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The "parts-per" notations are used to denote low concentrations of Chemical Elements . Also known as mixing ratios, they are often used to denote the relative abundance of trace Elements in the Earth's crust, trace elements in forensics or other analyses, dissolved minerals in water, or Pollutant s in the Environment . Parts-per notations (in particular ppm) are also used to specify the size of the errors of very precise measurements, such as gas pressure voltage stability [http://www.valhallascientific.com/calibrators/cal-2701c.shtml , and Oscillator frequency. The IEC suggests not to use the part-per notation to avoid misunderstanding. In most countries, a billion is 1012 (a million2) and a trillion 1018 (a million3); in the U.S. a billion is 109 and a trillion 1012 (being a billion in Europe). Nevertheless the notation is still widely used. TYPES OF PARTS-PER NOTATIONS
Caveats
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 particle count as above, Volume (used in particular for Gas es) or Mass . It can also be used as a mixed term, indicating mass per volume of liquid, as in mg/L, especially where the liquid density approximates that of water. The usage is generally quite fixed inside most specific branches of science, leading some researchers to believe that their own usage (mass/mass, volume/volume or others) is the only correct one. This, in turn, leads them not to specify their usage in their research, and others may therefore misinterpret their results. For example, Electrochemists often use volume/volume, while Chemical Engineers may use mass/mass as well as volume/volume. Many academic papers of otherwise excellent level fail to specify their usage of the part-per notation. The difference between expressing concentrations as mass/mass or volume/volume is quite significant when dealing with gases and it is very important to specify which is being used. It is quite simple, for example, to distinguish ppm by volume from ppm by mass or weight by using ''ppmv'' or ''ppmw''. 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.
USE Examples of situations where parts per million are an appropriate measure include:
INEXACT ANALOGUES
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 #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 only distinction being whether the litre is calibrated to the international standard kilogram or to the international standard meter , which are defined without reference to one another. #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. #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. #In Atmospheric Chemistry and in Air Pollution regulations, the notation is commonly expressed as parts per million by volume (ppmv). This is useful only for gas concentrations (e.g., ppmv of carbon dioxide in the ambient air). For concentrations of non-gaseous substances such as aerosols, cloud droplets, and particulate matter in the ambient air, concentration is commonly expressed as as mass per unit volume of air or other gas ( μg/m³ or mg/m³ meaning μg or mg of particulates per cubic metre of ambient air). # Note that using "parts per notation" as vol per total vol (ppmv), or any expression where the units in the numerator and denominator are identical, is technically not a concentration (eg. amount of substance per volume). More precisely, this is a mixing ratio. Mixing ratios can not be converted concentrations (eg. mol/volume, mg/L, etc) without additional knowledge of the system. For example to convert 1 ppmv to a concentration (say mol/volume), the pressure and temperature of the system must be taken into account. SEE ALSO |
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