bmse
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Cast Aluminum Digestion for ICP-AES
Hi, I'm fairly new to the analysis of metals with ICP-AES and need some help digesting aluminum samples with high silicon content. Does anyone have
any tips/methods that would digest this and get accurate results for silicon? I'm hoping to find a method that uses no hydrofluoric?
Thanks!
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Crowfjord
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If you want the silicon to dissolve, you're going to need hydrofluoric acid. At least, that's the only way I have learned how to do it in my three
years of analyzing metals via ICP-AES. For high silicon aluminum like AA 380 or similar, the way we do it involves digesting ~0.5 g of sample in a
mixture of 15 mL concentrated hydrochloric acid and 10 mL 78% nitric acid, followed by addition of about 2 mL of 48% HF.
Maybe ammonium bifluoride would work instead, but that still technically contains HF.
I guess you could quantitate the silicon gravimetrically by filtering and taking the mass after drying. You would need also to analyze the solution
for silicon by ICP-AES to get the tiny amount that dissolved and add that to the gravimetric result.
[Edited on 9-10-2015 by Crowfjord]
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bmse
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I thought it might be a lost cause to digest it without HF... For your method, do you heat the solution when you put the hydrochloric and nitric?
And after you add the HF you just bring it to 100, or 250 mL end vol?
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Crowfjord
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Whoops, I realized the mixture I cited is what we do for for steel and titanium. For aluminum, the nitric is cut back to 8 mL. The amounts I have
given are for a final volume of 100 mL.
I add the nitric first, then slowly add the hydrochloric. How fast you add the HCl and whether or not to heat depends on how finely divided the
aluminum is, and some alloys seem more reactive than others. I recommend to start adding HCl very slowly without added heat and see how vigorous the
reaction is. If you get all the hydrochloric in and there are still some pieces, then heat gently until the reaction gets going and remove from the
heat to allow the digestion to finish. Once the bubbling is done, hydrofluoric acid may be added. Alloys with 8-9% silicon need 2-2.5 mL of 48% HF to
completely dissolve all of the grey-black silicon precipitate.
This is all done with PTFE (Teflon) beakers and watch "glasses."
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Upsilon
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Would you not be able to use any bases like sodium hydroxide? These can apparently dissolve silicon to give water-soluble silicates.
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Crowfjord
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That's an interesting thought. I have never tried it. I can't immediately think of any reason it would not work, as long as the instruments' tubing,
nebulizer, spray chamber, and torch are compatible. Then again, I feel like there must be a reason that we don't do it this way. Maybe large amounts
of sodium salts cause too much crud buildup on the torch, or inside the torch compartment.
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Upsilon
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I hardly know anything about spectroscopy so that's why I was unsure whether or not it would work for this purpose. I haven't tried it but there is
strong indication that it would work for the purpose of simply dissolving silicon. I am actually in the process of preparing silicon metal right now -
I think I can spare some to experiment on this.
It seems like evidence is pointing to it behaving fairly similarly to silicon dioxide - resistant to all acids except HF and dissolving in
concentrated alkali. If this trend continues then one may predict that it would also dissolve in molten phosphoric acid since it too is capable of
dissolving glassware.
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bmse
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Quote: Originally posted by Crowfjord  | Whoops, I realized the mixture I cited is what we do for for steel and titanium. For aluminum, the nitric is cut back to 8 mL. The amounts I have
given are for a final volume of 100 mL.
I add the nitric first, then slowly add the hydrochloric. How fast you add the HCl and whether or not to heat depends on how finely divided the
aluminum is, and some alloys seem more reactive than others. I recommend to start adding HCl very slowly without added heat and see how vigorous the
reaction is. If you get all the hydrochloric in and there are still some pieces, then heat gently until the reaction gets going and remove from the
heat to allow the digestion to finish. Once the bubbling is done, hydrofluoric acid may be added. Alloys with 8-9% silicon need 2-2.5 mL of 48% HF to
completely dissolve all of the grey-black silicon precipitate.
This is all done with PTFE (Teflon) beakers and watch "glasses." |
Thanks so much! This method worked great for our high silicon aluminum. You said you use this same method for steel but increase the nitric?
I also had a couple other questions if you don't mind. Do you still use the hydrofluoric for low silicon steels or aluminum? And when trying to
analyze low phosphorus, I'm having trouble with getting accurate results. It seems that the background is so high and there are interferences at a
lot of those wavelengths. Do you have any suggestions?
Thanks again!
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Crowfjord
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| Quote: Originally posted by bmse |
You said you use this same method for steel but increase the nitric?
I also had a couple other questions if you don't mind. Do you still use the hydrofluoric for low silicon steels or aluminum? And when trying to
analyze low phosphorus, I'm having trouble with getting accurate results. It seems that the background is so high and there are interferences at a
lot of those wavelengths. Do you have any suggestions?
Thanks again! |
Indeed, for steel we use a little more nitric acid, as I noted above. We also do use some hydrofluoric acid in almost all preparations (copper being
pretty much the only exception), usually about 0.25 to 0.5 mL of 48% HF to a 100 mL prep for most steels. Even when a metal has low silicon and there
is no precipitate, the silicon numbers are always lower than the actual value when there has been no HF added. HF is also important when analyzing
for niobium or tantalum, or when a sample has a large amount of tin, but should be omitted when analyzing for yttrium.
Phosphorous is always a tricky one to analyze, as it is usually in very small amounts and there are many interferences. I usually have to use several
standards so I can pick and use the ones that work best. I find that the best wavelength for P quantitation is the emission at ~178 nm (I'm pretty
sure that's the one, I'm going off memory). All the other wavelengths tend to have too low intensity or have too much interference from other
elements to be of much use.
[Edited on 3-11-2015 by Crowfjord]
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bmse
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Thanks so much for the info!
The standards you pick and choose from to use for phosphorus, are those just single element standards?
And, would you happen to know of a good reference book for digestion methods of metal alloys? I'd like to read up more on digestion methods and all
the ins and outs of it/tricks for various elements, but I'm having a hard time finding much out there that is reliable?
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Crowfjord
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I work with metals, so my standards are just solutions of certified alloys. I try to use alloys with compositions as similar as possible to that which
I am testing. When all else fails, then I'll use something like a single element standard.
I don't know about any books like you describe. I think the information used at my work has been refined by years of practical experience by many
technicians. Much of what we know in the way of dissolution of various materials came about through trial and error, aided by chemistry knowledge.
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