15
$\begingroup$

I picked up a metal fence loop cap today and took it home just because I liked the shape. fence loopcap

(^ not mine) I cleaned it, but some of the paint is chipping off. I'm only vaguely thinking about lead, and I'm not going to chew on it, so I don't want to buy a test kit. Is there a way I could test for the presence of lead ions with common/ideally household chemicals? I'd be willing to wait until I'm at my school lab again if I needed HCl or something.

$\endgroup$
  • $\begingroup$ Since this doesn't answer your question about testing for lead, I'll leave a comment. The sale of lead-based paint is banned in the US and other countries. In the US, it has been banned since 1978, and fell out of common use before that. If you have reason to believe that your fence loop is newer than 1978 (and it probably is), and you found it in the US, then there is very likely any lead in the paint. $\endgroup$ – Ben Norris Jul 3 '12 at 10:53
  • $\begingroup$ Lifting and thinking should be enough. Second thought is about softness of lead, making such a cap made from lead useless. BTW, what is the purpose of the eye? $\endgroup$ – Georg Jul 10 '12 at 9:01
13
$\begingroup$

General remark: get a bit of lead (metal or salt - whatever you look for) to have a positive control for your experiments.

More non-chemical tests for lead (metal):

  • lead is soft: you can cut/scrape it quite easily with a normal knife.

  • sheets of lead are easily bent, and they stay so.

  • For bigger pieces: Hammering on it would leave a rather big impression of the hammer. The sound would be dull.

  • it usually looks grayish, not silvery-metallic, but the cut will be silvery.

If it is about the paint:

  • what colour is it?

  • white lead: $\ce{2 PbCO3 \cdot Pb(OH)2}$ I'm more familiar with that in the context of paintings or painting of wooden things (window frames)

  • $\ce{PbSO4}$ also white. I think it has also been used for things like window frames.

  • side thought: lead paints for wood may have been popular due to the fact that the toxicity to microorganisms helps very much with the anti-fouling effect. Anti-corrosion paints for metal need different properties.

  • lead yellow: $\ce{PbCrO4}$ Wikipedia says that was the original pigment for school buses.

  • $\ce{2PbO \cdot PbO2}$(Mennige) red anti-corrosion pigment that was at least here in Germany the most important lead based priming paint. Only difficulty with recognizing by color: lots of non-toxic replacements are produced with similar colour to have the familiar look...

  • Very sensitive but but not very distinctive test$^*$ for $\ce{Pb^2+}$: the sulfide is black and very insoluble. Kitchen chemistry: put some egg on and even the surface of the metal should turn black. Possibly some heat is needed (I didn't test, though). Otherwise, use acidic $\ce{Na2S}$ solution from the school lab. If the $\ce{Na2S}$ solution doesn't give a black precipitate, there could in theory still be lead in the paint which is not accessible by the sulfide due to the binding material of the paint. In that case, however, the toxicity will be less of a problem, too (i.e. unless you dissolve the paint somehow, burn it, ...)
    $^*$ OTOH it is highly improbable that there are substantial amounts of silver in the paint...

  • reductive borax bead should be gray, oxidative colourless.

  • Isolating Pb is quite easy, maybe you can do that in your school lab: precipitate $\ce{PbCl2}$ with cold HCl, filter. The dissolve the chloride in hot water. You can then precipitate PbS (black) or $\ce{PbCrO4}$ (yellow). Looking at the crystals under the microscope can help further. Qualitative inorganic textbooks often have microscope photos of different precipitates.

$\endgroup$
  • $\begingroup$ Google translate says Trennungsgang is "separation process", so maybe filtration or separation? Thank you for your detailed answer. $\endgroup$ – Maddy Byahoo Jul 4 '12 at 21:21
10
$\begingroup$

If you’re testing for whether it’s entirely made of lead, you can do it with a simple density calculation rather than anything chemical – an easy typical way to do this is to immerse it in a known volume of water and see how much volume the object adds, then divide the weight by that. Lead is the one of the heaviest common metals, with a density of just over 11 g/cm³.

If on the other hand it’s just lead content you’re looking for, I think you can go for, coincidentally enough, HCl.

NB: I have not tested the following procedure(s).

Lead (II) chloride isn’t very soluble (0.9 g/100 ml at 20 degrees Celsius), so if you just add a small quantity of strong HCl to a small sample of the metal and wait a while (leave it overnight or longer if you can, the reaction’s supposedly quite slow with lead: cf this video of trying it quickly and nothing happening, even with strong acid and strong peroxide. You might get a white precipitate of lead chloride forming.

If nothing happens, try adding a little potassium iodide. Lead (II) iodide is over ten times less soluble than the chloride, so you might get a precipitate with that if you don’t with the chloride. Note that adding hydrogen peroxide in the initial step will speed up the reaction of the acid with the metal, but hydrogen peroxide will also decompose dangerously rapidly if you then add iodide later (see the ‘elephant toothpaste’ demonstration, for example). Getting boiling acid everywhere is not something I recommend.

You can also then do a flame test – adding methanol to a lead salt and igniting should give you a somewhat ambiguous blue-ish flame.

$\endgroup$
  • $\begingroup$ Wrt the flame test: as methanol alone burns with blueish colour, you at least need a positive control. Also, a spectroscope to look at the lines would be good, as there are more things that cause blue flames, and AFAIK the color with lead is not very strong. Also, make sure you take $\ce{PbCl2}$, the paint most likely will cause a orangeish colour as organic binding material burns. $\endgroup$ – cbeleites Jul 4 '12 at 11:00
  • $\begingroup$ All the $\ce{PbCl2}$ I've precipitated was white. $\endgroup$ – cbeleites Jul 4 '12 at 11:06
  • $\begingroup$ @cbeleites: Ah, yes, oops. Edit time. $\endgroup$ – Aesin Jul 4 '12 at 16:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.