# Making a reference KH solution



## hoppycalif (Apr 7, 2005)

I have been misleading people. I passed on information I trusted about how to make a 4 dKH reference solution. That method doesn't work. It doesn't even get you close to 4 dKH. At this time I haven't found an agreed upon number for how many grams of sodium bicarbonate are needed in a liter of distilled water to end up with X dKH water. The only method that I would rely upon now is use of a KH test kit, and adding small amounts of bicarbonate of soda until the test kit says the dKH is 4.

I apologize if anyone was badly misled by my passing on unproven information.:sad:


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## Left C (Jun 14, 2005)

Hi Hoppy

I was just messing around with the baking soda/KH calculator. Buffering capacity and pH

Here's some amounts to try. I haven't tried them yet. I just got in. I've been at the hospital all night.

This is what the baking soda/KH calculator looks like when I punch in these numbers.

Calculate baking soda needed to raise KH by a
specified amount. 
Specify tank size and the KH increase you want to cause:

Gallons: 0.39

Desired KH change: 4 degrees
Which measure of KH? *dh* ppm

Teaspoons of Baking soda needed: 0.03 

________________________________________________________________

This is how I arrived at these numbers:

0.39 gallons - There's 3.7843 L in 1 gallon - So, 3.7843 L/gallon X .39 gallons = 1.4759 L or 1475.9 mL

1/32 tsp = 0.03125 tsp (or ~ 0.03 tsp) Baking Soda

Then: 0.03125 tsp / 0.03 tsp X 1475.9 mL = 1537.4 mL

_So, add 1/32 tsp Baking Soda to 1537.4 mL distilled water = 4dKH_

What do you think Hoppy?


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## dennis (Mar 1, 2004)

using NaHCO3....

By molecular weight, NaHCO3 is 84.0072g/mol. CO3 weights 60.0095g/mol so CO3 is 60.0095/84.0072= 0.71 or 71% of NaHCO3 is Co3, the part that will effect the hardness of your water.

Therefore, 1g NaHCO3 in 1l of water will add 0.71g, or 710mg/l of CO3. Since 1 degree of hardness as CaCO3 contains 10.716 ppm as CO3, then 710/10.716= 66.26 degrees.

So, for easy math you want the fist solution to be 40 degrees. Remember from before 10.719mg/l as CO3 in 1 degree, then 40*10.716=428.64 mg/l CO3 is needed to make 40 degrees. Since CO3 is 71% of NaHCO3, then 428.64/0.71= 603.72mg of NaHCO3 are needed. So, 0.6037g NaHCO3 will give 40 degrees in 1 liter of RO/DI water. 

To get a 4degree solution, mix 10ml of your standard solution with 90ml RO/DI (or any factor of those) to get a 10% solution. 

Alternately, you could mix 1g of NaHCO3 in 1 liter of water, for a total of 66.26 degrees. If you want a final solution of 4 degrees, then 4/66.26=0.0604 Multiply by 100 to get the % solution you need to make. If you make a final solution of 100ml then each percent equals 1ml. So, 6.04ml of 66.23degreee solution with 93.94ml RO/DI would give 66.23*(6.04/100)= 4.0003 degrees. If you can't be that precise then 66.23*(6/100)= 3.97degrees.

The hardest part of all this is weighting accurately to two or more decimal places, though for our purposes weighing 0.56g would probably be good enough. There would be to large a margin of error measuring to only 1 place since the volume of water we are using is so small that it greatly compounds any error.

Either of these two techniques should give you a way to "accurately" create a stock solution. Also, these method is the only accurate way to make a reference solution. You need to use math and chemistry concepts to do it. Even trusting to an expensive test kit to check you reference is fraught with chances for error. Correctly interpereting color changes in different lights, whether or not the kit is accurate, using 5.4 instead of 5.0ml in the test, age of reagents, temperature, etc can all effect the test. The ideal method is to make a know solution using know materials/weights/volumes and checking your test kit with that, not the other way around.

Its also important to remember,IMO, that you are making a reference. So, if your reference is different than mine, we can't possibly compare results; however, if your goal is to be consistent with your CO2 in your tank and not compare 40ppm vs 45ppm with me, then if your reference is off by 5-10% it won't matter as you are simply making something that will be consistent enough upon replacement to give you the same reading. You'll need to figure out what "green" looks like to you, in your light and if it is enough or two much CO2, but, once you do then you will always have the same baseline for your own, personal comparison. The concept of using kH and pH of the tank water itself is the same concept, though the drop checker (or reference solution in a pH probe) may remove some variables like additional acids in the water, changing water parameters, etc. 

Bottom line, there is a difference between accuracy, precision and consistency. On the individual level, consistency is enough as you will have a personal, developed baseline/reference. Its like watching the plants and fish but with a little more concrete measure. If you wish to compare your results with other hobbyists or in a scientific manner, then accuracy and precision are very important and consistency means something different. 

Also, don't forget to double check my math.


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## ed seeley (Dec 1, 2006)

I think part of the problem Hoppy was trying to solve with this method was that there was water in with the NaHCO3 that screwed up the calculations and needed heating to drive off the water? This is just what I gathered from reading the threads and people I know who have tried it on other forums.

That meant that a set weight couldn't be relied upon. Personally I test the KH of the water until I get a 4dKH solution and then use that, plants are pearling great.

But if the weighing method works for you then great!


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## dennis (Mar 1, 2004)

Yes, for complete accuracy one needs to dry the stuff before using it. I forget how hot and how long but there should be reference info for that on the net. Water can add a lot of weight and most of the chemicals we use are very hyroscopic.


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## hoppycalif (Apr 7, 2005)

If you bake sodium bicarbonate you start converting it to sodium carbonate, and it is only about 57% carbonate, not 71%. So, attempting to get more accuracy by baking out the small amount of water can actually destroy the accuracy by reducing the amount of carbonate per gram.

Yesterday I mixed up yet another batch of 4dKH water. I used 6 grams of baking soda, not baked, but an almost new box of it, so it should have contained very little water. I added it to one liter of distilled water, to get 200 dKH water. I mixed 10 ml of that with 490 ml of distilled water to get 500 ml of 4 dKH water. Then my test kit said I had a bit lower than 4 dKH, so I accept it as "my standard" 4 dKH water.

The quantity of baking soda needed to make even a liter of 40 dKH water is so small that trying to measure it by volume, using a measuring spoon, is futile. The stuff can occupy more or less volume depending on whether it is fluffy or packed down, and no measuring spoons are very accurate anyway. 

I agree with Dennis' analysis of the subject. (But, Dennis, where did you convert grams of CO3 to equivalent grams of CaCO3, which is what carbonate hardness is based on?) If you are doing this just to be able to monitor your own CO2 in the tank, you don't need much accuracy. The ppm of CO2 in the water is directly proportional to the KH of the water when you use a drop checker, so even a 20% error in KH means that if you think you have 30 ppm, you might, from that error alone, actually have 24 or 36 ppm - great accuracy. The real accuracy killer is the pH, where a .3 error in measuring gives you a +100% or -50% error. But, if you carefully judge the color of the water as being green and not yellowish or blueish green, you can hit +/- .1 pH, which leaves you with the 30 ppm really being about 25 to 40 ppm. That is the reason for trying for less than a 10% error in KH - just to limit the ppm CO2 error to the pH error only.


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## dennis (Mar 1, 2004)

hoppycalif said:


> If you bake sodium bicarbonate you start converting it to sodium carbonate, and it is only about 57% carbonate, not 71%. So, attempting to get more accuracy by baking out the small amount of water can actually destroy the accuracy by reducing the amount of carbonate per gram.


Your right about the baking. My understanding is that there is a temp where you are able to drive off the water without "cooking" the carbonates. I don't know what that is but and I could be wrong.

%carbonate in NaHCO3: By molar mass...
Na=22.9898 g/mol
H= 1.0079 g/mol
C= 12.011 g/mol
O= 15.9995 g/mol*3=47.9985 g

Total weight is 84.0072g/mol CO3=60.0095g/mol 
%carbonate=(part/whole)*100= (60.0095/84.0072)*100= 71.434%

[quote}I agree with Dennis' analysis of the subject. (But, Dennis, where did you convert grams of CO3 to equivalent grams of CaCO3, which is what carbonate hardness is based on?)[/quote]

Thanks

From previous reading on the web I "know" (as in only read it from the web) that kH is generally measured as the hardness equivalent to the amount CO3 in CaCO3 assuming it is all coming from CaCO3. Hence kH measurements are generally written as ppm/degrees as CaCO3. Also from my reading I "know" that one degree kH is the same as 17.86mg/l as CaCO3. Using mol weights of CaCO3 we find that CaCO3= 100g/mol and CO3 is 60g/mol so the CO3 portion of CaCO3 is 60%. 60% of 17.86mg/l= 17.86*0.6= 10.716 mg/l.

Basically that says that the amount of carbonates in water is 10.716mg/l for every degree. if you want 4 degrees in your sample, you need to add 428.64mg/l to your water. That is already in CaCO3 equivalent, I believe? Please, correct me if I'm wrong. (Not sarcasm or posturing, as I said I am new to this)


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## hoppycalif (Apr 7, 2005)

I wish I understood better what it means for something to be in ppm of carbonate as calcium carbonate, but I don't. I keep seeing the "experts" correct the calculations they make by using some kind of ratio of what looks like the amount of carbonate in sodium bicarbonate and the amount in calcium carbonate. This only concerns me because my use of the 4.99 grams of bicarb to 5 liters of water equaling 20 dKH (my memory says those were the numbers?) gave me a finished product of "4 dKH" that my test kit said was less than 3 dKH. That convinced me that I don't know how to calculate this, so I am seeking a simple explanation that I understand. (And that may be a futile goal.)

EDIT: before I forget. Tom Barr is trying to get Greg Watson to stock small bottles of certified known very high KH distilled water, so we can all mix 4 dKH or what ever KH water we want for our drop checkers or membrane/ref KH water/pH probe devices. If that goes thru, my interest in making my own will probably vanish.


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## dennis (Mar 1, 2004)

Hoppy,

In playing around with the kH calculator Tom posted to the APD, CNYKOI - Alkalinity (KH) calculator it seems that I am partly wrong. It seems you don't divide the final concentration of CO3 in the sample by 10.716 to get the degrees but rather by 17.86. This part does not make sense to me, but like I said, I am not so smart in this...

to use your example as an example...

4.99g = 4990mg of which 71.44% is CO3 so, 4990*.7144= 3564.856mg of CO3. Add that to 5liters of water, 3564.856/5= 712.9712mg/l of CO3. Divide that by 17.86 ppm per degree = 39.92 degrees. That matches Tom's link almost perfectly.

I guess that makes sense after all. You are accounting for the "as CaCO3" part by focusing entirely on the amount of CO3 you are adding. The divide by 10.716 means one is accounting for it twice. Seems logical...

I'd like someone more knowledgeable than I to verify that last part.

I guess in the end, for your situation, if the calculator is right and providing you measured accurately, your error is either in dilution or the test kit is wrong

How do you dilute that? Assume you get a 40degree solution and you want a 4 degree solution, you would use 10ml of the 40' and 90ml of DI.

Tom's post also said to dry the NaHCO3 at 400F for 30-45mins but he also says to drive off the water and CO2(?). I have no idea about that.

Also, here is a good percent mass calculator... Element Mass Percent Calculatorr


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## Matt Hewitt (Aug 30, 2012)

Here's what worked for me: I did not require a chemically exact ph or dKH number, all I needed to know was whether or not my co2 levels were conducive to a healthy environment for my plants (I do not have any fish/invertebrates). I added a 10 drops (roughly .5ml) of 4dkh solution to 1/2 cup distilled water. I then added a tiny* amount of sodium bicarbonate to the solution until I achieved a green color. This ensures a CO2 neutral solution that can then be used to measure (visually only) the CO2 content in your tank. This is NOT an exact measure by any means and should not be attempted by those with animals in the tank unless you are sure that the reagent is working somewhat accurately.

*tiny= the amount that stuck to my fingers when I touched the powder in the box of sodium bicarbonate.


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