Parts per notation

From Freepedia

Parts-per notation is a measure of concentration that is used where low levels of concentration are significant. These types of measurement units are also known as mixing ratios.

This is often used to denote the relative abundance of trace elements in the Earth's crust, trace elements in forensics or other analyses, or levels of pollutants in the environment.

• Parts per hundred (denoted by '%' and very rarely 'pph') - denotes one particle of a given substance for every 99 other particles. This is the common percent. 1 part in 10².
• Parts per thousand (denoted by '‰' [the per mil symbol], and occasionally 'ppt') denotes one particle of a given substance for every 999 other particles. This is roughly equivalent to one drop of ink in a cup of water, or one second per 17 minutes. 'Parts per thousand' is often used to record the salinity of seawater. 1 part in 10³.
• Parts per million ('ppm') denotes one particle of a given substance for every 999,999 other particles. This is roughly equivalent to one drop of ink in a 40 gallon drum of water, or one second per 280 hours. 1 part in 10⁶.
• Parts per billion ('ppb') denotes one particle of a given substance for every 999,999,999 other particles. This is roughly equivalent to one drop of ink in a canal lock full of water, or one second per 32 years. 1 part in 10⁹.
• Parts per trillion ('ppt') denotes one particle of a given substance for every 999,999,999,999 other particles. This is roughly equivalent to one drop of ink in an Olympic-sized swimming pool, or one second every 320 centuries. 1 part in 10¹².
• Parts per quadrillion ('ppq') denotes one particle of a given substance for every 999,999,999,999,999 other particles. This is roughly equivalent to a drop of ink in a medium-sized lake, or one second every 32,000 millennia. There are no known analytical techniques that can measure with this degree of accuracy; nevertheless, it is still used in some mathematical models of toxicology and epidemiology. 1 part in 10¹⁵.

Warning: although 'ppt' is usually used to denote 'parts per trillion', it is also on occasion used to denote 'parts per thousand'. If there is any chance of ambiguity, one should describe the abbreviation in full.

It is a term with several variants in meaning, so the meaning should be made clear if this term is used. In particular the ratio can be expressed in terms of particles as above, volume (used in particular for gases) or mass.

Contents 1 Examples of parts per notation 2 Use 3 Inexact analogues 4 NIST caution 5 Notes

Examples of parts per notation

The metric system is the most convenient way to express this since metric units go by steps of ten, hundred and thousand. For example, a milligram is a thousandth of a gram and a gram is a thousandth of a kilogram. Thus, a milligram is a thousandth of a thousandth, or a millionth of a kilogram. A milligram is one part per million of a kilogram thus, one part per million (ppm) by mass is the same as one milligram per kilogram. Just as part per million is abbreviated as ppm, a milligram per kilogram has its own symbolic form -- mg/kg, which unlike ppm is unambiguous.

• By mass:
  • one milligram in a kilogram is 1 ppm by mass.
  • one milligram in a metric tonne is 1 ppb by mass.
• By volume:
  • one millilitre (or cubic centimetre) in a cubic metre (or kilolitre) is 1 ppm by volume. For most gases (those behaving much like an ideal gas) this is numerically equivalent to µmol/mol on the basis of molecules (not atoms). See Avogadro's law.
• By mass/volume ratio for dilute aqueous solutions (ppm w/v or ppm m/v):
  • 1 litre (L) of water has mass of approximately 1 kg¹, so 1 milligram per litre (mg/L) is, loosely speaking, 1 ppm, for small concentrations in a water solution².
• By number of particles or moles:
  • one micromole per mole can also be called 1 ppm.
  • one nanomole per mole is 1 ppb.
  • one picomole per mole is 1 ppt.

Use

Examples of situations where parts per million are an appropriate measure include:

• relative abundances of trace elements in the earth's crust
• concentrations of pollutants in the environment

Inexact analogues

• one square centimeter in 1000 square feet is about .95 ppm
• one two-parent, two-child family in a city of about 4 million people is roughly 1 ppm
• one CD in the 1.57-million disc³ FreeDB catalogue is nearly 2/3 ppm

NIST caution

According to the U.S. National Institute of Standards and Technology (NIST) Guide for the Use of the International System of Units (SI), "the language-dependent terms part per million, part per billion, and part per trillion ... are not acceptable for use with the SI to express the values of quantities." NIST's Guide for the Use of the International System of Units (SI) has examples of alternative expressions. Acceptable SI units are:

1 millimole/mole = 1 part per thousand

1 micromole/mole = 1 part per million

1 nanomole/mole = 1 part per billion

1 picomole/mole = 1 part per trillion

Notes

1. Exactly one kg of pure water at maximum density (~4°C) and standard pressure was the definition of a litre from 1901 to 1964; today the litre is defined as exactly 1 dm³, the distinction being whether it is calibrated to the international standard kilogram or the international standard meter, which are defined without reference to one another.
2. Properly speaking it is approximately 1 ppm by mass or by weight in solution. When solids dissolve, they can increase or decrease the total volume they occupy, and even increase or decrease the total volume of the solution. Adding 1 ppm by weight will rarely produce a solution that is 1 ppm by volume to the same precision. The notation ppm w/v or ppm m/v demonstrates the exact nature of the ratio and is therefore the most precise.
3. The definition given above is that parts per notation refers to numbers of particles (equivalent to moles), but the parts per notation can also be used by mass or volume. Those using the notation need to state their usage to avoid confusion.
4. In atmospheric chemistry the parts per notation is commonly expressed with a v following, such as ppmv (or ppvm is some usages), to indicate parts per million by volume. In gases ppmv is equivalent to ppm by particles (Avogadro's law). This works fine for gases, but may have problems with cloud droplets and smoke or other atmospheric particulate matter.