Hach DR/850 Procedures Manual page 40

CHEMICAL ANALYSIS INFORMATION
Branch F
time required for the reagent in question. If the reagent(s) is defective,
repeat Branch B with new reagents. If the reagents are good, proceed with
Branch H.
Examples of non-uniform increments between standard additions are
shown below.
Example A
mL of Standard Added
0
0.1
0.2
0.3
Example B
mL of Standard Added
0
0.1
0.2
0.3
These examples show the effect of interferences on the standard addition.
Data plotted on the graph in Figure 10 for samples A and B show that the
four data points do not lie on a straight line.
The plot for sample A illustrates an interference that becomes
progressively worse as the concentration of the standard increases. This
type of interference is uncommon and may be caused by an error or
malfunction of the procedure, reagents or instrument. It is recommended
Branch B be performed to verify the supposed interference.
The plot for sample B shows a common chemical interference which
becomes less or even zero as the concentration of standard increases. The
graph shows the first addition was consumed by the interference and the
remaining additions gave the correct increment of 0.2 mg/L.
The apparent interference in Example B could be the result of an error
made in the standard addition. Repeat the analysis to see if an error was
made during standard addition. If not, the method is not appropriate for the
sample matrix. When these two types of interferences occur, try to analyze
the sample with a method which uses a different type of chemistry.
, continued
mg/L Standard Added
0
0.2
0.4
0.6
mg/L Standard Added
0
0.2
0.4
0.6
40
mg/L Found
1.0
1.10
1.18
1.23
mg/L Found
0
0
0.2
0.4