for at least 24 hours after the addition of AgN0₃.
Filter the solution to remove the precipitate and dilute to 1L with DI water."
I was wondering if this step was necessary for this prac or if a 0.1M solution would do.

Carol

[RosalieM]

I always just use fresh potassium chromate. No need for that extra step. It always works for me without it, although you need a really weak solution of silver nitrate. I think we use about 0.001M so maybe that's where your extra step comes in. Perhaps the kids can use a stronger silver nitrate?? I don't know... it's a mystery to me but for now I'd say don't waste your time and effort. It works with just the potassium chromate as is.

[Shas]

Hi Carol,
trying to trawl through my distant, blurred memory, for a time when i did zillions of titrations determining chloride, i can only guess that the AgNO3 added to the potassium chromate indicator precipitates out any chloride (as AgCl) present in the indicator. if there was any chloride dissolved in the indicator, your titration result would be more than it should be.

if you use distilled, deionised water to make solutions, my suggestion is that the chloride from the indicator soln is minimal. I use tap water for most solutions here except anything to do with silver nitrate. (junior school,tight budget and bucket chemistry works for me.)
You'll get a result without the fussing in prep for the potassium chromate, but you may not get an "accurate" result.

hope this helps
shas

[fibreweb]

Purely as a matter of interest, what topic are they doing this prac in as I have been in science now for 10 years and have never been asked for it.

I asked one of our chem teachers which topic it is likely to be and he couldn't work out which topic or dot point it related to. We always do corrosion and shipwrecks here as the Yr 12 elective so I was guessing it was in one of the other electives.

I always get facinated when others, especially those from NSW who obviously do the same sylabus, ask about pracs that we never do here, though some sound so complicated and need a lot of work that I'm glad my teachers don't do them.

Wendy

[RosalieM]

Hi Wendy, It is part of chemistry and water quality. Our kids do a field trip for a day collecting water and testing samples on site at 5 different locations along the peel river. They do other tests back in the lab, such as this one, which are too complicated to do out there. Chemical monitoring and management is the name of the topic (one of the teachers just walked in). It is for year 12 but sometimes it is also done in year 11 as their open ended assessment and I think they bring samples from wherever they want.
Rosalie

[smiley]

Umm stupid question really, but unless the titration is a "skills component" part of their work, why not use all the readily available chlorine test strips that are available in any water testing kit? Less mucking around with chemicals, and with a reasonable scale. Just a thought.

[Shas]

Hi Smiley,
i agree, why make things more complicated than they need to be. KISS!!!

But testing chloride is different to testing chlorine. chloride is something like the salt content of the water. maybe salinity or conductivivty meter could be used as an indicator. Chloride is usually pretty stable in the water sample.
chlorine is the smell that you get from the indoor pool that stops you catching all the bugs. Chlorine is added to water supplies as well but it 'disappears' from solution easily so if you want to test for this, it really needs to be done where you collect sample.

i think that's what they have the test strips for (not really sure on this point).

Shas