Sample Preparation - MACHEREY-NAGEL PF-12Plus Manual De Instrucciones

8. Sample preparation

Water samples are not always suitable for immediate analysis. In case of heavily polluted
water (or waste water) especially, it is often not possible to conduct analysis without pre-treat-
ing the sample first; otherwise, larger concentrations of organic or inorganic compounds can
interfere and lead to false negative or false positive test results.
Some of the following preparations may be necessary before analyzing the water (examples
in parentheses):
1. Dissolve undissolved compounds (metal oxides)
2. Release complex or adsorptive compounds (hexacyanoferrates)
3. Decompose polymer compounds (polyphosphates)
4. Change the state of oxidation (Cr(lll) → Cr(VI))
5. Remove interfering substances (nitrite in the case of nitrate determination)
6. Separate the substance to be determined by distillation (ammonium, cyanide)
7. Eliminate organic substances (waste water)
8. Filter turbid and suspended matter (sedimentation); e.g. with membrane filters
MACHEREY-NAGEL offers a number of methods for sample pre-treatment:
The crack set (REF 918 08) and NANOCOLOR ® NanOx are used for oxidative treatment
of samples in an acidic medium under normal pressure at 100–120 °C. These methods are
easy to handle and solve a large number of decomposition problems.
For samples with difficult matrices, but especially for rapid determination of total nitro-
gen, total phosphorus and total chromium, we recommend oxidative decomposition with
NANOCOLOR ® NanOx at elevated pressures in a microwave oven.
Very resistant samples (applicable to points 1, 2, 3, and 7) can be treated by wet decompo-
sition (oxidation) with nitric and sulfuric acid:
Instructions: Add 50 mL of sample, 2 mL nitric acid (65 %) and 2 mL sulfuric acid (96 %) to
a beaker, heat almost to dryness (use fume cupboard with fan on). As soon as a white SO -
3
fog appears, stop heating and allow the deposit to cool down to room temperature, then
add 20 mL distilled water. Neutralise with sodium hydroxide solution and pour sample into a
volumetric flask 50 mL, rinse out the beaker twice with 10 mL distilled water each time and
pour into the volumetric flask. Then fill the volumetric flask to the 50 mL mark with distilled
water. Almost all metals can be determined directly from this solution.
Depending on the specific problems, this method has to be adjusted or replaced by another
sample preparation method. Always keep in mind that after the sample preparation has
been completed, a defined volume of sample has to be present. This is critical to make an
accurate statement with regards to the concentration once the actual analysis has been
performed. However, it is also highly important to re-establish the sample's original chemical
milieu (pH value, redox potential, etc.) according to the specific analytical requirements of
the specific analysis method.
As can be deduced from this brief explanation, each analysis has to be treated individually
and the sample prepared accordingly in case of polluted samples. Only thus accurate and
realistic test results can be achieved. Please contact MACHEREY-NAGEL should you have
any questions.
For detailed instructions for the different decomposition methods and their applicability
please consult the respective instruction leaflet.
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