Zerochlor

[chem geek]

There's an important side effect from using sulfate-based products such as MPS or dry acid or sulfuric acid on a regular basis and that is the buildup of sulfates in the water. This PoolSan link gives an initial dosage rate of 200 grams per 10,000 liters. This is 20 mg/L of the triple salt that contains 43% MPS so the MPS portion is 8.6 mg/L and is probably what is measured in the PoolSan PoolTester as "oxygen level" where the desired level is between 3 and 8 mg/L. This initial dose gives an oxidizing power equivalent to 4 ppm FC. It also adds 12.5 ppm sulfate. This dosage is also added once a week when the pool is in use. Over 6 months this would be 325 ppm sulfate buildup if there were no water dilution. (NOTE: Typical MPS test strips measure in units of added product, so the triple salt level, where the usual weekly dose is 1 pound in 10,000 gallons is 12 ppm and equivalent to 2.4 ppm FC and adds 7.5 ppm sulfate).

There are two problems with the buildup of sulfates. First is increased stainless steel corrosion rates when chlorides are present as well. This EPA file describes the situation as follows:

Non-halide salts have little effect on stainless steels, but chlorides particularly tend to promote pitting, crevice corrosion, and stress-corrosion cracking. In some cases sulfates seem to aggravate the effects of chlorides. Chlorides present in amounts as little of 0.3% with sulfates present can produce severe corrosion. Even quite low concentrations of chlorides can cause corrosion when concentrated by occlusion in surface films. Oxidizing chlorides such as ferric or cupric chloride are specific for severe pitting, although halide salts can cause severe pitting and stress corrosion cracking. The austenitic stainless steels are, however, the most susceptible of all the stainless steels to “chloride” stress corrosion cracking.

This basically means that one should be careful about using sulfate-based products in saltwater pools such as those that use chlorine generators and typically have 3000 ppm salt levels or higher.

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[chem geek]

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The second concern with sulfate buildup is with corrosion of plaster, cement, concrete surfaces. In this case, a maximum limit of 300 mg/L sulfate is often quoted. Some references include Sulphate Attack, Sulfate attack in concrete and mortar, Mitigating Sulfate Attack, Sulfate Attack on Precast Concrete, External Sulfate Attack and Damage to concrete flatwork from sulfate attack and many other related articles/papers. Note that the sulfate does not go away except by water dilution so over several seasons it could easily buildup beyond this 300 mg/L limit.

[Teapot]

Just like to add, when testing the copper level, the test is for ionised copper (the active form) only when I bought my all singing pool tester did I test for total copper in the pool water. Over the 3 years this total copper level had reached 5mg/l (5ppm) turning the water into a contaminated waste product that should not be poured onto land or rivers/storm drains etc.