View Full Version : Relationship between pH/KH/CO2

Tin Pusher
11-23-2002, 05:46 PM
On trying to understand water chemistry a little more (ok a lot more) I am looking at the relationship between pH,KH, and CO2. There are several charts out (see link below) that show that if 2 out of the 3 are known the 3rd unknown can be fiqured out. http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html

My aged tap water for example is 7.0 pH, less than 1 degree or 17.9 ppm KH and that would be at a CO2 level equivelant to the air (0.5 ppm). Using the charts this fits the parameters. What I want is KH between 3 & 5 degrees to buffer pH levels and the CO2 levels are to stay the same. Punching those numbers into the chart I get a pH of close to 7.8.

1. If this is an exact chemical formula how can I get the desired 7.0 pH & 3 degrees KH?
2. How do chemicals that advertise stabilizing the pH levels without raising the pH work?
3. What do you use?

Hopefully this topic will lead to a debate which will lead to more questions.

11-23-2002, 10:23 PM
I'm no expert but if you tap water is 7ph after aging I wouldn't mess with it . Adding chemicals unless you have to can become expensive and destabalize the water if you don't fully understand what you are doing.
My ph is 10.2 out of the tap so I have no choice. I use seachem discus buffer to bring it down to 6.5ph.

I think to lower the kh and gh and keep a good ph you'll need an R.O. unit and mess with the mixture until you get what you want.

I'm pretty sure you can use baking soda (sodium bicarbonate) to buffer your water(Raise kh) but it will raise the ph in which case you will have to add an acid to bring it back down.

I'll let the experts help you out on that one. I hope I didn't mislead you. Take what I said with a grain of salt until someone who really knows what they are talking about helps you out. ;D

11-26-2002, 07:07 PM
Hi all, Tin pusher i know exactly what you mean ,my tapwater is very similar to yours and if not careful the ph can drop suddenly,but like you when one buffers the kh to a reasonable and hopefully stable level,the ph then tends to rocket up.Maybe somebody could enlighten us ?
cheers Alsbeth

11-27-2002, 11:43 PM
1. If this is an exact chemical formula how can I get the desired 7.0 pH & 3 degrees KH?

CO2 (ppm) = 3 * KH (as CaCO3) * 10^( 7-pH) - This is the formula used that correlates to the chart on thekrib.com. It assumes that the only buffering system used is carbonate (CO3). Use of phosphate buffers or peat will negate any correlation to the formula. With your values you would need to have 9 ppm of CO2 in the tank.

This chart is most commonly used by Planted Tank People. In an ideal planted tank there should be 15-30 ppm for good plant growth. This is why most Planted tank folks inject CO2 into the water column. In my tank I keep a kH = 4 and inject CO2 to get the ph down to 6.6 (= 30ppm CO2)

For your case I would not know if injecting CO2 would be warranted. Do you want to keep plants? If not just use peat or Sechem Buffers (My personal favorites)

2. How do chemicals that advertise stabilizing the pH levels without raising the pH work?

Most that lower the ph are phosphate buffer systems and the ones that raise ph are carbonate salts. Mixtures of each will stablize pH where ever you need it, but will also raise kH and GH

Sechem is the only company I found that makes an acid buffer that does not use phosphate. Do not know the chemical make up though - must be a weak acid like citric.

3. What do you use?

CO2 to lower ph and Baking Soda for kH (Na2CO3)

Wolf - Yes add Baking Soda for kH, but do not use a strong acid to bring the pH back down. You will pretty much be defeating yourself as adding the acid to bring back down the pH will precipitate the CO3 that you just added. This will result in a net effect of zero on the kH.

11-28-2002, 12:04 AM
Hey Paul,
I was hoping we could get a chemist to answer those questions. I was getting ready to try but I wouldn't have done them the justice that you did.

11-28-2002, 08:04 AM
Fantastic post!! API claim that their ph down contains no phosphates also.

Tin Pusher
11-28-2002, 07:55 PM
Is there any easy answers in the Discus world? ;)