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Discussion Starter · #1 ·
I wanted to see if a 10 year old API nitrate kit was still accurate so I made a 20ppm KNO3 solution by mixing 1.631g of KNO3 with 1000ml of pure RO water (TDS reads 1ppm) and then by mixing 1ml of that solution with 49ml of pure RO water. I did not need to make other references so I thought that a 50:1 dilution would make 20ppm using the original 1000ppm solution. Only did one test but it really did read 20ppm as the photos show. The LaMotte kit from 1992 read only 2ppm.
 

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Discussion Starter · #3 · (Edited)
I am not sure how you arrived at your conclusion. 1.631 grams in 1 litre of water gives a nitrate solution of 1631 ppm.
A few references to reference standards on the web. But if you are correct, wouldn't that mean that 1g of KNO3 is 100% NO3.

If am not a chemist but the amount of grams to start with in 1000ml of water seemed arbitrary to 3 decimal points. If what you say is true, then a 50 times dilution would have yielded 32.62 ppm???

OK found it

since a litre of water has a mass of 1 million mg's
100 ppm NO3-
has
100 mg of NO3)-1 per litre

using molar mass find moles of NO3)-1
0.100 grams NO3)-1 @ (1 mole NO3)-1 / (62.005 grams NO3)-1 = 0.001613 moles NO3)-1

since 1 mole of KNO3 has 1 Mole NO3)-1
( 0.001613 moles NO3)-1 comes from 0.001613 moles KNO3


using molar masses
0.001613 moles KNO3 @ 101.10 g KNO3 / mol = 0.163051 grams of KNO3

your answer, rounded to 3 sig figs would be
0.163 grams of KNO3


So this makes 100ppm, all I did was multiply .163 by 10 and then diluted by 50 times.
 

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A few references to reference standards on the web. But if you are correct, wouldn't that mean that 1g of KNO3 is 100% NO3.

If am not a chemist but the amount of grams to start with in 1000ml of water seemed arbitrary to 3 decimal points. If what you say is true, then a 50 times dilution would have yielded 32.62 ppm???

OK found it

since a litre of water has a mass of 1 million mg's
100 ppm NO3-
has
100 mg of NO3)-1 per litre

using molar mass find moles of NO3)-1
0.100 grams NO3)-1 @ (1 mole NO3)-1 / (62.005 grams NO3)-1 = 0.001613 moles NO3)-1

since 1 mole of KNO3 has 1 Mole NO3)-1
( 0.001613 moles NO3)-1 comes from 0.001613 moles KNO3


using molar masses
0.001613 moles KNO3 @ 101.10 g KNO3 / mol = 0.163051 grams of KNO3

your answer, rounded to 3 sig figs would be
0.163 grams of KNO3


So this makes 100ppm, all I did was multiply .163 by 10 and then diluted by 50 times.
Oho! You're right! Thanks for showing the math; had a brain fart earlier today.

It seems odd that the LaMotte is that off, but this is why there are expiration dates on test kits!
 

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I recently tested an old API nitrate kit against a new one, and both tests came out the same. Multiple times, on multiple tanks. The old kit had to be at least 10 years old, as well. I am glad to see it confirmed.
 

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Discussion Starter · #6 · (Edited)
Oho! You're right! Thanks for showing the math; had a brain fart earlier today.

It seems odd that the LaMotte is that off, but this is why there are expiration dates on test kits!
I wish it was my math, but sadly it is not. The LaMotte kit was over 20 years old but after looking for refills, I realized that it only measures up to 10ppm nitrogen so probably better off that it expired.

Bump:
I recently tested an old API nitrate kit against a new one, and both tests came out the same. Multiple times, on multiple tanks. The old kit had to be at least 10 years old, as well. I am glad to see it confirmed.
I argue with my wife when she goes to throw out all of the Advil and vitamins because they go 1 day past the expiration date, then leaves the turkey cold cuts for 3 weeks. Sort of like Will Ferrell in the LEGO movie when he tells his son that LEGO is for adults and the son replies that the box says ages 8 to 14, and Will Ferrell replies "that's just a suggestion"
 

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I argue with my wife when she goes to throw out all of the Advil and vitamins because they go 1 day past the expiration date, then leaves the turkey cold cuts for 3 weeks. Sort of like Will Ferrell in the LEGO movie when he tells his son that LEGO is for adults and the son replies that the box says ages 8 to 14, and Will Ferrell replies "that's just a suggestion"
There is some degree of degradation of drugs once they are significantly past their expiration date, so they might not necessarily be as effective. While this does not really matter (so much) for things that are non-life threatening, for other drugs, it is more important.

Of course, for things like food, a day or two past the expiration date is usually fine, though I usually let my nose decide. I've had milk spoil before the expiration date even though it was stored in the fridge.
 
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