Hi all,

If the result of the test reads 5mg/l as N, you multiply the result by 62 and then divide by 14 to express the result as NO3.

When the test measures in elemental nitrogen (N) it will give you a much smaller number than if it measures NO3.

The RAM of N = 14, and O = 16. In NO3 you have a RMM of 14 + (3*16) = 14 + 48 = 62. So this where the "14" and "62" come from.

So NO3 is 14/62 ~ 22.6% N.

If you divide your 5 ppm N by 0.226, it gives you 22.14 ppm NO3.

You don't really need the maths, if you multiply the N value by 4.5 you are pretty near the NO3 value (22.5 ppm).

If the reading was 75mg/l (i.e. higher than the 40 range of the kit) you dilute the test solution with de-ionised water at a rate of 1 to 1 - 50% test solution and 50% de-ionised water which results in a measurement of 37.5 x 2

When you have a sample that reads above your highest standard (in this case you don't have to create a calibration curve, Hach have already done it), you need to dilute that sample so that the value lies on your calibration curve. Because you've doubled the sample volume with DI water (which doesn't contain any NO3) you need to double the recorded value (37.5 * 2 = 75)

Curve is actually a bit of a misnomer, because ideally you want to read the sample value near the origin of the curve, in the initial linear section.

This is one of the problems, if you did as suggested your value (right at the top of the curve) would probably still be an underestimate of the true NO3 value. You can carry on dividing your sample by 2 with DI water (1:2,1:4 (multiply value by 4),1:8 (multiply by 8)) until you get a repeatable value, but in the case of

*off the scale nitrogen* a 1:10 dilution (1 part tank water, 9 parts DI water) would be more likely to give a more accurate result.

The question really has to be is this process likely to give a better estimation of NO3 values than visual estimation of nitrate provided by the "Duckweed Index" and a conductivity reading? For me the answer is probably not.

I get quite a few PM's on other forums that start with a statement like,

I know I don't have any nitrate because my test kit says so, but I have to take out handfuls of Duckweed every week, what is happening?

and I tell them to trust the plants.

cheers Darrel