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Author: Subject: Extremely beginnery question (sorry)
Daffodile
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[*] posted on 1-2-2017 at 15:36
Extremely beginnery question (sorry)


Hey guys I have to prepare some extremely pure Tripotassium Phosphate, without mono or di phosphate.

Can I just mix stoichiometric quantities of Phosphoric Acid and Potassium Hydroxide? My partner says I can't, so I'm coming to you guys to dispell doubt. This sounds silly, but I have had problems before when mono di and tri phosphate were not differentiated.
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Hexavalent
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[*] posted on 1-2-2017 at 16:35


An old paper in "Industrial and Engineering Chemistry", (Vol. 23, No. 5, pp 470) describes the industrial preparation of trisodium phosphate. They use sodium carbonate with phosphoric acid to afford the dibasic salt, then use sodium hydroxide to form the tribasic salt. This is said to be due to economic reasons, and so I can't see why just using 3 equivalents (possibly a slight excess?) of KOH wouldn't also work. They perform the final reaction in hot solution, hot filter, and then cool to allow the tribasic compound to crystallise out. They make no mention of the presence of the monobasic or dibasic species in the product.

It's advised to crystallise in the presence of another salt to reduce caking. They mention that sodium fluoride, chloride and borate have been used. Whether you feel this is necessary, and in fact possible, depends on what you need your product for (and why it needs to be so pure).


[Edited on 2-2-2017 by Hexavalent]

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nezza
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[*] posted on 2-2-2017 at 02:11


You can.



If you're not part of the solution, you're part of the precipitate.
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