Checking the Checkers

Spike Cover

Have you ever thought of the virtues of checking your test kits for accuracy? well maybe you should, not only will this show you if they are past their sell by date and useless , but in the past i have found some test kits leave a lot to be desired for accuracy.
If you know the error in the kit, this will give you the luxury of allowing for that error when testing your water parameters, this can only be a good thing.

Testing the test kits

By Spike Cover, April 7, 2003

Word definitions

Precision – reproducibility

Sensitivity – smallest increment that can be measured - resolution

Accuracy – variance from actual

Stock solution – ammonia test

Prepared by mixing 3.14 grams of ammonium chloride (NH₄Cl) into1 liter of distilled water – the 3.14 grams was arrived at by taking the atomic weights of the chemical and then determining how much of the ammonium chloride is required to yield the desired amount of ammonia (NH₃).

The relevant numbers are (ref: periodic chart):

N = nitrogen, atomic weight of 14.0067

H = hydrogen, atomic weight of 1.0079

Cl = chlorine, atomic weight of 35.453

So NH₄Cl has an atomic weight of 14.0067 + 4x1.0079 + 35.453 = 53.49

And NH₃ has an atomic weight of 14.0067 + 3x 1.0079 = 17.03

If we desire 1000 ppm of NH₃, we need 1000 mg of NH₃ per 1 liter of water

But since we have NH₄Cl and not NH₃, we will need 53.49/17.03 x 1000 mg = 3141 mg of NH₄Cl to yield 1000 mg of NH₃, or 3.141 g of NH₄Cl

If we desire to make up 1 liter of ‘stock solution,’ we will need to add 3.141 g NH₄Cl or 3.14 g when rounded to two decimal places, i.e., this is the amount of NH₄Cl that would be added to 1 liters of distilled water to make a 1000 ppm solution of NH₃.

Exercise - Test ammonia kit

Using the pre-made stock solution of 1000 ppm ammonia, make diluted solutions to test your ammonia test kit.

Example: take 1 ml of the stock solution and add to 1 liter of distilled water to make 1 ppm or 1 mg/l, stock solution of NH₄Cl,

2 ml of stock added would make 2 ppm or 2 mg/l

0.5 ml to 1 liter would make 0 .5 ppm or 0.5 mg/l, etc:

Then fill the test tube of your kit to the kits required level and test

Use at least 4 data points (5 would be better), including zero ammonia, to generate a curve of actual vs. readings to show the spectrum of error, (use graph paper) for your (individual) ammonia test kit. Make sure you span the manufacturer’s stated range of your kit.

On your graph, label the x and y axes, actual and reading, respectively and show values in ppm or mg/l which ever you prefer.

Additional tests for NO₂ and NO₃ - using sodium nitrite (NaNO₂) and potassium nitrate (KNO₃), respectively

Stock solution – nitrite test, the relevant numbers are (ref: periodic chart):

Na = sodium, atomic weight of 22.9898

N = nitrogen, atomic weight of 14. 0067

O = oxygen, atomic weight of 15.9994

K = potassium, atomic weight of 39.0983

Sodium nitrite, (NaNO₂) has an atomic weight of 22.98978 + 14.0067 + 2x 15.9994 = 31.9988. = NaNO₂ atomic weight of = 68.9953

Nitrite alone (NO₂), has an atomic weight of 14.0067 + (2X 15.9994) = 46.0055

If we desire 1000 ppm of NO₂, we need 1000 mg of NO₂ per 1 per liter of water.

But since we have NaNO₂ and not NO₂, we will need, 68.9953 / 46.0055 x 1000 mg = 1482.1755 mg of NaNO₂ to yield 1000 mg of NO₂.

If we desire to make up 1 liters of ‘stock nitrite solution,’ we will need to add 1.48 g of sodium nitrite, when rounded to two decimal places, i.e., this is the amount of NaNO₂ that would be added to 1 liters of distilled water to make a 1000 ppm solution of NO₂.

Stock solution – nitrate test, the relevant numbers are (ref: periodic chart):

K= potassium, atomic weight of 39.0983

N= nitrogen, atomic weight of 14.0067

O = oxygen, atomic weight of 15.9994

Potassium nitrate (KNO₃₎ has an atomic weight 39.0983 +14.0067 + (3X 15.9994) = 101.1032.

Nitrate alone (NO₃₎ has an atomic weight of 14.0067 + (3X 15.9994) = 62.0049

If we desire 1000 PPM of NO₃, we need 1000 mg of NO₃ per 1 liter of water.

But since we have KNO₃ and not NO₃, we will need 101.1032 / 62.0049 x 1000 mg = 1630 mg of KNO₃ to yield 1000 mg of NO₃.

If we desire to make up 1 liters of ‘stock nitrate solution,’ we will need to add 1.63 g of sodium nitrite, when rounded to two decimal places, i.e., this is the amount of KNO₃ that would be added to 1 liters of distilled water to make a 1000 ppm solution of NO₃.

Sodium hydrogen carbonate; sodium acid carbonate; baking soda; bicarbonate of soda
Molecular Weight: 84.01
NaHCO3

Solubility: 7.8g/100g water @ 18C (64F).
pH: 8.3 (0.1 molar @ 25C (77F))

Add 7.8 gm of Sodium Bicarbonate to 100ml of tap water this should settle to give a pH of 8.3-8.4 check your test kit using this buffered solution.