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Hey folks, just got off my night shift and tested my pool water before bed. My CH test turned purple instead of blue! I know high chlorine can mess with tests, but why purple? My Taylor test kit says blue means calcium is present. Is the chlorine reacting with the dye? Pool's at 5 ppm chlorine right now (a bit high after shocking).

Whoa, purple pool chemistry! 🍇 As a health nut, I think about how chemicals interact in our bodies too. Chlorine is an oxidizer - it might be oxidizing the indicator dye in your CH test. The dye is probably eriochrome black T or something similar that changes color based on calcium ions. High chlorine could alter its molecular structure, shifting from blue to purple. Have you tried adding a chlorine neutralizer drop first? That usually fixes interference issues!

GummyGuru is on the right track! The chemistry here is fascinating. Most CH tests use eriochrome black T indicator that forms a blue complex with calcium ions at pH 10. Chlorine (hypochlorous acid/hypochlorite) is a strong oxidizing agent that can degrade the indicator molecule itself. The purple color likely represents a partially oxidized form of the dye or an intermediate species. The oxidation changes the chromophore's conjugated π-electron system, altering light absorption from blue to purple wavelengths. Try adding sodium thiosulfate (chlorine neutralizer) before testing - it reduces chlorine without affecting calcium measurement.

Great explanations! From a natural science perspective, this is a classic redox interference. Chlorine (Cl₂/HOCl/OCl⁻) has a standard reduction potential of +1.49V, making it powerful enough to oxidize many organic dyes. The eriochrome black T indicator has azo groups (-N=N-) that are particularly susceptible to oxidation. When those break, you get different colored products. Purple suggests partial oxidation - complete oxidation would probably give a colorless result. This is why test instructions always say to neutralize chlorine first with reducing agents like ascorbic acid or thiosulfate.

Thanks everyone! That makes sense - chlorine is basically bleaching the dye. I added the neutralizer drop and retested: nice blue at 250 ppm CH. So the purple wasn't measuring wrong calcium, just a messed up indicator. Science is cool even at 3 AM after a long shift! Should I worry about my 5 ppm chlorine level though? Fiberglass pool here in CA.

5 ppm is fine for fiberglass! 🏊‍♂️ Vinyl liners might get bleached, but fiberglass handles it well. Just make sure pH is balanced (7.4-7.6) since high chlorine + low pH can be harsh. The neutralizer trick works for most tests - I use it for alkalinity too when chlorine is high. Your pool's probably just oxidizing organics after a shock treatment. Test again in 24 hours!

Exactly! The neutralizer (usually sodium thiosulfate, Na₂S₂O₃) reduces chlorine to chloride ions: 2Na₂S₂O₃ + Cl₂ → Na₂S₄O₆ + 2NaCl. This stops interference without affecting calcium ions. For your 5 ppm chlorine: that's actually within safe swim limits (up to 10 ppm for many guidelines), though 1-3 ppm is typical maintenance. Fiberglass is inert, so no corrosion concerns. Monitor pH as GummyGuru said - chlorine effectiveness depends on pH. Cool chemistry lesson from a simple color change!