Hach DR/850, Procedures Manual, Page: 39 / 460

39
CHEMICAL ANALYSIS INFORMATION
, continued
The data show several points:
•
The chemicals, instrument, procedure/technique and standards are
working correctly because the iron added to the water sample was
completely recovered in the same uniform steps that match the
standard addition increments.
•
Because iron added to the deionized water was recovered, but iron
added to an actual sample was not recovered (Branch A), the sample
contains an interference which prevents the test reagents from
working properly.
•
An iron analysis previously done on the actual sample using this
method gave an inaccurate result.
If the results of multiple standard additions give the correct increment for
each addition, proceed to Branch C.
If the results of multiple standard additions do not give the correct
increment for each addition, go to Branch D.
Branch C
If interfering substances are present, the analysis may be incorrect.
However, with multiple standard additions, it may be possible to arrive at
an approximate result if the increases are uniform.
Suppose the sample result for iron was 1.0 mg/L. Because interferences
may be present, a standard addition of 0.1 mL of a 50 mg/L iron standard
to a 25 mL sample is made. The expected increase in the iron
concentration is 0.2 mg/L, but the actual increase is 0.1 mg/L. Then 0.2
and 0.3 mL of the same standard are added to two more 25 mL samples
and analyzed for iron.
If there is a uniform increase in concentration between each addition (i.e.,
0.1 mg/L difference between each addition), use Branch G. If the increase
in concentration is not uniform (i.e., 0.1, 0.08, 0.05), go to Branch F.
Branch D
Carefully check the instructions for the test. Make sure to use the correct
reagents in the correct order. Be sure the glassware in use is what is
required. Be sure time for color development and the sample temperature
are as specified. If the procedure technique was incorrect, repeat Branch
B. If the procedure was correctly followed, proceed to Branch E.
Branch E
Check the reagent performance. This may be done by obtaining a fresh lot
of reagent or by using a known standard solution to run the test. Make
sure the color development time given in the procedure is equal to the