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There is an article about tripotassium phosphate that states the following:

Consumers may have health concerns about why this cleaning agent can be used in food, but that is the technical grade, not the food grade. When used as a food additive, it almost has no side effects and its safety has been approved by the FDA...

There is an answer here that nicely addresses the difference between lab-grade and food-grade purities but does that answer my question? My question specifically is what is the difference between "technical-grade" and "food-grade" tripotassium phosphate? Is it just a matter of impurities? At the atomic level, K3PO4 is K3PO4, whether it's being used as a commercial degreaser, a stain remover, or a food additive, no?

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    $\begingroup$ Food grade must have acceptably low levels of impurities, such as heavy metals, as dictated by applicable authorities. Technical grade does not necessarily have to meet this requirement because it is not intended for consumption. $\endgroup$
    – Ed V
    Oct 31, 2022 at 18:02

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It is just a question of purity. Usually the manufacturer of chemicals prints their composition on the label. For example, the purest form of potassium triphosphate produced by VWR is

Minimum purity : > $98$ %; Free alkali : < $1.0$ %; Dipotassium hydrogen phosphate : < $1.0$ %; Sodium : < $0.5$ %; Chloride < $0.003$ %; Iron : < $0.001$ %; Lead : < $0.002$ %; Total nitrogen : < $0.001$ %

I don't have the result of the analysis of the same product, in a technical-grade quality. But I am sure that the same values are significantly higher. Technical products are cheaper. But they contain more impurities.

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    $\begingroup$ Correct my lay understanding here. When a chemical like tripotassium phosphate is made, compounds are concocted (in this case, to neutralize phosphoric acid) that, by the nature of the concoction, yield a chemical with impurities. This chemical then undergoes purification. When the purified chemical is tested for human safety, as a food additive, do the researchers simply look for the presence of side effects or do they try to determine if the side effects are caused by the chemical itself (in its purest form) or caused by its impurities? And can we even test this chemical with 0% impurities? $\endgroup$
    – trndjc
    Oct 31, 2022 at 22:21
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    $\begingroup$ Potassium triphosphate is made by adding enough $\ce{KOH}$ to neutralize phosphoric acid. But both these substances ($\ce{KOH}$ and $\ce{H3PO4}$) are themselves not 100% pure. They contain some impurities, that usually remain in the potassium triphosphate. Of course, these impurities can be removed by dissolution and reevaporation. But this treatment costs and is often not necessary, depending on the use of the product. If a phosphate is used as a fertilizer, the purity is not important. If a phosphate is used as a food additive, the product must be tested for side effects of impurities. $\endgroup$
    – Maurice
    Nov 1, 2022 at 9:01
  • $\begingroup$ So when the phosphate is tested for human safety as a food additive no distinction is made between side effects caused by the chemical compound itself (in its purest form) versus its impurities? $\endgroup$
    – trndjc
    Nov 1, 2022 at 15:04
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    $\begingroup$ @trndjc if it was at some point tested for safety as a food additive, an even purer form may have been used. Individual batches aren't tested for safety, though quite possibly are for purity, or at least for the presence of known contaminants. Certain impurities may be totally benign while others are harmful; we in food the concern is far more about the latter. The other potassium phosphates, for example, are generally recognised as safe meaning they're so well established that modern tests aren't needed. $\endgroup$
    – Chris H
    Nov 1, 2022 at 15:49

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