morris

From what i understand about kh & ph is that it goes hand in hand:
if you raise kh, ph will raise to and visa versa

then how is it possible that some people say there kh is say 1 deg, and ph say 7.2 and other's kh is say 4 deg, and ph say 6.8

Isn't it suppose to be the same all over the world, cause if you've got a higher ph, you'll oatomatically have a higher kh, same goes for lower values?

where i live in South Africa, our values are ph-7, kh-2
After adding co2 the ph goes down to +-6, and kh to 0.5

Now some people recomend to raise the kh, add baking soda, wich again raise the ph, but then they also add to raise kh, add baking soda, and add more co2 not to raise the ph, but by adding more co2, won't that again lower the kh?

I'm seriously confused, or am i just missing something?

ashappard

your getting confused when you follow the idea that adding CO2 has a significant effect on KH.
theres a few threads on it, good question and its been up several times. Like this one for example

in the case where KH is observed at 2.0 and then 0.5, attributed to CO2 -- how reliable is the measurement?

ray-the-pilot

This is a confusing issue so here is some help (I hope).

You can think of KH as the total amount of CO2 that will dissolve in your water. It has nothing to do with the actual amount of CO2 that is dissolved in your water.

A good KH test should give you the same result no matter how much CO2 is dissolved in your sample. Unfortunately, must kits are designed for fairly high KH and low dissolved CO2. They titrate the sample with an acid to a pH of about 4.3. The assumption is that all of the CO2 will outgas at that pH (which is not true). It takes quite a long time for the CO2 to outgas and typically you need to boil the sample for a few minute and then add an additional amount of acid to find the KH. For samples with a high KH and low CO2 this additional amount is not significant. Your sample has both low KH and high CO2 so the standard test will be unreliable in this range.

KH has nothing to do with pH. At a given KH the pH will depend on the actual amount of CO2 dissolved in the sample water.

If you start with a solution of NaOH and check it with your KH test kit, it may say that the KH is 10 deg. The pH will be something like 13. Now if you bubble CO2 into this solution the pH will gradually go down as more and more CO2 dissolve in the sample. If you stop at some pH, say 7.0, and check the KH accurately (ie boiling to drive off the CO2) it is obvious that the KH will still be 10 deg.

Besides CO2, pretty much any other acid that you add to your water will reduce KH and any other base that you add to your water will increase KH. Things like NaOH NaHCO3 Na2CO3, Na3PO4 etc will increase KH. Things like HCl, NaHSO4, HNO3 will decrease KH. Note that these chemicals will also lower or increase pH as well but you can still adjust the final pH of your sample by adding or subtracting CO2 from the water.

That is probably more KH information than you need.

saulat

I feel like I should understand this already but I am having some issues. I made up a 4dKH solution for my drop checker and it has turned pale yellow. I turned down my CO2 a bit and checked the water finding a KH of just over 2dKH. On my water change day I added some baking soda boosting it up to around 5 or 6dkh. I also have CaCl2 and MgSO4. My GH is 180ppm or higher while my pH is around 6.5 to the best I can tell by the test strips I have. Will adding either Ca or Mg raise the KH or only the GH? Will raising the KH automatically affect the pH? My drop checker is still showing yellow after raising the KH. The solution is only about a week old. Is baking soda really all I need to regulate KH?

ray-the-pilot

It looks like your KH and GH are pretty good. You should try to reduce the CO2 level so that your drop checker is green. This will cause your pH to go up to 6.9 (that is a guess so don't use it as a marker). I would not add any Ca or Mg until your CO2 stablilzes. Baking soda will increase KH. To decrease KH you need to add an acid. Ca and Mg only increase the GH.

saulat

Cool, thanks. Thought Ca and Mg had some overlap with KH or that GH had somve overlap with KH.