KH Standard / How to
Here is the information needed to make a KH standard. When I started using the drop checker method for measuring CO2 there was a lot of confusion as to the proper way to make a KH standard. After a lot of searching and asking questions I found the correct way to make a good KH standard.
I have made the KH standards with the following.
If you want a more accurate standard than that you could use the alkalinity standard solution. The reason for the increase in accuracy is because all that needs to be done is the dilution part, the weighing process is removed, which eliminates any errors from moisture in the sample or an inaccurate scale.
To make a KH standard using baking soda you would need the following.
1. Measure 3000 ml of distilled water using the 500 ml graduated cylinder and put it in a clean container. The container could be an empty 1 gallon distilled water bottle. (I found it easier to use 3000 ml, instead of the often suggested 5000 ml because you can make your standard with 1 gallon of distilled water instead of having to have a larger container or multiple containers)
To read the precise water level in the graduated cylinder.
-Place the cylinder on a flat surface.
-Make sure your eye is on a level plane with the of the meniscus.
-The meniscus is the half-moon curve formed at the surface of liquid.
-Water should be read from the bottom of the meniscus
2. Weigh 3.60 grams of baking soda, add it to the 3000 ml of distilled water and mix well, this will make a 40 dKH / KH standard.
3. Pour 450 ml of distilled water into the 500 ml graduated cylinder.
4. Pour 50 ml of the 40 dKH standard into the 50 ml graduated cylinder
5. Pour the 50 ml of 40 dKH standard into the 500 ml graduated cylinder and mix well, this will give you 500 ml of a 4 dKH standard.
6. If you want a 5 dKH standard instead of a 4 dKH - change step 3 from 450 ml to 400 ml and you will end up with 450 ml of a 5 dKH standard.
A few addition items:
1. Donít heat your baking soda to dry it out or it will change from sodium bicarbonate into sodium carbonate. You could then end up with a mixture of sodium bicarbonate and sodium carbonate, this would change the sample weight needed because it takes less sodium carbonate to make the KH standard than sodium bicarbonate.
2. You can use sodium carbonate anhydrous instead of baking soda. When I found out that heating baking soda turns it to sodium carbonate I thought this might be a good alternative to using baking soda. I bought Lab grade sodium carbonate that is 99.95% pure, thinking it might make a more accurate standard. While in theory it should, I really canít say for sure that it does because when I follow the above instructions both the baking soda and sodium carbonate standards come out correct.
3. Not all of the calculators on the web are correct for calculating how much baking soda or sodium carbonate to use.
If you want to calculate your own weights here is the calculator to use.
'Reef Chemistry Calculator FV'
This calculator is correct, it was off when calculating sodium carbonate but Jose
changed it in the last couple of weeks and now itís right.
This calculator rounds everything up because it was made for calculating
aquarium water not small samples. To get precise weights just add some zeroís to the sample size.
For example: if you wanted to know how much baking soda to add to 3 liters of water to get a standard that was 40 dkh the calculator would give you a weight of 3.6 grams to add. We want to be accurate to at least .01 grams so instead of putting in 3 liters of water you can put in 300 liters, the calculator will then give you a weight of 360 grams to add which you know would be 3.60 grams.
It has been said that there are a lot of assumptions built into these calculators but thatís not correct statement for this calculator when calculating how much sodium bicarbonate or sodium carbonate to use for KH. This calculator will give you the same answer you would get if you calculated long hand, except that it rounds up the numbers (the solution for that is in the above paragraph).
Although there is one assumption in this calculator when using sodium carbonate, it assumes that the sodium carbonate is 100%. You will need to calculate the difference between your sodium carbonate and 100%
I purchased lab grade sodium carbonate anhydrous and it is 99.95% pure. After I calculate how much to use I have then multiply the answer given by the calculator by .5% and add that to the weight given by the calculator.
4. I have made these KH standards several times with sodium bicarbonate and sodium carbonate. When I follow the instructions listed above the standards has always came out right.
I have also made the KH standards using a lab certified Alkalinity Standard Solution as NA2CO3 in 10mL Ampules. As long as the accurate dilutions are done this should make the most accurate standard. This is what the EPA uses as an alkalinity standard when they are monitoring and assessing water quality.
I hope this helps when others when they are making their KH standards and that they wonít have to do all of the research I had to do just to get a good KH standard.
If you don't won't to make your own KH Standards or if you want a standard that's more accurate than the baking soda standard. You can go to the swap and trade forum or to Aqua Bid under "test kits" and buy one of several different KH Standards.
I've got an accurate pH meter. Would it be possible to use this to make up a 4dKH solution?
If I let the distilled water degas to the level of the air (outside an enclosed room of course)I wonder what the pH level of 4dKH should be?
Making up 3L for me will take up a lot of room and I've got to buy it! :)
I do not believe a pH meter will aid you in making up a 4 dkH reference solution.
All you need is a scale that can measure to (at least) 0.1 grams.
I tried using 1 litre of de-ionised water, some Tesco Bicarbonate of soda and a API KH liquid test kit. Oh and I used some electronic kitchen scales accurate to 1 gram.
Water weights 1Kg per 1Litre so dead easy to calculate volume using weight.
I put a tiny amount of Bicarb into 100ml of the water and it went to 20dKH so I added double amount of water. Then I halved it and added double again to get 5dKH with 200ml then I added 40ml to dilute it by 1dKH.
Only took me a few minutes to get a 240ml of 4KH.
But I guess it's not going to be that accurate? But will it be accurate enough do you think for a drop checker?
I took a double sample size of water using my API KH test kit and used twice the amount of drops. I got 8 drops to make it turn yellow. :)
As long as your initial calculation was correct, then your dilutions seem OK.
Your last dilution is incorrect, however.
To 200 mL of 5 dkH, adding 40 mL of distilled water will yield 240 mL of 4.16 dkH reference solution, not a solution of 4 dkH.
KH test kits can be inaccurate too. Here is a way to calibrate that test kit, or make 4 dKH water, that has worked pretty well for at least a few people: http://www.plantedtank.net/forums/fe...tml#post800612 The fortunate thing is that you don't need very good accuracy on the 4 dKH standard. Since the ppm of CO2 you determine with a drop checker is directly proportional to that KH, a 25% error only makes a 25% error in ppm of CO2. But the drop checker isn't that accurate anyway. When it tells you you have 30 ppm, you may have anywhere from 20 to 45 ppm, depending on how good you can interpret the color of the drop checker liquid.
What am I forgetting?
Dilutions do not work that way. You must take into account that by adding 40 mL of dilute water, you are changing the final volumes and concentrations. The proper way to calculate this:
(200 mL)(5 dkH) = (x dkH)(240 mL)
Solving for x = 4.167 dkH.
I measured out 6 grams of sodium bicarbonate from a brand new box of arm and hammer on a laboratory scale that measured to .0001 (I settled for .001, I was fine tuning by the spec). I mixed that with 5 liters of distilled water measured out with a 1000 ml graduated cylinder, in a new carboy rinsed out with DI water. Then I diluted it 1:9 with distilled water using a 100 ml graduated cylinder and the 1000 ml (the last solution I was very diligent measuring by the drop with a pipette).
So, I was hoping for a liter of 4 KH, but when I tested it with an API KH tester, it took 5 drops to turn it from blue to yellow:icon_frow. Would it make sense to remix, and try again, or dial it in to the drops by diluting it?
I would just make it again, if you have the reagents to spare.
If you have access to an analytical scale (which I assume you mean by "laboratory scale"), then you can scale it down from 6 grams to (say) 0.6 grams and make 500 mL of 40 dkH water only.
From there, just take 100 mL of the 40 dkH water and add to 900 mL of DI water to get your 1 L of 4 dkH water.
You probably do not need a Pasteur pipette, as it will not add any more accuracy if you are using graduated cylinders only. The only time it may be useful is if you were using volumetric flasks to make up your solutions.
It's been 30 years since I did Chemistry. A lot has vaporised. :hihi:
I measured it out again. Still coming up with 5 kh. I do get the blue to be pale greenish at four drops, but it does take 5 to seal the deal and be yellow. I guess the arm and hammer is suspect? It is a new box tho, and any added moisture would give me a lesser amount to the mixture, so a lesser concentrated mixture. I checked the scale, and it has been calibrated recently. I carefully measured the water with plastic disposable eye droppers. I checked the TDS in the mixture, and it came out at 74 ppm (which I suppose would be the most accurate way to measure it). Oh, :icon_ideajust checked my conversion chart. It should be 71.6 ppm. It is good to write things out sometime isn’t it? That will be a good way to dial it in…
The thing is 5 kh is 89.5 ppm. So, I suspect the integrity of the API dropper test (drop size could vary). The room in my lab (not a chemistry lab) that I test in is very well lit with fluorescents, and I do it on white paper for color fidelity. The instructions say the solution should be bright yellow (not pale green). I guess I would have to calibrate my TDS know for sure, what’s going on.
As KevinC mentioned, you can easily increase your precision by increasing the volume of water to be tested.
The gH and kH test kits work on the basis of titration. As KevinC also mentioned, if your 4 dkH reference solution is slightly off (i.e. 4.1 dkH), then it will require that 5th drop to change the colour to a bright yellow.
As for where your error is coming from, I would suspect the sodium bicarbonate as well. Despite it being a brand new box, it is not completely air tight, and there are surely impurities left over from the manufacturing process, etc. However, since it is going to go into a drop checker, which is not the most precise tool anyway, you can probably get away with the reference solution you already have made up.
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