Making Standard KH solutions

[VaughnH]

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.

 

[Tom Barr]

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

 

[shane]

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.

 

[VaughnH]

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!

 

[VaughnH]

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.

 

[Tom Barr]

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 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

 

[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

 

[shane]

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?

 

[Tom Barr]

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/watersa archive/2001/October/1428.pdf



Regards,
Tom Barr

 

[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

 

[Tom Barr]

Okay, I now have NIST referenced alkalinity solutions. They are high concentration, 25,000ppm as CaCO3 measurment from Na2CO3 and 10mls. So one will make 3.49944 liters of 4 KH solution: 71.44ppm.

Since the accuracy is high at high concentrations, the dilution downstream errors will be very low.

I have a nice Volumetric flask as well and some smaller ones and pipettes that are accurate to 0.01ml for the topping off part or I could snatch a micropipette from work.

But I do not think I need that mucn accuracy, this will get me well below the 0.1ppm range and that will not influence CO2 much(much less than 1ppm).

Greg Watson might sell it also. We shall see.
Given they are small etc and make a lot, they would be well worth it since most folks will have to buy a scale, and deal with the mixing and other unverifiable issues.

This way it's a lab grade solution you dilute.
Vaugh, you want 1 liter of certified 4KH solution?


Regards,
Tom Barr

 

[Mr G]

Having read this thread, I decided to check my ref solution against a Nutrafin KH test kit. I did the check using 2 x the volume of water (solution) and 2 x the number of drops.

The test kit says my solution is 3.36dKH. I'm fairly happy with that, given that there will be inaccuracies with test kits.

I made the reference solution up according to Toms suggestion of cooking off the soda and mixing up an initial 40dKH solution then diluting that down.
The solution was made about 6 weeks ago and has been stored in an airtight bottle since...

I refilled my drop checker about 2 days ago and it still seems to be working fine.

 

[JamesC]

I've been looking at this a bit. If as the krib states 30mg NaHCO3 in 1 litre gives 1dKH then we'd need 6g of it to raise 5 litres of water to 40 dKH. If though as wikimedia states it requires 50mg then we'd use 5g to raise the 5 litres to 40dKH. This wikimedia figure then is what we have been using. But the strange thing is I've done the calculations and the Krib figure seems to me to be correct. My chemistry has been rusting away for over 20 years so I may have got something wrong here. I'd be glad if someone could shed some light on this.

James

 

[Bill]

When all of this started weeks ago (I guess were into month's ago now).

I bought a digital scale, calibration weights, graduated cylinders, volumetric flask and some pipettes.

Then I was ready to make my own KH standard but I ran into the same problem. All of the calculators gave different amounts of sodium bicarbonate to add and there were issues with heating the sodium bicarbonate and it changing to sodium carbonate.

I went searching the web and posting on some chemistry/ lab forums looking for answers.
I took a couple of weeks and a lot of questions asked but I did find the correct answers.

I was going to post what I had found out about making the KH reference solution but I just haven't had the time.
Later this morning or in the early afternoon I will post how to make the KH reference solution and I think it will clear everything up.

I also have 500ml bottles of 4 or 5 dKH standard for sale, if you don't want to go to all of the trouble of buying the equipment and making your own solution.

Bill

 

[neil1973]

I remember i looked into this a while back and have been using the krib figure (6g will give a kh of 200 in 1 litre) ever since. If this figure is correct then Vaughn's dilutions would give a kh of 3.33 this is closer to what Vaughn has been getting from the test kits but there is still quite a difference.

 

[shane]

I would be interested in some Bill.

 

[VaughnH]

I keep coming back to the equation for ppm of CO2 vs KH and pH:
ppm = A*KH*10^(B-pH)

The ppm of CO2 in the tank water is proportional to the KH of the water, so any percent error in KH gives an equal percent error in ppm. If I think I have 4 dKH water, and I really have 3.5 dKH water, my ppm error will be 100* .5/4 in percent, or 12.5%. I think most of us hobby types would kill to get that great an accuracy on our CO2 concentration.

If my point isn't obvious, we are flogging a dead horse here! Or, to put it another way we are on the wrong road to redemption. KH measurement isn't our problem.

Our problem is the pH measurement. That problem is solved by using the membrane type KH reference pH probe that Tom has posted about. Since the average hobby person isn't too interested in investing in such a device, and doesn't really need that kind of accuracy anyway, the colored water drop checker is preferable. And, for that a test kit determined KH reference solution is just what the doctor ordered. Unless the KH test kit is off by a factor of 2 or thereabouts, that is all we need.

Those, like Tom, who are into more accurate measurements of CO2, with very rapid response, should be looking hard at the pH probe based "drop checker" method. For them, getting a very accurate KH reference solution is a worthwhile goal. What the heck would I do with knowledge that I have 32.4 ppm of CO2 and not 25 - 40 ppm?

I purchased a 100 gram scale accurate to +/- 0.1 gram, a one liter measuring flask, a 100 ml graduated cylinder, a new KH test kit, a calibration weight, 2 gallons of distilled water, and, lest we forget, a box of Arm and Hammer baking soda - fresh, unopened. I could have used my existing KH test kit, my remaining half gallon of distilled water, my old box of Arm and Hammer soda, and made a batch of 4 dKH water and gone back to my experimenting. (But, I admit, it is fun to play Chemist!)

 

[Bill]

Here is the information that I found on how to make a KH standard.
I did a lot of searching and this seems to be the correct way to make a KH standard solution. Thanks, Tom Barr, Randy Holmes and Jose from helping to clear this up.

After I got all of the supplies in to make the KH standard I went to this thread and got Tom’s instructions.

http://www.barrreport.com/co2-aquat...ethods-drop-checker-ph-probe-adaptations.html

>>>Cooked/baked baking soda: 400F for 45 min.
Use about 2x what you think you need.

10 grams may yield 6 grams after dehydrating.
Sometimes 8 if you live in drier areas.

I add 4.99 grams to 5 liters of DI water.
This makes 40KH.

Add 10mls of this to 90 mls of DI water to make a 4KH solution.

Regards,
Tom Barr

 

[Tom Barr]

I suggested using Sodium Carbonate to resolve the baking soda H2O and CO2 thing/issue.

I also have a lab verified reference sample at very high concentration, thus making shipping easier, as well as more accuracy when dilutions are done.
This provides higher confidence levels and it's easy, the solutions are pre made, all you do is dilute accurately with a flask with DI water.

The best test method would involve tritration pH meter end point and a digital tritrator like the HACH.

One thing is clear, test kits vary in this hobby, as do the dosing calculators for KH measurements. The range of accuracy is fairly wide and in most general cases, this is not an issue.

But we should all try hard to maintain a standard that's something we have confidence in. All these calculators have assumptions built into them.

You'll note Rnady and others do the chemistry and seldom rely on calculators. I am pre calculator in my area, so I do it the old fashion method as does Paul Sears or Randy at RC.

But for accurate critical measure of CO2, it is important.

For the drop checker method, having a lab verified reference solution that you can count on truly, is a nice option for a few $. I think it'll be about 10$+ shipping or so and 3.5 liters ought to last a few years and allow you to send all the KH ref solution you want to the friends etc

From there, a pH probe will be the thing that's the issue but ref solutions and their accuracy is easier to check for pH than KH.

I think this discussion is good as it outlines the issues with KH, it's measure, the web's various opposing measures/dosing calculators, different measurements etc, and methods available, test kit issues as well as a lab verified reference stock solution that is now available.

Before the myths start to amplify on the web, some definitive resolution should be made.

It seems that for lower Ref accuracy, the baking soda is fine.
For higher confidence/accuracy levels, go a lab grade vertified ref sample, they are relatively cheap, get a Volumetric flask((10$ or so for a 500ml) and you'll have plenty for years.

This will help understand and variation we see and give a new group of folks some back ground on the other types of alkalinity beside the carbonate type.

The system is more complex than some might assume on the Krib or various web sites.

And each step, going after such issues critically and in great detail, helps to provide a "closer to the truth" approach.

We may never arrive at the truth for most things, but we will know more about the issue and be better able to use it to help us in the hobby/life. I'd prefer that route versus mere belief
That choice is clear, at least to me.

Regards,
Tom Barr

 

[VaughnH]

Okay, I now have NIST referenced alkalinity solutions. They are high concentration, 25,000ppm as CaCO3 measurment from Na2CO3 and 10mls. So one will make 3.49944 liters of 4 KH solution: 71.44ppm.

Since the accuracy is high at high concentrations, the dilution downstream errors will be very low.

I have a nice Volumetric flask as well and some smaller ones and pipettes that are accurate to 0.01ml for the topping off part or I could snatch a micropipette from work.

But I do not think I need that mucn accuracy, this will get me well below the 0.1ppm range and that will not influence CO2 much(much less than 1ppm).

Greg Watson might sell it also. We shall see.
Given they are small etc and make a lot, they would be well worth it since most folks will have to buy a scale, and deal with the mixing and other unverifiable issues.

This way it's a lab grade solution you dilute.
Vaugh, you want 1 liter of certified 4KH solution?


Regards,
Tom Barr

Yes, I would love to have some known 4 dKH solution. Can I Paypal you for some? I wish Greg would add something along this line to his store.

Today I mixed up some using my KH test kit - that isn't easy either. I ended up with about 3.75 dKH +/- .25. It takes a very steady hand to get right on the money, or some terrific good luck. And, I haven't spent $ on any pipettes yet either. My Chemist amusement only extend so far when I am spending money.