I usually cringe when some of us refer to it as "baking soda". We should be saying "bicarbonate of soda", or "sodium bicarbonate". Then there would be no confusion. And, there is no reason to bake bicarbonate of soda unless you are depending on measuring a specific weight to be added to get a specific KH. Since it absorbs water it can consist of almost as much water as sodium bicarbonate when you measure out a few grams of it. Baking it drives off the water, leaving nearly pure bicarbonate of soda.

thats thrue, we should call things by there names to not be confused. sometime I have that problem with english (forget words all the time).
What I used was carbonate of soda. I used the recipe Tom gave to make a kh ref solution on the post about the ph drop checker.

so it could be the reason why kh goes up and not the ph?

thx for your help

Something is amiss. If you have X ppm of CO2 dissolved in water, and add bicarbonate of soda to raise the KH, the pH will have to go up too, unless the ppm of CO2 drops at the same time. One possible answer is that the KH you get with added sodium bicarbonate is not stable immediately. If you measure KH right after adding it, you get a higher number than if you wait a few hours and remeasure it. I am not a chemist, so I don't know why that is true.

thx I'll make a few more try. by the way, do you know what quantities of bicarbonate of soda it's need to increase the kh value in a certain volume?

Check here:

Buffering capacity and pH

great toll! thank you!

I'me glad and sad at the same time.

sad beacause I have test how good is my kh test kit and it showed that this nutrafin test kit is off by 3.
Glad beacause I found out about it. beter later than never.

now, I am gonna get a new test kit! a good one this time!

great! it worked! I raised my kh by 4 and the ph went up.... a little!

so I know have a kh of 7.5 for a ph of 5.6 (was 5.5 before so no big change). I'll wait until tomorow and see if ph goes up a little more like it should be (according to the link you gave me).
by the way, what should be the kh limit to reach? I like to make experience but do not want to kill what's in the tank.

thats odd. when I have used baking soda in the past it raises the ph as well as the kh. eg. In my 90g with a ph of 6.2 and a low kh of 3 I used a measured 7 tspns and my kh went up to 8 and my ph up to 7.4 . also 5.6 seems pretty low especially for fish. I would think your fish to be stressed at that level. I know when I have turned up my CO2 to anythything above 40ppm with a PH of 6 I see dramatic stress to the fish, to the point of laying on the bottom on their side. So I keep my CO2 at 35ppm and PH of 6.8. fish seem fine and the plants are happy.

there is no fish in this tank and it's a co2 tank. I have dose more bicarbonate of sodium and my kh went up lot but the ph did not follow as I thought.