Kh And Co2 Questions

[gt568]

Right I'm struggling to understand the relationship between KH/CO2/and PH. I'm at 6.8 ph and thought I'd do some testing this morning. My KH came out at 13 which would make my CO2 somewhere between 47-70ppm. Bit too high so if I lower my bubble rate, is this going to make the KH go down thus reducing co2ppm or make the ph go up or a mixture of the two? Can anybody explain? Ta.

 

[gt568]

Or maybe its because my kits are quite old, how long do they last?

 

[gt568]

Anybody???

 

[jimbooo]

hi there, i'm no chemist but i'll try be it in laymens terms

firstly KH is the hardness of your water. ie the amount of calcium carbonate dissolved in it (i think). for some reason the greater amount of this in the water the less co2 will effect PH.

so if you put 2 bubbles per second into two tanks. one KH 3 and one KH 13 then the KH3 tank will have a higher co2 content after x amount of time than the KH 13 tank.

when co2 disolves in water you are actually adding carbonic acid (some how!?!) this obviously will raise the PH level

KH tells you the buffering capacity of the water

hence PH has a direct correlation to CO2 levels

so you can combine those figures using a mathmatical formula the result being the co2 levels in the water.

this is just blind faith on my part. i do not know the specific detail of all that just the rough outline. but hopefully that will clear things up for you

google "chucks planted tank calculator" and download the program. your co2 content is 62ppm with that KH and PH. to lower it you need to either lower the co2 rate going into the tank or raise the KH of the water (co2 adjustment is obviously the easier way to go)

on a final point just to throw all that out the window... PH test kits are notoriously very rough. i have never been able to pinpoint PH using test kits the colour of the solution will look different under different lighting or how far from the colour chart you look at it. the angle you hold the tube at etc.... the error margin is enough to take you below or above good co2 readings once you calculate the readings. this is another reason why drop checkers are good. they do not give you an exact reading they just tell you if there is not enough / enough / too much co2 in the water. removing the worrying over test kits etc.. at a quick glance at a drop checker you are reassured of the approx levels. this is more than enough indication for me and it saves all the faffing about with test kits.

 

[Themuleous]

A very quick comments, Jim you're wrong the CO2/KH/pH relationship is fixed. I.e for a given CO2 and KH you get a set pH. I dont have time to go into it in more detail so I nicked this from Chuck

Myth: A Low KH results in a larger pH swing when adding CO2.
Many people are under the mistaken impression that a low KH results in large pH swings when adding CO2, while raising the KH will result in smaller pH swings. This is not the case. The KH will move the start and end pH values, but the pH swing will be the same for a given level of CO2. You can see this in the chart below, or using the calculation:

Case 1: Assume a KH of 15 degrees, and a starting CO2 level of 4.5ppm, which would result in a pH of 8.0. If we then add CO2, to increase the CO2 level to 28ppm, that would drop the pH down to 7.2, for a pH shift of .8.
Case 2: Assume a KH of just 1.5 degrees, and a starting CO2 level of 4.5ppm, which would result in a pH of 7.0. If we then add CO2, to increase the CO2 level to 28ppm, that would drop the pH down to 6.2, for a pH shift of .8, the exact same as in case 1.

Sam