Summary for fluoride-phosphate bath analysis Discussion
fluoride-phosphate conversion bath analysis is described as a tricky analytical problem in AMS2486. Don Matthews asks for methods to analyze trisodium phosphate, potassium fluoride, and 49% hydrofluoric acid in the bath. In reply, A. explains that the bath is more complicated than most and suggests only a general approach: use a colorimetric procedure for phosphate, similar to method 39.02 in Aubrey Knowles’ book; titrate hydrofluoric acid with standard NaOH to a phenolphthalein endpoint, while correcting for HF already neutralized by the trisodium phosphate; and determine total fluoride by another fluoride method, then subtract the fluoride attributable to HF to estimate KF. A. warns that the calculations are error-prone because one bad result affects the others, and says the best practical advice is to rely heavily on the coating’s physical properties rather than chemical analysis alone.
Q. Looking for methods to analyze for the baths constituents in the Fluoride-Phosphate Conversion Bath specified in AMS2486. Bath Constituents/Control –
1) trisodium phosphate
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(Na3PO4-12H20) 6.5 to 6.9 oz/gal
2) potassium fluoride
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(KF-2H20) 2.3 to 3.2 oz/gal
3) Hydrofluoric Acid (49% HF) 2.6 to 4.4 fl oz/gal
Any suggested methods for analysis is appreciated. Thank you.
Don Matthews
surface finishing – Holyoke, Massachusetts
2006
2007
A. Don-
From an analytical point of view, this bath is more complicated than most. In order to be relatively brief, all I can do is give you a hint at what needs to be done which assumes that you are a chemist and have access to some standard metal finishing analytical reference materials. For phosphate (TSP), work out a colorimetric procedure similar method 39.02 in Aubrey Knowles’ Colorimetric Analysis of Metal Finishing & Metal Working Solutions & Effluents
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endpoint, but that only determines part of the HF concentration since some of the HF has been neutralized by the TSP. You will need to account for the HF that was neutralized by TSP in order to get your total HF. This will require quite a bit of number crunching to express your final result in % of 48 wt% HF. For KF, you need to run total fluoride by any number of methods (search on this site) and do the math to subtract the result from the fluoride due to HF. As you can see, there is plenty of room for error as a bad result in one determination will also yield a bad result in another. Because of this, my best advice is to rely heavily on the physical properties of the coating rather than on the chemical analysis of the solution. Good Luck.
Jon Barrows, MSF, EHSSC
Independence MO
Analysis Methods for Phosphate Fluoride conversion coatings
Q. Does anyone have any analysis methods to control these baths?
Greg Payne
– Belfast Northern Ireland
April 1, 2018