Re: phenolphthalein and methyl orange alkalinity

>Just before Christmas, I ordered a number of LaMotte and HACH test kits.
>The LaMotte kits arrived with no problem and I was very impressed with
>them. Today I received a package which I was hoping would contain a HACH
>Calcium and total Hardness Kit. Unfortunately, the supplier made a mistake
>and sent me the HACH Alkalinity Kit (AL-AP MG-L) by mistake. While looking
>at the instructions which came with the kit, I notice that, unlike the
>LaMotte Alkalinity Kit which measures Total Alkalinity, the HACH kit
>measures both phenolphthalein alkalinity and total (methyl-orange)
>alkalinity. Both measurements are in units of mg/l of CaCO3.
>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.
I am not familiar with these kits, but, if you are titrating with an acid,
phenolphthalein turns from pink to colorless over a pH range of 10 to 8.3,
with the last trace of color disappearing around 8.3.  Methyl orange turns
from yellow to red over a pH range of about 4.5 to 3.0.  So, it would seem,
with a given water sample, that you would get a much higher value (number
of drops of acid) with methyl orange than with phenolphthalein.    My guess
is that the end points of these two acid-base indicator dyes are an
agreed-upon standard for measuring and characterizing water samples.

Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174
In Jackson, Mississippi, where we had some freezing rain this morning.