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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Monoamine on March 22, 2021, 04:01:15 AM

Title: Concentration of HBr solution whose pH is < 0 (how do you measure negative pH?)
Post by: Monoamine on March 22, 2021, 04:01:15 AM

Hi, so I made this solution of HBr, but its pH is less than 0 (tested ectronically and with pH strips). So then how am I supposed to determine the concentration of the HBr solution? (I was going to calculate the HBr concentration from its pH...

One thing I was thinking was to take a small amount of the solution and add known quantities of NaOH to it until its pH is = 7. I think that would mean that there are the same amount of moles of HBr in the solution as moles of Na. This would give me a mol/mL value of the HBr in solution.

From this I could also determine how pure the solution is, since once I know the HBr concentration, I could calculate how the solution should weigh, and if it matches up, then this would suggest that the HBr solution is pure and doesn't contain residual H₂SO₄ or something...

Would this work? Or is there a simpler way of doing this?

Cheers.

Title: Re: Concentration of HBr solution whose pH is < 0 (how do you measure negative pH?)
Post by: Borek on March 22, 2021, 06:06:02 AM

pH is a very poor measure of the acid concentration.

Simple titration with a base gives much more better results (which is more or less what you are thinking about doing): http://www.titrations.info/acid-base-titration

Checking the solution purity requires meticulous tests of contaminants, every other approach will give just an approximate answer which is typically useless.

Title: Re: Concentration of HBr solution whose pH is < 0 (how do you measure negative pH?)
Post by: Monoamine on March 22, 2021, 06:16:22 PM

Thank you for the tip and for the link. I'll try to do it by titration in that case.