Lead in Tin Lead Solution: Reliable Fix with AAS or ICP

January 24, 2013

Q. Hello,

I am having real trouble with an analysis method for determination of Lead in 90 10 tin/lead solution with an MSA acid system.

This is the method I am using;

Reagents
1. 0.1M EDTA.
2. 0.1M zinc sulphate

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(ZnS047.H20).
3. Buffer Solution – add 71 grams of ammonium chloride to 600 ml
of concentrated ammonium hydroxide and dilute to 1 litre with
water.
4. eriochrome black t

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Indicator – Dissolve 0.4 g of Eriochrome Black
T in 500 mls of IPA and 500ml of ethanolamine.
5. 30% hydrogen peroxide solution.
6. 20% Rochelle salt

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solution

Method
1. Pipette

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a 25 ml sample of the plating solution into a 250 ml
Erlenmeyer Flask.
2. Add 25 ml of 0.1M EDTA.
3. Add 50 ml of water and 3 ml 30% hydrogen peroxide solution.
4. Add 50 ml of the 20% Rochelle salt solution.
5. Add 25 ml of the buffer solution to a pH of 10.0.
6. Add 4 ml of the indicator solution.
7. Add 5 ml of formaldehyde solution and titrate immediately with
the 0.1M zinc sulphate to a deep blue endpoint.

Calculations
(25 – no. of ml 0.1M zinc sulphate required) x 0.825 = g/l lead.

Basically what happens when I add the Ammonium Chloride buffer solution the solution turns blue without a titration. I have tried using more EDTA to complex the lead, using more solution, using less solution making up the buffers/indicators again and again.

I have completed this analysis before with satisfactory results.

Does anyone either have another method please or perhaps have an idea of where I am going wrong!!

Emily Gardiner
Chemist – Poole, Dorset, UK

January 29, 2013 A. If you are titrating with standard Zn the endpoint is going to be pink, not blue. Try using less sample. Published methods often recommend using absurdly huge sample sizes (5 ml, when 2 or even 1 would be plenty) and it adds to the time, and wastes titrant. Dave Wichern Consultant – The Bronx, New York January 29, 2013 A. Hi Emily, Do you have access to Atomic Absorption Spectrophotometry (AAS) ⇦ on eBay or Amazon [affil link] or Ion Coupled Plasma Spectroscopy (ICP) ⇦ on eBay <!– & Amazon –> [affil link] ? Both of these will definitively tell you the metal ion concentration. If you don’t have either on site I’m sure that around the Poole/Bournemouth area there will be an independent lab that has one or the other technique available. Do you do an analysis for tin, or do you rely on the tin level being 100% minus lead content? It may well be worth checking your tin levels, not that it helps with your analysis problem, but it should give you some confidence about the lead level. Brian Terry Aerospace – Yeovil, Somerset, UK