I'm really confused about this subject. I have now made up 4 batches of 4 or 5 dKH distilled water, and all of them test way off with my KH test kit. Today I got disgusted with the process and bought a new test kit - same problem. I made up a 4 dKH solution, but the kit says it is less than 2 dKH.

Here is what I did:
Baked a tsp of freshly opened sodium bicarbonate at 230F for about 10 minutes. Used my 100 gram capacity scale to measure 4.99 grams of that baking soda. Added it to a one liter measuring flask. Added one liter of distilled water to it, making sure all of the soda was dissolved - none left on the sides of the flask. That should have give me a liter of 200 dKH water.

I mixed 10 ml of that water with 90 ml of distilled water, which should have given me 100 ml of 20 dKH water. Then I mixed 80 ml of that water with 20 ml of distilled water to get 100 ml of 16 dKH water.

I added 300 ml of distilled water to that to get 400 ml of 4 dKH water.

But, two different API test kits say it is less than 2 dKH water. So, what did I do wrong?

Some of us do not have much background in mixing up chemical solutions, measuring weighed amounts with known volumes of water, etc. so what seems easy to some isn't to others.

I had calibrated the scale with a 100 gram and 50 gram weight, and it was right on. I also tried it once with a very light piece of paper to hold the soda as I weighed it, and a second time with a heavier piece of plastic and reset the scale to zero to account for the tare weight.

Several sources I've looked at suggest quite different standards as well.
I arrived at the 4.99 grams from the reef side (namely ReefCentral folks)which tend to be anal about such things.

Using something other than baking soda(heat CO2/water issues) might be tougher to dissolve, but would not entail as much errors in other areas.
I asked Greg Morin and there's some discussion on various reef forums.

I'm looking at other standard base chemicals to get the bicarb HCO3 other than baking soda for a KH solution.

I have not gotten around to it, nor the pH membrane probe test either, but in due time I will.

Regards,
Tom Barr

As with many things in life, what may seem easy may in fact not be. I don't see anything wrong with the logic Hoppy. What type of test kit do you use? My small batch of KH solution was done by trusting my Lamotte KH test kit. I did an initial calculation of how much KH+ I need and then I tweaked the solution by calculating how much RODI water I needed to get to 4 degrees. Then I retested the solution with my test kit to confirm 4 degrees. But, I am sure that is not all that accurate either.

I would much rather just buy some KH reference from Seachem or something if they made it. I don't want to mess with getting all this precision measuring equipment.

I decided to do some research with Google:
Water Hardness is one reasonable sounding article. It says 30.02 mg/liter is the amount of NaHCO3 needed to get 1 dKH, because dKH refers to carbonate from CaCO3, not from just any old source. (This is in the right direction to explain my problem.)

Carbonate hardness - Wikipedia, the free encyclopedia is Wikipedia's explanation. It says 50 mg of NaHCO3 in a liter of water gives about 18 mg/l of carbonate hardness, which is about 1 dKH. (That too is in the right direction to explain my problem.)

KH reference solution/ baking soda is where one of our members is asking similar questions. No answers, of value, yet.

I suppose I need to take the 30.02 number in the first reference, make up my 4 dKH solution based on that and test to see if it works. But, that doesn't really make any case for doing more than just adding sodium bicarbonate until the same test kit says it is at 4 dKH.

Consider the equation that is used to determine ppm of CO2 by measuring KH and pH: ppm = A*KH*10^(B-pH) (A and B can have various values, but the ones used the most are A=3 and B=7) In any case, the ppm calculated is proportional to KH. So, a 10% error in KH gives a 10% error in ppm. That is much, much less significant than a .3 error in pH, which gives an error in ppm of 2X or 1/2 X (+100% or - 50%). I am having a hard time seeing why I need great accuracy in setting my reference KH. Even a 20% error would seem to not be that big a deal.

In a chemistry lab, where a measurement of CO2 ppm really, really has to be dead accurate, a very carefully mixed reference KH solution would be essential. But, we (at least I, if not we) are trying to establish that the ppm in our aquarium is in the ballpark of 20-40 ppm, and not 5 ppm or 60 ppm. A micrometer isn't always the best way to measure a length!

Just to belabor this a bit more: NaHCO3 will add H2O to its crystal structure if it is in a humid place, so when we weigh a bit of NaHCO3 the result is the total weight of both the NaHCO3 and the H2O. We can bake our NaHCO3 to drive off the H2O, but in doing that we are also converting some of the NaHCO3 to Na2CO3. The former is 71% CO3, but the latter is only 57% CO3. So, by baking we are losing accuracy, not gaining it. I don't see any possibility of making an accurate KH solution by weighing NaHCO3. And, for sure I don't see a way to equal the accuracy of a fresh KH test kit.

I totally agree, this is where SeaChem could sell ref solutions and make some $.
I think if we all bother Greg Morin, he might.

Regards,
Tom Barr

Due to H2O and heating losses of CO2 with baking soda, I think side stepping baking soda all together may be wiser.

All that's needed is something can be dissolved and has KH.

Regards,
Tom Barr

Vaughn,

What you are saying then through your research is to do the following:

1) Get a good KH test kit
2) Estimate how much KH+ you may need for a certain solution to get let's say 4 degrees (I would think a larger solution would be more accurate)
3) Mix the above KH+ with RODI water.
4) Test the solution with the KH test kit
5) Adjust the KH solution either up or down (add KH+ or RODI water) to get closer to 4 degrees
6) Retest the KH reference solution
7) Repeat steps 5 and 6 if necessary.

Is this correct?

But that method is only as good as the non calibrated test kit

I do not trust test kits, we need to have a simple standard reference solution, not a test kit.

That's the idea behind the reference KH solution after all

Lamotte's test kit is good to 4.4ppm.
I'm not sure of the accuracy though.
Better than most test kits though.

Please read this link, it will help you:

Reference.com/Encyclopedia/Alkalinity

Now go back and read it

Reference.com/Encyclopedia/Carbonate alkalinity

Randy suggest the Hach Test kit might be best since the measure is sharper than others.

Chemistry and the Aquarium - Randy Holmes-Farley

Please read all of this.
It will answer 99% of your questions.

And it will raise many more in their place

Note, doing a pH endpoint tritration, you can use a pH probe/meter to measure the exact end point where all the KH is consumed by the strong acid.

Sodium carbonate or potassium carbonate etc will work and have less volitility than baking soda.

Here's an interesting paper of low KH water measurements:
http://www.wrc.org.za/archives/water...tober/1428.pdf



Regards,
Tom Barr

Now look, there are many ways to define alkalinity, but a darn good test kit/method should be in place if you plan on doing this right.

Folks from years past screwed up NO3 and PO4 using cheapo test kits and poor assumption. About 11 years Steve Dixon got fed up and adopted the Lamotte abnd Hach line to get around this issue. I followed suit and then questioned whether the Lamott and Hach kits where accurate as well.

Here's a digital Alk test:

Hach - Alkalinity Test Kit, Model AL-DT, Digital Titrator, 10-4000 mg/L, 100 tests

Here'a a cheaper HACH test kit:

Hach - Alkalinity Test Kit, Model AL-AP, Drop Count Titration, 100 tests

Here's the Lamotte:
Alkalinity Test Kit

Look, don't monkey around all hodge podge here, do it right and then you can feel better about it.

The Chemistry Section: Alkalinity

And for the UK folks: Hanna Instruments HI-3811 Alkalinity Test Kit

1ppm ain't bad

Hanna makes a one for the USA yanks as well.

Regards,
Tom Barr