One of the challenges in performing a nitrate analysis using the Cadmium Reduction procedures (Hach Method's 8039, 8171, and 8192) is that the chemistry is highly sensitive to differences in technique. Variations of as much as 50% can be observed simply by changing how hard and how long one shakes the vials during mixing. Now, you can avoid all this by using the Chromotropic Acid chemistry for Nitrate analysis (Hach Method 10020, using reagent set #2605345.) But if your range or your equipment limits you to the Cadmium Reduction method, then you will have to find a way to increase your accuracy with what you have.The Cadmium Reduction procedures give you the advice to normalize your technique using a standard solution, practicing until you can get the numbers to come out right. The problem with this, of course, is maintaining consistency from one sample to the next. However, there is a way you can get true numbers without practice. It just requires that you run a standard with your samples, and that you use a little math.
