🧵 Untitled Thread

Anonymous at Tue, 5 Nov 2024 19:25:23 UTC No. 16463980

Teaching a class and I'm looking for advice regarding the quantitative analysis of phosphorus lab. The assignment in the chem 2 class reacts phosphate with ammonium molybdate, making the yellow solid. Then, they measure absorbance of the yellow solution. So, they're making the yellow solid (which is a qualitative method) and then trying to use absorbance to determine concentration (a quantitative method). The solution they make is slightly yellow, I think because there's excess yellow solid just floating around in it. So, spectroscopy might let you determine "amount of stuff floating around", but "amount of stuff floating around" is not a function of phosphate concentration, so long as there is any phosphate present.

I could be wrong in my thinking that the slightly yellow solution just has excess solid floating around, and there is some additional reaction happening making the solution yellow, or the solubility of the yellow solid is known and can be used to back calculate concentration, for example. However, I cannot find any literature or textbook to support the method their lab manual uses, or anything suggesting one of my alternative counter examples is valid. I think someone confused/blended the two methods and it's just a bad procedure? Or, there is some information I don't have?