What is the difference between PH and phS in the parameters of circulating water?

When calcium carbonate is at saturation in water, calcium bicarbonate does not decompose into calcium carbonate, nor does calcium carbonate continue to dissolve. The pH value at this point is called the saturated pH value, denoted as PHs. If the pH value of the water sample is greater than PHS and the S.I. is positive, calcium carbonate will precipitate from the water; such water is considered scale-forming water ； When S.I. is negative, the existing scale layer is dissolved, exposing the raw material to erosion in water; such water is known as erosive water ； When S.I. is equal to zero, the water is in a saturated state, and such water is stable. The concepts of saturated PH and saturation index were proposed by Langelier to determine whether calcium carbonate scale will precipitate in water. Based on this, it is proposed to use acid or alkali addition as treatment methods to control the precipitation of scale.

Definition of pH: The negative logarithm of the hydrogen ion activity (concentration), which indicates the acidity of a solution. A pH value less than 7 indicates acidity, 7 indicates neutrality, and a value greater than 7 indicates alkalinity. Because pH is used to measure dilute solutions, allowing the activity to be calculated based on concentration; its range is 1–14, and values that are too high or too low are usually expressed in terms of alkalinity and acidity. When calcium carbonate is at saturation in water, calcium bicarbonate does not decompose into calcium carbonate, nor does calcium carbonate continue to dissolve. The pH value at this point is called the saturated pH value, denoted as PHs. That is, the pH value of an aqueous solution when it is saturated with salts, as determined through calculation, is denoted by pHs; the subscript s indicates a saturated state (the same applies hereafter).