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    How exactly would you go about detecting the concentration, for example lead, in a sample of water using a spectrophotometer?
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    (Original post by I<3Econs)
    How exactly would you go about detecting the concentration, for example lead, in a sample of water using a spectrophotometer?
    The best way is to use atomic absorption spectrophotometry (AAS)

    This detects and measures mg/dm3 (ppm) concentrations.
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    First you need a sample of that metal with known concentration. You make up solutions with different concentrations (like 0.2M, 0.4M etc) and run them through the colorimeter. Plot the graph of absorption rate/concentration with the data you have and draw a line of best fit.

    Run the sample of unknown concentration through the colorimeter. Check the value on the graph and compare with the line of best fit to get the concentration.
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    (Original post by piszczel)
    First you need a sample of that metal with known concentration. You make up solutions with different concentrations (like 0.2M, 0.4M etc) and run them through the colorimeter. Plot the graph of absorption rate/concentration with the data you have and draw a line of best fit.

    Run the sample of unknown concentration through the colorimeter. Check the value on the graph and compare with the line of best fit to get the concentration.
    The water with lead in it would be colourless wouldn't it? So what wavelength should I use?
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    (Original post by I<3Econs)
    The water with lead in it would be colourless wouldn't it? So what wavelength should I use?
    You could add something that would amplify the colour or react with led to produce a coloured solution, I'm sure of that, but I just don't know what it is :p:
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    (Original post by I<3Econs)
    The water with lead in it would be colourless wouldn't it? So what wavelength should I use?
    Like I said before...

    AAS is the way to go...
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    (Original post by charco)
    Like I said before...

    AAS is the way to go...
    Unfortunately I'm limited to a simple spectrophotometer. Any ideas on what I could react the lead with?
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    (Original post by I<3Econs)
    Unfortunately I'm limited to a simple spectrophotometer. Any ideas on what I could react the lead with?

    Lead salts are usually neither coloured nor soluble.

    Maybe you could do it in reverse...

    Set up the colorimeter to detect chromate ions. Prepare a calibration curve. The ions are yellow, so you would use the complementary colour filter.

    Add a quantity of your lead solution. Filter and read the new chromate ion concentration off the calibration curve.

    By calculating the loss in chromate ions you can relate it to the concentration of lead ions in the solution.

    NOTE: This is just an 'off the top of my head' idea.

    BEWARE: chromates are very toxic, carcinogenic and so are lead salts...

    If you decide to test this, be very aware of the hazards.
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    If you only have a simple spectrophotometer, you have to be a bit smart. First, lead is indeed colourless and doesn't have any absorptions in the uv either. So you can't detect directly. Second, it won't have the sensitivity to detect lead, or almost any other metal, at realistic water concentrations.

    The classic way of doing it is to react the metal of interest with something which complexes it and forms a very strongly coloured species, then measure the intensity of the colour. This is called colourimetry and, for lead, a complexant called dithizone (1,5-diphenylthiocarbazone) was often used.

    The lead chromate suggestion above is a neat idea but would probably suffer from a lack of sensitivity. 1 ppm of lead is only 5 micromolar so you'd be measuring a very small change in chromate concentration, which is difficult.
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    (Original post by Cora Lindsay)
    If you only have a simple spectrophotometer, you have to be a bit smart. First, lead is indeed colourless and doesn't have any absorptions in the uv either. So you can't detect directly. Second, it won't have the sensitivity to detect lead, or almost any other metal, at realistic water concentrations.

    The classic way of doing it is to react the metal of interest with something which complexes it and forms a very strongly coloured species, then measure the intensity of the colour. This is called colourimetry and, for lead, a complexant called dithizone (1,5-diphenylthiocarbazone) was often used.

    The lead chromate suggestion above is a neat idea but would probably suffer from a lack of sensitivity. 1 ppm of lead is only 5 micromolar so you'd be measuring a very small change in chromate concentration, which is difficult.

    Yes Cora, as you say, there are a number of compleximetric agents that are actually used to do this, but I was struggling to find a school lab based idea, presupposing that the concentrations of lead are relatively high.
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    Are there any common reagents that would form a complex coloured solution with the Lead Nitrate other than dithizone?
 
 
 
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