Finding Heavy metals... (1 Viewer)

[CHUDYMASTER]

I have a water sample from my local creek.
Now basically, I have to somehow quantitatively find the % composition of heavy metals in the sample.

I've filtered out the large impurities. I've added the nitric acid and sodium sulfide. The precipitate of various heavy metal sulfides have formed. NOW, how the hell would I accurately show the % comp. of the former heavy metal ions.

 

[McLake]

Do you have to use the method you described, because there is no easy way of telling how much of a particlar metal is in the solution.

If we can assume that all heavy metals have valency 2 (which I doubt you can) then you could filter it and determine the mass, assuming the formula "MS" where M is all of the valent two metals. Use calculations of moles to determine mass of METALS, and hence % composition ....

 

[CHUDYMASTER]

Actually, what I really meant to say was the % composition of the overall heavy metal content. And yeah, there's a big problem- I can't just presume they're all valence 2.

Hmm, can any suggest a way to quantitatively determine the amount of heavy metal in the water sample?

Mclake...perhaps?

 

[McLake]

Are there any particular heavy metals you are looking for (say Hg) or is it ALL of them?

 

[CHUDYMASTER]

Well it would actually be harder to find one of them specifically, as most tests, such as the addition of sodium sulphide will actually result in the formation of more than one precipitate.

 

[McLake]

That's true ....

Can you post the actual question? It seems like a really hard task ...

 

[CHUDYMASTER]

Ok, it seems like my far-out and different idea is taking me no where.

I basically have to look for factors influencing water quality and make up an experimental test to determine such factors. What's more, I have to perform the experiment.

Now, I've done pH, which was easy. I think my anion experiment is all right. I need one more and if heavy metals is out, then i need an idea and no, don't suggest turbidity or something ludicrous like that.

 

[McLake]

What about TDS? (Total Disolved Solids)

This can be done either by evaporating off the water or testing the elecrical conductivity.

Or Hardness?

React with Na2CO3 of known concentration, percipitate out and weigh, then assume all solids are CaCO3 (which you ARE allowed to do) and preform mole calculations ...

 

[CHUDYMASTER]

The TDS experiment is kinda dodgy. I don't want to do it the electricity way because that doesn't show me anything except a value which I can look at.

I don't want to evaporate it because the yield of solids will be quite small.

The hardness idea you got there I have not heard of.

And how can you assume only CaCO3 forms? Plus, Mg also attributes to hardness. You're gonna have to elaborate on that experiment, I'm not sure I follow.

 

[McLake]

OK, Conquering Chem has some stupidly long and complicated experiment for hardness, but HSC Online will tell you a much easier way to do it!!!

From HSC Online
Hardness is due to the presence of calcium and magnesium ions in the water. These form
insoluble compounds with soap ions, resulting in a scum on the water surface and around sink
basins. The removal of soap ions from solution reduces the ability of soap to lather.

The test for hardness involves precipitating the calcium and magnesium ions from a known
volume of the water sample with a solution of sodium carbonate (of known concentration),
followed by filtering and drying of the precipitate. Most of the insoluble salt is assumed to be
calcium and the concentration of calcium ions is calculated and reported in parts per million
(ppm).

The Mg is not used 'cause it isn't neccesarry ...

 

[CHUDYMASTER]

But what about the Mg ions? Why doesn't the Na2CO3 form precipitates with other dissolved cations?

 

[McLake]

Originally posted by CHUDYMASTER
But what about the Mg ions? Why doesn't the Na2CO3 form precipitates with other dissolved cations?

I'm not sure

I think it does, but not in signifigant amounts ...

 

[CHUDYMASTER]

Tsk Tsk, you call yourself the "HSC Guru"...

 

[McLake]

I'm sorry. Never said I was a Chemistry Guru, now did I?

I figure if it says it on the HSC Online site it must be true. And i've used it in test and it's been acceptable, so just trust that it is true ...

 

[CHUDYMASTER]

Well thanks, but to be on the safe side, I shall consult some of my sources to verify it.

And you didn't say you were the chem guru, you said you were the HSC guru, which ENCOMPASSES chemstry...HAW HAW HAW HAW HAW!!!

 

[McLake]

Fine, don't trust the all-powerfull HSC Guru ....

I should be worried that I can't answer this queation, since chem was probably my best subject ...

Blame the holidays / heat / general laziness on me losing all of my chem knowledge ...

 

[d00d]

i did an assignment that had a bit to do with pollution. i believe u use atomic absoprtion spectroscopy (AAS) to detect heavy metals in water system.

 

[d00d]

oh yeah, especially for mercury and lead

 

[kini mini]

AAS is THE heavy metal test. Why you would bother to learn the reagent approach is beyond me, remember the quantities concerned are minute in any likely water supply.

 

[kini mini]

Originally posted by CHUDYMASTER
Ok, it seems like my far-out and different idea is taking me no where.

I basically have to look for factors influencing water quality and make up an experimental test to determine such factors. What's more, I have to perform the experiment.

Now, I've done pH, which was easy. I think my anion experiment is all right. I need one more and if heavy metals is out, then i need an idea and no, don't suggest turbidity or something ludicrous like that.

No-one is expected to actually perform the experiment, use your imagination like the rest of us did . The turbidity experiments are actually quite good, I suggest the description in Thickett's Pathways. You use a light source of known intensity shone thru the sample and a light detector on the other side. Comapre to standard samples. You can also do it nephelometrically.

On the hardness issue - both Mg and Ca cause hardness. But it is always measured in units of one or the other (can't remember which) because they are functionally identical.