Mixing up an Alkalinity Standard

[jacko]

To calibrate some alkalinity test kits (homemade and other), I would like to mix up an alkalinity standard, but I have a couple questions.

I assume a meq/L is = mmol/L, correct?

Should I use NaHCO3 or Na2CO3 ? Is the solution just going to be a ratio of these two, based on the pH (Henderson-Hasselbalch relationship) ?

I will target pH 8.1 to match the average of my tank, But how does the pH of the test solution affect the titration in the test kit?

[Randy Holmes-Farley]

I assume a meq/L is = mmol/L, correct?


That depends. If you use baking soda, yes. If you use something else that provides more than 1 unit of alkalinity per mole, then no. Washing soda, for example has 2 meq/L per 1 mmol.

If you have the capability, I'd measure alkalinity by pH titration, not with a standard solution. I show that here:

What is Alkalinity
http://www.advancedaquarist.com/issues/feb2002/chemistry.htm

Note that there are some issues about what the endpoint should be for fresh vs salt water, so that complicates the issue of standards a bit, IMO. That too is discussed in the article.