It's a bit of a long intro, please bear with me.
As part of a water testing project, I am using a color method to test for Soluble Reactive Phosphorus or SRP. The idea is this: you filter a water sample, then you add in order sulfuric acid, then a solution of molybdate and antimony and finally ascorbic acid. The molybdate gives a nice blue complex with phosphorus (of a certain type, hence "reactive" in the name) at can be read at 880 nm.
I can also test for Total Phosphorus (TP). It is basically the same, but you don't filter and before adding the molybdate and ascorbic, you add the sulfuric + persulfate and autoclave it in a "digestion" step.
I've been observing a weird phenomenon, where SRP often overshoots in color development during a run (not all samples) and I have to repeat the sample, but TP never does. I've been washing the glass tubes with acid between runs and still - always SRP, never TP. Can anyone think of a valid reason for something like that?
Residues on the glass would have been digested with the TP and overshoot as well, so I don't think it's that. But I might be wrong.
Two more points that might help: 1) an "old" molybdate solution will result in higher background overall 2) There is another method where you combine all solutions together and add once. This one also statistically has never been observed to overshoot but the overall reading is consistently lower (recovery). Strange.