serial dilution
/ SEER-ee-ul dy-LOO-shun /
Suppose you need a solution a thousand times weaker than your stock, but measuring out one part in a thousand directly would mean pipetting an impossibly tiny drop. Instead you dilute tenfold, then take some of that and dilute tenfold again, and again — a staircase down to the target.
Serial dilution is the stepwise dilution of a sample, where the diluted product of each step becomes the starting material for the next. Each step multiplies the previous dilution factor, so a few modest steps reach extreme dilutions accurately.
It is the standard way to build calibration standards across a wide range and to count microbes too dense to count directly. The catch is that errors compound: a small pipetting slip early on is carried into and magnified by every later step, so technique must be careful and consistent.
Three successive tenfold dilutions — taking 1 mL into 9 mL of solvent each time — turn a stock into a solution one thousand times weaker, far more reliably than one direct 1:1000 step.
Stepwise tenfold dilutions multiply: 10 × 10 × 10 = 1000.
Mix each tube thoroughly before drawing the next aliquot, and change pipette tips between steps; carry-over and incomplete mixing are the commonest sources of a skewed dilution series.