Alkaline compounds

7.5.1 General characteristics

These compounds are effective against a wide range of pathogens. Strong alkalis, at pH 12 or more, have excellent activity against all categories of viruses.

Alkalis retain their effectiveness in the presence of heavy burdens of organic matter and assist the penetration of soiling. They are particularly useful for decontaminating ponds, drains, effluent waste pits and carcase disposal pits. They may be used on concrete surfaces.

Alkalis are corrosive to some metal alloys and irritant to skin and mucous membranes. Care should be taken when treating metal or painted surfaces. Staff must be supplied with appropriate safety equipment.

The activity of alkalis is relatively slow compared with other disinfectants such as hypochlorites, peracid solutions and chlorine dioxide. Raising water temperatures and increasing concentration can enhance the effect of most alkali compounds, but this should be done with extreme caution because of risks to workplace safety and the risk of corrosion. Alkalis can form complexes with ions in hard water when used at high temperatures, and these precipitates can be difficult to rinse off equipment. An acid rinse to remove residues may be required.

Section 7.5 and Tables 4.6 to 4.9 contain further details on alkali disinfectants. Table 7.1 compares the disinfection activity of various alkalis. The recommended doses for various disinfectant applications are shown in Table 7.3.

7.5.2 Advantages

  • Effective against a wide range of pathogens, especially viruses.

  • Not affected by the presence of organic matter; therefore useful in the disinfection of pond bases.

  • Useful for the decontamination of carcases and organic matter in burial pits.

  • Relatively cheap and available in bulk.

7.5.3 Disadvantages

  • Corrosive on metallic structures, especially aluminium and soft metal alloys.

  • Irritant to skin and mucous membranes. Sodium hydroxide and calcium oxide are highly corrosive and irritant.

  • Use requires experience and appropriate personal protective equipment.

  • Run-off into waterways should be avoided.

  • May be corrosive to painted surfaces.

7.5.4 Environmental and workplace safety considerations

  • Adhere to recommendations and safety advice from the manufacturer and in the material safety data sheet.

  • Alkalis may affect the pH of surface waters, especially fresh water, if large amounts of run-off occur, affecting aquatic life in localised regions.
Caution notes:
B3   Sodium hydroxide is highly irritant and should be used with care. Appropriate safety equipment, including waterproof clothing, hats, boots and eye protection, should be worn.

B4   Sodium hydroxide is corrosive to some surfaces.

B5   Under some circumstances, sodium hydroxide may make surfaces slippery. Care
should be taken to ensure its use does not result in a slipping hazard.

7.5.5 Availability

Sodium hydroxide (caustic soda), calcium hydroxide (slaked lime), calcium oxide (quicklime or burnt lime) and sodium carbonate (anhydrous or hydrated forms — soda ash or washing soda) are available from chemical wholesalers. Calcium carbonate is readily available from agricultural suppliers as crushed limestone.

Table 7.1

Comparison of the relative activity of alkali compounds
Chemical
Disinfection activity
   Comments
CaO (quicklime) 
High
Produces heat in contact with water
Often used to disinfect carcases
Ca(OH)2 (slaked lime)
Very high
Stains surfaces
NaOH (caustic soda)
High
Produces heat when reacting with water
Na2CO3 (washing soda or soda ash)
Moderate
Anhydrous form is more active
CaCO3 (limestone)
Poor
Particles need to be small (<0.24 mm)

7.5.6 Applications: sodium hydroxide

  • Against a wide range of bacteria and viruses, including spores and mycobacteria (using high concentrations that produce pH 12 or more).

  • Disinfecting earthen ponds when mixed in a solution with a wetting agent (‘Teepol’) and lime.

  • Disinfecting concrete structures and plastic structures (including tanks).

  • Disinfecting footpaths.

  • In footbaths.

7.5.7 Applications: calcium oxide

  • Disinfecting earthen ponds, particularly for cases of whirling disease (infection with Myxobolus cerebralis).

  • Disinfecting carcases and organic material at the time of burial.



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