I bought this product at Home Depot to use to lower the pH of my spa, which I had overdosed with sodium carbonate pH increaser. I was disappointed to find that the product label did not provide any of the critical information needed to estimate the dose needed to correct my spa pH. After reading this thread I set about to do a little research on muriatic acid to see if I could estimate the HCl concentration in the product. Here is what I did: I measured out 4 fluid ounces (118.4 cc) as precisely as I could and then weighed the contents on a precision scale with a result of 135.9 grams. These measurements give a density of 1.147 gm/cc. Per the hydrochloric acid table in the CRC Handbook of Chemistry and Physics aqueous HCl has a density of 1.147 gm/cc at a concentration of 29.6%. My second area of research was based on the product claim that it produces 90% less fumes than some other unspecified product. Presumably this relates to relative vapor pressures of the two products. I found vapor pressure data on Wikipedia for aqueous HCl concentration from 10% up to 38% (aqueous HCl can go as high as 40% concentration at sea level, but not higher because the vapor pressure equals atmospheric pressure at that concentration). Muriatic acid is produced at a maximum concentration of 38% to keep fumes manageable and allow for use at higher elevations. The vapor pressure for 38% HCl is 28.3 KPa (.27 atmospheres). By interpolation I found the concentration corresponding to a vapor pressure 90% lower than this (or 2.83 KPa), and the result was 30.9%. So I conclude that the Klean Strip Green muriatic acid product has an HCl concentration in the range 29-31%.
