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Titration for acid with Ni and Ti present

2005

I am currently working on NiTi polishing and am using a acid/methanol (sulfuric acid)based electrolyte. In characterising the system I would like to monitor the level of acid. When the electrolyte is new (i.e., no Ni or Ti ions) this is a relative straight forward titration with NaOH. However when the electrolyte is used the Ni and Ti ppt out (as hydroxides) during the titration. Not knowing the exact amount of Ni or Ti this leaves the acid determination difficult because the amount of NaOH could be to neutralise acid but also to cause the PPT.

Does anybody know of a method to determine the acid level accurately? Is it possible to complex the Ni and Ti ions so they will not interfere in the titration?

Any help appreciated.

Gerry McCaffrey
Medical Device - Galway, Ireland

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First of three simultaneous responses --

Titrate against methyl orange ⇦ this on eBay or Amazon [affil links] endpoint. This will give free acid before metals precipitate.

Jeffrey Holmes, CEF
Spartanburg, South Carolina
2005

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Second of three simultaneous responses --

Probably the easiest way would be to add a known excess of sodium hydroxide to fully precipitate the Nickel and Titanium and react with the acid, then filter to remove precipitate and back-titrate the solution using a known strength acid.

This way you remove the interference of the metal ions, while keeping the method relatively simple and not involving the expense of complexing agents.

Brian Terry
Aerospace - Yeovil, Somerset, UK
2005

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Third of three simultaneous responses -- 2005

One answer is to keep the endpoint pH below pH of 6. methyl orange ⇦ this on eBay or Amazon [affil links] and methyl red ⇦ this on eBay & Amazon [affil links] will work ok. You can look at other indicators and try them in the lab to see if you can find one that works better for you. Since it is a strong base-strong acid titration a pH of 4 is close enough and avoids much of the precipitation problem.
Probably the secret is to use a weaker NaOH solution. If you are using 1N try 0.1N. Consider using 0.05 N.
A Teflon spin bar agitation is a must and use a quite vigorous agitation with a relatively slow addition of the NaOH. You need to avoid having any areas of high pH as it will precipitate out metals which are very very slow to come back into solution. You may want to try adding another drop of indicator as you approach the end point. Play around with it in the lab and you will find something that works for you.

James Watts
- Navarre, Florida

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If the precipitated metals are a problem, why not complex them? Maybe a little KF? Another way would be to pick your indicator (try methyl orange ⇦ this on eBay or Amazon [affil links] , it changes about pH 3.8) to a range on the acidic side.

James Totter, CEF
- Tallahassee, Florida
2005

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2005

Thanks for the advice on this issue to date. To follow up on this further I have used conductivity as a measure of the endpoint in addition to an indicator. I had expected that on addition of the base the conductivity would decrease to a minimum and the increase indicating the endpooint. However on addition of the NaOH initially the conductivity increased before decreasing. The endpoint for indicator and conductivity coresponded well. However I have concerns that as the electrolyte is non aqueous (all atempts are made to minimise water) that the addition of the NaOH is having some "effect". Considerations that I have given are 1. The addition of NaOH introduces water that allows further dissocation of acid and hence increase in conductivity. 2. The reaction of methanol and sulfuric acid can give dimethyl sulphate (!). This may react with the water to give acid again hence increase in conductivity and not accurate level of acid determination.
Can anyone recommend a non aqueous base (tetramethylammonium hydroxide?) Would it be best to use an aprotic solvent?
Oncea again any thoughts/advice appreciated.

Gerry McCaffrey
- Galway, Ireland

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Have you considered dissolving sodium hydroxide in methanol [affil links]? Dependent on the concentration that you wish to use, you can then calibrate the solution against standard acid and adjust your calculation for the exact concentration of NaOH that you are using.

We have used this method successfully for the determination of the acid number for degreaser solutions (Trichloroethylene, perchloroethylene and methylene chloride) and for the determination of the acid content of various hydraulic and engine oils.

Brian Terry
Aerospace - Yeovil, Somerset, UK
2005

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The advice to use a methanolic titrant solution is a good one. You can buy ready-made solutions of KOH in MeOH (1N, 0.5N or - preferably - 0.1N), e.g. from Aldrich. That may clarify the conductivity picture. In addition, if you work in a nearly purely alcoholic solution the second pKa of sulfuric acid may get high enough (i.e. HSO4(-) may be a weak enough acid) to enable you to accurately define the end-point pertaining to the first sulfuric acid proton only. That way you stay far away from Ni and Ti precipitation. For total sulphate, if needed, you can then use an independent method such as turbidimetry (or turbidimetric titration).

Emanuel I. Cooper
- Yorktown Heights, New York
2005