Temperatures in the lab appear in three scales: kelvin in physical chemistry, Celsius on almost every instrument, and Fahrenheit on some equipment and in older protocols. Converting between them is simple, and getting it wrong can cost a sample.
The three scales
- Kelvin (K) is the SI base unit of thermodynamic temperature. Zero kelvin is absolute zero. Kelvin takes no degree sign: write
298.15 K. - Celsius (°C) is defined from the kelvin:
t/°C = T/K − 273.15. A step of 1 °C is exactly a step of 1 K. - Fahrenheit (°F) has smaller degrees (1 °F = 5/9 K) and a different zero. Water freezes at 32 °F and boils at 212 °F at standard pressure.
Conversion formulas
K = °C + 273.15 °F = °C × 9/5 + 32 °C = (°F − 32) × 5/9
Note that −40 °C equals −40 °F, the one point where the two scales meet.
Worked examples
−20 °C → °F: −20 × 1.8 + 32 = −4 °F. In kelvin: −20 + 273.15 = 253.15 K.
−80 °C → °F: −80 × 1.8 + 32 = −112 °F.
37 °C → °F: 37 × 1.8 + 32 = 98.6 °F.
Storage temperature bands
| Band | °C | °F | Typical equipment |
|---|---|---|---|
| Controlled room temperature | 15 to 25 |
59 to 77 |
Bench, cabinet |
| Refrigerated | 2 to 8 |
36 to 46 |
Lab refrigerator |
| Frozen | −20 (approx.) |
−4 |
Standard freezer |
| Ultra-low | −80 (approx.) |
−112 |
Ultra-low freezer |
| Cryogenic | −150 to −196 |
−238 to −321 |
Liquid nitrogen vapour or liquid |
General storage practice for peptides
Always follow the storage conditions on the supplier’s certificate of analysis. As general laboratory practice:
- Lyophilised solids are usually kept cold (often −20 °C or below), sealed and dry. Let a cold vial reach room temperature before opening, so moisture does not condense on the powder.
- Solutions are less stable than solids. Divide a stock into single-use aliquots and freeze them, so each aliquot is thawed only once.
- Sensitive residues such as Cys, Met and Trp can oxidise, and Asn and Gln can deamidate over time, especially in solution and at higher pH.
- Freezer cycles. Frost-free freezers warm periodically to defrost. For long-term storage, a manual-defrost or ultra-low freezer is more stable.
Common mistakes
- Writing “°K”. The kelvin has no degree sign.
- Converting a temperature difference with the +32 offset. A difference of 10 °C is a difference of 18 °F, not 50 °F.
- Reading a −20 °F setting as −20 °C. That is −28.9 °C, colder than intended, and the reverse mistake leaves samples too warm.