Hallo Jochen und Tilman,
ich danke Euch!
Nun ist es sicherlich auch für jeden verständlich und zeigt sich doch als Alternative auf.
Die Vorschrift kommt aus diesem Papier: (es wird tatsächlich darauf hingewiesen, dass das Entkalken nur bei der Oxidation mit Säuren notwendig ist, wie z.B. Schwefelsäure)
A Methods Manual for the Collection,
Preparation and
Analysis of Diatom Samples
Report to the
Water Research Commission
by
JC Taylor*, WR Harding**
and CGM Archibald***
* School of Environmental Sciences and Development
North-West University (Potchefstroom Campus)
** DH Environmental Consulting [DHEC]
*** KZN Aquatic Ecosystems [KZNAE]
1.3.2 Hot HCl and KMnO4 method (recommended technique)
This method is recommended by the authors as it has yielded good results with samples taken
from throughout South Africa, which usually have a high content of organic material. In
addition, there is no need to remove calcium (1.3.1) before processing the samples as in the
other techniques below (1.3.3 – 1.3.4).
1.3.2.1 Allow the diatom sample to settle for 24 hours after return to the laboratory;
1.3.2.2 Decant the clear supernatant liquid from the sample bottle taking care not to loose any
of the diatom material;
HEALTH AND SAFETY
Hydrochloric acid is CORROSIVE. CHLORINE GAS is emitted when combined
with potassium permanganate. Potassium permanganate is an OXIDATIVE
AGENT. Do not perform any analysis using these chemicals outside of a fume
cabinet. When handling HCl wear acid resistant gloves, goggles and a lab coat.
31
1.3.2.3 Shake the sample well and pour 5 to 10 ml (depending on the concentration of the
material) of thick suspension into a heat-resistant beaker;
1.3.2.4 Mark the beaker clearly with the sample number in several places;
1.3.2.5 Add 10 ml saturated potassium permanganate (KMnO4) solution, mix and leave for a
period of 24 hours;
1.3.2.6 In a fume cabinet, add 5-10 ml concentrated HCl (32%), taking care not to inhale
the gasses released. Cover the beaker with a watch glass and heat on a hot plate at
90°C for 1 to 2 hours until the solution becomes clear (usually the solution will have a
yellowish colour when clear);
1.3.2.7 After oxidation of organic material, add 1 ml of hydrogen peroxide to check if the
oxidation process is complete and no organic material remains, in which case the
hydrogen peroxide will not cause lasting foaming;
1.3.2.8 When oxidation is complete, the samples are allowed to cool and are then transferred
to 10 ml centrifuge tubes. Before pouring the diatom and acid samples from the
beakers, the beakers are vigorously swirled, the aim of the rotary movement being to
re-suspend the diatoms, whilst causing the stone and heavier sand particles to fall to
the bottom of the beaker;
1.3.2.9 The samples are rinsed by centrifuging with distilled water at 2500 rpm for 10
minutes;
1.3.2.10After centrifugation the supernatant is decanted and the washing is repeated a further
4 times until the sample is circumneutral;
1.3.2.11The supernatant should be poured off in a single movement, and care should be taken
not to lose any diatom material. After pouring off the supernatant fluid the diatoms
and small particles of sand at the bottom of the tube are loosened by means of a jet of
distilled water from a wash bottle. More water is then added until reaching the
required volume in the centrifuge tube;
1.3.2.12After the last wash, the diatoms are again loosened by means of a jet of distilled water
and then poured into small glass storage vials bearing the necessary sample
information. It is important to store diatom samples in glass as opposed to plastic
vials, as glass releases silica, which counters the dissolution of diatom valves.
Alternatively, the excess acid and soluble chlorides can be washed out by a series of timed decantations.
lg
anne
