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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Parts-per notation | 2/2 | https://en.wikipedia.org/wiki/Parts-per_notation | reference | science, encyclopedia | 2026-05-05T07:24:44.742123+00:00 | kb-cron |
One part per billion (ppb) denotes one part per 1,000,000,000 (109) parts, and a value of 10−9. This is equivalent to about three seconds out of a century.
One part per trillion (ppt) denotes one part per 1,000,000,000,000 (1012) parts, and a value of 10−12. This is equivalent to about thirty seconds out of every million years.
One part per quadrillion (ppq) denotes one part per 1,000,000,000,000,000 (1015) parts, and a value of 10−15. This is equivalent to about two and a half minutes out of the age of the Earth (4.5 billion years). Although relatively uncommon in analytical chemistry, measurements at the ppq level are sometimes performed.
== Criticism == Although the International Bureau of Weights and Measures (an international standards organization known also by its French-language initials BIPM) recognizes the use of parts-per notation, it is not formally part of the International System of Units (SI). Note that although "percent" (%) is not formally part of the SI, both the BIPM and the International Organization for Standardization (ISO) take the position that "in mathematical expressions, the internationally recognized symbol % (percent) may be used with the SI to represent the number 0.01" for dimensionless quantities. According to IUPAP, "a continued source of annoyance to unit purists has been the continued use of percent, ppm, ppb, and ppt". Although SI-compliant expressions should be used as an alternative, the parts-per notation remains nevertheless widely used in technical disciplines. The main problems with the parts-per notation are set out below.
=== Long and short scales ===
Because the named numbers starting with a "billion" have different values in different countries, the BIPM suggests avoiding the use of "ppb" and "ppt" to prevent misunderstanding. The U.S. National Institute of Standards and Technology (NIST) takes the stringent position, stating that "the language-dependent terms [...] are not acceptable for use with the SI to express the values of quantities".
=== Thousand vs. trillion === Although "ppt" usually means "parts per trillion", it occasionally means "parts per thousand". Unless the meaning of "ppt" is defined explicitly, it has to be determined from the context.
=== Mass fraction vs. mole fraction vs. volume fraction === Another problem of the parts-per notation is that it may refer to mass fraction, mole fraction or volume fraction. Since it is usually not stated which quantity is used, it is better to write the units out, such as kg/kg, mol/mol or m3/m3, even though they are all dimensionless. The difference is quite significant when dealing with gases, and it is very important to specify which quantity is being used. For example, the conversion factor between a mass fraction of 1 ppb and a mole fraction of 1 ppb is about 4.7 for the greenhouse gas CFC-11 in air (Molar mass of CFC-11 / Mean molar mass of air = 137.368 / 28.97 = 4.74). For volume fraction, the suffix V or v is sometimes appended to the parts-per notation (e.g. ppmV, ppbv, pptv). To distinguish the mass fraction from volume fraction or mole fraction, the letter m (for mass, but is ambiguous with mole fraction) or w (standing for weight) is sometimes added to the abbreviation (e.g. ppmw, ppbw). Expressions such as "parts per (million) by volume" and "parts per (million) by mass" are sometimes used.
== SI-compliant expressions == SI-compliant units that can be used as alternatives are shown in the chart below. Expressions that the BIPM explicitly does not recognize as being suitable for denoting dimensionless quantities with the SI are marked with !.
Note that the notations in the "SI units" column above are for the most part dimensionless quantities; that is, the units of measurement factor out in expressions like "1 nm/m" (1 nm/m =1 × 10−9) so the ratios are pure-number coefficients with values less than 1.
== Uno (proposed dimensionless unit) == Because of the cumbersome nature of expressing certain dimensionless quantities per SI guidelines, the International Union of Pure and Applied Physics (IUPAP) in 1999 proposed the adoption of the special name "uno" (symbol: U) to represent the number 1 in dimensionless quantities. In 2004, a report to the International Committee for Weights and Measures (CIPM) stated that the response to the proposal of the uno "had been almost entirely negative", and the principal proponent "recommended dropping the idea". To date, the uno has not been adopted by any standards organization.
== Footnotes ==
== See also == International Electrotechnical Commission (IEC) Milligram per cent Percentage (%) 1 part in 100 Per mille (‰) 1 part in 1,000 Permyriad (‱) 1 part in 10,000 Per cent mille (pcm) 1 part in 100,000 Per-unit system
== References ==
== External links ==
Media related to Parts-per notation at Wikimedia Commons National Institute of Standards and Technology (NIST) International Bureau of Weights and Measures (BIPM)