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What is the purpose of additive and accelerator in zinc phosphating?

January 20, 2009

what is the purpose of additive and accelerator in zinc phosphating? as I know, additive is added when the free acid is too high. But for my last and first phosphating process, I never add in the additive for 20 hours operation for my 2000 Liter of bath. and it is too hard for me to check the free acid pointage by using bromophenol blue indicator. as the color changes also not so clear. does it any problem with my process?

Process Study - Taiping, Perak, Malaysia

January 21, 2009

When you think that you are just short of the end point, add another drop or two of the indicator.
To learn the endpoint, overtitrate the sample while watching the color changes and occasionally adding one drop of indicator. Then back titrate it with a dilute acid and watch the colors change. Then forward titrate it and look for a specific color change. Once you know the color, it is easy to replicate on a normal titration.
The alternative is to use a very good pH probe and instrument.You need to take excellent care of the probe and it will give you long life with excellent accuracy. Titrate slowly as you approach the endpoint pH or you will overshoot.
You need excellent agitation as you do the tiration. Use of a magnetic stirrer with a 1 inch spin bar at a medium speed is the best.

James Watts
- Navarre, Florida

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