What can be done to improve accuracy and reproducability of nitrate analysis by cadmium reduction?

Document ID

Document ID TE515

Published Date

Published Date 04/09/2019
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Question
What can be done to improve accuracy and reproducability of nitrate analysis by cadmium reduction?
Summary
Inconsistent results with Nitrate tests.
Answer
The nitrate cadmium reduction method is very dependent on how the sample is shaken. When the sample is shaken, cadmium particles in the reagent reduce some of the nitrate to nitrite. The nitrite then reacts with the indicator to form a color.  If the sample is not shaken hard enough or long enough, not enough nitrate will be reduced, and results will be low. If the sample is shaken too hard, results could be high. Shaking must be consistent with each sample after adding the NitraVer® 5 or NitraVer® 6 reagent.

The best way to ensure correct results is to analyze a nitrate standard solution in place of the sample. Adjust the shaking technique if theresults are higher or lower than the standard concentration.
  • Be sure to measure a reagent blank and subtract this reading from your test results. The medium and high range nitrate methods especially have high reagent blank values.
  • Consider alternate nitrate methods which have detection limits well below the sample concentration or nitrate methods which don't use cadmium.

Another way to improve accuracy involves running a standard along with the samples and doing a few calculations. This is described below:
 

Each time samples are analyzed a standard solution should be analyzed at the same time. Be sure to shake the sample cells with the same consistency by putting them in a rack and shaking with both hands together. This is very important. If shaking causes the standard to be measured incorrectly by a certain percentage , then assume that the unknown sample will also be measured incorrectly by the same percentage. The calculations are as follows:

Given the following:

A = True Concentration of your unknown sample

B = Concentration of the Reagent Blank (what you get when you run deionized water as a sample)

C = Observed Concentration of the sample

D = True Concentration of the standard

E = Observed Concentration of the standard

Then:

A = (C - B) * [D / (E - B)]

Details and Example:

To improve  accuracy, run a reagent blank for the new lot of reagent. This is done by running the test on deionized water as if it were a sample. It is often best to do this in triplicate, noting the average as your Reagent Blank, or B in the equation above.

For this example, use 0.5 mg/L as the value of the Reagent Blank.

Then, take a nitrate standard solution and run it through the test along with the sample.

For this example, say a  10 mg/L standard was used and a  value of 12 mg/L was measured. The True Concentration of the standard, D in the equation, would be 10 mg/L. The Observed Concentration of the standard would be 12 mg/L. For this example, say the sample was measured at 15 mg/L. This is Observed Concentration of  the sample.

Now, using the equation above:

A = (15 - .5) * [10 / (12 - .5)]

A = (14.5) * [10/11.5]

A = 14.5 * .87

A = 12.6 mg/L

In conclusion, the sample, which initially read 15 mg/L, has a true concentration of 12.6 mg/L once the reagent blank and the effect of shaking has been accounted for.

 

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