I usually use "eq" but I have been told to use "equiv." in order to avoid confusion with "equation". Is there a reference I can check?
The first place to check would be standards that are used frequently globally, such as CAS. According to the American Chemical Society page CAS Standard Abbreviations & Acronyms, the abbreviation for the unit of measurement 'equivalent' is
equiv, while that of the adjective 'equivalent' is
equiv. (with a dot).
Also, the ACS page Standard Abbreviations and Acronyms, shows equation abbreviated to
eq and equivalent as
However, some journals, such as the Journal of Biological Chemistry page Table I: Abbreviations of units of measurement and of physical and chemical quantities state that the abbreviation of equivalent is as you used,
eq, so if you are intending to publishing, just be aware of what the journal expects in terms of abbreviations.