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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Schwarz107 on November 09, 2017, 04:38:08 PM

Title: Why does a higher pH mean less hydrogen cations?
Post by: Schwarz107 on November 09, 2017, 04:38:08 PM
Hi,

I looked up the etymology of pH, and according to one source (https://www.etymonline.com/word/ph) it's the following:

pH, from P, for German Potenz "potency, power" + H, symbol for the hydrogen ion that determines acidity or alkalinity

Okay, so a potent solution of hydrogen would naturally be a solution with many hydrogen cations, right?

But when we look at the pH scale it looks like this. So, given that the etymology is correct, wouldn't it make more sense if the scale were reversed? So, a potent solution of hydrogen cations would have a high number, say pH 14.


(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.all-about-ph.com%2Fimages%2Fhoh-relationship-ph.jpg&hash=c80ea0f217cc6990b942ad89bdeec14549f32674)
Title: Re: Why does a higher pH mean less hydrogen cations?
Post by: Corribus on November 09, 2017, 06:23:51 PM
You have to consider the mathematics of the log function and the effective concentration range encountered.   Over most realistic concentrations the log function returns a negative value. 
Title: Re: Why does a higher pH mean less hydrogen cations?
Post by: Enthalpy on November 10, 2017, 09:24:15 AM
Hallo Schwarz107 and the others,

Experimental sciences (notably chemistry) are difficult enough that linguistics shouldn't distract your efforts.

The ancestors found one number to characterise the acidity, it was negative so they removed the sign but then it varies in the wrong direction. Live with that.
Title: Re: Why does a higher pH mean less hydrogen cations?
Post by: Schwarz107 on November 10, 2017, 02:30:37 PM
Quote from: Enthalpy on November 10, 2017, 09:24:15 AM
Hallo Schwarz107 and the others,

Experimental sciences (notably chemistry) are difficult enough that linguistics shouldn't distract your efforts.

The ancestors found one number to characterise the acidity, it was negative so they removed the sign but then it varies in the wrong direction. Live with that.

If I could have it make sense, it'd be easier to live with. So, I suppose we could say that pH 3 is shorthand for pH -3? Because the pH logarithm is indeed negative.

Thanks for you input, by the way. Appreciated!
Title: Re: Why does a higher pH mean less hydrogen cations?
Post by: mjc123 on November 13, 2017, 07:03:59 AM
pH 3 is not shorthand for pH -3. They mean different things. pH -3 means a hydrogen ion concentration of 103 M (supposing that to be achievable practically!). pH is defined as the negative of the logarithm of the hydrogen ion concentration. That's just because it's more convenient to deal (most of the time) with positive numbers than negative ones.