Hach DR/850 Procedures Manual page 37

CHEMICAL ANALYSIS INFORMATION
Branch A
Branch B
c) Are you using the correct glassware?
d) Is the glassware clean?
e) Does the test need a specific sample temperature?
f) Is the sample's pH in the correct range?
Hach's written procedure should help you to answer these questions.
2. Check your reagents. Repeat the Standard Additions using new, fresh
reagents. If your results are good, the original reagents were bad.
3. If nothing else is wrong, the standard is almost certainly bad. Repeat
the Standard Additions with a new standard.
If the check list does not determine the problem, use the decision tree
(Figure 9) and explanation of each branch, below, to identify the problem.
Suppose a single standard addition to the sample did not give the correct
concentration increase. A possible cause could be interferences. Other
causes include defective reagents, incorrect technique, a defective
instrument/apparatus or defective standard used for the standard addition.
If interferences are known or assumed to be absent, proceed to Branch B.
If interferences are known to be present, proceed to Branch C.
Perform multiple standard additions on a sample of deionized water as in
the following example using iron as the analyte of interest:
1. Pour 25 mL of deionized water into a 25-mL sample cell.
2. Add 0.1 mL of a 50-mg/L iron standard solution to a second 25 mL
sample of deionized water.
3. Add 0.2 mL of the same standard to a third 25 mL sample of
deionized water.
4. Add 0.3 mL of the same standard to a fourth 25 mL sample of
deionized water. Analyze all these samples for iron.
5. Tabulate the data as shown below:
mL of Standard Added
0
0.1
0.2
0.3
, continued
mg/L of Standard Added
0
0.2
0.4
0.6
37
mg/L of Iron Found
0
0.2
0.4
0.6