Free caustic and total alkalinity

Free caustic content depends on the initial concentration and on the sulfide and phenolate concentration rate. 

Caustic loss = phenol input X - + HjS input X — + mercaptan input X —. 

Experience has shown that the loss calculated in this way is always lower than the effective measured loss. 

For a maximum possible effectiveness of 90% for the initial caustic, actual effectiveness ranges in fact from 40 to 75%, Part of the free alkalinity is in fact carbonated into Na2C03. It is not usually measured, but may account for 10 to 25% of the total alkalinity. It is advisable to state the total alkalinity of a spent caustic rather than the free caustic, which only represents 75 to 95% of the total allmlinity. 

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