however it might not be phosphoric, because i tried precipitating it with the Fe3+, and it didn’t precipitate. That might be because the solutions were too dilute.
Okay fair point about the instructor saying triprotic, but what other triprotic acids are there besides citric (which has an even worse curve with only one visible eq point)? I suppose they could grab some crazy organic acid which no one has ever hear of…
Why couldn’t we have been given pH meters 
he said it was a mineral acid found in soda so i was assuming phosphoric
“…pKa1=2.15, pKa2=7.20 and pKa3=12.35. That means the titration curve contains only two inflection points and phosphoric acid can be titrated either as a monoprotic acid or as a diprotic acid.”
Source: http://www.titrations.info/acid-base-titration-phosphoric-acid
Basically, the problem is that the phosphate ion is too strong of a base to detect.
We must be missing something. USNCO intentionally made it impossible to detect the third equivalence point using more simple methods. Any ideas? As a reminder, all we had, includig the unkown acid, was:
- Fe(III) solution
- Iodine solution
- Thiosulfate and starch solution
- Stopwatch
- NaOH solution
- Deionized water
- Universal indicator (pH range 4.0 - 10.0)
- 2 beakers
- 2 graduated cylinders
whoa that’s a lot of chemistry
we could precipitate with Fe3+, but I tried it and it was clear (maybe solutions are too dilute). but then again, its ksp is 2.3 x 10-18, so it might not even be phosphoric acid.
It was definetly phosphoric acid because my lab procter checked the materials list they were given. I got yellow, green, then pink for the color changes.
@youngdslr The thing is, phosphoric acid doesn’t even have a third color change… in fact, the third equivalence point occurs around pH = 12.
How did you get two color changes in the lower pH range? I think you mistook one of the color changes for an equivalence point.
I mean they weren’t necessarily in the lower region? I just added NaOH and observed sudden color changes. I added straight NaOH with some indicator and saw that the solution turned very pink so I assumed the last pink color I saw was the third equivalence point.
I just got the test itself back. Let me know if there any specific questions you want me to reprint / discuss / give answers to.
anyone knows when acs will post the national exam on the website?
@ericliu760 Sometime next week.
When will camp results come out?
I don’t think so
Based on past experience, you have to add a few days to the dates USNCO publishes.
don’t jinx it
Anyone know if the test and key will be posted online tommorow?
Has anyone received a camp call yet?