Metal finishing Q&As since 1989

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Determination of phosphate (TSP) in neutralization bath

Hello I'm new to the industry and I have a titanium etch bath (HNO3/HF) that involves a soak in a neutralization bath that contains HCl/NaF/TSP. I have methods for figuring out the NaF and HCl (I hope) but I can't seem to find a good method to find the phosphate. I've looked on this board and found a colometric procedure from a book by Aubrey Knowles but this book looks like it's out of print. Hach also has a method but that involves me buying a whole lot of supplies that I don't want to buy and a massive dilution. Any suggestions?

Geronimo Cebrero
Chemist - Oxnard, California, USA
April 23, 2010

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First of three simultaneous responses -- April 26, 2010

Once upon a time, I spent a fair amount of time streamlining and validating this method for the similar bath solution. I am puzzled about your use of HCl instead of the usual HF. It shouldn't matter for this determination, though. For me it was convenient to modify Knowls' method like this:

Step 1----------------
250-mL Volumetric Flask A: 1.0 mL bath sample
250 mL Volumetric Flask B: Blank (no sample)

Step 2----------------
Both flasks: add 10mL of a solution containing 2.500 g/L ammonium metavanadate and 170 mL/L concentrated nitric acid

Step 3----------------
Both flasks: and 20 mL of a solution containing 25.000 g/L ammonium molybdate tetrahydrate.

Step 4----------------
Both flasks: Dilute to 250.0 mL and mix well

Step 5----------------
Wait 25 minutes then measure at 465 nm against the blank in flask B

Result----------------
TSP (Na3PO4*12H2O) g/L= Absorbance x 131.6

You should always make sure that you understand the chemistry involved and then run standards to verify your method rather than rely on any advise given over an internet site.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

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Second of three simultaneous responses --

As another chemist (retired or retarded?) what is the reaction (s) that allow you to call it a neutralizing bath, The F is the active component in the etch and not the nitric.

James Watts
- Navarre, Florida
April 26, 2010

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Third of three simultaneous responses --

You might want to try analyzing for phosphate content using Ion Chromatography - that's what I use for my analysis and it works really well!

Nigel Gill, BSc CChem MSC MIMF
- Glasgow, Scotland
April 27, 2010

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Thanks for the responses.

My spec called for the use of HF and they called it an acid neutralization bath so I just kept the name the same.

Geronimo Cebrero
- Oxnard, California
April 28, 2010