Diluição serial (fator final)

Serial dilution by factor

A serial dilution is a chain of dilutions, each one using the same factor, which gives you a concentration scale that drops off exponentially. With a factor of r per step over n steps, the final concentration is C_final = C_original / rⁿ and the overall dilution factor is rⁿ. Example: a 1:10 dilution means 1 part sample plus 9 parts diluent. Run that 5 times and the sample ends up diluted 10⁵ = 100,000-fold, so the final concentration is original/100,000. Classical microbiology rests on this for counting viable colonies (CFU/mL) once the density is low enough to count, and serological titrations in virology and immunology use it too (1:2, 1:4, 1:8…).

Applications

It's routine in microbiology for CFU/mL counts on agar plates, in virology for HAI titrations and PFU work, and in the clinical lab for antibody titres and serological dilutions. You'll also find it in analytical chemistry, building standard curves that span a wide concentration range, and in the QC standardisation of biological reagents.

FAQ

What does 1:10 mean? 1 part sample and 9 parts diluent, 10 parts in all, leaving the sample at 1/10 of its original concentration. Don't mix this up with 1+10, where 1 part plus 10 parts diluent gives 1/11.

Why serial dilution instead of a single one? Because it lets you hit very high dilution factors (10⁶, 10⁸) using sensible volumes and keeping the accuracy good. Doing 10⁶ in one shot would force you to pipette microlitres, and the precision suffers badly.

Does the propagated error grow at each step? It does, since pipetting errors pile up along the chain. Work with calibrated pipettes and, where you can, prepare each dilution from the one before it using volumes ≥ 100 µL to keep the relative error small.