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topic 10210

Function of Boric acid in Nickel Plating


 

Could anyone please advise me of the function that Boric Acid plays in Nickel Plating? Does it act as a buffer for pH control?

Andrew Elliott
- Sheffield, UK


 

Yup. A portion of the water in the solution hydrolizes as H2 gas and 2OH-. The H2 evolves as a gas, leaving the 2OH- behind to try to lower^ COLOR="#AF0000">raise the pH from where you wanted to hold it.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


 

Ted and I rarely disagree, but he had an "OOPS" here. The remaining OH, as stated, is a hydroxyl group and would actually raise the pH.

Boric acid, H3BO3, has the capability of ionizing with an H+ and a H2BO3- which provides one hydrogen ion to react with an available OH-. Actually, you can titrate a saturated boric acid solution with 0.1N NaOH and done carefully, find three separate pH's that it will try to hold.

The purpose of all this is: hydrogen forms nascent hydrogen, H2, at the surface of the plated part, leaving a two free OH- groups which raises the pH at the surface of the part and a couple of millionths of the solution (which is the diffusion or barrier layer). This higher pH typically forms a black deposit or at least, a nasty one. The H+ from the boric acid reacts with the OH- to hold the pH stable.

The beauty of a buffer is it is bi directional, resisting movement in either direction, thus it takes an available hydrogen and goes back to the original ion. This is why agitation is so important in most baths and you have to add acid to a nickel tank.

Drag out is by far the biggest loss of boric from a tank. Actual use or destruction is very minimal.

This is also why it is so hard to change a pH on a tank. You add and add and very little happens and then it abruptly changes a huge amount.

It is much more complicated, but that is the short version of it.

James Watts
- Navarre, Florida


 

Indeed it was an "oops". The OH- is left behind as I said, but this does indeed raise the pH, not lower it. Thanks for your far more detailed explanation as well, Jim.

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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