Calibrating Test Kits - for non-Chemists

[Hoppy]

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

[plakat]

IMO this needs to be a sticky. I would check but I am procrastinating doing homework.

[Dave k]

[quote=plakat;800312]IMO this needs to be a sticky.


I second the motion

[ColeMan]

Sticky, sticky, sticky - 5 stars from me! Excellent work, Hoppy. Now that you've made this so simple, I'm going to have to find something else to do with my digital scale!

[plantbrain]

It's sort of EI for test kit calibration.


Regards,
Tom Barr

[Hoppy]

Originally Posted by plantbrain

It's sort of EI for test kit calibration.


Regards,
Tom Barr

I was planning to call it the Julia Child calibration plan.

[jjp2]

Nice work. Check the phosphate, it says to add KNO3 not kh2po4 or other compound

[clmntch]

Thanks Hoppy...

[Hoppy]

Originally Posted by jjp2

Nice work. Check the phosphate, it says to add KNO3 not kh2po4 or other compound

Thank you! I corrected it.

[epicfish]

Originally Posted by manofmanyfish

Humm...1/2 cup plus 3/4 cup = one cup.....that's like creating matter from nothing....that's cool. I smell a Nobel Prize...

You mean making matter disappear?

1/2 + 3/4 = 1 1/4

[Hoppy]

Originally Posted by manofmanyfish

Humm...1/2 cup plus 3/4 cup = one cup.....that's like creating matter from nothing....that's cool. I smell a Nobel Prize...

I ran this through Einstein's Relativity equations, and you are right. So I corrected it.

[Hoppy]

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.

[tazcrash69]

Originally Posted by plakat

IMO this needs to be a sticky. I would check but I am procrastinating doing homework.

I agree, great write up Hoppy. Thanks

Stickified.

[Hoppy]

KH Test Kit

First, buy one gallon of distilled water from your local grocery store. Set a reasonably accurate thermometer on the kitchen counter near where you will be working. Get an empty plastic bottle that contained bottled non-carbonated drinking water. Wash the bottle with plain tap water a few times, then rinse a couple of times with distilled water.

1. Add enough distilled water at near room temperature, or slightly above, to the bottle, to get it about 2/3 full, or at least 1 cup of water.
2. Add about 5 teaspoons or more of bicarbonate of soda, sodium bicarbonate, baking soda to the water and gently swirl it around a few seconds.
3. Make sure there is still some undissolved baking soda at the bottom of the bottle.
4. Squeeze the bottle slightly to expel some of the air, and cap it to seal the bottle.
5. Leave it beside the thermometer for a few hours, gently swirling it a few times during that time. Make sure there is always some undissolved baking soda left at the bottom of the bottle. (Vigorous shaking of the bottle will cause some of the carbonates to break down, leaving CO2.)
6. Record the thermometer reading in degrees Celsius.
7 If the temperature is 18 degrees, you will need 1.5 ml of that water, at 19-20 degrees you will need 1.4 ml, and at 21 degrees you will need 1.3 ml.
8. Measure 2 cups of distilled water into a very clean bottle that can be capped.
9. Use a cleaned syringe, that has been rinsed with distilled water, to suck up from well under the water surface of the baking soda saturated water more than 2 ml of water. (Make sure you don’t suck up any of the undissolved baking soda from the bottom of the bottle.)
10. With the syringe “needle” under the surface of the distilled water, add the amount per step 7 of saturated solution to the distilled water, and cap the bottle. Gently swirl it to mix. You now have a bottle of 10 dKH water.
11. Add 1/2 cup of the 10 dKH water to 1/2 cup of distilled water to get one cup of 5 dKH water.
12. Add 1 cup of the 10 dKH water to 1 1/2 cups of distilled water to get 2 1/2 cups of 4 dKH water.
13. Add 1/4 cup of the 10 dKH water to 1 cup of distilled water to get 1 1/4 cups of 2 dKH water.
14. Use your KH test kit to test the KH of the 2, 4, 5, and 10 dKH samples. Record the results.
15. If the results agree with the known KH of each sample, your kit is calibrated. If not, you can make a graph of the kit readings on the “X” axis versus the known values on the “Y” axis, and use that to convert the kit readings to correct readings. Your kit will then be calibrated.
16. For future calibrations, store these standard solutions in tightly sealed bottles, labeled with their KH values.
17. For best accuracy, calibrate the KH test kit every time you use it.

This procedure will be accurate to about +/-10-20%, depending on how carefully you do the measuring.

[Regloh]

Thanks Hoppy,
I appreciate the work you put into this. I don't have an accurate scale to use other described methods, so this is making my day
It actually is similar to what I do to add a known amount of Calcium and Magnesium to my water. Eventually I may have done the math myself some day, but now I'll just use your method