Polyprotic Acids

[Lauren To 1E]

For the questions involving finding the pH of polyprotic acids, the textbook says the second ionization can be ignored when Ka2<<Ka1. Is there a specific cutoff for this difference?

[Sophia Kalanski 1A]

i don't think there is a specific cut off but we need to know and assume that the second reaction will be way weaker and the K value will be extremely small

[Jenny Lee 2L]

I also agree; I think if Ka2 is extremely small, we can disregard this because most of the ionization has occured from Ka1. But based on the lectures, it doesn't look like we know much about polyprotic acids yet.

[Sami Siddiqui 1J]

There really isn't a specific cutoff. However, keep in mind that H2SO4 is the only exception when it comes to neglecting the Ka2 value in pH calculations. Because the acid is strong and polyprotic, H+ protons will not only be released to form HSO4-, but also SO4-2 in quantities that we just can't ignore. That in itself is mostly a reflection of the fact that the Ka2 value is abnormally high (10^-2).

[Alexandra Salata 2L]

There is no specific cutoff, but as people said if the Ka2 is very small, then we can assume that all the ionization is from ka1 and not focus too much on it.

[Kamille Kibria 2A]

I also don't think there are specific numbers, but i think it'll be rly obvious on a midterm or a test. in my discussion section the example given was the Ka1 and Ka2 of (COOH)2 were 5.9E-2 and 6.5E-5 respectively. there is a rly obvious difference between the two and i don't think there would be any ambiguity about it when we are being tested

[Pranav Daggubati 3C]

This is usually the case when the -1 ion is not as strong of an acid/base as the neutral acid/base. I don't think that there is a specific cutoff, however, and it just depends on the individual properties of the acid/base.

[SavannahScriven_1F]

The textbook does give a cutoff: K_a2 must be less than about K_a1*10^-3 for K_a2 to not affect the pH.

[Leyla Anwar 3B]

Does polyprotic mean that it can donate more than one proton? And therefore make monoprotic mean there can only be a subtraction of one proton? (or addition of many/one electron)