RogueRose
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Tri-Sodium Phosphate - is contamination with Sodium Carbonate normal?
I've seen a number of TSP powders sold where the MSDS lists it as about 75% TSP and the rest is Sodium Carbonate, yet the product is soley named TSP.
This product is sold in the painting section of many hardware stores in the US.
Is there something in the process of making TSP that doesn't allow for a pure product or is the carbonate something that is added for performance
reasons (or as a less expensive filler).
Is this something that can be expected in other products that state it is pure TSP, like from a pottery supplier?
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Dr.Bob
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Many states have water pollution rules that limit the use of phosphates in products such as soaps, laundry detergent and dish detergent. So that
often limits the amount of phosphate in many products. The other reason is that TSP is used as a cleaner, so it may be that sodium carbonate helps
in that function. You can buy TSP that is pure, as well, many things labelled TSP are ALL sodium carbonate or other things, depending on where you
buy it. I would look to buy it on the web, as there you can find purer material, and also ask what the contents are before buying. Many stores
have employees that cannot even read the label, much less tell Na2CO3 from Na3PO4 on an MSDS. And sadly, disodium phosphate is much more common in
the lab, at least where I have worked, so it is hard to find surplus.
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RogueRose
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Thanks for the reply. That makes sense on a number of levels and I'm guessing that the carbonate is less expensive and helps being a base (as it is
for cleaning walls before painting or driveways).
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byko3y
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Trisodium phosphate is slightly more basic than sodium carbonate (12.12 vs 11.26 for 0.1M), and order of magnitude more basic than sodium bicarbonate.
It's a common problem for trisodium phosphate to become not a trisodium phosphate over time. The reaction is Na3PO4 + CO2 + H2O -> Na2HPO4 +
NaHCO3.
Are you sure MSDS tells exactly about sodium carbonate, or maybe it's something like "sodium carbonateS"?
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woelen
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Production of pure Na3PO4 is very difficult. Na3PO4 is quite a strong base and you get something like byko3y describes.
In solution you get absorption of carbon dioxide in a multistep process:
PO4(3-) + H2O <--> HPO4(2-) + OH(-)
The OH(-) in turn reacts as follows:
CO2 + 2OH(-) --> CO3(2-) + H2O
The latter reaction takes away OH(-) and this leads to completion of the conversion of phosphate to hydrogenphosphate ion.
In practical production processes, the phosphate solution is in contact with air for quite some time and hence it absorbs a lot of CO2 and indeed many
percents of it can be converted this way.
I myself have had a similar problem in making Na2WO4. I had impure brown MoO3 (with iron and other transition metals in it). I managed to get rid of
all those other metals by dissolving the MoO3 in a calculated amount of a solution of NaOH and filtering the turbid solution. On evaporation of the
liquid I obtained a nice white solid, but unfortunately it absorbed quite some CO2. When I add this white powder to acid, then it fizzles while
dissolving.
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