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P-Alkalinity and T-Alkalinity

Phenolphthalein is less alkaline than hydroxides or carbonates, and when it is 
added to a sample containing hydroxides and or carbonates it will first neutralize the 
hydroxides forming salts, it will turn pink in color. The acid used after this coloration will first  neutralize the hydroxides forming salts, it will then react with the carbonate molecules 
present forming bicarbonate molecules. Bicarbonate molecules are less alkaline than 
phenolphthalein, hence, the pink coloration disappears once all the hydroxides and 
carbonates have been dealt with by the acid. One bicarbonate molecule is formed from two 
carbonate molecules, hence in the test the quantity of acid used is a measure of the 
alkalinity due to the hydroxides (caustic) present and half the carbonates. 

Methyl-orange indicator is less alkaline than phenolphthalein and bicarbonates. 
It can be used initially in place of phenolphthalein or in continuation after the alkalinity to 
phenolphthalein test. If no yellow coloration results when the methyl-orange is added to the 
alkalinity to phenolphthalein sample no bicarbonates are present. Hence no carbonates are 
present. Therefore, the alkalinity as determined in the alkalinity to phenolphthalein test has 
been due to hydroxides alone.


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