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topic 4485

Orthophosphate levels in Pyrophosphate tanks


Hi There:

I'm a chemist working at an electroplating/forming company and I am having difficulty with the orthophosphate levels in our pyrophosphate tanks. We have been adding ammonium hydroxide in a vain attempt to alleviate the problem but it was to no avail. It also appears that the ammonia which is added into the tank via a tube evaporates off in less than a day. If anyone knows any other methods to deal with this problem please let me know.

Jim Boulanger
- Woodbury, Connecticut USA


Just from a chemical standpoint you will always get the condensed phosphates (suck as pyro) degrading to the ortho form. In the case of TKPP (tetrapotassium pryophosphate) degradation to DKP (dipotassium phosphate) is accelerated by elevated temperature and lower pH. The pH of TKPP solutions are so high, adding ammonium hydroxide won't move the pH much. It is also volatile (NH4OH is a gas dissolved in water). With time and heat the gas wants to go back to the lower state which is out of the solution. You may want to try the alkali of the base salt, KOH for potassium systems or MaOH for sodium systems.

If you need to adjust to the acid side in a pyrophosphate system, you might want to consider using a polyphosphoric acid with a low ortho content. 115% polyphosphoric acid only has 5% orthophosphoric acid content.

Lee Branch
Richmond, Virginia

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