i remember reading recently that it was discovered that a lot of studies regarding microplastics are likely wrong. because nitrile gloves used to operate in the laboratory gives off microplastics, so there is basically no uncontaminated samples.

so, always check your presumably "clean" samples too!

Not only the CO2, but also the glassware you put the water in for measuring can very significantly alter the pH. Some scientists I know systematically screened different, presumably clean, containers because of this effect before progressing with their experiments.

it will vary, just after distillation (or RO/ion exchange) it should be closer to 7 then it goes down as carbon dioxide gets absorbed. that's why it's buffered everywhere where it matters

"We switched brands of litmus paper and now everything's in shambles!"