Hach DR/700 Procedures Manual page 25

If the results of multiple standard additions gave the correct increments
between additions, proceed to Branch C. If the results of multiple
standard additions gave other than the correct increments between
additions, proceed to Branch D.
Branch C
If interfering ions are present, the analysis may be incorrect. However, it
may be possible, with multiple standard additions, to arrive at a close
approximation of the correct result. Suppose the result of a sample
analyzed for iron was 1.0 mg/L. The analyst, knowing interfering ions
could be present, made one standard addition of 0.1 mL of 50-mg/L iron
standard to 25.0 mL of sample. Rather than finding an increase of
0.2 mg/L as expected, the analyst found an increase of 0.1 mg/L. The
analyst took a third and fourth water sample and added a standard
addition of 0.2 and 0.3 mL, respectively. Samples were analyzed and
results tabulated. If steps between each addition are roughly uniform
(i.e., 0.1 mg/L difference between each addition), proceed to Branch G.
If the results are not uniform (i.e., 0.1, 0.08, and 0.05 mg/L), proceed to
Branch F.
Branch D
Carefully check the instructions or directions for use of the test, making
sure the proper reagents are used in the proper order and time, the
colorimeter is adjusted for the correct wavelength and calibration and
the glassware in use is that specified. Be sure time for color
development and the sample temperature are exactly as specified. If the
procedure in use is found to be in error, repeat Branch B using the
correct procedure. If the procedure is found to be correct, proceed to
Branch E.
Branch E
Check the performance of the reagents. This may be done easily by
obtaining a new 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 or greater than the time required for the reagent
in question. If it is determined reagents are defective, repeat Branch B
with new reagents. If the reagents are proven in good condition, proceed
with Branch H.
Branch H
Check operation of the instrument and/or apparatus used in the
performance of the test. Perform the wavelength and linearity checks
1-7