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7
5. Operation of the Instrument
5.1 Sample Preparation
5.1.1. Preparation of the NaOH solution
The concentration of NaOH solution shall be 10 % - 30 %.
Na
2
CO
3
solution can be used as a substitute for NaOH solution and formulated in two ways:
● Dilute 600 g Na
2
CO
3
with 3 L warm distilled water.
● Dilute 1.7 kg Na
2
CO
3
·10H
2
O with 3 L warm distilled water.
5.1.2 Add pH indicator to NaOH solution:
Indicator shall be added to the NaOH solution. Normally 100 mg bromothymol blue shall be added to
per 3 L NaOH solution. As a pH indicator, bromothymol blue is insoluble in water. It is freely soluble in
ethyl alcohol, taking on sandy beige and freely soluble in diluted alkaline solution, taking on blue. The
color ranges from yellow, via green to blue, with the pH value ranging from 6.0 to 7.6.
Other pH indicator can be used on condition that meets the requirements.
5.1.3
The three tanks shall be cleaned and dry if they will not be put to use for a long time after the
experiment.
5.1.4 Soda lime is made of no reusable particles. Thus it shall be replaced for new one after complete
reaction.
5.1.5 The active carbon in the tank shall be replaced for dry one, and the replaced active carbon
may be put into drying oven for reuse.
5.1.6 When the knob is turned to the maximum position and the indication from the flowmeter is below
1m
3
/h, please screw off the filter cap in the active carbon tank. Then put it under the running water
tap for 5 minutes for washing in order that the filter cap is not blocked with solid particles. (Be sure not
to hold tightly the filter section in the cap in case it is damaged.)
5.2 Operating procedure of the instrument
5.2.1 First, connect the gas inlet of Exhaust System to the gas outlet of the digestion waste discharge
system properly.
5.2.2 Fill the inner tank with soda lime and active carbon until they reach the scale. Shake the tank
while filling in order that the soda lime and active carbon is tamped.
5.2.3 Check every tank cover for leakproofness and the piping connection for accuracy. (The tank
cover may be put into the neck in the case and screwed tight for leakproofness after tightening.)
5.2.4 Turn on the main switch for the Exhaust System to enable the pump.
5.2.5 Turn the knob on the front and bottom side of the instrument until the required pumping speed is
achieved.
5.2.6 Put the digestor into operation and the sample digestion starts.
5.2.7 Shut down the digestor and leave the Exhaust System running in non-load condition for 10
minutes after completion of the sample digestion, which serves as protection for the pump.
5.2.8 Switch off the power and Exhaust System stops running.
Notice: Two ways of filling the neutralizing tank:
1. Pour the NaOH solution with the indicator into the neutralizing tank directly.
2. Fill the inner tank with soda lime and then put it into the outer tank.