# Molarity of hydrogen gas in standard hydrogen electrode

I am unable to understand the folowing statement of Wikipedia it is about standard hydrogen electrode :

The concentration of both the reduced form and oxidised form is maintained at unity. That implies that the pressure of hydrogen gas is 1 bar.

But why if I apply ideal gas equation then I am getting molarity of nearly 0.004M .this is condradiction to the statement in Wikipedia. N/V=P/RT THEREFORE M=N/V~0.004

The standard hydrogen electrode is a "reference electrode." It is supposed to be used with ultra-low currents.

In operation, hydrogen gas is bubbled through a 1 N acid solution, but there is little hydrogen gas in the solution. In order to get the hydrogen gas to bubble through the solution, a pressure of a bit more than 1 bar must be used.

• We know that 1bar pressure is less than atmospheric pressure. The reduced form ie. Gaseous form must have concentration unity. So how then? – JM97 Dec 18 '15 at 23:12
• In order for the hydrogen gas to bubble out in the solution the pressure MUST be a bit above atmospheric pressure. With constant bubbling the solution is saturated with hydrogen gas. – MaxW Dec 19 '15 at 1:36

The $[H^+]$ ion concentration is unity, i.e. 1M. What you are obtaining by ideal gas equation is number of moles of Hydrogen gas per unit Volume.

Hope this helps!

• Yes the H+ ion concentration is unity which is the oxidised firm of hydrogen in the redox couple but as from statement both oxidised and reduced forms of redox couple must be a one molar concentration but after applying ideal gas equation iam getting less concentration of reduced form of redox couple – JM97 Dec 18 '15 at 15:28
• The reduced form is $H_2$ whose concentration or we would rather say Pressure is 1bar (unity) and the oxidised form that is $H^+$ has concentration 1M. The term concentration for gas here implies its pressure and not the actual concentration. – Vaibhav Dec 18 '15 at 15:49
• So one bar pressure of a gas is equivalent to one molar solution? – JM97 Dec 18 '15 at 15:55
• Its just the way of writing. Pressure and concentrations are different things. These is no equivalence between them – Vaibhav Dec 18 '15 at 16:31