Swede
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Quantitative determination of Ti ion
I created an acidic solution of TiCl3 by simply dissolving an excess of Ti sponge in HCl. The leftover Ti solids were filtered, leaving a purple,
acidic solution of TiCl3 in HCl.
The question... how can I determine the concentration of Ti in this solution? I thought I could simply add a base like NaOH, which would yield
(hopefully) insoluble TiO2, leaving behind salt and water. The solids would be filtered, dried, and weighed.
When I attempted this, the addition of NaOH yielded not the yellow/white ppt I had hoped for, but a gluey, blue-black solid.  I think I made Ti III oxide hydrate.
If I cannot identify the solid, then yield calculations will fail. How can I convert the TiCl3 to a known, insoluble titanium compound?
[Edited on 6-1-2013 by Swede]
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kristofvagyok
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Titrate it with phosphate and measure the participated solid mass.
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unionised
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If you made it from Ti and acid, it's probably mainly Ti(III) in which case you can titrate it with an oxidising agent.
For example, you could add a known excess of iodine and then titrate the leftover iodine with thiosulphate.
Or, you could add an oxidant to the solution, then add base to precipitate the Ti as TiO2(? H2O).
You would need to heat the product to drive off water before you weighed it.
Bleach is a cheap oxidant.
Peroxide would sort-of work, but Ti forms odd complexes with peroxide which would confuse things.
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woelen
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I would indeed oxidize the Ti(III) and then make the solution alkaline, such that the Ti precipitates as hydrous TiO2. You then need to filter the
TiO2 and strongly heat the material to drive off any water. Altogether, it is not easy to do this kind of measurements, most likely due to mechanical
losses in the process. Some TiO2 may remain stuck in the filter, material may stick to spatulas and so on. It is very hard to do this kind of
measurements accurately, especially if you work on microscale.
Another option may be to titrate the liquid. Measure a certain amount of liquid as precisely as possible. Take some very dilute hydrochloric acid
(e.g. 2% HCl) and boil this for a while to get rid of dissolved oxygen. Add the liquid to the dilute hydrochloric acid, such that you get a manageable
amount of liquid (e.g. 20 to 50 ml) with just a weak color. Add some starch and potassium iodide to the liquid as well and then titrate with an
oxidizer (not H2O2, because this creates highly colored and quite stable complexes with titanium), until the strong color of starch/iodine appears.
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Swede
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Thanks for the suggestions, guys. If none of these work, I can always make a new batch by carefully weighing the mass of an excess Ti sponge, both
before and after dissolving in HCl, to determine how much Ti went into solution.
Frankly I'm surprised it's turning out as messy as it is. I was thinking the conversion to the Ti (IV) oxide was going to be rather simple, but it's
not.
H2O2 definitely creates the bright orange pertitanate. Got to figure out another way to oxidize it.
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