Water balance question

[McDuff]

Something is perplexing me, at least about the terminology. If ph is a measure of acidity in the water, and the ph is high, why do we add acid to lower the ph?

I'm sure it is just me, but what is my pea brain missing on this one? Thanks!

 

[McDuff]

I got it figured out. Low ph = acidic........duh.

 

[Donldson]

As simply as possible: because acid has a low pH.

The reason you are confused is you are assuming the scale goes from low to high acidity. Rather start in the middle at 7, as it gets lower it becomes more acidic. Each digit lower is 10x more acidic than the last. If you start at 7 and go up then it becomes more basic, 10x from one digit tho the next. Don't think of it as a scale from 0-14 measuring acidity, think of it as a scale that starts at 7 and goes infinitely in either direction.

 

[JoyfulNoise]

pH is technically the measure of how much hydronium ion (H+) is in the water. It's based on the concentration of that ion in solution and it is mathematically expressed as a negative logarithm in base 10.

H₂O <----> H⁺ + OH^- pKa = 14

The above reaction is the most basic equation of chemistry.

When water has a pH of 7, there is an exact balance of [H+] and [OH-] ions. When water has a pH less than 7, there is more [H+] than [OH-] and we humans describe that with the adjective "acidic". When the water has a pH greater than 7, there are more [OH-] ions than [H+] and we humans use the word "alkaline" or "caustic" to describe it. When you add more acid to water, you add more [H+] ions and the the pH decreases. The reason why it decreases is because of the mathematics we use to describe what is happening in the equation above.

 

[McDuff]

You guys are great. Thank you.