When working in the lab testing different pHlevels of acids and bases what safety rule must you always follow?
The first rule is most obvious, and the most basic: WEAR SPECTACLES.
Your eyes are the most vulnerable part of your body, and a splash or conc. acid or base could mean you lose sight in 1 eye. You therefore must wear safety spectacles in a lab. And we learn this rule from the first chemistry class we take. If you are a speccy (i.e. you wear optical spectacles) these are an acceptable substitute; but there must be a glass or perspex surface between the chemicals and your mince pies.
And the other rule that applies to acids, is that if you spit in acid it will spit back. And thus, you always add acid to water, and never the reverse. So don't spit in the acid.
Bases are generally more hazardous than acids. If you get a splash of acid, it will sting your skin and you wash it off with copious water. A base burn is not so obvious; you generally don't know there is a problem until after some damage has been done, and your skin has become soapy and slimy and burnt.
Labcoats are worn to protect your clothing. If you get an acid splash on fabric, it weakens the fabric, and the next time you wash that garment it develops a hole.