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Cobalt chloride hexahydrate


I have some questions about 'Reagent chemicals 10th edition' test method.

When I tested 'Cobalt chloride
hexahydrate-Nitrate test', Part 4. Monographs for reagent chemicals. book page

It refers to measure the absorbance of sample, control, blank. and calculate
by followed formula.


After tested that, absorbance value was like below.

* Sample value : -0.0508 (at 410 nm)

* Control value : 0.1526 (at 410 nm)

So I input this data to calculation fomula.

%NO3 = sample / (control - sample) = -0.0508 / (0.1526-(-0.0508)) = -0.24%

Here is questions,

I wonder sample value can be minus quantity (Is it possible and acceptable

Also %NO3 could be shown minus percent? does that mean nitrate is not
exist (or little) in cobalt chloride hexahydrate?

We followed reagent chemicals test recipe well.


Please, answer this questions. it will be helpful to test this compound.

I hope to receive your respond soon.


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1 Reply
New Contributor II

Re: Cobalt chloride hexahydrate

The sample value can't really be a negative number, as it's supposed to reflect the amount of nitrate in it. The blank is adjusted to 0, and the sample should have some absorbance greater than 0. Practically speaking, errors in measurement and impurities can give you results like yours. Ideally, it's supposed to work like this:

Blank = 0 adsorbance

Sample absorbance = some positive value "A" due to Nitrate ion in the sample

Control absorbance =  some positve vaule "B", due to Nitrate in sample ("A") PLUS absorbance due to  0.01% Nitrate.

The difference between A and B should reflect the absorbance due to 0.01% nitrate, and A should be due only to the nitrate in the sample. Since you have interference or errors from someplace, giving a less than 0 absorbance for the sample, I'd run the test again; the -0.05 sample absorbance is a bit big. If you get similar results, but the sample absorbance is close to 0 (+ or - 0.02, maybe) just note that the nitrate concentration is below the detection limit of the method.

And no, you can't have a negative percent of Nitrate, except on philosophy classes.

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