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topic 59134

Problems with determination of lead in 90:10 Tin lead Solution

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;

1. 0.1M EDTA.
2. 0.1M zinc sulphate (ZnS047.H20).
3. Buffer Solution - add 71 grams of ammonium chloride to 600 ml
of concentrated ammonium hydroxide and dilute to 1 litre with
4. Eriochrome Black 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 solution

1. Pipette 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.

(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

simultaneous 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
Dave Wichern
Consultant - The Bronx, New York

January 29, 2013

A. Hi Emily,

Do you have access to Atomic Absorption Spectrophotometry (AAS) or Ion Coupled Plasma Spectroscopy (ICP)? 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

February 1, 2013

thumbs up signThanks Dave. It turns out there is very little lead in our solution. I made up a standard solution of lead and the test worked fine.

Thanks Brian, we are having this confirmed by external analysis. The % results we are getting on XRF are showing slightly lower than usual.

I just want to say a big thank you to everyone at finishing.com. This is actually my last day as a Plating Chemist. This resource has been fantastic throughout my plating career and I will be sure to pass on the resource to my replacement.

Emily Gardiner
Chemist - Poole, Dorset, UK

February 1, 2013

thumbs up signThanks Emily, and happy trails.


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