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Offline eglaud

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Experiment Design
« on: March 20, 2016, 03:13:24 PM »
I am to design an experiment to test for pb2+ in water. I've noticed in my lab book that pb2+ plus OH- creates a solid from the two aqueous reactants. Will adding hydroxide actually create a solid product that would prove there is lead in the water, or will the hydroxide react with the water instead of the pb2+?

Offline eglaud

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Re: Experiment Design
« Reply #1 on: March 20, 2016, 03:19:11 PM »
If it makes a difference, here is the full question:

"Describe an experiment you could perform to determine whether pb2+ has leached into water drawn from plumbing in which lead was used to solder to join copper pipes"

We don't have much prior knowledge on this, but the lab involved precipitates, so I feel like that would be the answer. I could be wrong of course

Offline Arkcon

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Re: Experiment Design
« Reply #2 on: March 20, 2016, 03:27:53 PM »
You've done a good first step, you've identified a chemical test for the analyate of choice.Now you have to ask:  How much lead are you asked to look for?  and How can you prove your chemical test can see that much.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline eglaud

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Re: Experiment Design
« Reply #3 on: March 20, 2016, 03:45:03 PM »
It doesn't give us the amount, so i'd say they're just looking for the process in generalities.

So I would add an appropriate amount of hydroxide to this solution, as well as to a control of just water and another control of just aqueous lead. Then I will look for any reactions that would happen. I assume the hydroxide will react with the lead to form a solid, so that's what I will look for, comparing it to the two controls. I would hope that there would be enough to be visible, but again i'm assuming that since they don't give much information with the question that they are assuming there won't be this sort of problem (hopefully).

But before I put any more thought into this, I just want to know if the hydroxide would react with the water as well?

Offline Burner

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Re: Experiment Design
« Reply #4 on: March 20, 2016, 08:46:36 PM »
But before I put any more thought into this, I just want to know if the hydroxide would react with the water as well?

Basically no.

I am to design an experiment to test for pb2+ in water. I've noticed in my lab book that pb2+ plus OH- creates a solid from the two aqueous reactants. Will adding hydroxide actually create a solid product that would prove there is lead in the water, or will the hydroxide react with the water instead of the pb2+?

I am not sure if it is important, but - OH- forms precipitate with most metal cations, not only Pb2+. I personally think that you need more than one chemical test to confirm that Pb2+ has leached out but not only other metals. Remember that solder is an alloy composed of many metals.
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Offline mjc123

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Re: Experiment Design
« Reply #5 on: March 21, 2016, 10:49:02 AM »
Try adding some KI solution. PbI2 is an insoluble intensely yellow solid.

Offline AWK

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Re: Experiment Design
« Reply #6 on: March 21, 2016, 02:03:23 PM »
Quote
Try adding some KI solution. PbI2 is an insoluble intensely yellow solid.
PbI2 test is rather useless here. This test needs solid PbI2. Solubility of this compound is ~10-3 mol/dm3 and is much greater than concentration of Pb2+ expected in this problem (tap water). Solutions of lead iodide are colorless.
I think that the only method for detecting concentration Pb2+ in tap water is using ion selective electrodes.
AWK

Offline eglaud

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Re: Experiment Design
« Reply #7 on: March 21, 2016, 03:25:17 PM »
I am not sure if it is important, but - OH- forms precipitate with most metal cations, not only Pb2+. I personally think that you need more than one chemical test to confirm that Pb2+ has leached out but not only other metals. Remember that solder is an alloy composed of many metals.

That could be important... The reason I chose the hydroxide was because that was given as an example in our prelab but that was just given for a sample equation. Ion selective electrodes are not an option I'd say, since that's not really what is expected from us as students.

In the lab notebook it also mentions that the combination of Pb(NO3)2 and CuCl2 will form a blue solution... So If I add No3 and allow the Lead Nitrate to form, then add some Copper Chloride, shouldn't this create something that will allow me to see that there is lead?

Offline AWK

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Re: Experiment Design
« Reply #8 on: March 21, 2016, 03:51:57 PM »
The lowest solubility shows lead sulfide PbS.
AWK

Offline eglaud

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Re: Experiment Design
« Reply #9 on: March 21, 2016, 06:53:07 PM »
The lowest solubility shows lead sulfide PbS.

If you're replying to me I don't know what you're referring to

Offline Burner

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Re: Experiment Design
« Reply #10 on: March 21, 2016, 09:12:23 PM »
https://en.wikipedia.org/wiki/Solubility_chart

This chart should give you some more options. Personally, to confirm that the detected ion is Pb2+, I will prepare two samples of the tap water to be tested, in one of them I will add excess NaOH to see if a white precipitate is formed and then redissolve(As far as I know only Al3+ and Pb2+ can have this observation), then in the other sample I will add Na2SO4 as PbSO4 is insoluble.

Like @AWK said, tap water will not contain a lot of Pb2+ for precipitation tests to be carried out effectively.

Side note: World Health Organization suggests that the concentration of Pb2+ must not exceed 10μg/L. (Which is 4.82625483 x 10-8 M)
Year 1 science student in HKUST and a Chemistry geek.
If I make any mistakes in the forum, please don't hesitate to correct me as I want to learn.

Offline eglaud

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Re: Experiment Design
« Reply #11 on: March 21, 2016, 09:33:44 PM »
https://en.wikipedia.org/wiki/Solubility_chart

This chart should give you some more options. Personally, to confirm that the detected ion is Pb2+, I will prepare two samples of the tap water to be tested, in one of them I will add excess NaOH to see if a white precipitate is formed and then redissolve(As far as I know only Al3+ and Pb2+ can have this observation), then in the other sample I will add Na2SO4 as PbSO4 is insoluble.

Like @AWK said, tap water will not contain a lot of Pb2+ for precipitation tests to be carried out effectively.

Side note: World Health Organization suggests that the concentration of Pb2+ must not exceed 10μg/L. (Which is 4.82625483 x 10-8 M)

Okay thanks a ton, but are you saying don't test for a precipitate? Since it won't be carried out effectively? Because isn't that what you're suggesting

Offline Burner

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Re: Experiment Design
« Reply #12 on: March 21, 2016, 10:39:54 PM »
I mean you can't expect precipitate tests will give you very obvious/observable results. However, as you have said before precipitation tests seems like your only available choice.

Side note: World Health Organization suggests that the concentration of Pb2+ must not exceed 10μg/L. (Which is 4.82625483 x 10-8 M)

Forgot to mention that this standard is for drinking water.
Year 1 science student in HKUST and a Chemistry geek.
If I make any mistakes in the forum, please don't hesitate to correct me as I want to learn.

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