MPS shows up as Combined Chlorine in the DPD test but as Free Chlorine in the FAS-DPD test. While MPS does not react with DPD dye directly, it acts as an oxidizer to the potassium iodide in the R-0003 reagent for the CC test when using DPD and it reacts with the R-0871 FAS-DPD reagent when using the FAS-DPD test. Either way, the interference can be removed by the
Taylor K-2042 kit.
As for how long MPS lasts, it depends on what there is to oxidize, but it is also affected by sunlight though not nearly as much as chlorine. It is not true that so long as there is chlorine present that the MPS will not oxidize anything. They are each selective oxidizers. For example, chlorine very quickly reacts with ammonia to form monochloramine (and more slowly oxidizes this eventually to nitrogen gas and some nitrate) while MPS does not react quickly with ammonia (or ammonium ion) as described in
this paper. On the other hand, chlorine reacts rather slowly with certain organics while MPS reacts more quickly and in more varied ways. For example, chlorine forms combined chlorine through substitution reactions at nitrogen sites while MPS reacts with carbon-carbon double bonds to form epoxides. 2.2 pounds of (the triple salt of) MPS in 12,000 gallons is equivalent in oxidizing power to 4.4 ppm FC. Lasting around 10-14 days implies a loss of around 20-25% per day to end up with 0.08 (25% daily loss over 14 days) to 0.47 ppm (20% daily loss over 10 days).
It is primarily the sodium persulfate contaminant that is irritating though this tends to react and therefore dissipate in a day or two. Yes, very high MPS levels themselves can be irritating, but as noted by Pb2Au that happens when not responsibly using MPS -- it is not a substitute for chlorine except when used with Nature2 in spas (as noted in
this post, the silver ion of Nature2 reacts with the sodium persulfate so normally prevents it from being irritating).
As for sulfate being food for algae, this is not a concern because it is not usually a limiting nutrient in pools. There is far more carbon, hydrogen, oxygen and nitrogen required compared to phosphorous and sulfur. Sulfur is primarily found in a couple of amino acids and therefore proteins/enzymes containing such amino acids (as opposed to phosphorous used in the phosphate backbone of DNA, RNA and in the ATP energy carrier and in phospholipids in cell membranes). Nevertheless, actual sulfur content in algae is roughly similar to that of phosphate so though a macronutrient, far less is needed than carbon, hydrogen, oxygen and nitrogen. There are many pools that have never used MPS or dry acid products and have very low sulfate levels in fill water that readily get algae if there is too little sanitizer and there are sufficient phosphates (and possibly nitrates). Also, most fill water has more than enough sulfates to supply sulfur to algae and bacteria. Remember that for phosphates to be limiting enough to prevent algae growth, the level needs to be below around 100 ppb which is 0.1 ppm while the typical sulfate level in fill water in the U.S. has a median of 4.6 ppm (mg/L) and is even 0.5 ppm or higher in rain (see Figure 6 in
this link). There are concerns with high sulfate levels (above 300 ppm) with regard to plaster as noted in
this thread.
The three numbers for fertilizer are for nitrogen, phosphorous, and potassium (N-P-K). It is only when there is a fourth number where that represents the sulfur content (N-P-K-S).
