[ajax0604]

Can someone help me with this question? I am assuming scientists didn't have ultra sensitive pH meters in the past.

[Borek]

Why do you think you need an ultrasensitive pH for determining these concentrations?

[Sophia7X]

They stick a pH meter in, even one decimal place is fine, and calculate the pH. No need for super accuracy?

at 25 C it's pH = 7.0 so [H+] = 10^-7 = [OH-]. Ka = [OH-][H+] = (10^-7)^2 = 10^-4.

Similarly,
at -35 C I believe the pH is 8.5. so [H+] = 10^-8.5 = [OH-]. (10^-8.5)^2 = Ka = 10^-17

10^-7 may be a small number but a pH measuring that would say 7 and a super accurate one might say 7.000000 (for pure water). Not necessary because it's still 7!

[ajax0604]

I see, thank you for your explanation. I thought that scientists determined the concentration of hydronium/hydroxide first, then established the pH but it was the other way round.

[Borek]

First pH electrode was constructed by Haber & Klemensiewicz in 1908, pH scale was proposed by Søren Sørensen in 1909, so these things happened almost at the same time. You don't need "highly sensitive pH meter" to determine pH - you may need a precise and accurate one to be sure the result is correct, but that is another thing (actually first pH meters and pH electrodes were plagued by problems).

No idea who was the first to determine Kw and what method was used. Interesting question!

[AWK]

Arhenius around 1890 from hydrolysis of sodium acetate,
Kohlrausch and Heydweiller 1894 from conductivity of the purest water.
Works of Arrhenius and Kohlrausch published in Z. Phys. Chem.
Cited after "The Electrical Conductivity of Aqueous Solutions" by Arthur A. Noyes from 1907 year.