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Hexavalent chromium compounds include oxo acid salts ( chromate and dichromate), chromium trioxide (CrO 3 ) and chromoyl chloride (CrO 2 Cl 2 )
.
1.
Existence and transformation
Add acid to the CrO 4 2- solution to convert it into dimerized dichromate Cr 2 O 7 2- .
When the acid concentration is large, red needle-like CrO 3 crystals are precipitated , and CrO 2 2+ is formed in the concentrated strong acid
.
If alkali is added to Cr 2 O 7 2- solution, it will be converted into CrO 4 2- solution
The dichromate dimer is formed by tetrahedral CrO 4 co-apex oxygen, as shown in Figure 16-1
.
Figure 16-1 Configuration of dichromate ion
When preparing the lotion, mix concentrated H 2 SO 4 with saturated K 2 Cr 2 O 7 solution, and red needle-like crystals of CrO 3 are precipitated
.
CrO 2 2+ is called chromium oxy or chromium acyl
.
Chromyl chloride CrO 2 Cl 2 is a dark red liquid, which is volatile
The hydrolysis of CrO 2 Cl 2 produces H 2 Cr 2 O 7 and HCl
.
2.
Oxygenates
The most important salts of hexavalent chromium are yellow Na 2 CrO 4 and K 2 CrO 4 , orange-red Na 2 Cr 2 O 7 and K 2 Cr 2 O 7 , dark red (NH 4 ) 2 Cr 2 O 7 , They are all easily soluble in water
.
Since sodium salt is easy to absorb water and deliquescence, potassium salt is often used in laboratories
Na 2 CrO 4 is widely used in the production of paints, inks, rubber, ceramics, and can also be used as an oxidant for sterilization and organic synthesis reactions
.
The common insoluble chromates of hexavalent chromium are: brick red Ag 2 CrO 4 and yellow PbCrO 4 , BaCrO 4 , SrCrO 4
.
H 2 CrO 4 is a strong acid but is easy to polymerize, its secondary dissociation constant is very small, and the insoluble chromate is easily soluble in strong acid
.
Ag 2 CrO 4 is soluble in nitric acid, and is converted into Ag 2 O by reaction with NaOH solution , and converted into AgCl by reaction with hydrochloric acid
.
PbCrO 4 is soluble in both nitric acid and strong alkali
.
BaCrO 4 is soluble in hydrochloric acid and nitric acid, but insoluble in strong alkalis, and can be converted into BaSO 4 by reacting with sulfuric acid
.
Adding Ba 2+ , Pb 2+ , Ag +, etc.
to the CrO 4 2- or Cr 2 O 7 2- solution all produced chromate precipitation, indicating that the solubility of chromate is lower than that of dichromate .
3.
Oxidizing
In acidic media, hexavalent chromium has strong oxidizing properties
.
K 2 Cr 2 O 7 is a commonly used oxidant.
It can oxidize many substances under acidic conditions.
It reacts quantitatively and quickly with Fe 2+ to analyze the iron content
.
CrO 3 and (NH 4 ) 2 Cr 2 O 7 have strong oxidizing properties and are easily decomposed by heat
.
(NH 4 ) 2 Cr 2 O 7 thermal decomposition produces Cr 2 O 3 , which is neither soluble in acid nor alkali due to high temperature
.
Acidify the K 2 Cr 2 O 7 solution with dilute sulfuric acid , and add the H 2 O 2 solution to produce blue CrO 5 , the chemical formula of which is CrO(O 2 ) 2
.
This is a sensitive reaction for testing Cr(VI) or H 2 O 2
.
CrO 5 has poor stability in water and quickly decomposes to produce green Cr 3+
.
If ether or pentanol is added to the CrO 5 solution , the organic layer will be blue .
CrO 5 is relatively stable in organic solvents and has a slower decomposition rate (do not understand that CrO 5 can only be produced in ether ) .
Related links: trivalent chromium compounds
