Units

The basic mass unit is moles (mol) or, where appropriate grams (g).

1 mole of a substance is equal to its molecular weight expressed in grams. i.e. the molecular weight of sodium chloride (NaCl) is 58.44 therefore 58.44 g of NaCl = 1mole

The basic volume unit is litres (l or L) or liters if you are American

Concentration = Mass / Volume therefore the basic concentration unit = moles per litre (mol/l) (or g/l). Molar solutions may also be (and usually are) denoted by M

length is in metres (m) and time in seconds (s) or minutes (min)

Very large and very small values should be expressed in standard form i.e. multiplied by the appropriate power of 10 or by using the appropriate prefix.

The prefixes are:

multiplier | Standard form | prefix | symbol |

1 000 000 000 000 | 10^{12} | tera | T |

1 000 000 000 | 10^{9} | giga | G |

1 000 000 | 10^{6} | mega | M |

1 000 | 10^{3} | kilo | k |

1 | 0 | - | - |

0.1 | 10^{-1} | deci | d |

0.01 | 10^{-2} | centi | c |

0.001 | 10^{-3} | milli | m |

0.000 001 | 10^{-6} | micro | µ |

0.000 000 001 | 10^{-9} | nano | n |

0.000 000 000 001 | 10^{-12} | pico | p |

0.000 000 000 000 001 | 10^{-15} | femto | f |

0.000 000 000 000 000 001 | 10^{-18} | atto | a |

Therefore:

0.00053g/l *should* be written as 5.3 x 10^{-4} g/l or as 530 µg/l

76000000 l *should* be written as 7.6 x 10^{7} l or as 76 Ml (mega litres)

Common Abusage

There are still some funny values and measurement systems floating around in old textbooks and practical schedules that need updating or proofreading properly.

Percentage solutions (%) means weight (grams) divided by volume (ml, usually) expressed as a percentage. Therefore 10 g in 100 ml is a 10% solution; 8.8 g of NaCl in 1000 ml is a 0.9% solution, which is isotonic with plasma i.e. the same as 150 mM. Medics (at least old medics) seem very fond of percentage solutions and also of expressing concentrations in g/100 ml or g/dl (which is perverse). Most recently, medics and in particular American medics, have decided that µ is a bit confusing and instead recommend "mcg" as an abreviation for micrograms.

Furthermore:

l is sometimes written as dm^{3} (cubic decimetres; which is correct but pedantic)

ml is sometimes written as cm^{3} (cubic centimetres)

ml is sometimes written as cc (cubic centimetre; which is old fashioned)

µl is sometimes written as mm^{3} (cubic millimetres)

g l^{-1} (etc.) means the same as g/l

Megs & Gigs

The prefixes for very large numbers have moved into common usage for use in describing the ever increasing capabilities of PCs. Unfortunately, Mega and Giga etc. don't mean exactly the same to geeks as they do to the digitally challenged.

A kilobyte (KB) is not 1000 bytes but rather 1024. Computers count in powers of 2 (binary) and geeks can manage in binary and powers of 10 (decimal). Obviously it is better to use both and so cause maximium confusion to non-geeks.

Therefore 1K is 2^{10}

Similarly, a megabyte (MB) is not 1,000,000 bytes but rather 1,048,576 bytes which is 1024^{2} or 2^{20}

A gigabyte (GB) is 1,073,741,824 bytes which is 1024^{3} or 2^{30}

A terabyte (TB) is 1,099,511,627,776 bytes which is 1024^{4} or 2^{40}

This should be good for a few years, after which I will revise it to include petabyte, exabyte, zettabyte and (my personal favourite) yottabyte.