[cliffclof]

I struggle with chemistry. Can you please help?

The chemicals I have are repackaged fertilizers that might be labeled incorrectly.  I am trying to find out what they are by adding controlled amounts in a solution and testing the solution.  Then i cross reference the molecular weight calculations with my test results and some tests don't add up or even come close.  The problems:

I have KNO3 labeled as 13.5-0-46.2.  By molecular weight I would assume that if I add 100 mg of KNO3 to 1 Liter of Deionized water it would result in a solution of ~53.87 mg/L of NO3 and ~33.97 mg/L of K.  The problem is when I test using a Hach DR890 Colorimeter my Nitrates (NO3) read 88.2 mg/L. (Cadmium Reduction Method)₁

Similar to this problem I have Mono-potassium Phosphate (KH₂PO4) labeled 0-51-34.  When I mix .2 mg of MKP in 1 Liter of water₂ my equations would suggest ~1.396 mg/L P as PO4 and ~.57 mg/L of K.  My actual test results are 0.96 PO4 using the HACH DR890 colorimeter (Ascorbic Acid Method)₃  This led me to believe the product I am using is actually Di-potassium Phosphate (K₂HPO4)


Can you please advise why my testing does not match my equations?

If I am going about this the right way, what could the product labeled KNO3 be?

Was I correct to assume the second fertilizer is Di-potassium Phosphate?




1. Cadmium metal reduces nitrates present in the sample to nitrite.  The nitrite ion reacts in an acidic medium with sulfanilic acid to form an intermediate diazonium salt which couples to gentisic acid to form an amber-colored product.

2. To make this solution I mixed 2 mg of MKP in 1 Liter of water and then diluted 1:10 with DI water.

3. Orthophosphate reacts with molybdate in an acid medium to produce a Phosphomolybdate complex. Ascorbic acid then reduces the complex, giving an intense molybdenum blue color.

[DevaDevil]

By molecular weight I would assume that if I add 100 mg of KNO3 to 1 Liter of Deionized water it would result in a solution of ~53.87 mg/L of NO3 and ~33.97 mg/L of K.

53.87 + 33.97 = 87.84 mg.
where is the rest of your 100 mg?

When I mix .2 mg of MKP in 1 Liter of water2 my equations would suggest ~1.396 mg/L P as PO4 and ~.57 mg/L of K.

1.396 + 0.57 = 1.966 mg, and apparently that is without even counting the oxygen (if I read your line right)



You are not quite right in the mass balances it seems...

[cliffclof]

I struggle with chemistry. Can you please help?

I was using a spreadsheet and didn't adjust it to 100 mg before referencing it. Sorry.  KNO3 has a molecular weight of 101.1.  K 39.098 and NO3 62.0049  so i guess 62.0049 + 39.098 = 101.1029 mg.  And in case you ask this is for ~101.1 mg.

But my test would still read 88.2 mg/L NO3 for this amount.  Do you know why?

1.396 + 0.57 = 1.966 mg, and apparently that is without even counting the oxygen (if I read your line right)

Once again I wrote this wrong.  I will edit the post and fix my obvious mistakes in writing this.  It should read 2 mg/L MKP and that solves the decimal problem. It is PO4 not P. Sorry.  And in the subnotes I actually mixed 20 mg of this and diluted it 1:10.

Thanks for the reply and pointing this out.


Does anyone know why my test results are still off?

[cliffclof]

Can you please use your greater mind to tell me if this is possible.  I appreciate it.

The only way I can get the NO3 to work out is if the product contains KNO3 and and an extra N with something else.  If that something else is Na which would be impossible right, it would equate to 20.347 mg/L NO3-N or 89.5268 mg/L NO3?

Maybe the Mono Potassium Phosphate is actually Di-Potassium Phosphate which comes closer, but why is it marked Mono Potassium Phosphate?

Is this correct or am I messing up the good name of Chemistry again?

[DevaDevil]

if it were pure sodium nitrate you would get 73 mg/l out of 100 mg of the compund.
if it were pure Lithium nitrate your would get 90 mg/l

if it were pure magnesium nitrate your would get 84 mg/l (but it would have to be dry, which would not be very stable as it is hygroscopic; the hexahydrate would give only 48 mg/l)


lithium nitrate is the closest, but it isn't a fertilizer, it is used in fireworks 



my guess is for you to redo the analysis, and verify the number.

[DevaDevil]

as for the phosphate problem; it may indeed be labelled wrong.

you would need a reading of 1.1 mg/l if you dissoluted 2 mg for it to be pure dibasic potassium phosphate, so what you get is close.
As for why it is labelled wrong, I cannot know of course 



what you should also not forget is that we assume here that you have PURE compounds, which may not be the case at all...

[cliffclof]

Wow, thanks so much.  I will do a few more control tests.  I have before, but not so carefully as now.

I'm beginning to think that the KNO3 is definitely not pure.  Is there any way to have K (NO3)2  maybe by adding some other element? like KMg(NO3)2  I know that is a silly question.

I know from tests that it does not have Ammonia or Ammonium in it, No Nitrites, No significant Mg or Ca (less than 5 ppm hardness as CaCO3).  I know K does not interfere with the test I am running and I think it is calibrated, I will check that.

Thanks again, I'll report back when I figure it out.

[cliffclof]

The more obvious choice is that my tests are inaccurate.  I guess I either have bad regents, Colorimeter is messed up or dirty, Glassware isn't clean, operator error.

It is just odd that so many of my tests are inaccurate with standard solutions.  Oh well, I'm done.

[cliffclof]

SOLVED

My final report.  I called Hach and talked to a lovely tech there.  She said to always trust the instrument because the calibration curves don't go bad and the colorimeter lights don't degrade they fail.  She said if the regents are not clumpy then they should be fine too.  She was confused that I was getting higher than normal NO3 readings because usually lower readings are more common in mistakes.  She suggested I order some standard solutions from Hach and run multiple tests for control.

If the meter isn't wrong and the regents look okay and I trust the fertilizer package labels then everything should be fine.  So I mixed my own standard solutions based on package labels.  Then read through the NO3 procedures very carefully and found one little thing in the fine print italic notes.  It admonishes in bold to shake vigorously for one full minute with a built in timer on the device which I was following to the T.  Then in little itty bitty letters it says to adjust this to your liking...    So I busted out my solutions and tested about 5 - 10 times per no3 solution logging the results and trying different times.  If I shake for about 4 - 6 seconds I get consistent and very close results.  Shaking a full minute gives high results every time.

As for the KH2PO4 solution I had to mix a 697.9 ppm PO4 solution then dilute to 70 ppm and 7 ppm then made 2 gallons of 1.5 ppm and .3993 ppm Standard Solutions.  They tested fine. The testing accuracy also seemed better with less shaking.

I guess be careful because your vigorous shake may be too vigorous.