[tomtailford]

Practical question...

Someone we work with etches stainless steel using ferric chloride, as a result, they get a high content (44mg / L) of Nickel in the water they wash the stainless with.

What we have suggested, to decrease the amount of nickel is to first rinse with sodium hydroxide solution - to form a nickel hydroxide precipitate in a drum, then rinse for a second time with clean water to remove any caustic from the stainless steel plate. The precipitate could be collected in the container used for the first rinse, and the water they discharge after the second rinse should have a lot less nickel in it.

I wanted to know...
Would the alkalinity decrease over time as the nickel is ppt'd out? If so, at which point would the Nickel no longer ppt out?
What strength sodium hydroxide would be necessary to drop out the nickel from the water?

If anyone can think of any other practical issues relating to this, would be great to hear from you lot!

[tomtailford]

Anyone have any suggestions on this one?

[hobobot]

This type of issue is generally called Acid/Base Controlled Solubility.

The solubility constant of Ni(OH)₂ is 5.48*10^-16.

So the pH of the solution will go down until the equation

[Ni²⁺] * [OH^-]² = 5.48*10^-16

is satisfied. (If you didn't already know this : when chemists put things in brackets, they're referring to the molarity. Take note that the hydroxide concentration is squared.  You should really be using something called Activities, but that's a whole 'nother can o worms)

Because the solubility constant is so small, your NaOH solution is probably doing a very good job.

Or:
You could get some major cool points and use an electrochemical process to precipitate out solid Nickel metal.

The equation

Ni(OH)₂ + 2 e ⇌ Ni + 2 OH^–

has a standard voltage of –0.72V

[tomtailford]

Thanks Hobobot... I fed you a mole snack!

Further to this, I need to find if it is possible to use the solubility constant to calculate the pH at which the Nickel will form the soluble ion instead of the insoluble nickel hydroxide?

If people have some information on how to do this, I can try and work out myself... or if someone can give a pH that's lazy, but will achieve the same thing.

Ideally I would like to know how to do this, just in case I have to apply it again!

Thanks hobobot!

Tom

[Borek]

For a given Ni²⁺ concentration use Ksp to calculate [OH^-] at which precipitation starts, then convert [OH^-] -> pOH -> pH.

[tomtailford]

[Ni²⁺] = 44 mg/L = 0.044 g/L = 7.49*10^-4 mol/L

[Ni²⁺] * [OH^-]² = 5.48*10^-16

[5.48*10^-16 / 7.49*10^-4 mol/L]^-2 = [OH^-]

[7.31*10^-13]^-2 = [OH^-] = 8.55*10^-7

pOH = -log10[OH-] = -log10 8.55*10^-7 = 6.07

pH = 14 - pOH = 7.93

So... if my calculations are correct... the nickel will ppt out providing that the pH of the water is above pH 7.93??

I would appreciate if people can confirm / check my workings!!

[Borek]

Your calculations are OK. Reality can be a little bit more complicated, as Ni²⁺ will probably react with OH^- creating some soluble complexes. But pH 8 as the border case should be a good starting point.