Hach DR/850, Procedures Manual, Page: 42 / 460

42
CHEMICAL ANALYSIS INFORMATION
, continued
Branch G
Examples of uniform increments between standard additions are
given below.
Example C
The plot for sample C illustrates a common interference with a uniform
effect on the standard and the substances in the sample. The four data
points form a straight line which may be extended back through the
horizontal axis. The point where the line meets the axis can be used to
determine the concentration of the substance you are measuring.
In this example, the first analysis gave 0.4 mg/L. After extrapolating the
line to the horizontal axis, the graph shows the result should be much
closer to the correct result: 0.8 mg/L.
Apparent interferences may also be caused by a defect in the instrument or
standards. Before assuming the interference is chemical, check Branch B.
Example D
The plot for sample D illustrates a problem for the analyst. The
increments are uniform and the recovery of the standard was complete.
The result of the first analysis was 0 mg/L and the line extrapolates back
through 0 mg/L. If interferences are known to be present, the
interferences may be present in an amount equal to the substance in
question, preventing the analyst from finding the substance. This would
be an uncommon situation.
mL of Standard Added mg/L Standard Added mg/L Found
0 0 0.4
0.1 0.2 0.5
0.2 0.4 0.6
0.3 0.6 0.7
mL of Standard Added mg/L Standard Added mg/L Found
0 0 0
0.1 0.2 0.2
0.2 0.4 0.4
0.3 0.6 0.6