Hach BioTector B3500C, User Manual

Page: 68 / 165

Page 68
6.1.2 Sodium Hydroxide Reagent
There are a number of factors which affect the purity of the sodium hydroxide solution used by the BioTector.
These can take the form of inorganic or organic contamination. Inorganic contamination has two main
sources: Sodium Carbonate (Na
2
CO
3
), which is present in the concentrated sodium hydroxide, or dissolved
CO
2
, which can be in the water used to make up the solution. The level of Na
2
CO
3
in the concentrated sodium
hydroxide will be specified by the manufacturer. For the BioTector, this should be 0.1% (or lower if possible).
Carbonates appears to be the main cause of contamination found in sodium hydroxide solutions, and
generally the level of Na
2
CO
3
is found to be lower in concentrated liquid sodium hydroxide than in sodium
hydroxide pellets.
To remove any CO
2
dissolved in the deionized water used to make up the sodium hydroxide solution, sparge
the water with a CO
2
free gas, for example nitrogen. Ideally, the level of organics should be less than 100

g/l
(ppb).
Mixing Procedure for the preparation of 4.0N Sodium Hydroxide solution, from 50% concentrated
Sodium Hydroxide solution:
Concentrated sodium hydroxide is dangerous. The preparation of sodium
hydroxide solutions should only be carried out by persons properly trained in
the handling of chemicals.
Use eye protection and gloves.
The instructions below are for 25 liters of solution.
1. Use deionized water, free from organic and inorganic carbon. The conductivity of the deionized water
used should be less than 0.5 µS/cm.
2. Fill 60% of the container with deionized water.
3. Blow through the deionized water with a CO
2
free gas such as Nitrogen or Oxygen, to purge the water
of any dissolved CO
2
.
4. a) In order to prepare 1 liter of 4.0 Normal base solution, add 320 grams of 50% purity sodium
hydroxide. Mix gently and add enough deionized water to make it exactly 1 liter.
b) To prepare a 25 liters base solution, add 8000 grams of 50% purity sodium hydroxide in stages.
Adding the sodium hydroxide in stages will prevent the solution from heating up. Gently mix and
then add enough deionized water to make it exactly 25 liters.
5. Seal the container.
6. Gently shake the container to mix the base with the water.
7. Allow the container to stand for one hour, and gently shake again to ensure a good mix.
It is important to purge the deionized water (used to make sodium hydroxide base
reagent) with a CO
2
free gas prior to the mixing procedure.
As it is not possible to purge the carbonates (CO
3
-
) from the base reagent by means of
any CO
2
free gas, prepared base reagent should never be purged further with any gas
after preparation.
When mixing the sodium hydroxide solution, care should be taken to minimize the
amount of atmospheric CO
2
gas being absorbed in the solution. The base container
should be fitted with a CO
2
filter to prevent the reagent coming in contact with air.
Failure to do so will result in increased background CO
2
readings.
WARNING
Caution