Primary standard
A primary standard in metrology
Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...

 is a standard that is accurate enough that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 and time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

. Primary standards are used to calibrate other standards referred to as working standards.

In chemistry

Primary standards are used in analytical chemistry
Analytical chemistry
Analytical chemistry is the study of the separation, identification, and quantification of the chemical components of natural and artificial materials. Qualitative analysis gives an indication of the identity of the chemical species in the sample and quantitative analysis determines the amount of...

. Here, a primary standard is typically a reagent which can be weighed easily, and which is so pure that its weight is truly representative of the number of moles of substance contained. Features of a primary standard include:
  1. High purity
  2. Stability (low reactivity)
  3. Low hygroscopicity and efflorescence
    In chemistry, efflorescence is the loss of water of crystallization from a hydrated or solvated salt to the atmosphere on exposure to air.-Examples:...

  4. High solubility
    Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...

     (if used in titration
    Titration, also known as titrimetry, is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of an identified analyte. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the...

  5. High equivalent weight
    Equivalent weight
    Equivalent weight is a term which has been used in several contexts in chemistry. In its most general usage, it is the mass of one equivalent, that is the mass of a given substance which will:...

  6. Non-toxicity
  7. Ready and cheap availability
  8. Eco-friendliness

(The last three are not as essential as the first five.)

Some examples of primary standards according to the European Pharmacopoeia
European Pharmacopoeia
The European Pharmacopoeia of the Council of Europe is a pharmacopoeia, listing a wide range of active substances and excipients used to prepare pharmaceutical products in Europe...

 5, ch. 4.2:
  • Arsenic trioxide
    Arsenic trioxide
    Arsenic trioxide is the inorganic compound with the formula As2O3. This commercially important oxide of arsenic is the main precursor to other arsenic compounds, including organoarsenic compounds. Approximately 50,000 tonnes are produced annually...

     for making sodium arsenite
    Sodium arsenite
    Sodium arsenite is a compound with formula NaAsO2. It is the sodium salt of arsenous acid. It is a crystalline solid consisting of sodium cations, Na+, and catena-arsenite anions, [AsO2], which are infinite -O-As- chains, similar in structure to selenium dioxide, SeO2.It is a pesticide....

     solution for standardisation of sodium periodate
    Sodium periodate
    Sodium periodate is the sodium salt of periodic acid. It can refer to two different chemical compounds, sodium metaperiodate , which has the formula NaIO4, and sodium orthoperiodate , which has the formula Na2H3IO6...

     solution (until Ph. Eur. 3, Appendix 2001 also for iodine
    Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

     and cerium(IV) sulfate
    Cerium(IV) sulfate
    Cerium sulfate, also called ceric sulfate, is a yellow to yellow/orange chemical compound. It exists as the anhydrous salt Ce2; a few hydrated forms are also known: Ce2 ⋅ xH2O, with x equal to 4, 8, or 12. Ceric sulfate is available commercially.-Properties:Ceric sulfate is moderately soluble in...

     solutions, since Ph. Eur. 4, 2002 standardised by sodium thiosulfate
    Sodium thiosulfate
    Sodium thiosulfate , also spelled sodium thiosulphate, is a colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3•5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.”...

  • Benzoic acid
    Benzoic acid
    Benzoic acid , C7H6O2 , is a colorless crystalline solid and the simplest aromatic carboxylic acid. The name derived from gum benzoin, which was for a long time the only source for benzoic acid. Its salts are used as a food preservative and benzoic acid is an important precursor for the synthesis...

     for standardisation of waterless basic solutions: ethanol
    Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

    ic sodium and potassium hydroxide
    Potassium hydroxide
    Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...

    , TBAH
    Tetrabutylammonium hydroxide
    Tetrabutylammonium hydroxide is the chemical compound with the formula 4NOH, abbreviated Bu4NOH with the acronym TBAOH or TBAH. This species is not readily obtainable as a pure compound, but it employed as a solution in water or alcohols. It is commonly used as a base in organic chemistry...

    , and alkali methanolates in methanol
    Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

    , isopropanol, or DMF
    Dimethylformamide is an organic compound with the formula 2NCH. Commonly abbreviated as DMF , this colourless liquid is miscible with water and the majority of organic liquids. DMF is a common solvent for chemical reactions...

  • Potassium bromate
    Potassium bromate
    Potassium bromate , is a bromate of potassium and takes the form of white crystals or powder.-Uses in baking:Potassium bromate is typically used as a flour improver , strengthening the dough and allowing higher rising. It is an oxidizing agent, and under the right conditions, will be completely...

     (KBrO3) for standardisation of sodium thiosulfate
    Sodium thiosulfate
    Sodium thiosulfate , also spelled sodium thiosulphate, is a colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3•5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.”...

  • Potassium hydrogen phthalate
    Potassium hydrogen phthalate
    Potassium hydrogen phthalate, often called simply KHP, is an acidic salt compound. It forms white powder, colorless crystals, a colorless solution, and an ionic solid that is the monopotassium salt of phthalic acid...

     (usually called KHP) for standardisation of aqueous base and perchloric acid
    Perchloric acid
    Perchloric acid is the inorganic compound with the formula HClO4. Usually encountered as an aqueous solution, this colourless compound is a strong acid comparable in strength to sulfuric and nitric acids. It is a powerful oxidizer, but its aqueous solutions up to appr. 70% are remarkably inert,...

     in acetic acid
    Acetic acid
    Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

  • Sodium carbonate
    Sodium carbonate
    Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...

     for standardisation of aqueous acids: hydrochloric
    Hydrochloric acid
    Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

    , sulfuric acid
    Sulfuric acid
    Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

     and nitric acid
    Nitric acid
    Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...

     solutions (but not acetic acid
    Acetic acid
    Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

  • Sodium chloride
    Sodium chloride
    Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...

     for standardisation of silver nitrate
    Silver nitrate
    Silver nitrate is an inorganic compound with chemical formula . This compound is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides...

  • Sulfanilic acid
    Sulfanilic acid
    Sulfanilic acid is a grey crystalline solid produced from sulfonation of aniline.It readily forms diazo compounds and is used to make dyes and sulpha drugs....

     for standardisation of sodium nitrite
    Sodium nitrite
    Sodium nitrite is the inorganic compound with the chemical formula NaNO2. It is a white to slight yellowish crystalline powder that is very soluble in water and is hygroscopic...

  • Zinc
    Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

     powder, after being dissolved in sulfuric or hydrochloric acid, for standardisation of EDTA
    Ethylenediaminetetraacetic acid, widely abbreviated as EDTA , is a polyamino carboxylic acid and a colourless, water-soluble solid. Its conjugate base is named ethylenediaminetetraacetate. It is widely used to dissolve limescale. Its usefulness arises because of its role as a hexadentate ligand...


Primary standards are used in titration and are essential for determining unknown concentrations.

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