Why use stocks
A stock solution is a concentrated solution you dilute when you need it. Weighing a few milligrams accurately for every batch of media is hard; weighing a gram once for a concentrated stock is easy, and adding a set volume of stock is quick and reproducible. Concentrated stocks also keep longer and take less freezer space.
The × notation
A 1000× stock is a thousand times stronger than its working concentration, so it is diluted 1:1000: 1 µL of stock per mL of final solution. 10× and 100× stocks work the same way.
A worked example: ampicillin
LB plates use ampicillin at 100 µg/mL.
- Stock at 1000×: 100 µg/mL × 1000 = 100 mg/mL.
- For 10 mL of stock, dissolve 100 mg/mL × 10 mL = 1 g of ampicillin in water and filter-sterilize it.
- For 500 mL of agar, add 500 ÷ 1000 = 0.5 mL, or 500 µL, of stock once the agar has cooled to about 50 to 55 °C.
Common antibiotic working concentrations
- Ampicillin and carbenicillin: 100 µg/mL (stocks in water)
- Kanamycin: 50 µg/mL (water)
- Spectinomycin: 50 µg/mL (water)
- Chloramphenicol: 25 µg/mL (ethanol)
- Tetracycline: 10 µg/mL (ethanol, kept dark)
These are common values; follow your own protocol where it differs.
Storing stocks
Split stocks into aliquots, label them with name, concentration and date, and keep them at −20 °C. Thaw one aliquot at a time rather than refreezing the same tube again and again.
Calculators
The Stock Solution Calculator gives the mass to weigh and the volume to add for any × stock. For other dilutions use the Dilution Calculator, and for molar stocks the Molarity Calculator.