In brief "locked up" means that it should be stored in a secure place. If your chemical store is a lockable store which cannot be accessed by unauthorised people, then that would satisfy the security requirement. The GHS precautionary statement "P405 Store locked up" is applicable to a range of chemical hazards, including the classification Skin Corrosion/Irritation Category 1A(1) which applies to sodium hydroxide.
Sodium hydroxide is also listed as a Category 3 chemical in illicit drug manufacture.(2) However, the Code of Practice(2) does not specify security measures for Category 3 chemicals (only Category 1 chemicals are required to be stored securely on this basis).
It is essential that your school enforces a policy regarding the security of chemicals, as well as general security in the science area. See our information sheet which considers this topic AIS: School science area security.
Regarding the other questions that you have raised about the storage of sodium hydroxide, sodium hydroxide is:
- required to be stored in a corrosion resistant container.(3) It is best stored in the container in which it is supplied.
- incompatible with acids.(3) Therefore, it should be stored in your corrosives cabinet with other bases and compatible corrosive chemicals, and segregated from acids.
Segregation should be such that
"it is sufficient to prevent the mixing of incompatible chemicals should two containers break at the same time; or the spill catchments for areas in which compatible goods are kept are separated."(4)
- hygroscopic,(3) which means that it absorbs moisture from the air. Therefore, ensure that the lid is kept tightly closed. It may be stored in a desiccator, ensuring that no incompatible chemicals are stored along with it in the desiccator.
Bunding (secondary containment) is required for corrosive liquids but not normally for corrosive solids. Containers with the plastics identification code 5 PP (polypropylene) generally have good chemical resistance(5) and are suitable for trays and storage containers that can be used for bunding. These can also assist with the segregation of incompatible chemicals by providing an impervious barrier in the event of a container breaking.
Further details regarding the handling and storage of sodium hydroxide can be found in section 7 of the SDS and a list of incompatible chemicals is listed in section 10. Science ASSIST is currently developing a Chemical management handbook for school chemicals which is due for release late 2016. Also a previous question on Chemical Storage Timeframes has some helpful information on using desiccators.
(1) United Nations. 2009. Globally Harmonized System of Classification and Labelling of Chemicals (GHS) Third revised edition, p328, http://www.unece.org/trans/danger/publi/ghs/ghs_rev03/03files_e.html
(2) ‘Code of Practice for Supply Diversion into Illicit Drug Manufacture’. The Plastics and Chemical Industries Association website http://chemistryaustralia.org.au/ or www.pacia.org.au/DownFile.Aspx?fileid=291
(3) 'Sodium hydroxide SDS', Chem Supply website, https://www.chemsupply.com.au/documents/SL1781CH6F.pdf (September 2014)
(4) Standards Australia. 2004. AS/NZS 2243 Safety in Laboratories, Part 10: 2004 Storage of chemicals. Sydney, Australia. Reproduced with permission from SAI Global Ltd under Licence 1407-c117
(5) The Plastics and Chemical Industries Association (PACIA). 2005. 'Plastics Identification Code'. http://chemistryaustralia.org.au/