pH is the acidity scale, and the low end usually eats copper.
What it is
pH measures how acidic or alkaline a water is on a scale from 0 to 14, with 7 in the middle. The scale is logarithmic, so a pH of 6 is ten times as acidic as a pH of 7. Well water in sandy ground with little limestone usually reads on the low side, and the EPA secondary range for drinking water is 6.5 to 8.5.
What it usually does
Water below about 7 usually keeps dissolving a little of whatever pipe it runs through. In a copper system that shows up as blue-green staining in sinks and tubs, a metallic taste, and eventually pinhole leaks in the older runs. pH also decides how well iron and manganese treatment works, since most oxidizing media needs the pH in a particular band to do anything at all.
What it does not prove
pH is not a contaminant and the reading does not say what is dissolved in the water. A low pH makes it more likely that metals are being picked up from the plumbing, but it does not establish that any particular metal is present at any particular level. Only a metals test does that.
When to test
Usually worth reading before sizing a neutralizer, before sizing any iron filter, and whenever blue-green staining appears. pH changes as water sits and off-gasses, so it is one of the numbers usually best read on site rather than days later.
See what this usually means for your water.
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