NaHCO3 isn't giving me the calculated KH increase

[Lmuhlen]

Hello,

I'm trying to increase the KH of a small shrimp planted tank and it is not working as calculated. My tap water comes with a KH of around 0.5 DKH and I would like to increase it by 4 points. I'll list the procedure I'm using and would like your best guess as to what is most likely the problem.

I'm using NaHCO3 bought in a 1kg package, from a lab materials store;
I'm making a solution using tap water in a 200ml container and adding 5,6g of NaHCO3, it dilutes completely;
I weight the powder with a cheap chinese electronic scale with a 200g capacity and a 0.01g resolution;
Using a 30ml plastic pipette, basically a graduated turkey baster, I add 30ml of this solution to 7L of water, which is the volume of the water change;
I measure KH of the water change water using a test from Prodac with water volume that gives a 1 DKH resolution, the result is 2 drops indicating 2 DKH, half of what was intended.

Do you think the problem is the powder, the scale, the KH test, the math... or what?

 

[Allwissend]

Math checks out. Could it be that the water change water wasn't completely well mixed. Then I would look into having a KH test from another company and compare the 2 results.

You can calibrate the scale before every use, most of them come with a calibration weight. It may not be accurate to the 100mg range but wouldn't expect to be so much off.

 

[SDdave]

Well, a great biochemist said one time never to use tap water because you never know what contaminates is in it. Always use distilled water.

I do, 100% in my tanks. No side effects and my chemistry is always the same.

Btw, What is your substrate, because your substrate has a big effect on those water parameters.

Maybe you need to do a biology experiment : take a few glass jars, and put a few inches of substrates of different kinds in each jar, fill them with distilled water. Wait one week, then test the water parameters for each different substrate. This is a test of how a substrate mixture effects the ph parameters (PH,KH) and I always suggest this to the serious hobbyist that wants a certain result so that they can start correctly with their tank setup.

Otherwise, you going to fight a loosing battle turning your tank in a chemistry set.