Scientific references

This page lists the numbers you look up at the bench. It has the periodic table, the SI prefixes, the units a lab writes on a bottle or a pipette, a few constants, the amino acids, the nucleotides, buffer pKa values at 25 °C, and the temperatures we set most often. The figures are reference values. If a mass or a pH has to be right, check the bottle and measure it yourself. The calculations are in the calculators.

Last updated: 24 October 2026

Periodic table

Pick an element. The molar mass is the one the molarity calculator uses. A number in brackets is the mass number of a long-lived isotope. Click an element to see the further details in the data block below.

SI prefixes

The prefixes a lab uses most.

Symbol	Name	Factor
T	tera	10¹²
G	giga	10⁹
M	mega	10⁶
k	kilo	10³
m	milli	10⁻³
µ	micro	10⁻⁶
n	nano	10⁻⁹
p	pico	10⁻¹²
f	femto	10⁻¹⁵

Units

The mark on the label, and what it counts. An ångström is not an SI unit. The same length is 0.1 nm. When a conversion and the label disagree, the figure on the bottle wins.

Length

Symbol	Name	What it counts
	metre	The SI base unit of length. On the bench a length is usually a millimetre or a centimetre.
	centimetre	One hundredth of a metre. Cuvettes, tubing and the size of a plate.
	millimetre	One thousandth of a metre. The usual unit for glassware, tubing and an instrument bed.
	micrometre	One thousandth of a millimetre. Filter pore size, particle size and the size of a cell.
	nanometre	One thousandth of a micrometre. Wavelengths, nanoparticles and the finer pores of a membrane.
	ångström	One tenth of a nanometre, so 10⁻¹⁰ m, or 100 pm. Bond lengths, atomic and ionic radii, X-ray wavelengths and crystal lattice spacings. Not an SI unit. The SI name of the same length is 0.1 nm. Crystallography and older spectroscopy still print the ångström.
	picometre	One hundredth of an ångström. Newer tables of ionic radii use the picometre.

Volume

Symbol	Name	What it counts
	litre	One cubic decimetre. The catalogue also prints l for the same unit.
	millilitre	One thousandth of a litre, and one cubic centimetre.
	microlitre	One thousandth of a millilitre, and one cubic millimetre. The unit of a pipette. A label that prints uL means µl.
	nanolitre	One thousandth of a microlitre. Some dispensers and droplet systems are quoted in nanolitres.

Mass

Symbol	Name	What it counts
	kilogram	A thousand grams. Balances and shipments.
	gram	The usual mass on a reagent bottle.
	milligram	One thousandth of a gram.
	microgram	One thousandth of a milligram.
	nanogram	One thousandth of a microgram. Immunoassays and standards.
	picogram	One thousandth of a nanogram.
	dalton	The same size as the unified atomic mass unit: 1 Da = 1 u. A molar mass of 180 g/mol is a molecular mass of 180 Da. The residue masses further down this page are in dalton.
	kilodalton	A thousand dalton. The usual mass of a protein.

Amount and concentration

Symbol	Name	What it counts
	mole	The SI unit of amount of substance. mmol, µmol, nmol and pmol are the steps of a thousand that a protocol actually writes.
	molar	One mole per litre. 1 mM is 0.001 mol/L. 1 µM is 10⁻⁶ mol/L. 1 nM is 10⁻⁹ mol/L.
	normality	Equivalents per litre, on older bottles. One normal is not always one molar. For sulphuric acid, 1 N is 0.5 mol/L. The bottle states which.
	mass fraction	Grams per 100 g of mixture.
	mass per volume	Grams per 100 ml of solution.
	volume per volume	Millilitres per 100 ml of solution.
	parts per million	In dilute water, 1 ppm is taken as 1 mg/L. That conversion does not hold for every matrix.
	parts per billion	In dilute water, 1 ppb is taken as 1 µg/L.
	enzyme unit	One micromole of substrate per minute, under the conditions named on the bottle. A unit of one enzyme is not a unit of another.
	katal	One mole per second. 1 U is 16.67 nkat. Rare on a bottle. It is the SI unit of catalytic activity.
	international unit	Defined for that substance by a reference preparation. There is no universal size.
	equivalent	One mole of reactive units, as the reaction defines them. Normality counts equivalents per litre.
	osmolality	Milliosmoles per kilogram of solvent. Cell-culture media are often quoted in mOsm/kg.

Temperature and pressure

Symbol	Name	What it counts
	degree Celsius	The bench scale.
	kelvin	0 °C is 273.15 K. The degree is the same size.
	pascal	The SI unit of pressure.
	kilopascal	1 kPa is 1000 Pa.
	bar	100 kPa, exactly 10⁵ Pa.
	millibar	One thousandth of a bar, 100 Pa. Vacuum gauges and some autoclave charts.
	atmosphere	The standard atmosphere, 101.325 kPa.
	torr	1/760 of a standard atmosphere. A bench gauge marked mmHg reads the same number for ordinary work.

Spin, light and water

Symbol	Name	What it counts
	revolutions per minute	The force depends on the rotor radius, so the number does not transfer to another rotor.
	relative centrifugal force	Multiples of standard gravity. This is the number that transfers. With the radius r in centimetres and the speed n in revolutions per minute, × g = 1.118 × 10⁻⁵ × r × n².
	svedberg	10⁻¹³ seconds. Sedimentation, as in a 30S subunit.
	absorbance	Dimensionless. The log10 of the ratio of the intensities at a stated wavelength.
	optical density	Used as the absorbance at a named wavelength, for example OD600.
	conductivity	Ultrapure water at 25 °C is 0.055 µS/cm.
	resistivity	The same water is 18.2 MΩ·cm at 25 °C.
	hertz	One cycle per second. A shaker is more often quoted in revolutions per minute.
	second, minute, hour	The time on a protocol. s is the SI unit.

Molecules, counts and scales

Symbol	Name	What it counts
	base pair	A duplex is counted in base pairs. kb is 1000 bp. Mb is 10⁶ bp.
	nucleotide	A single strand is counted in nucleotides, a duplex in base pairs.
	colony-forming unit	A viable cell that forms one colony. Not a count under the microscope.
	mesh	Openings per inch. Not an SI length. The opening in micrometres depends on the wire. Compare sieves by the micrometre opening.
	needle gauge	A scale, not a millimetre. A higher gauge is a thinner needle. The millimetre size is the one on the product.

From rpm to × g (relative centrifugal force)

A protocol should state the force, not the speed: rpm depends on the rotor radius, × g does not.

RCF = 1.118 × 10⁻⁵ × r × n², with r the rotor radius in centimetres (centre to the bottom of the tube, r_max) and n the speed in rpm. The other way round: n = √(RCF / (1.118 × 10⁻⁵ × r)).

Worked example. A rotor with r = 10 cm at 3 000 rpm: 1.118 × 10⁻⁵ × 10 × 3 000² = 1.118 × 10⁻⁵ × 10 × 9 000 000 ≈ 1 006 × g. A protocol that asks for 1 000 × g on a rotor with r = 8 cm needs n = √(1 000 / (1.118 × 10⁻⁵ × 8)) ≈ 3 344 rpm.

Constants at the bench

The SI values, rounded the way the bench uses them.

Constant	Value
NA	6.022 × 10²³ mol⁻¹
R	8.314 J·mol⁻¹·K⁻¹
F	96 485 C·mol⁻¹
0 °C	273.15 K
Vm	22.4 L·mol⁻¹ at 0 °C and 101.325 kPa

Amino acids

Average residue mass in a protein chain, in dalton.

Code		Residue mass (Da)
Ala	A	71.08
Arg	R	156.19
Asn	N	114.10
Asp	D	115.09
Cys	C	103.14
Gln	Q	128.13
Glu	E	129.12
Gly	G	57.05
His	H	137.14
Ile	I	113.16
Leu	L	113.16
Lys	K	128.17
Met	M	131.19
Phe	F	147.18
Pro	P	97.12
Ser	S	87.08
Thr	T	101.11
Trp	W	186.21
Tyr	Y	163.18
Val	V	99.13

Source: average residue masses calculated from the IUPAC standard atomic weights; they match the residue masses tabulated in the CRC Handbook of Chemistry and Physics (Properties of amino acids). Add 18.02 Da (one water) for the free peptide.

Nucleotides

Residue mass in a chain, in dalton.

Base	DNA	RNA
A	313.2	329.2
C	289.2	305.2
G	329.2	345.2
T / U	304.2	306.2

Source: calculated from the IUPAC standard atomic weights for the neutral nucleoside monophosphate minus one water, as used in common oligonucleotide mass tables.

Buffer pKa at 25 °C

The same pKa values as the buffer calculator. The useful window sits about one unit around the pKa.

Buffer	pKa	Useful range
Tris	8.07	7.1–9.1
HEPES	7.56	6.8–8.2
MOPS	7.20	6.5–7.9
Phosphate	7.21	5.8–8.0
PIPES	7.14	6.1–7.5
MES	6.27	5.5–6.7
Acetate	4.76	3.6–5.6

Sources: pKa values at 25 °C as tabulated in the CRC Handbook of Chemistry and Physics (Dissociation constants of organic acids and bases; Buffers for biological research) and in Goldberg, Kishore & Lennen, "Thermodynamic quantities for the ionization reactions of buffers", J. Phys. Chem. Ref. Data 31 (2002) 231 (NIST). PIPES: 7.14 at 25 °C per NIST (Goldberg et al. 2002). Published values differ by a few hundredths depending on ionic strength and method.

Molar extinction coefficients (UV, 280 nm)

The absorbance of a protein at 280 nm comes mainly from tryptophan, tyrosine and cystine (disulfide bonds).

Contributor	ε at 280 nm (M⁻¹·cm⁻¹)
Trp (W)	5 500
Tyr (Y)	1 490
Cystine	125

Beer–Lambert law: A = ε × c × l, with A the absorbance, ε the molar extinction coefficient (M⁻¹·cm⁻¹), c the concentration (mol/L) and l the path length (cm). For a protein: ε₂₈₀ = (n_Trp × 5 500) + (n_Tyr × 1 490) + (n_cystine × 125). Free cysteines do not count. The linear range of most photometers ends around A = 1 to 1.5.

Worked example. A 25 kDa protein with 2 Trp, 4 Tyr and 1 disulfide bond: ε = 2 × 5 500 + 4 × 1 490 + 125 = 17 085 M⁻¹·cm⁻¹. A measured A₂₈₀ of 0.50 in a 1 cm cuvette gives c = 0.50 / 17 085 ≈ 2.93 × 10⁻⁵ M = 29.3 µM, or 29.3 µM × 25 000 g/mol ≈ 0.73 mg/mL.

Source: Pace, Vajdos, Fee, Grimsley & Gray, "How to measure and predict the molar absorption coefficient of a protein", Protein Science 4 (1995) 2411–2423.

Henderson–Hasselbalch equation

The ratio of base to acid that gives a buffer its pH.

pH = pKa + log₁₀([A⁻] / [HA]), with [A⁻] the base form and [HA] the acid form. When both are equal, pH = pKa. It holds best within about one unit of the pKa and at moderate ionic strength.

Worked example. A 0.1 M phosphate buffer at pH 7.4 (pKa2 = 7.21): [HPO₄²⁻] / [H₂PO₄⁻] = 10^(7.4 − 7.21) = 10^0.19 ≈ 1.55. So [HPO₄²⁻] = 0.1 × 1.55 / 2.55 ≈ 0.061 M and [H₂PO₄⁻] ≈ 0.039 M. Check the pH with a calibrated meter and adjust; the buffer calculator does this arithmetic.

Dilution: C1V1 = C2V2

The amount of solute stays the same when you add solvent.

C1 × V1 = C2 × V2, with C1 the stock concentration, V1 the volume of stock, C2 the target concentration and V2 the final volume. Use the same units on both sides.

Worked example. 50 mL of 20 mM from a 1 M stock: V1 = (20 mM × 50 mL) / 1 000 mM = 1 mL. Pipette 1 mL of stock and make up to 50 mL, so add 49 mL of solvent. For large dilution factors, a serial dilution (for example two steps of 1:100 for 1:10 000) keeps pipetting error down. The dilution calculator does the same.

Temperatures at the bench

Usual set-points. Not a rule.

Where	Temperature
Ice bath	0 °C
Room	20–25 °C
Incubation, body	37 °C
Heat shock	42 °C
PCR, denaturation	95 °C
PCR, extension	72 °C
Autoclave	121 °C
Freezer	−20 °C
Ultra-low	−80 °C
Liquid nitrogen	−196 °C

Sources

• IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW): standard atomic weights, used for molar masses and residue masses.
• NIST / CODATA recommended values of the fundamental physical constants (N_A, R, F) and the SI Brochure (BIPM) for units and prefixes. (BIPM):
• CRC Handbook of Chemistry and Physics (CRC Press, current edition): amino-acid properties, dissociation constants and biological buffers.
• Goldberg, Kishore & Lennen, J. Phys. Chem. Ref. Data 31 (2002) 231 (NIST): buffer ionization constants.
• Pace et al., Protein Science 4 (1995) 2411: protein extinction coefficients at 280 nm.
• IUPAC Green Book, "Quantities, Units and Symbols in Physical Chemistry": RCF, absorbance and unit symbols.