stock solution
A stock solution is a concentrated master batch you mix once and keep, then dilute as needed to make the weaker working solutions you actually use. Think of frozen concentrated juice: you make up a strong can, store it, and add water later to fill each glass. Mixing one strong batch is easier and more consistent than weighing tiny amounts again and again.
Concretely, it is a solution of accurately known, relatively high concentration, prepared carefully and stored. To get the lower concentrations you need for calibration standards, you measure out a small volume of the stock and dilute it by a known factor, calculating the new concentration from the dilution.
It matters because it saves work and improves consistency: every standard descends from the same well-characterized parent, so they all share the same starting accuracy. The caveats are practical — a stock solution can change with time through evaporation, adsorption to container walls, or decomposition, so it carries an expiry, and any error in the stock propagates into everything diluted from it.
A 1000 mg/L copper stock solution is prepared and stored. To make a 10 mg/L working standard, 1 mL of stock is diluted to 100 mL — a hundredfold dilution.
One concentrated parent solution is diluted to make many working standards.
An error in the stock solution is not random — it shifts every standard diluted from it by the same proportion, producing a systematic bias rather than scatter. That is why stock solutions are prepared with extra care and verified or remade periodically.