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Sodium hexametaphosphate (SHMP)

Sodium hexametaphosphate (SHMP) generally performs very well at close-to-neutral pH ranges, while tetrasodium pyrophosphate (TSPP) and sodium tripolyphosphate (STPP) work best under alkaline conditions. Monosodium phosphate (MSP) is often used together with SHMP for more acidic pH environments. The so-called threshold effect refers to the ability of some phosphate compounds to inhibit the formation of carbonate or sulfate scales well below the amount that would be required for a stoichimetric 1:1 combination with the metal ions. This apparently results by the phosphate interfering with early crystal growth. In the case of SHMP, only 2-4 ppm is all that is required to inhibit scale formation in water with relatively high calcium levels.

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