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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: tim__jones on September 10, 2007, 06:16:13 AM

Title: Making set pH solutions of H2SO4
Post by: tim__jones on September 10, 2007, 06:16:13 AM

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

Title: Re: Making set pH solutions of H2SO4
Post by: Borek on September 10, 2007, 07:31:53 AM

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).

Title: Re: Making set pH solutions of H2SO4
Post by: tim__jones on September 10, 2007, 09:06:53 AM

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.

Title: Re: Making set pH solutions of H2SO4
Post by: Borek on September 10, 2007, 09:16:45 AM

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

Title: Re: Making set pH solutions of H2SO4
Post by: tim__jones on September 10, 2007, 09:32:30 AM

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

Title: Re: Making set pH solutions of H2SO4
Post by: Borek on September 10, 2007, 12:37:43 PM

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.

Title: Re: Making set pH solutions of H2SO4
Post by: tim__jones on September 10, 2007, 01:19:39 PM

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

Title: Re: Making set pH solutions of H2SO4
Post by: Borek on September 10, 2007, 01:35:03 PM

Quote from: tim__jones on September 10, 2007, 01:19:39 PM

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.