Limit value (chemistry)

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Limit values are numerical information in specifications , standards , ordinances, laws, contracts and other agreements that determine the tolerance in chemistry with regard to a desired quality ( target value ). In contrast to the actually measured actual values , they indicate the acceptable measured values .

properties

Limit values ​​are divided into lower limit values ​​(minimum values) and upper limit values ​​(maximum values). Both limit values ​​include a target value range, e.g. B. a concentration or an absolute or relative (mostly in percentages) mass or amount of substance . Depending on the method and the accompanying measurement error , relative limit values ​​can also reach values ​​below 0% or above 100%. It is important to define an exact test method (including sample preparation) for each limit value, since different analytical measurement methods often result in different actual values ​​for one and the same sample.

The American Chemical Society regularly publishes a directory for currently around 500 chemicals of the quality "ACS Reagent Chemicals" for which limit values ​​and test methods are precisely described. Pharmacopoeia (e.g. European Pharmacopoeia and in the United States Pharmacopeia) contain a large number of limit values ​​and test methods for substances , materials and methods for the manufacture of drugs. When evaluating the test results, the rounding rules according to DIN 1333 must always be observed.

Examples of limit values

  • lower limit values ​​for content information, e.g. B. ≥ 97.0% content for acetic anhydride in the quality "ACS Reagent Chemicals",
  • lower and upper limit values ​​with tolerated maximum different deviations from the target value (100%), e.g. B. drug purity of acetylsalicylic acid from the lower limit value of 99.5% to the upper limit of 101.0% (dried substance), Purity of the drug cortisone acetate 97.0% to 103.0% (dried substance), corresponding to 100.0 % (± 3.0%),
  • lower and upper limit values ​​for characterizing the enantiomeric purity of the chiral natural product D- glucose in the quality “ACS Reagent Chemicals”, e.g. B. by specifying the lower limit value G u and the upper limit value G o for the optical rotation value α.
    Scheme to illustrate a specification limited on both sides (e.g. for the optical rotation value α). The tolerance range of the specification is marked in green . It begins with the lower limit value G u and ends with the upper limit value G o .
  • Mixtures of flammable gases, vapors or dusts with air and thus the oxygen they contain are explosive in certain mixing ratios typical of the substance . These mixing ratios determine the explosion range, which is described by its two explosion limits, the upper and lower explosion limits (UEG and LEL). These limits are also known as ignition limits.
Explosionsgrenzen.svg

Explosion limits
  • upper limit values ​​(maximum values), e.g. B. Chloride content ≤ 0.15% for sodium cyanide in the quality "ACS Reagent Chemicals",
  • Upper limit values ​​(maximum values) for solvent residues in pharmaceuticals , auxiliary substances and drugs , specified in pharmacopoeias .
  • Upper limit values ​​for substances in the air at the workplace at the workplace limit value (AGW), previously also the MAK value .
  • Upper limit values ​​for substances in a biological material at the biological limit value (BGW).
  • Upper limit values ​​in analytical chemistry : the limit concentration (in g ml −1 ) is the minimum concentration at which the proof is positive in a qualitative detection reaction,
  • In physical chemistry , the limit concentration (saturation solubility) is the maximum concentration with which a solid substance is soluble in a liquid,
  • In the food industry , maximum or minimum values ​​(e.g. pH value ) must be specified and monitored for the critical control points (CCP) identified by a hazard analysis in order to prevent, eliminate or reduce a risk to the consumer to a tolerable level . These principles are regulated in the HACCP procedure.

criticism

  • In the building materials atlas (editor Manfred Hegger ) it is criticized that limit and target values ​​are agreed according to the reasonableness and maximum permissible load and less according to the technically achievable minimum values ​​or the results of scientific procedures. Basically, they would only be attempts to assess trigger mechanisms or effects about which little is known. Limit values ​​would not optimize environmental conditions, but would be "at best the normative determination of the tolerability and risks of an apparently unchangeable condition."
  • In the case of pesticides , even concentrations that are up to 10,000 times below today's limit values ​​are problematic for sensitive people .

Individual evidence

  1. Reagent Chemicals: Specifications and Procedures. 10th edition. American Chemical Society, Washington, DC 2006, ISBN 0-8412-3945-2 .
  2. ^ Mission and Preface. (PDF; 673 kB) USP, archived from the original on December 2, 2013 ; accessed on February 19, 2015 (English).
  3. Reagent Chemicals. 7th edition. American Chemical Society, Washington, DC 1986, ISBN 0-8412-0991-X , pp. 76-78.
  4. European Pharmacopoeia. 6th edition. Deutscher Apotheker Verlag, Stuttgart 2008, ISBN 978-3-7692-3962-1 , pp. 1561–1562.
  5. European Pharmacopoeia. 6th edition. Deutscher Apotheker Verlag, Stuttgart 2008, ISBN 978-3-7692-3962-1 , pp. 2215-2217.
  6. Reagent Chemicals. 7th edition. American Chemical Society, Washington, DC 1986, ISBN 0-8412-0991-X , pp. 309-314.
  7. ^ Roy Bergdoll, Sebastian Breitenbach: Die Roten Hefte, Heft 1 - Burning and Extinguishing . 18th edition. Kohlhammer, Stuttgart 2019, ISBN 978-3-17-026968-2 .
  8. Reagent Chemicals. 7th edition. American Chemical Society, Washington, DC 1986, ISBN 0-8412-0991-X , pp. 591-593.
  9. ^ A b Gerhard Hilt, Peter Rinze: Chemical internship for physicians. Chemistry study books . Springer-Verlag, 2009. ISBN 978-3-8348-0667-3 . P. 6, 7.
  10. Norbert Hochheimer: Das kleine QM-Lexikon: Terms of quality management from GLP, GCP, GMP and EN ISO 9000. John Wiley & Sons, 2011. ISBN 978-3-527-33076-8 . P. 105.
  11. Bernhard Adler and others: Analytikum - Methods of analytical chemistry and their theoretical foundations. VEB Deutscher Verlag für Grundstofftindustrie, Leipzig 1971, OCLC 256201747 , p. 64.
  12. Hermann Kienitz: Determination of the solubility. In: Eugen Müller (ed.): Methods of Organic Chemistry (Houben-Weyl). Volume 3: Physical Methods. Part 1, Georg Thieme Verlag, Stuttgart 1955, OCLC 310562623 , p. 226.
  13. ^ Manfred Hegger: Atlas as a building material. Walter de Gruyter, 2006, ISBN 978-3-034-61448-1 ( limited preview in the Google book search).
  14. ^ Matthias Liess, Sebastian Henz, Saskia Knillmann: Predicting low-concentration effects of pesticides. In: Scientific Reports. 9, 2019, doi : 10.1038 / s41598-019-51645-4 .
  15. Dangers from pesticides underestimated. In: biooekonomie.de. November 12, 2019, accessed November 20, 2019 .