Hach DR/850, Procedures Manual, Page: 35 / 460

35
CHEMICAL ANALYSIS INFORMATION
, continued
One of the greatest aids is knowing what is in the sample. You don’t need
to know exactly what is in each sample, but be aware of substances that
are likely to interfere in the analysis method you use. When using a
method, it may be helpful to determine if those interferences are present.
pH Interference
Many of the procedures in this manual only work within a certain pH
range. Hach reagents contain buffers to adjust the pH of the typical
sample to the correct pH range. However, the reagent buffer may not be
strong enough for some samples. This occurs most often with highly
buffered samples or samples with extreme sample pH.
The Sampling and Storage section of each procedure usually gives the
proper pH range for the sample.
Adjust the sample to the proper pH range before testing. If this
information is not given, follow these steps:
1. Measure the pH of your analyzed sample with a pH meter. For
measuring Ag
+
, K
+
or Cl
-
, use pH paper.
2. Prepare a sample using deionized water. Add all reagents called for in
the procedure. Timer sequences, etc., may be ignored. Mix well.
3. Measure the pH of the reagent blank with a pH meter.
4. Compare the pH values of your analyzed sample with the reagent blank.
5. If there is little difference in the values of your analyzed sample and
the reagent blank, then pH interference is not the problem. Follow the
Accuracy Check given in the procedure to help identify the problem.
6. If there is a large difference between the value of your analyzed
sample and the reagent blank, adjust the sample pH to the value of the
reagent blank. Adjust the sample pH to this same pH for all future
samples from the same source before analysis. Use the appropriate
acid, usually nitric acid, to lower the pH (do not use nitric acid for
nitrate or nitrogen testing). Use the appropriate base, usually sodium
hydroxide, to raise the pH. Adjust the final result for any dilution
caused by adding acid or base; see
Correcting for Volume Additions .
7. Analyze the sample as before.
8. Some purchased standards may be very acidic and will not work
directly with Hach procedures. Adjust the pH of these standards as
described above. Adjust the final concentration of the standard for the
dilution. The Hach standard solutions suggested in the procedures are
formulated so that no pH adjustment is necessary.