Product Notes

Limitations of Test

Phenolphthalein (P) and Methyl Orange (M) Alkalinity

The alkalinity measured from the titration to phenolphthalein end point or P alkalinity is considered to be equivalent to the sum of all the hydroxide ion present plus one half of the carbonate ion present.  The total alkalinity by titration to the methyl orange endpoint or M alkalinity includes the other half of the carbonate ion plus any bicarbonate that was originally present.  The presence of phosphate, silica, organic or other buffers may affect the titration and the calculation of the form of alkalinity present may be in error.    

The presence of bases other than hydroxide, carbon and bicarbonate will interfere directly.
Common bases titratable by acids include sulfide (S=  ) , hydrosulfide ( HS- ), cyanide ( CN-)  and ammonia (NH3) . All these will add to the apparent concentration of one or more of the alkalinities .

measured.  Care should be exercised in interpreting alkalinities found in these water strictly in terms of scaling tendency.  However, under normal circumstances in plant control, expression of result only as the P and M alkalinity actually determined is satisfactory and is preferred from the standpoint of simplicity. 

Hydroxide Alkalinity

The method for hydroxide alkalinity is considered accurate only to within 10 per cent.  High silica, organic matter and aluminates can interfere with the determination, as will the bases mentioned above.  High sulfate content in the sample can produce turbidity which may mask the endpoint.  However, the method for hydroxide alkalinity is satisfactory for routine control and can be valuable in the control of lime-soda softening. 

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