Silver chromate

The chromates of the alkali metals and of magnesium and calcium are soluble in water the other chromates are insoluble. The chromate ion is yellow, but some insoluble chromates are red (for example silver chromate, Ag2Cr04). Chromates are often isomorph-ous with sulphates, which suggests that the chromate ion, CrO has a tetrahedral structure similar to that of the sulphate ion, SO4 Chromates may be prepared by oxidising chromium(III) salts the oxidation can be carried out by fusion with sodium peroxide, or by adding sodium peroxide to a solution of the chromium(IIl) salt. The use of sodium peroxide ensures an alkaline solution otherwise, under acid conditions, the chromate ion is converted into the orange-coloured dichromate ion ... 

The chloride is white, the bromide pale yellow and the iodide deeper yellow. These are examples (uncommon) of a coloured compound being obtained from colourless ions. The silver(I) ion intensifies colour in other cases, for example silver chromate(VI), Ag2Cr04, is brick-red while potassium chromaie(VI). K2Cr04. is yellow. 

Potassium chromate(VI) gives a brick-red precipitate of silver chromate(VI) in neutral solution. 

Silver Chromate. Silver chromate, Ag2Cr04, is prepared by treating silver nitrate with a solution of chromate salt or by heating a suspension of silver dichromate [7784-02-3]. 

No systematic study of the minimal required amount of lead tetraacetate has been made. In cases where the product of the hypoiodite reaction is an iodo ether (20-hydroxy steroids) the reaction can be interrupted at the iodohydrin stage by reducing the amount of iodine to about 0.5 mole. For the oxidation of iodo ethers to lactones, chromium trioxide-sulfuric acid in acetone has been used. Silver chromate is often added to the reaction mixture but comparable yields are obtained without the addition of silver salt. 

The residue (12 g) which contains the 18-iodo-18,20-ether is dissolved in 200 ml of acetone, 5 g of silver chromate is added Note 3) and after cooling to 0°, 11.8 ml of a solution of 13.3 g of chromium trioxide and 11.5 ml of concentrated sulfuric acid, diluted to 50 ml with water is added during a period of 5 min. After an additional 60 min, a solution of 112 g of sodium acetate in 200 ml of water is added and the mixture diluted with benzene (400 ml), filtered and the benzene layer separated. The aqueous phase is reextracted with benzene, washed with half-saturated sodium chloride solution, dried and evaporated to yield 11.2 g of a crystalline residue. Recrystallization from ether gives 7.2 g (72%) of pure 3/5, 1 la, 20/5-trihydroxy-5a-pregnan-18-oic acid 18,20 lactone 3,11-diacetate mp 216-218°. 

Note 3. Although in principle the chromate ester can be formed directly from the 18-iodo-l 8,20-ether with silver chromate, hydrolysis and oxidation with aqueous chromic acid sulfurc acid is equally efficient. 

N. Curtis, D. G. Leaist. Interdiffusion of aqueous silver nitrate and potassium chromate and the periodic precipitation of silver chromate Liesegang bands. Ber Bunsen-Ges—Phys Chem, Chem Phys 702 164, 1998. 

No dimensional change Proprietary brands of alkaline cleaners may be used provided these do not attack metal Soda ash with washing soda and soap are suitable for preparing alkaline cleaning baths No dimensional change unless much impurity is present For small delicate parts 0 1-0-5% silver chromate may be added to reduce attack caused by impurities such as chloride in corrosion product... 

Note in making up the chromic acid solution it is advisable to dissolve the silver nitrate separately and add it to the boiling chromic acid to prevent excessive crystallisation of the silver chromate. The chromic acid must be free from sulphate to avoid attack on the zinc. Immerse each specimen for 15 s in a 6% solution of hydriodic acid at room temperature to remove the remaining corrosion products. Immediately after immersion in the acid bath, wash the samples first in tap water and then in absolute methanol, and dry in air. This procedure removes a little of the zinc and a correction may be necessary. 

Magnesium alloys Dip for approximately 1 min in boiling 15% chromic acid to which has been added with agitation 1 % silver chromate solution. 

Write the equation for the dissolving of silver chromate, AgjCrO, and the solubility product expression. Silver chromate dissolves to give Ag+ and Cr04 2 ions. 

Example 2. Calculate the solubility product of silver chromate, given that its solubility is 2.5 x 10-2gL-1. 

The theory of the process is as follows. This is a case of fractional precipitation (Section 2.8), the two sparingly soluble salts being silver chloride (Xsol 1.2 x 10 10) and silver chromate (Kso] 1.7 x 10 12). It is best studied by considering an actual example encountered in practice, viz. the titration of, say, 0.1M sodium chloride with 0.1M silver nitrate in the presence of a few millilitres of dilute potassium chromate solution. Silver chloride is the less soluble salt and the initial chloride concentration is high hence silver chloride will be precipitated. At the first point where red silver chromate is just precipitated both salts will be in equilibrium with the solution. Hence ... 

FIGURE l.l When a solution of yellow K2Cr04 is mixed with a colorless solution of AgNOj a red precipitate of silver chromate, Ag2Cr04, forms. 

Self-Test I.2A Write the net ionic equation for the reaction in Fig. 1.1, in which aqueous solutions of colorless silver nitrate and yellow potassium chromate react to give a precipitate of red silver chromate. 

The molar solubility of silver chromate, Ag2Cr04, is 6.5 X 10 mol-L. Determine the value of Ksp for silver chromate. 

The common ion effect alters the amount of solid that will dissociate in solution. The addition of solid silver chromate to an aqueous solution of potassium chromate will affect the silver chromate s solubility because —... 

B the ion chromate is common to both solutions and will decrease silver chromate s solubility... 

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

Silver chloride electrodes standard potential, 3 413t Silver chromate(VI), 6 537... 

To illustrate your answer, calculate and compare the molar solubility of silver chloride, AgCl (Kgp = 1.8 x 10 ), with silver chromate,... 

Mohr titration analychem Titration with silver nitrate to determine the concentration of chlorides in a solution silver chromate precipitation is the end-point indicator. mor tT tra-shon ... 

Elemental composition K 40.26%, Cr 26.78%, O 32.96%. An aqueous solution of the salt is analyzed for potassium and chronium (see Potassium and Chronium). Potassium chromate may be identified by its physical properties and by x-ray methods. Also, an aqueous solution of the salt forms a red precipitate of silver chromate when treated with a solution of silver nitrate. The chromate content may be determined stoichiometrically by weighing the dry precipitate. 

Silver chromate is a catalyst in conversion of alcohol to aldol. It s formation signals the end point in argentometric titration in measuring halides. 

Silver chromate is prepared by slowly adding a solution of potassium chro-... 

The brick-red precipitate was identified as silver chromate thus, flask X must be AgNOj and flask A must be K2Cr04. 1 point for correct identification of X and 1 point for correct identification of A. 

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