Re: P and M alkalinities

James Purchase asked....

>My confusion is caused by uncertainty over just what is being measured by
>each of these tests. As I understand it, Alkalinity in freshwater is caused
>by basic ions (predominantly but not necessarily exclusively HC03- and
>CO3--). Neither the LaMotte nor the HACH Alkalinity Test Kits give much
>background as to what is actually being counted by their titrations.
>What is phenolphthalein alkalinity and how does it differ from total
>alkalinity (I measured my tank's water with both test kits and while the
>LaMotte and HACH test kits give identical readings for Total Alkalinity,
>the HACH kit indicates that my water has zero phenolphthalein alkalinity. 

Alkalinity measures bicarbonate, carbonate and hydrate ions primarily.
Phenolphthalein alkalinity is assumed to equal all of the hydrate and
one half of the carbonate alkalinity. Methyl orange alkalinity is equi-
valent to total alkalinity. The endpoints of titration for the P and M
alkalinities approximate pH 8.3 and pH 4.3 respectively. In natural
waters phenolphthalein alkalinity is usually absent which is why the
Hach kit registers zero.

Dave Whittaker
ac554 at FreeNet_Carleton.ca