Rising KH shouldn't lower pH ?

jbrazio

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Jun 14, 2009
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This was wierd.. at least I couldn't understand it !

Yesterday my KH was 6 and pH 7,0, I added some sodium bicarbonate to rise the KH to 8 because I want to inject more CO2 (push it to 30ppm) but keeping the same pH.

I've got pressurized CO2 injection. I DID NOT change the amount of CO2 being injected.. my deduction was that the KH would raise and so pH..

Today I did some more measures, KH was 8 as expected.. but pH was still 7,0..

If I deduct CO2 values from the KH/pH table, the amount of CO2 dissolved in the water increased.. Once again, I didn't increased the injection rate.
 

VaughnH

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First, don't worry about pH, it isn't a significant parameter in a planted tank. Just increase the bubble rate on the CO2, and the amount dissolved in the water will go up, unless you are doing something that causes very fast loss of CO2 from the water.

KH doesn't have to meet any minimum value either. It is better to have some KH, 2 dKH for example, than to have none, which is nearly impossible in an aquarium, but increasing the KH beyond what it will be with the tap water you do water changes with isn't normally necessary or even desirable.

Don't try to measure CO2 using that table of ppm CO2 vs KH and pH. The table is only good for water containing nothing that affects pH or alkalinity except carbonates and CO2, and our aquarium water just isn't that perfect. The table normally gives you a value for ppm of CO2 that is much too high.

You can use the drop checker method to get close to a good concentration of CO2 in the tank water, but eventually you need to watch the plants and the fish to zero in on the optimum bubble rate for the CO2. Then to complicate life some more, as the plants grow more mass, their usage of CO2 increases, so you can't just set the bubble rate once and forget it. You have to keep an eye on the plants and fish and make adjustments when needed. This is a good reason for regular, consistent pruning of the plants.
 

jbrazio

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Hello,

Yes, I know that the infamous pH/KH table reports wrong values..
But nevertheless a relationship between the three variables exists, I kept the CO2 variable constant and changed the KH variable, mathematically the pH variable should change.. or am I traing to put some logic where there is none ? ;-)

From my, limited, understanding:
- One of the effects of CO2 injection is that the pH will get more acid
- Higher KH values would mean that the tamper effect will increase, lowering pH variations
- Higher KH would also mean that I can get more ppm of CO2 in the water column but keeping the pH at a desired level

So thats why I used the table, I would like to keep a pH of 7 and have CO2 at 30ppm, this would mean that I must have a KH value of 10.

I'm not on the right track here ?

Thanks.
 

jbrazio

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Oh ! I forgot, if memory does not fail me, I read somewhere on another post that you use a "4dKH solution" on the drop checker.. what does that mean ?
 

aaronnorth

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Well imagine you had a kh of 4, and pH 6.6 out of the tap. According to a relationship chart that equals 30ppm.

how can you possibley have 30ppm without even injecting CO2 into the water!! That is the major flaw in the charts, because they dont take into account any other acidic substances, even NO3 & PO4 can affect the reading.
This is also why you shouldnt use tank water in the drop checker.
This is why 4dkh is needed in the DC, because it is a known reference with no other acidic compunds in it which wont affect the result. As CO2 gasses from the aquarium & into the DC, it lowers the pH and the colour changes, when it is pH6.6 the DC will be green.

Thanks, Aaron
 

jbrazio

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I completely agree with "tap water doesn't have CO2, but the table reports 30ppm", but that was not my point.. maybe I'm not geting trough..

I believe that KH acts like a tamper protection, more KH.. less pH with more CO2.
If the injection of CO2 changes the acidity of the water, raising the KH would allow to have more CO2ppm with the same pH.

Thanks for your 4dKH explanation, if I correctly understood I should create a solution base on RO/DI water with the addiction o Sodium Bicarbonate in order to get a 4dKH solution.