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

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.

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

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

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.

I would be interested in some Bill.

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!)

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.

KH reference methods, the drop checker and the pH probe adaptations

>>>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<<<

I was going to make my standard 5 dKH instead of 4 dKH, so I went to the KH calculators below to see how much baking soda (sodium bicarbonate) I would need to add. That’s when I first found out that something was wrong because the calculators gave different results.

I made larger samples and I was always planning on making larger sample sizes as Tom had suggested but just for comparison purposes I was calculating how much sodium bicarbonate to add to 1 liter of water to get 5 dKH.

The first calculator said to add:
100 mg from Jose’s calculator / 'Reef Chemistry Calculator FV'
(Note the above calculator now shows the correct amount of sodium bicarbonate to add 149.9 mg – it was changed due me asking about the difference. This is the calculator to use it is accurate)

The second calculator said to add:
125 mg from web calculator / CNYKOI - Alkalinity (KH) calculator

The second problem I ran into was the heating of the baking soda, I had read on the web site below that if you heat sodium bicarbonate at 392 degree's F that the sodium bicarbonate changes to sodium carbonate. This was causing more problems because you could end up with all sodium carbonate or a mixture of sodium bicarbonate and sodium carbonate.

General Chemistry Online: FAQ: Introduction to inorganic chemistry: What happens when sodium bicarbonate is heated?


At this point I started going to the chemistry/lab forums looking for answers and while they helped some they didn’t give me the information that I was looking for.

The real help came from the forum called “The Reef Chemistry Forum” over at Reefcentral.com
Randy Holmes and Jose really helped clear things up, below is the link to the discussion. If you have any doubts about there numbers check out Randy’s resume of work.

Reef Central Online Community - Heating baking soda

So here are the basics of making the KH standard. I have tried both the sodium bicarbonate and sodium carbonate with these numbers and everything comes out just right.

The weights I posted here is the amount of sodium carbonate or sodium bicarbonate needed to make a 4 dKH and a 5 dKH standard in one liter of water. I just did it this way because it was easier for me to compare. Although the numbers are accurate I made my sample sizes larger as Tom has suggested for better accuracy.

To make the KH standard solution.

Use distilled water

add baking soda (sodium bicarbonate) or sodium carbonate
-

• sodium carbonate can be bought from a lab supply
• -the sodium carbonate anhydrous is 99.5% pure, compared to baking soda which is 97% pure. The sodium carbonate anhydrous would give you a little more accurate sample.

This is the calculator to use,

• -again it was wrong but Jose has changed it and it works great.
• -you can select to use either sodium bicarbonate or sodium carbonate and it will give you the correct weights
• -when using sodium carbonate anhydrous the calculator calculates this as 100% -the sodium carbonate anhydrous that I bought was 99.5% so I needed to multiply the weight the calculator gave by .005 to get the correct amount to add. So if you buy your own sodium carbonate anhydrous keep this in mind and calculate accordingly
• -The calculator rounds everything up because it was made for calculating aquariums not small samples. To get accurate weights just add some zero’s to the sample size.
• For example: when I enter in 1 liter of water and 5 dKH of baking soda it gives me a weight of .1 grams of baking soda to add. I want a more accurate amount to add so I increased the water from 1 liter to 1000 liters and I now get a weight of 150 grams. By doing this I know to add .15 grams instead of .1 grams

weight of baking soda to add for standards

• -1 liter of distilled water with a 5 dKH .15 grams (actually .14994)
• -1 liter of distilled water with a 4 dKH .12 grams
• -5 liters of distilled water with a 40 dKH 6.00 grams

Bill

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

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.