I went through a lot of this in my first job. They used a store bought isopropyl alcohol as their equipment sanitizer, and the production manager had been buying it from a drug store near his house for over 10 years or something crazy. Cases of the stuff were special ordered for him weekly. It worked until it didn't. Auditors finally pushed back I think in our third year, that a drug store is not a proper supplier for a chemical directly affecting food safety. We can't evaluate them, we can't get their audits, can't get specifications, etc. Also, no guarantee the employee sent to purchase them buys the correct thing while they're there.
I've never found issue with maintenance purchasing standard or food grade chemicals through Grainger or other OTC places to be an issue, so long as you go back and list all of the chemicals they're buying and make sure you have specs and SDS on them. But for anything related to your food production, sanitation, pest control, etc., they've always told me it needs to come from approved suppliers. It becomes a business continuity thing as well: if you're buying critical need items over the counter, no guarantee they'll have it on site and now your company is left in a pickle.
I agree with most above that the code isn't super clear about approving chemical suppliers (even I'm admitting I pick and choose which chemicals I have approved suppliers for). I decided to take a run at all the possibly relevant code sections as to why you would want to have them on your list. I'll say it has made my life easier to just have them approved, less questions from auditors about ensuring we have the right cleaners in the right concentrations with the right specification data, so on and so forth.
Definition from Glossary: Supplier: The entity that provides a product or service to the SQF certified site.
2.3.4.1 The responsibility and procedure for selecting, evaluating, approving, and monitoring an approved supplier shall be documented and implemented. A current record of approved suppliers, receiving inspections, and supplier audits shall be maintained.
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2.3.2.2 Specifications for all raw materials and packaging, including, but not limited to, ingredients, additives, hazardous chemicals, processing aids, and packaging that impact finished product safety shall be documented and kept current.
2.3.2.10 Specifications for raw materials and packaging, chemicals, processing aids, contract services, and finished products shall be reviewed as changes occur that impact product safety. Records of reviews shall be maintained.
11.2.5.2 Detergents and sanitizers shall be suitable for use in a food manufacturing environment, labeled according to regulatory requirements, and purchased in accordance with applicable legislation. The organization shall ensure:
i. The site maintains a list of chemicals approved for use;
ii. An inventory of all purchased and used chemicals is maintained;
11.6.4.1 Hazardous chemicals and toxic substances with the potential for food contamination shall be:
i. Clearly labeled, identifying and matching the contents of their containers;
ii. Included in a current register of all hazardous chemicals and toxic substances that are stored on-site;