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Alkaline Zinc chemistry question

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Q. Good day,

I have been wondering about the chemistry behind an alkaline zinc bath. What I am confused is it's often common to do titrations to measure zinc and caustic (NaOH) levels, and they can vary independently from each other hence the need to test.

If the contents of the bath are Zn + NaOH + H20 that would chemically react to create Na2Zn(OH)4. This is where I can't understand how caustic and zinc levels can be dissolved in a bath independently from each other.

If there was no reaction to form Na2Zn(OH)4, I would understand Zn would simply exist dissolved in water floating around by itself, and same for NaOH - and as such the levels "gram per litre" vary.

Does a alkaline zinc bath once made up only contain Na2Zn(OH)4 which is the dark brown we see, or does it contain the Na2Zn(OH)4 molecule and also Dissolved Zn and NaOH in the water as well that can't react with each other, hence being able to have varying zinc and caustic levels.

Any explanation to understand better welcome. Thanks

Boris Siljanoski
- Perth, Western Australia
September 10, 2022

A. Hi Boris. I'm not a chemist and can't claim that Na2Zn(OH)4 is or isn't the only form in which zinc is dissolved in an alkaline zinc bath, but there might be a bit of overthinking here ...

If you have an alkaline zinc plating bath where you've just analysed the zinc and the caustic, there is nothing preventing you from arbitrarily adding caustic to it, such that you now have the same amount of zinc as you just measured, but more caustic than you just measured; and nothing prevents you from operating the line, plating some zinc out from it, and having less zinc than you just measured.

Alkaline zinc plating tanks often employ a 'generation' tank -- a side tank with zinc anodes (actually zinc slabs), wherein zinc dissolves to its 'natural' concentration in the caustic, and from which the operator can control the bleed of these contents into his plating tank such that the zinc concentration in the plating tank can be limited.

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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