| | Re: Test kits: the good, bad and ugly
Believe it or not it doesn’t matter much which kit you use if you follow some basic chemistry.
Firstly, you have to realize that the color reference you have with your kit may be unreliable because color development depends on a lot of factors besides the concentration of the substrate. Things totally out of your control like temperature, age of the reagents, printing errors in the package and a gazillion other things make using the reference colors in any kit questionable.
So how do you get around this problem? You always need to run a minimum of three tests at the same time.
The first is a blank. This uses purified (ie distilled) water in place of the sample.
The second is a control. This uses a sample with a known concentration of substrate at the concentration you expect in place of the sample.
The third is, of course, your sample.
You have to run all three tests at the SAME TIME and under the same conditions. When the colors develop you can expect that the blank should have no color and the control will be the same color as the equivalent reference on your color chart. If this is not true than you cannot use the color chart for your estimate directly.
What you have to do is correct the color chart to your control. If the known concentration of the control is 20 ppm and the reading on the reference chart is 30 ppm then your chart is off by 20/30. Then when you check your sample and it reads 40 ppm you have to multiply this result to get the corrected answer:
20/30 * 40ppm = 27 ppm.
If you are consistent when running all three tests, you can use some really bad reagents and still get good results.