citric acid question 2

[josephn16]

Basic question on citric acid. I am a ChemE, but working in ceramics industry, so I am the "chemical expert" in the plant, but I am having a hard time figuring this one out. I have a specification for buying Citric Acid and it reads: "C8H8O7 (on anhydrous basis) 99.5%-100.5%" The question will undoubtedly come to me, how can you have 100.5%? I know I should know the answer, but I don't. Please help me refresh my memory.

 

[25362]

Primo. Citric acid has 6 C atoms, not 8.
Secondo. A mix of anhydrous (MW 192.13) and monohydrate (MW 210.14) may contain not less than 99.5% and not more than 100.5% expressed as anhydrous acid.

I hope this helps.

 

[josephn16]

Yes, that helps, thanks.

I never even noticed that there were too many C atoms, but I just checked the spec, and that's exactly what is says. I only started here recently, so I didn't write the specs for any of the chemicals, looks like they could use a once-over.

 

[25362]

to josephn16, what I didn't say is that the purity is measured by titrating the acidity of a given water solution of a sample against a change of colour with phenolphthalein using NaOH. In this manner one can get an equivalence of "more than 100%" anhydrous acid C₆H₈O₇.