Yellow chromating

Fusion of any chromium compound with a mixture of potassium nitrate and carbonate gives a yellow chromate(VI). ... 

A reducing agent (for exampie sulphur dioxide) reduces the yellow chromate or orange dichromate to the green chromium(III) state. 

Chromium in the +6 state forms two different oxoanions, the yellow chromate ion, Cr042-, and the red dichromate ion, Cr2072- (Figure 20.11). The chromate ion is stable in basic or neutral solution in acid, it is converted to the dichromate ion ... 

If excess base is added to this solution, it turns yellow, and yellow chromate salt may crystallize out. Thus, as mentioned above, in an aqueous solution of CrOs, there is an equilibrium between two Cr " species, namely, the chromate and dichromate ions ... 

Prepare about 30 g. of normal lead chromate, PbCr04 (Exercise 23), by the double decomposition of lead acetate and potassium dichromate and wash the precipitate until it is free from soluble lead salts. Suspend the yellow chromate in a dilute solution of sodium hydroxide (1 20) and boil until the color has changed to a bright brick red. This material is a basic chromate known as chrome red, having the composition represented by the formula PbCr04-PbO ... 

In 1 M HC1, the formal potential is just 1.00 V and, in 2 M H2S04. it is 1.11 V so dichromate is a less powerful oxidizing agent than Mn04 or Ce4+. In basic solution, Cr20 is converted into yellow chromate ion (CrO -), whose oxidizing power is nil ... 

The two major forms of chromium(VI) in solution are yellow chromate, CrO r, and orange dichromate, Cr20. The latter predominates in acidic solution, as shown by the following reaction, the equilibrium of which is forced to the left by higher levels of H+ ... 

Blue copper(II) green nickel, iron(II), chromium(III), manganates yellow chromates, hexacyanoferrate(II), iron(II) orange-red dichromates purple permanganates pink cobalt, manganese(II). 

Test for chromate If the soda extract is colourless, chromate is absent. If yellow, chromate may be present hexacyanoferrate(II) and (III) also impart a yellow colour to the solution. The presence of chromate will have been indicated by the precipitation of green chromium(III) hydroxide in the silver nitrate solution tests (Table V.31) and also in the analysis for cations. 

Chromate and Dichromate Ions. In basic solutions above pH 6, Cr03 forms the tetrahedral yellow chromate ion CrO between pH 2 and 6, HCr04 and the orange-red dichromate ion Cr2Or are in equilibrium and at pH values <1 the main species is H2Cr04. The equilibria are the following ... 

Chromium(VI) forms two different oxyanions, the orange dichromate ion, Cr207 -, and the yellow chromate ion, Cr042-. (See the photos.) The equilibrium reaction between the two ions is... 

In aqueous solution normal chromates are yellow in colour on treatment with acid they are converted into the orange-red dichroma.tes the yellow chromate is regenerated on treatment of an aqueous solution of a dichromate with an alkali. Alkali chromates and dichromates may be supposed to dissociate in solution primarily in accordance with the equations ... 

Chromates of the Rare Earth Metals.—A series of isomorphous yellow chromates, sparingly soluble in water, and of general formula E2 (Cr04)3.8H30, where R=Lanthanum, Praseodymium, Neodymium, or Samarium, has been prepared. ... 

While at the end of this test zinc was completely corroded, galvanoaluminum and cadmium showed no sign of red rust. The corrosion-protective effect of the anodized layer is less than that of the nonanodized galvanoaluminum layer, because of the missing eathodic corrosion protection. Yellow chromated 12 pm thick galvanoaluminum and cadmium coatings exhibited no trace of red rust after 1000 h. 

Galvanoaluminum layers thicker than 12 pm are extraordinarily corrosion resistant to exposure to SO2, in which they surpass the performance of cadmium and zinc layers [31]. After 20 rounds of the SFW 2.0 L, DIN 50018 Kesternich test, in contrast to the ca. 12 pm thick, yellow chromated galvanoaluminum layers, the zinc and cadmium coatings were already heavily corroded. The SO2 resistance of anodized galvanoaluminum can also be considered excellent in comparison to cadmium and zinc coatings. 

From such solutions orange dichromate salts can be crystallized after adding a stoichiometric amount of base. Addition of excess base produces yellow solutions from which only yellow chromate salts can be obtained. The two anions exist in solution in a pH-dependent equilibrium. 

Chromium is known in all oxidation states between 0 and VI, but is commonly found in oxidation states III and VI. Cr(III) species in acidic solution exist as Cr(H20)f, ions and in concentrated alkali have been identified as Cr(OH)ft and Cr(0H)5(H20)2". These ions are regular octahedra. Cr(VI), in basic solution above pH 6, exists as tetrahedral yellow chromate, Cr04 , ion. Between pH 2 and 6, HCr04 the orange-red dichromate, Cr207, are in equilibrium. Linder strongly acidic conditions, only dichromate ion exists. Addition of alkali to dichromate results in chromate. 

MULLEN K (1985) The contrast sensitivity of human colour vision to red-green and blue-yellow chromatic gratings, J. Physiol, 359, 381-400. 

Potassium chromate 4 186 K2 4 Chromate of potassa monochromate of potassa neutral chromate of potassa potasJtc chromate salt of chrome yellow chromate of potassa. 

Bichromate of Potaasa. The rcfl chromate of potash is obtained from a concentrated solution of the yellow chromate, by adding. sulphuric for, still better, acetic) acid in quantity equal to half that required for the noutralization of tho salt. (See jVo. 83. j The liquid is then concentrated by evaporation, and slowly cooled, so that crystals may form. 

E. J. Houston and E. Thomson, in that the maximum intensity, with rise of temp., moves towards the red. The basic chromates also become a deeper red when heated. Conversely, as the yellow chromate is cooled to —30° to —40°, it becomes yellowish-green and M. Bamberger and R. Grengg said that both the natural and artificial crystals are decolorized at —190°. 

---