KH

Carbonate Hardness (KH), also known as alkalinity, is the measure of carbonate (CO32) and bicarbonate (HCO3) ion concentrations dissolved in water. These minerals are present in municipal, well, and bottled spring water. The level of carbonate hardness in tap and bottled water depends on the source of the water and the treatment process it has undergone. Carbonate hardness helps stabilize pH. Water with a low KH level (50 ppm or less) tends to be acidic and can cause rapid pH shifts. A KH level of 100-150 ppm is recommended. One drop of the testing solution is equal to 17.5 ppm. 

(Ref. 9)

(Ref. 10), (Ref. 11)