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Application of potassium dichromate
Potassium dichromate has few major applications, as the sodium salt is dominant industrially. The main use is as a precursor to
potassium chrome alum
, used in
), potassium dichromate has been used to prepare "
" for cleaning glassware and etching materials. Because of safety concerns associated with hexavalent chromium, this practice has been largely discontinued.
It is used as an ingredient in
in which it retards the setting of the mixture and improves its density and texture. This usage commonly causes
Potassium dichromate has uses in
and in photographic
, where it is used as an oxidizing agent together with a strong mineral acid.
discovered that paper treated with a solution of potassium dichromate was visibly tanned by exposure to sunlight, the discoloration remaining after the potassium dichromate had been rinsed out. In 1852,
Henry Fox Talbot
discovered that exposure to ultraviolet light in the presence of potassium dichromate hardened organic
, making them less soluble.
These discoveries soon led to the
, and other photographic printing processes based on differential hardening. Typically, after exposure, the unhardened portion was rinsed away with warm water, leaving a thin relief that either contained a pigment included during manufacture or was subsequently stained with a dye. Some processes depended on the hardening only, in combination with the differential absorption of certain dyes by the hardened or unhardened areas. Because some of these processes allowed the use of highly stable dyes and pigments, such as
, prints with an extremely high degree of archival permanence and resistance to fading from prolonged exposure to light could be produced.
Dichromated colloids were also used as
in various industrial applications, most widely in the creation of metal printing plates for use in photomechanical printing processes.
uses potassium dichromate together with equal parts of concentrated
diluted down to approximately 10% v/v to treat weak and thin negatives of black and white photograph roll. This solution reconverts the elemental silver particles in the film to
. After thorough washing and exposure to
light, the film can be redeveloped to its end-point yielding a stronger negative which is able to produce a more satisfactory print.
A potassium dichromate solution in
can be used to produce a reversal negative (i.e., a positive transparency from a negative film). This is effected by developing a black and white film but allowing the development to proceed more or less to the end point. The development is then stopped by copious washing and the film then treated in the acid dichromate solution. This converts the
, a compound that is insensitive to light. After thorough washing and exposure to actinic light, the film is developed again allowing the previously unexposed silver halide to be reduced to silver metal. The results obtained can be unpredictable, but sometimes excellent results are obtained producing images that would otherwise be unobtainable. This process can be coupled with
so that the end product resembles a negative and is suitable for printing in the normal way.
CrVI compounds have the property of
when exposed to strong light. This quality is used in photographic
a fine screen of bolting silk or similar material is stretched taut onto a frame similar to the way canvas is prepared before painting. A
sensitized with a dichromate is applied evenly to the taut screen. Once the dichromate mixture is dry, a full-size photographic negative is attached securely onto the surface of the screen, and the whole assembly exposed to strong light – typically about half an hour in bright sunlight – hardening the exposed colloid. When the negative is removed, the unexposed mixture on the screen can be washed off with warm water, leaving the hardened mixture intact, acting as a precise mask of the desired pattern, which can then be printed with the usual
Because it is non-hygroscopic, potassium dichromate is a common reagent in classical "wet tests" in analytical chemistry.
The concentration of ethanol in a sample can be determined by
with acidified potassium dichromate. Reacting the sample with an excess of potassium dichromate, all ethanol is oxidized to
CH3CH2OH + 2[O] → CH3COOH + H2O
Full reaction of converting ethanol to acetic acid:
3C2H5OH + 2K2Cr2O7 + 8H2SO4 → 3CH3COOH + 2Cr2(SO4)3 + 2K2SO4 + 11H2O
The excess dichromate is determined by titration against
. Subtracting the amount of excess dichromate from the initial amount, gives the amount of ethanol present. Accuracy can be improved by calibrating the
solution against a blank.
One major application for this
is in old police
tests. When alcohol vapor makes contact with the orange dichromate-coated crystals, the
changes from orange to green. The degree of the color change is directly related to the level of alcohol in the suspect's breath.
When dissolved in an approximately 35%
solution it is called Schwerter's solution and is used to test for the presence of various metals, notably for determination of silver purity. Pure silver will turn the solution bright red,
will turn it dark red, low grade
(0.800 fine) will turn brown (largely due to the presence of copper which turns the solution brown) and even green for 0.500 silver. Brass turns dark brown, copper turns brown, lead and tin both turn yellow while gold and palladium do not change.
Sulfur dioxide test
Potassium dichromate paper can be used to test for
, as it turns distinctively from orange to green. This is typical of all redox reactions where hexavalent chromium is reduced to trivalent chromium. Therefore, it is not a conclusive test for sulfur dioxide. The final product formed is Cr2(SO4)3.
Potassium dichromate is used to stain certain types of wood by darkening the tannins in the wood. It produces deep, rich browns that cannot be achieved with modern color dyes. It is a particularly effective treatment on
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