[tim__jones]

Hi

I need to make up separate solutions of H2SO4 with pH values of 3.0, 3.1,..., 4.0, from a stock solution of 98% H2SO4. Is there a calculation or website that can help me work out how much stock solution and water I need to make up these solutions?

Many thanks

Tim

[Borek]

That's not an easy task. Most important things, you need to take into account that sulfuric acid is diprotic, with relatively low 2^nd dissociation constant; and 98% stock acid is not 98% but something around, most likely a little bit less than 98%.

Two other things worth of addressing: ionic strength of the solution and solution densities, although for about 0.001 M acid these can be probably neglected.

You may try to make calculations easier using my pH calculator and my concentration calculator (see signature). Still, even with these tools, IMHO you can't prepare your solutions properly without pH meter (or at least narrow range pH stripes).

[tim__jones]

Thank you very much, I will try that. I've worked out that the molarity of pH 4.0 H2SO4 is 5.0 x 10-5. Is there a simple way of working out the concentration of sulphate in this solution, without running it through an IC machine?

Thanks.

[Borek]

What do you mean by sulphate - equilibrium concentration of SO₄^2-?

[tim__jones]

I think so - I'm not sure what equilibrium concentration means though, I'm a biologist!

I just need to know what the SO₄^2- concentration (in mg/L) of a certain molarity of H₂SO₄ is.

Tim

[Borek]

It can be calculated as a function of pH and total concentration... see equation 9.13 here:

http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-acid-base

But if 8% precision is OK with you, you can assume all sulphuric acid is fully dissociated. Or - you can use BATE, it displays full detailed information about the solution.

[tim__jones]

Not sure I quite understand sorry, my chemistry isn't very good! I've tried the calculation but got a strange number.

Could you possibly explain how to do the calculation please? I've used BATE to get the data for pH 4 H₂SO₄ (Ca 5.0 x 10^-5, [H] 9.952 x 10^-5 mol/L)

What units is the value of A^2-?

Thanks for your advice.

Tim

[Borek]

I've used BATE to get the data for pH 4 H₂SO₄ (Ca 5.0 x 10^-5, [H] 9.952 x 10^-5 mol/L)

What units is the value of A^2-?

All concentrations are given in mol/L - in 5x10^-5 mol/L sulphuric acid concentration of SO₄^2- is 4.952x10^-5 mol/L.