jgourlay
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Detecting lead in brass?
Asking this question on behalf of someone over at backyardmetalcasting.com
Is there a (cheap and convenient) way to test for the presence of lead in scrap bronze and brass prior to melting it?
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JohnWW
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Without having to dissolve the metal, arc emission spectroscopy would usually be the quickest way of analyzing alloys. X-ray fluorescence would be
another non-destructive method; Pb would emit X-rays at the shorter end of the wavelength range. For some specific metals, neutron activation analysis
can be used, if it results in the formation of certain radio-isotopes which emit radiation of characteristic energies.
But if you have to, or want to, go the "wet way", HNO3 would be needed to dissolve the stuff. Then it could be analyzed for Pb and other metals by
atomic absorption spectrometry, or by various colorimetric/spectrophotometric methods, as for metals in aqueous solutions. Gravimetric analysis is a
possibility, because of PbSO4 being only slightly soluble, while CuSO4 and SnSO4 are much more soluble.
Pb was often used by the Germans in WW1 and WW2 as a substitute for Sn to add to Cu to make bronzes, for military purposes such as heavy gun barrels
and ship propellors. There are, or were, deposits of Cu and Pb ores in Germany (in the mountains in the southeast) and its allies (Austria-Hungary,
Romania, Bulgaria, Italy); but very little in the way of Sn ores, although in some sulfur-containing Cu minerals Sn also occurs (e.g. redruthite). In
WW2, the Germans also used large cargo submarines to transport Sn and rubber from Japanese-occupied Malaya and Dutch East Indies, sending manufactured
items needed in Japan such as ball-bearings on return journeys.
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merrlin
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Quote: Originally posted by JohnWW  |
Pb was often used by the Germans in WW1 and WW2 as a substitute for Sn to add to Cu to make bronzes, for military purposes such as heavy gun barrels
and ship propellors. . |
Pb is insoluble in Cu in the solid phase, and will not substitute for Sn in the copper lattice, nor does it form intermetallic compounds with Cu. Pb
is used as an additional alloying agent to modify wear properties, particularly in bearing bronzes. The addition of Pb (as a second phase) to pure Cu
is done to improve machinability, but will not improve the mechanical properties. It will degrade the mechanical properties of Cu at elevated
temperatures.
[Edited on 6-8-2009 by merrlin]
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merrlin
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If you have a microscope available, you could try polishing and etching a sample, and look for the lead.
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12AX7
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It does dissolve to a small extent though, enough that you'd have a hard time spotting it in a lump of, say, 85-5-5-5.
Copper.org has this micrograph of 4.5% Pb, balance Cu (leaded copper):
http://www.copper.org/resources/properties/microstructure/im...
Tim
[Edited on 8-6-2009 by 12AX7]
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garage chemist
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No one here who ever had to do the classic wet separation and detection of cations in inorganic chemistry? I am disappointed. Three people posting
mindless rambling about instrumental analysis and metallography without a suggestion that anyone can do without instruments.
Lead forms the characteristic insoluble sulfate.
Dissolve a few turnings of your alloy in HNO3 (or KNO3 + hot 50% H2SO4 if no HNO3 is available), dilute with dH2O, filter from the undissolved SnO2
and add dilute H2SO4 (or another solution containing sulfate ions) to the filtrate. Lead produces a white precipitate.
If quite a lot of lead is present, it will also make a precipitate of PbCl2 with chloride solution.
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merrlin
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| Quote: Originally posted by 12AX7 | It does dissolve to a small extent though, enough that you'd have a hard time spotting it in a lump of, say, 85-5-5-5.
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To be precise, the metastable solubility of Pb in Cu can be as high as 10% to 12%, but at equilibrium the maximum solubility of Pb in solid Cu is
.007%
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not_important
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| Quote: |
Dissolve a few turnings of your alloy in HNO3 (or KNO3 + hot 50% H2SO4 if no HNO3 is available), dilute with dH2O, filter from the undissolved SnO2
and add dilute H2SO4 (or another solution containing sulfate ions) to the filtrate. Lead produces a white precipitate.
If quite a lot of lead is present, it will also make a precipitate of PbCl2 with chloride solution. |
Can't use the KNO3+H2SO4 in this, as PbSO4 will drop out in the dilution step.
You could use hot HCl and slowly add KNO3 or strong H2O2 until the sample dissolves, keep the solution hot during dilution and filtering, and add
iodide (KI/NaI) to the filtrate, flake-like crystals of yellow PbI2 will form as the solution cools.
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garage chemist
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How stupid of me with the H2SO4. Yes, HCl would be better if you keep it hot all the time.
But iodide might be oxidised by the nitric oxides in the solution, making the PbI2 color hard to see. Also, the yellow PbI2 flakes only become visible
if crystallising from a hot saturated solution, otherwise it's a fine yellow powder.
And Cu(2+) ions react with iodide, giving CuI and I2, so this test isn't applicable anyway.
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not_important
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True, which is why I said to add the oxidiser slowly, so as not to overshoot much. It could be boiled for a time to drive off excess oxidiser.
If iodide is added to the hot solution the CuI will drop out there, the free iodine following it or volatilising; filtering removing them. And with
slow cooling I've seen flakes of PbI2 form from solutions that were not near saturated when hot, so that's why I so described it. Adding alcohol or
acetone to the cold solution will dissolve free iodine.
[Edited on 7-8-2009 by not_important]
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merrlin
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| Quote: Originally posted by garage chemist | I am disappointed. Three people posting mindless rambling about instrumental analysis and metallography without a suggestion that anyone can do
without instruments.
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Mindless? How many people engaged in backyard casting are going to have the apparatus and reagents for the analysis? Starting from scratch, a
microscope will be safer and cheaper. You showed such conviction in your post and I am not confident that I could name all of the elements that can be
found in bronze and brass alloys. Could you tell me what they are? Surely you must know if you are assuming that the precipitate is due to lead and
not some other element.
[Edited on 7-8-2009 by merrlin]
[Edited on 7-8-2009 by merrlin]
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Magpie
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from garage chemist:
| Quote: |
No one here who ever had to do the classic wet separation and detection of cations in inorganic chemistry?
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I agree. Pb, Ag, and mercurous Hg can be identified from a solution of cations via Group I Qualitative Analysis testing. Since PbCl2 is slightly
soluble trace amounts may not yield a precipitate of PbCl2. A confirmatory test of the solution with 1 M K2CrO4 will show a yellow precipitate of
PbCrO4 if Pb is present.
A Google search should bring up the standard procedures.
The single most important condition for a successful synthesis is good mixing - Nicodem
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chloric1
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| Quote: Originally posted by merrlin | | Quote: Originally posted by garage chemist | I am disappointed. Three people posting mindless rambling about instrumental analysis and metallography without a suggestion that anyone can do
without instruments.
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Mindless? How many people engaged in backyard casting are going to have the apparatus and reagents for the analysis? Starting from scratch, a
microscope will be safer and cheaper. You showed such conviction in your post and I am not confident that I could name all of the elements that can be
found in bronze and brass alloys. Could you tell me what they are? Surely you must know if you are assuming that the precipitate is due to lead and
not some other element.
[Edited on 7-8-2009 by merrlin]
[Edited on 7-8-2009 by merrlin] |
Merrlin, If you perform multiple tests you will discover if you have lead by the process of elimination. For instance, you dissolve a sample of
your alloy in a nitric acid solution. Then you add a sulfate. If a white precipitate forms, you know it is either lead, silver, barium, strontium,
or calcium. Now the barium, calcium, and strontium are highly unlikely in this instance so it must be lead or silver. Silver sulfate dissolves in
strong ammonia but lead sulfate does not. Lead sulfate dissolves in strong fixed alkalies. These alkaline solutions can be oxidized by hypochlorites
to give the chocolate brown lead dioxide. Silver sulfate darkens on exposure to light.
If you precipitate chlorides, lead chloride will just melt at 500 Celcius or therabouts where silver chloride, mixed with sodium carbonate, is heated
strongly, silver metal is left as a button. Silver chloride dissolves in ammonia, lead chloride does not. Copper will displace the silver from its
ammoniacle solution. I think you get the idea.
If anyone in backyard casting wants to know the composition of alloys then they should have at least the common acids ( nitric, sulfuric, and
hydrochloric) or the means to produce them. It is advisable to procure ammonia, sodium hydroxide and possibly sodium sulfide or some other sulfide
precipitant.
Fellow molecular manipulator
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merrlin
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@chloric1, I do not dispute that wet chemical analysis can be applied to the problem. The question concerns a "cheap and convenient approach."
Considering that brasses and bronzes can have Sn, Zn, Pb, Fe, Ni, Al, Mn, Si, Sb, etc., as alloying elements, dissolution in pure nitric may not be
straightforward. As indicated by Not_Important, heated acids and filtration may be required. If someone involved in backyard casting does not have any
training or experience in handling heated acids and dealing with noxious vapors, I would not send them down that path if all they want to know about
is the presence of lead. I think that if there were a thread here about lab accidents and errors involving acids, particularly those involving a
novice, you would have no shortage of posts.
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not_important
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If you want convenient, purchase commercial lead test kits, or something like this : http://www.niton.com/Default.aspx
If you want cheap, you'll have to put up with traditional wet chemistry methods as used from the mid-nineteenth to mid-twentieth century, or be
chemistry savvy enough to make your own modern lead test reagents from cheaper starting materials (this was discussed in a thread some months ago).
There's plenty of scanned copyright free books from that time frame that will detail procedures for wet analysis of alloys.
It's the classic fast, cheap, good triangle with 'fast' being replaced by 'convenient', and 'good' being one fixed requirement so you get to pick
between the other two.
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