Potassium dichromate

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Structural formula
Structure of potassium dichromate
General
Surname Potassium dichromate
other names
  • Potassium dichromate
  • Potassium double chromate
  • Potassium pyrochromate
  • Potassium chromium
  • Potassium pyrochromate
Molecular formula K 2 Cr 2 O 7
Brief description

Orange-red, non-hygroscopic, triclinic panels

External identifiers / databases
CAS number 7778-50-9
EC number 231-906-6
ECHA InfoCard 100.029.005
PubChem 24502
Wikidata Q239729
properties
Molar mass 294.19 g · mol -1
Physical state

firmly

density
Melting point

398 ° C (α modification)

boiling point

thermal decomposition:> 400 ° C

solubility
  • good in water (115 g l −1 at 20 ° C)
  • insoluble in ethanol
safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary
03 - Oxidising 06 - Toxic or very toxic 08 - Dangerous to health
05 - Corrosive 09 - Dangerous for the environment

danger

H and P phrases H: 272-301-312-330-314-317-334-335-340-350-360FD-372-410
P: 201-221-273-280-301 + 330 + 331-302 + 352-304 + 340-305 + 351 + 338-308 + 310
Authorization procedure under REACH

of particular concern : carcinogenic, mutagenic, toxic for reproduction ( CMR ); subject to approval

MAK
  • Germany: repealed because it is carcinogenic
  • Switzerland: 5 μg m −3 (calculated as chromium)
Toxicological data
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Potassium dichromate , K 2 Cr 2 O 7 , is the potassium salt of dichromic acid , which is unstable in its free form . Other names are: potassium dichromate, double chromate potassium, pyrochromate potassium, chromium potassium; English: Potassium dichromate .

Historical

In 1856 was by W. H. Perkin in the oxidation of impure aniline the first dye man-made, that with potassium dichromate Mauvein (also Perkinviolett) discovered.

Occurrence

Potassium dichromate occurs naturally in the form of the mineral lopezite .

Extraction and presentation

The technical production takes place by melting together chrome iron stone , potassium carbonate (potash) and calcium carbonate and oxidation with blown air. The yellow potassium chromate K 2 CrO 4 is initially formed , which is leached and, after the addition of acid and recrystallization, yields potassium dichromate.

On a laboratory scale, potassium dichromate can be obtained by melting together potassium nitrate and chromium (III) oxide .

Chromium oxide reacts with potassium nitrate to form potassium dichromate and nitric oxide

It is also possible to display it by reacting a sodium dichromate solution with potassium chloride :

properties

Potassium dichromate

Potassium dichromate is bright orange-red crystals without water of crystallization which have a melting point of 397 ° C and 500 ° C under oxygen delivery to potassium K 2 CrO 4 and chromium (III) oxide, Cr 2 O 3 to decompose.

Potassium dichromate is a strong oxidizing agent , especially in acidic solutions. In an alkaline medium, the yellow chromate CrO 4 2− is mainly present, which has a far less oxidizing effect. For example, when potassium hydroxide is added, the orange-red color of a potassium dichromate solution changes to light yellow due to the formation of potassium chromate:

Potassium dichromate is readily soluble in water. The dichromate anion Cr 2 O 7 2- is in aqueous solution in equilibrium with HCrO 4 - , CrO 4 2- . Therefore, with barium , lead and silver ions (Ba 2+ , Pb 2+ , Ag + ), yellow, poorly soluble chromates and not dichromates are formed. Many acid anions react to form other complex anions , for example CrO 3 Cl - is formed in hydrochloric acid , which can be crystallized out as a potassium salt. Bromide and iodide also react in a similar way , because although (thermodynamically) the oxidation to the elementary halogens should actually take place, the oxidation is prevented by the low reaction rate (kinetic inhibition). Primary alcohols are oxidized with sulfuric acid to aldehydes or carboxylic acids, whereby the dichromate changes into green, trivalent chromium sulfate. The chromium in potassium dichromate has the oxidation number + VI.

The compound usually has a triclinic crystal structure with the space group P 1 (space group no. 2) and the lattice constants a = 13.367, b = 7.376, c = 7.445 Å , α = 90.75 °, β = 96.21 °, γ = 97.96 ° with four K 2 Cr 2 O 7 units per cell. The monoclinic , metastable at room temperature modification β-K 2 Cr 2 O 7 ( space group C 2 / c (space group no.15) ) was first developed in 1960 by Ulrich Klement and Georg Maria Schwab in addition to a high temperature form with the monoclinic space group P 2 1 / n (No. 14, position 2) . Some sources also indicate a metastable intermediate structure. The monoclinic form changes from 258 ° C to the triclinic form. Template: room group / 2 Template: room group / 15Template: room group / 14.2

use

In the laboratory, potassium dichromate is because of the good storability and ponderability as a common oxidizing agent , reagent (detection of hydrogen peroxide by blue staining) and volumetric standard used.

In engineering it is used in tannery , electroplating and for the production of chromosulfuric acid . In match heads it is used as an additive to regulate the burning rate. In analog photography and the fine printing process , potassium dichromate is used as a light-sensitive, colloid-tanning substance, especially in collotype and rubber printing . In neurohistology, Camillo Golgi developed a method for staining individual nerves and neurons. Known as the “black reaction”, nerve cells are stained down to the finest structures with the help of potassium dichromate and silver nitrate . Disposable tests for the determination of alcohol in the breath used to contain potassium dichromate, which is why they should be disposed of as hazardous waste. The production of the Alcotest tubes containing potassium dichromate at Dräger was discontinued at the end of 2016 after 53 years of production.

Both use and storage are prohibited at schools in Germany (list of substances for DGUV rule 113-018, substance 559, page 71).

Potassium dichromate is often used to measure the chemical oxygen demand (COD) of a water sample in sewage treatment plants. Corresponding applications for approval to the European Chemicals Agency (ECHA) after the inclusion of the compound in Annex XIV of the REACH regulation were made in 2015.

Hazard warnings

Although the substance was classified as Xi (irritant) until a few years ago, potassium dichromate is now classified as very toxic, oxidising, dangerous for the environment, mutagenic, toxic to reproduction and carcinogenic .

Potassium dichromate is highly toxic locally and systemically. It irritates the skin, respiratory organs and the eyes and is one of the most important contact allergens. With repeated skin contact, sensitization occurs and allergies can develop. In an extrapolation for the entire population of Germany with best-case, medium-case and worst-case assumptions, the rate of sensitizations for chromium (VI) (potassium dichromate) was estimated at 0.6% based on a “medium case scenario” . With a population of 82 million, this corresponds to about 514,000 cases. In 2013, potassium dichromate was included in Annex XIV of the Ordinance on the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) because of its carcinogenic, mutagenic and reproductive effects. Due to its occurrence in cement, the compound used to be a frequent trigger of eczema , which is now generally referred to as a chromate allergy as wall itch, masonry eczema, chrome eczema or cement eczema . Measures to reduce chromium (VI) in cement (adding iron (II) sulfate) have reduced the number of sensitizations. However, chromium allergies are often chronic, which is problematic due to the occurrence of potassium dichromate in almost all leather products.

With organic, flammable compounds, reducing agents, concentrated sulfuric acid, metals in powder form (especially: magnesium, iron), violent reactions up to self-ignition or explosion are possible. Potassium dichromate is not flammable, but it is fire-promoting. The substance should be brought to the hazardous waste collection facility or a disposal company in its original container. Potassium dichromate can be disposed of with iron sulfate , whereby it is reduced to Cr 3+ , which when precipitated as insoluble chromium hydroxide is much safer. Illnesses caused by chromium and chromates after occupational exposure (for example eczema or bronchial carcinoma in tilers, bricklayers and miners) are occupational diseases that are subject to notification and compensation (BK 1103).

Web links

Commons : Potassium Dichromate  - Collection of images, videos and audio files

Individual evidence

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