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Discussion Starter · #1 ·
Following on from advice after my algae bloom, I now have a CO2 drop checker. I've searched the forums for experience with setting these things up accurately and I followed Tom's advice of using 4.99g Na-bicarbonate with 5L DEIONISED water to form a 40dKH batch, then dilute 10ml of this with 90ml DEINOISED water to form a 4dKH solution for use in the drop checker.

I've highlighted deionised as this I believe is where I am having issues....

When I created the 4dKH solution, I checked the KH with a test kit, and as predicted, bang on 4dKH.
Then I checked pH. Now everyone else experiences indicate that it should be blue (no CO2 present). Mine went yellow/green (pH <6-6.6) indicated good levels of CO2 present. The DI water before changing the KH of course has the same pH.

When I looked into this further, I question whether is it possible to use DI water for this purpose:

In theory, deionised water doesn't have a pH value, but in practice, it is usually considered by convention to be pH 7.0. pH is a logarithmic measurement of relative ion presence. Since there are no ions, there is nothing to measure. In practice, both chemical pH measuring systems and electronic pH meters will indicate a pH value. The indication from chemical indicators can give a value of usually between pH 5.0 and pH 9.0 depending on the indicator used (the indication being the ions introduced by the indicator itself, its solvent and its impurities).

Deionized water will quickly acquire a pH while in storage. Carbon dioxide, present in the atmosphere, will dissolve into the water, introducing ions and giving an acidic pH of around 5.0.


Even with doubts I continued and filled the dropper with this solution anyway. As predicted, the colour has not changed in 36 hrs (with CO2 on and off).

So, has anyone actually used deionised water for this purpose? In reality, does it have to be distilled (or RO) water and not deionised in order to measure a true pH?

Hope someone with some experience on this can sort out the confusion for me!

Thanks :proud:
 

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Since distilled water contains no ions either I don't see why DI water wouldn't work. As soon as we add the sodium bicarbonate we do have ions in the water. PH is a measure of excess H+ ions or a deficiency in H+ relative to the OH- ions, as I understand it. Distilled or deionized water should have equal amounts of H+ and OH- ions, therefore a pH of 7. (I hope it is obvious that I am not a chemist.) The atmospheric CO2 that dissolves into the water will certainly give a less than 7 pH, but as soon as you add bicarbonate ions that pH should go to the value that corresponds to the KH/pH/ppm CO2 table.

It seems to now be accepted that 4.99 gm of sodium bicarbonate in 5 liters of water doesn't give 40 dKH water, but something less than that, more like 3 dKH. The now accepted number of grams needed is a hair over 6 grams of bicarb per 5 liters of water to get to 40 dKH. I don't think that explains why your indicator water remained yellow green, but it might encourage you to try another batch of 4 dKH water.
 

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Discussion Starter · #3 ·
Well arrording to the KH test kit (if you trust it), when the 40dKH is diluted, the solution is bang on 4dKH.

But are you saying that experience has now shown you to need ~6g per 5l to make 40dKH soltion, irrespective of what the test kit says? I haven't read this before?

Still, as you said, this doesn't explain why the colour never changes?!?
 

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Here's the math:

40dkH * (17.9mg/L CaCO3 / 1dKH) * (1mol CaCO3/100.08g CaCO3) * (2 mol NaHCO3/1 mol CaCO3) * (84.00g NaHCO3/1 mol NaHCO3) * (1g/1000mg) * 5L = 6.01g NaHCO3 in 5L to make a 40dKH solution.

If you use 5g NaHCO3, you get a 33dKH solution. So after 3 drops of titrant (kH test kit) in a 1:10 diluted solution (3.3dKH), it will not have changed color (still 0.3dKH to titrate), but will on the next drop.

Your 3.3dKH solution will have a lower pH than a 4dkH solution when the SAME amount of CO2 is dissolved. So yes, your solution could change color when you have less than ideal CO2 dissolved.
 

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Discussion Starter · #5 ·
This makes sense...! Thanks.

Looks like I'm going to have to buy another 5L of DI water and some more accurate scales. Thanks again.
 

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It isn't essential to use 5 liters of water. If you mix 6 grams of bicarbonate of soda in 1 liter of water you get 5 times the KH, or 200 dKH. Now, mix 10 ml of that with 490 ml of water and you have 4 dKH. That is the way I mixed mine. I bought a 100 ml graduated cylinder and a 1 liter measuring flask on ebay just to do this. Please don't ask me how much I have paid so far to get 500 ml of 4 dKH water! (I also had to buy a 100 gram +/- .01 gram digital scale.) I am rationalizing my expenditures by noting how much fun I have playing chemist.
 

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hey hoppy. Since it sounds like you have all the hardware for this stuff. why don't you mix up a patch of some 4dKH water and sell it by 1/4 ltr. then you can make back a few bucks. some of us kinda suck at making this stuff. lol
 

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I actually thought of doing something like selling this stuff, even going so far as to buy a couple of little plastic bottles at Tap Plastics to see if they would work. Then I ran into the fact that I just don't know what I am doing with this stuff.

Tom Barr is trying to convince Greg Watson to sell little bottles of very high KH water, with the KH certified as being dead on accurate. We need to pester Greg to make sure he understands that this is the road to unimaginable wealth!
 

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Yea, I've thought of trying something like that (selling ready-made solutions of various types), but the bottles are the most expensive part! I priced out making the kH and pH indicator solutions myself, filling bottles, labeling, and selling and came up with a price above what you can get it for at local big-box pet stores (without shipping)!

The bottles needed for a buffer solution would be cheaper (no dropper top needed), but still the most expensive part of the deal (besides labor).

How much would you pay a chemist (me) to make 250ml of a precisely 40dkH solution for you?? It would only take about 10 minutes for me to do it.
 

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So if you had access to an analytical balance, am I correct in saying that 30mg of NaHCO3 in 250ml of water would yield a kh4 solution?

Math:
6g/5L = 40kh
.6g/5L = 4kh
.12g/L = 4kh
.06g/500ml = 4kh
.03g/250ml = 4kh
.03g = 30mg
 

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Discussion Starter · #11 ·
All's looking good using the above info - thanks.

However one question for those that use a drop checker. When I make the drop checker up, works as expected - blue, then after ~2hrs, turns green (even slighly yellowish so maybe above 30ppm). CO2 and lights go off at midnight. In the morning (so no CO2 for ~8hrs), the solution is the same colour (yellow/green). I would have assumed the CO2 would dissipate overnight and would be blue again - is that wrong?

Thanks.
 

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So if you had access to an analytical balance, am I correct in saying that 30mg of NaHCO3 in 250ml of water would yield a kh4 solution?

Math:
6g/5L = 40kh
.6g/5L = 4kh
.12g/L = 4kh
.06g/500ml = 4kh
.03g/250ml = 4kh
.03g = 30mg
Well when you put it like that I guess even an idiot like me should be able to figure it out.
lol

EDIT- Or not. apparently my little scale does not read lower then .1
 
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