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Discussion Starter · #1 · (Edited)
I received a request to write something like this, so: (I hope someone will check this for accuracy.)

Calibrating Test Kits

Calibrating a test kit means using that kit to measure some water samples with known concentrations of the substance being tested for, and using those test results to verify that the test kit is accurate, or to train yourself to recognize the colors that correspond to the concentrations you want to test for. Hobby test kits are not laboratory quality tests. That means we don’t need extreme accuracy in the standard test solutions we use for calibration. If we have a good quality gram scale, with +/-.01 gram accuracy, and good laboratory glass graduated cylinders to measure water volume, there are other articles that tell how to make very accurate standard solutions. The methods described here are for use with ordinary kitchen measuring equipment, measuring spoons and cups. And, the Fertilator calculator on APC was used to easily calculate how to mix these.

Nitrate Test Kits

First, buy a gallon of distilled water from your local grocery store. Use that to make the test standard solutions.

1. Add 1/4 teaspoon - a level measure, not a heaping measure - of KNO3 to 4 cups of distilled water (one quart). This gives you 4 cups of 800 ppm nitrate water.
2. Mix 1/4 cup of that 800 ppm water with 1 3/4 cups of distilled water. This gives you 2 cups of 100 ppm nitrate standard water.
3. Mix one cup of that 100 ppm water with one cup of distilled water. This gives you 2 cups of 50 ppm nitrate standard water.
4. Mix one cup of that 50 ppm water with one cup of distilled water. This gives you 2 cups of 25 ppm nitrate standard water.
5. Mix 1/2 cup of that 25 ppm water with 3/4 cup of distilled water. This gives you 1 1/4 cups of 10 ppm nitrate standard water.
6. Mix 1/4 cup of 25 ppm water with 1 cup of distilled water. This gives you 1 1/4 cups of 5 ppm nitrate standard water.
7. Use your test kit to measure the nitrate concentration in each of the 5,10,25, and 50 ppm nitrate standards. If you wish, add the 100 ppm standard to that set.
8. Compare the colors of those to the color card for your kit, and either verify the accuracy of the kit, or use those colors to train yourself to recognize the colors.

Your nitrate test kit is now calibrated. You can store the standard solutions in tightly sealed bottles for an indefinite period of time for future calibrations. Ideally, you calibrate the kit each time you use it.

Phosphate Test Kits

First, buy a gallon of distilled water from your local grocery store. Use that to make the test standard solutions.

1. Add 1/4 teaspoon - a level measure, not a heaping measure - of KH2PO4 to 4 cups of distilled water (one quart). This gives you 4 cups of 1000 ppm phosphate water.
2. Mix 1/4 cup of that 1000 ppm water with 2 1/4 cups of distilled water. This gives you 2 1/2 cups of 100 ppm phosphate standard water.
3. Mix one cup of that 100 ppm water with one cup of distilled water. This gives you 2 cups of 50 ppm phosphate standard water.
4. Mix one cup of that 50 ppm water with one cup of distilled water. This gives you 2 cups of 25 ppm phosphate standard water.
5. Mix 1/2 cup of that 25 ppm water with 3/4 cup of distilled water. This gives you 1 1/4 cups of 10 ppm phosphate standard water.
6. Mix 1/4 cup of 25 ppm water with 1 cup of distilled water. This gives you 1 1/4 cups of 5 ppm phosphate standard water.
7. Mix 1/4 cup of 5 ppm water with 1 cup of distilled water. This gives you 1 1/4 cups of 1 ppm phosphate standard water.
8. Use your test kit to measure the phosphate concentration in each of the 1,5,10, and 25 ppm phosphate standards. If you wish, add the 50 ppm standard to that set.
9. Compare the colors of those to the color card for your kit, and either verify the accuracy of the kit, or use those colors to train yourself to recognize the colors.

Your phosphate test kit is now calibrated. You can store the standard solutions in tightly sealed bottles for an indefinite period of time for future calibrations. Ideally, you calibrate the kit each time you use it.

Other Test Kits

To follow, maybe:confused1:
 

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Discussion Starter · #12 ·
Yesterday I made some 4 dKH water, using 14 dKH water made from an ampule of lab standard water. When I used my KH test kit to measure the KH of that water, it said 3 dKH. So, that raised a lot of doubts in my mind about the KH test kits. Calibrating that kit is a much harder job. But, it finally occurred to me that I should be able to take advantage of a property of sodium bicarbonate to do that. The property is the solubility of NaHCO3 in water, which varies with temperature. I converted the graph of that solubility to read in solubility of carbonate instead of sodium bicarbonate, getting this graph:


The technique will be to start with distilled water, add a lot of baking soda to that, let it sit quietly on the kitchen counter (where else?) for an hour or so, with only very gentle stirring, to saturate the water with baking soda. It won't matter that my baking soda will contain some water in the crystal structure, since that water just joins the distilled water. Then I use a syringe to get a sample of that water, 1 ml, as closely as I can get, and quickly add it to enough distilled water to end up with 20 dKH water, by calculation. Then dilute that to get my standard solutions. Will this work? I know the saturated solution will be losing CO2 to the atmosphere, but if I do this rapidly I should still have saturated solution being added to the distilled water. Am I missing something?

If this will work, it is another way to make 4 dKH drop checker water.
 

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Discussion Starter · #14 · (Edited)
Calibrating KH test kit

KH Test Kit
This method, compliments of Cardinal's Keeper (http://www.plantedtank.net/forums/diy/178491-another-diy-4-dkh-solution.html)
(Note: You will need a whole gallon of distilled water to do this.)
1. Start with 6 cups of distilled water in a clean measuring container
2. Add 1/8 teaspoon Sodium Bicarbonate (baking soda, freshly opened) to the 6 cups of water and mix
3. Pour out 3 cups of this mix and discard
4. Add back 3 cups of distilled water and mix
5. Pour out 3 cups of this mix and discard
6. Add back 3 cups of distilled water and mix
7. Pour out 1 cup of this mix and discard
8. Add back 1 cup of distilled water and mix
7. Water comes out to a 4 dKH solution
8. To make a 2 dKH solution, mix one cup of the 4 dKH solution with one cup of distilled water.
9. To make a 1 dKH solution, mix one cup of the 2 dKH solution with one cup of distilled water.
 

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Discussion Starter · #18 ·
Oops, I'm very late seeing this. Sorry!

It isn't that the test kit results vary from day to day, it is that they are made with organic chemicals, with organic dyes, and those are not stable chemicals. Eventually they deteriorate. So, if your test kit sat on the dealer's shelf for a year before you bought it, or if the dealer allowed his store to get very hot for a few days, like over the weekend, the kits may never be accurate after you buy them. Then, if you are like me, you buy the kit, stash it under the aquarium and when you finally use it, you don't remember how long you have had it, so it may deteriorate on your shelf too. Now, add to that, that these are very cheap kits, so it is highly unlikely that there is good quality control on their manufacture.

Even if none of the above apply to you, just remember that professionals who do testing of any kind always calibrate their test equipment before use, or, at a minimum every few months. And, that calibration is done with an accuracy far better than the test equipment is designed to meet. No one who is paid to test things ever uses anything for measurements unless it is calibrated first and often. Even professional mechanics get their vernier calipers calibrated on a routine schedule.
 

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Discussion Starter · #22 ·
Thread Resurection!!!!!!:icon_eek:


Any new calibrations???

Thanks,
Wes
None, and I don't expect to do any additional ones. Remember, the only purpose for doing this type of calibration is to find out if your test kit readings mean anything. The accuracy you can get this way is very limited. But, it is better than thinking your tank has 50 ppm of PO4 in it, when it really has less than 4 ppm. I don't use test kits at all now, and never did make much use of them, so this was mostly just a challenge for me. I did try to make 4dKH water using the method here, and it measured 3 dKH with my test kit. So, that "proves" that either this calibration method works, but with crude accuracy, which is what I aimed for, or it proves that my KH test kit readings are off. Being an optimist, I assume the former :icon_bigg
 

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Discussion Starter · #26 ·
This looks supurb, and is probably just what I need to help resolve the difference between my tap water (25ppm Nitrate) and the time for a critical water change (50ppm? Nitrate). Two questions please:

1) Can the reference solutions be kept for a few weeks, or will the colours deteriorate?

2) Any chance that somebody could translate the measurements in the recipes into SI units? Many of us don't use cups as volume measures and don't use volume measures (teaspoons) to precisely measure chemical powders.

Edit: Trying to answer 2) myself. Presumably the 'cups' used in the recipes are US cups, i.e. 236.6ml. Now, all I need to do is work out the weight of a teaspoon on Potassium Nitrate (etc)

Peter
I admit I didn't give any thought to the rest of the world where "cups" and "teaspoons" are not easy to measure. All I wanted to do was make it easy for someone with a typical American kitchen to determine if his/her test kit reading meant anything at all. A similar approach could be taken with common metric measuring equipment. I will leave it to you to enjoy working that out:biggrin:
 

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Discussion Starter · #28 ·
[/INDENT]Did I get it right, teacher? :icon_bigg

Peter
Unfortunately I didn't keep my scratch pad calculations. As I recall I used a round number for the mass of the fertilizer chemicals per teaspoon. It may have been 4 grams or 5 grams, but I can't recall for sure. So, I don't know if you got it right. It does look like you are using the right logic, so it is probably right.
 

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Discussion Starter · #32 ·

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Discussion Starter · #34 ·
There are several posts in forums that tell how to use grams and liters to make reference solutions. My contribution was a way to do well enough using just what an average American kitchen would have - measuring spoons and cups.
 

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Discussion Starter · #38 ·
Hi all of you, and Hoppy in particular. I am a newbie and want to do CO2 injection in an 80G planted aquarium. I don't want my plants to be completely slow-growing and low-tech, and I don't want them to be growing too vigorously either, as with 30ppm of CO2. [Yeah, I'm that Great Moderation guy! :redface:] So, I'm thinking I'll go for moderation, say about 15ppm of CO2, moderate lighting. I have two questions:

(1) Does this approach seem reasonable, or am I forgetting/ignoring something important?
(2) Now the more "techy" question. MOst standard drop-checkers, set up with 4dKH, turn green at 30ppm. This works fine for those who want to keep their tanks at 30ppm CO2, but it wouldn't work well for me. I need an indicator that'll tell me (say go from blue to green) at 15ppm instead of 30ppm. So, looking at Hoppy's above post, I thought, why don't I make a 2dKH solution to put in the drop checker? The charts [ http://freshwateraquariumplants.com/carbondioxidechart.html ] seem to say that 2dKH will get to pH=6.6 at about 15 ppm instead of 30ppm, so this should work for me, right? All I do is put 2dKH standard solution and a few drops of the indicator into the drop-checker, and voila! I calibrate CO2 to keep the solution green without going blue or yellow, just like everybody else does, but I'm keeping it at 15ppm, not 30ppm, right? What am I missing?

Thanks for your help in advance!

-aadro
That would work fine, but keep in mind that a drop checker with 4 dKH fluid will be "green" not at 30 ppm, but at something between about 20 and 40 ppm. There is a lot of built in uncertainty with this method, based on the limits of our eyes in seeing green as a specific color, and on the inaccuracy in measuring pH.

I experimented a lot with using a drop checker where yellow was the indication of the right amount of CO2. This was primarily for DIY CO2, where the battle is about getting enough CO2, not about avoiding too much CO2. I ended up using 1.7 dKH fluid. It worked pretty well for me when I was using it, but making 1.7 dKH fluid is hard to do without starting with a good, known KH solution. http://www.plantedtank.net/forums/showthread.php?t=129720
 

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Discussion Starter · #41 ·
I have tried 2 drop checkers numerous times, with various KH fluids in them, hoping that would improve the accuracy. It never did. The basic problem is the difficulty in judging the color of the solution - a slight mistake in judging what is "green" makes a big mistake in the ppm of CO2 you think you have. I ended up the happiest when I used yellow as the desired color, and DIY CO2 so I could be sure I wouldn't ever have too much CO2. Then I just tried to keep the DC color yellow, which was relatively easy.
 

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Discussion Starter · #43 ·
Atmospheric CO2 is enough to make a 4 dKH standard reference solution be dark blue. Adding an acid to lower the pH would only introduce more inaccuracy. The only improvements I know of for drop checkers are:
Use a pH meter instead of the pH reagent.
Improve the time response of the DC by increasing the contact area between the tank water to trapped air, and the trapped air to DC solution, plus reducing the volume of the DC solution.
Both of those improvements are doable if you use a semi-permeable membrane instead of trapped air to separate the tank water from the DC water. But, it isn't an easy design to do, and have it work well.
 

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Discussion Starter · #48 ·
Standard pH solutions are much more difficult and complicated than the "salt" solutions this thread is about. If someone is a good chemist, and can design a kitchen measuring equipment way to make standard pH solutions please do so, and I will put them in the first post in the thread.
 

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Discussion Starter · #61 ·
You can't mix a known sample of nitrate in water, for example, then add the test reagent to get the color, then save that colored sample to refer back to later. The color comes from dyes in the reagents, and dyes are not stable chemicals. You might be able to store them for a week or so, but definitely not for years.
 
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