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ANNEX 7. LIVE FEED PRODUCTION SECTION: PREPARATION OF THE MOST COMMONLY USED CHEMICALS


A - Determination of active chlorine content in commercial grade bleach (hypochlorite solution)

Principle:

Active chlorine will release free iodine from potassium iodide solution at pH 8 or less. The released iodine is titrated with a standard solution of sodium thiosulphate, using starch as the indicator.

Reagents:

- dissolve 25 g of sodium thiosulphate in 1 l of freshly boiled distilled water;

- improve its stability by adding 0.1 g of Na2CO3;

- store in a clean glass bottle in the dark,

- mix 5 g starch with a little cold water and grind in a mortar, pour into 1 l of boiling distilled water, stir and let settle overnight;

- use the clear supernatant preserved with 1.25 g salicylic acid and store in a dark bottle.

Procedure:

- dissolve 0.5 to 1 g KI in 50 ml distilled water, add 5 ml acetic acid,

- add 1 ml from sample to be checked,

- titrate away from direct sunlight: add 0.1 N thiosulphate from a burette until the yellow colour of the liberated iodine is almost disappearing, add 1 ml starch solution and titrate until the blue colour disappears.

Calculation:

- 1 ml of 0.1 N thiosulphate equals to 3.54 mg active chlorine.

B - Preparation of a disinfecting 500 ppm hypochlorite solution from commercial grade bleach

WARNING:

  • This is a practical method, not a stoichiometric preparation.
  • When using a disinfecting hypochlorite solution for a prolonged time, it becomes essential to check regularly its active chlorine content (see above).

C - Chemical methods to sterilize seawater (for algal, rotifer and artemia cultures, both strain and mass level)

D - Identification of active chlorine residue

The same process indicated in the section A can be repeated in a simplified way for a quick check of residual active chlorine in water. To one ml sample add 1 ml of KI solution, 1 ml of 0.1 N thiosulphate solution and 1 ml of starch indicator. If a blue colour develops it is a proof of presence of active chlorine residues.

E - Preparation of disinfecting 10% (v/v) hydrochloric acid solution

Reagents:

Procedure:

- add 100 ml of HCl to 900 ml of tap water

WARNING

- fill the vessel first with water to drive out the acid fumes
- pour the acid without splashing to avoid acid splashes
- wear protective gloves, glasses and cloths when handling concentrated acids
- avoid breathing acid fumes

F - PREPARATION OF LUGOL SOLUTION

Lugol is a fixative staining solution for easier counting of brine shrimp nauplii.

Reagents:

Procedure:

Use:


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