Silver chromate precipitates

Gold Chromates. —A red solution of auric chromate, Au2(CrO.,).j, is obtained treating a solution of auric chloride with a large excess of freshly precipitated silver chromate. On evaporation, first metallic gold is precipitated, and then the compound Au2(Cr04)3.Cr0g crystallises out. 

When yellow potassium chromate and colorless silver nitrate solutions are mixed, a red precipitate, silver chromate, results. 

N. A. OrlofE 1 prepared an orange-coloured soln. of gold Chromate, Au2(Cr04)3, by adding a soln. of auric chloride to an excess of freshly precipitated silver chromate, and filtering ofi the silver chloride. If the cone. soln. be evaporated in a desiccator, crystals with the composition Au2(Cr04)3.Cr03 are formed. 

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

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. 

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 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. 

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. 

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. 

Other silver compounds include Silver chromate [CAS 7784-01-2] (Ag2Cr04), yellow to red to brown precipitate by reaction of silver nitrate solution and potassium chromate solution. 

The appearance of a yellow precipitate, lead chromate (PbCr04), indicates the presence of lead ions. To separate silver ions and mercury I ions, ammonium hydroxide (NH4OH), which converts to NH3, is added to the precipitates ... 

Ag+ preferentially reacts with the analyte to form a soluble salt or complex. During this addition, Ag+ reacts with the analyte only, and not the indicator. But when all the analyte is completely consumed by Ag+ and no more of it is left in the solution, addition of an excess drop of silver nitrate titrant produces an instant change in color because of its reaction with the silver-sensitive indicator. Some of the indicators used in the argentometric titrations are potassium chromate or dichlorofluorescein in chloride analysis and p -dime thy la m i nobe nzalrho da n i nc in cyanide analysis. Silver nitrate reacts with potassium chromate to form red silver chromate at the end point. This is an example of precipitation indicator, where the first excess of silver ion combines with the indicator chromate ion to form a bright red solid. This is also known as Mohr method. 

Silver nitrate Brownish red precipitate of silver chromate soluble in dilute nitric acid and in ammonia solution insoluble in acetic acid... 

Reddish brown precipitate of silver chromate Yellow precipitate of silver iodide Black precipitate of silver sulfide Yellow precipitate of silver phosphate Yellow-white precipitate of silver carbonate, forming a brown oxide upon heating... 

These relationships can be used for more than just describing the status of a solid and its constituent ions. Another useful application is to determine if a precipitate will form from two different solutions. For instance, silver nitrate is soluble in water. Potassium chromate is also soluble (as are all potassium salts). If these two solutions are mixed together, two possible products can form potassium nitrate and silver chromate. Potassium nitrate is soluble, but silver chromate is not. But what if very dilute solutions of each were added together Is there a point at which the solutions would be so dilute that no precipitate would form The ion product tells us that the answer is a definite, Yes. In order for a precipitate to form, the value of Q must exceed Ksp. If it does not, no precipitate will form. Let s take a look at a problem that shows this. 

The precipitate must be Ag2Cr04 because all potassium and nitrate salts are soluble. An important thing to notice is the formula of silver chromate. Each chromate will combine with two silver ions. Since both start out with 0.1 mol, silver is a limiting reactant. Only 0.05 mol of Cr042 will be able to precipitate. At the end of the reaction then, there will be ... 

Silver chromate, Ag2Cr04, is insoluble. It forms a brick-red precipitate. Calculate the mass of silver chromate that forms when 50.0 mL of 0.100 mol/L silver nitrate reacts with 25.0 mL of 0.150 mol/L... 

You need to find the mass of silver chromate that precipitates. 

Write a balanced chemical equation for the reaction. Find the amount (in mol) of each reactant, using its volume and concentration. Identify the limiting reactant. Determine the amount (in mol) of silver chromate that forms. Calculate the mass of silver chromate that precipitates. 

Fractional precipitation The calculation as to which of two sparingly soluble salts will be precipitated under given experimental conditions may be also made with the aid of the solubility product principle. An example of great practical importance is the Mohr method for the estimation of halides. In this process a solution of chloride ions is titrated with a standard solution of silver nitrate, a small quantity of potassium chromate being added to serve as an indicator. Here two sparingly soluble salts may be formed, viz. silver chloride (a white precipitate) and silver chromate (which is red) ... 

This expression shows that under equilibrium conditions the concentration of chromate ions in the solution is always much greater than that of the chloride ions. If therefore to a mixture of chloride and chromate ions, silver ions are added, these will combine with chloride ions, forming silver chloride precipitate until the concentration of chloride ions in the solution decreases to such an extent, that the ratio expressed in equation (iii) is achieved. From then onwards the two precipitates will be formed simultaneously. If a 01m solution of sodium chloride is titrated with silver nitrate in the presence of 0 002m potassium chromate, the concentration of chloride ions at which silver chromate starts to precipitate can be expressed from equation (iii) ... 

Potassium chromate in neutral solution red precipitate of silver chromate... 

Spot test place a drop of the test solution on a watch glass or on a spot plate, add a drop of ammonium carbonate solution and stir (this renders any mercury(I) or lead ions unreactive by precipitation as the highly insoluble carbonates). Remove one drop of the clear liquid and place it on drop-reaction paper together with a drop of the potassium chromate reagent. A red ring of silver chromate is obtained. 

Silver nitrate solution brownish-red precipitate of silver chromate, Ag2Cr04, with a solution of a chromate. The precipitate is soluble in dilute nitric acid and in ammonia solution, but is insoluble in acetic acid. Hydrochloric acid converts the precipitate into silver chloride (white). 

A reddish-brown precipitate of silver dichromate, Ag2Cr207, is formed with a concentrated solution of a dichromate this passes, on boiling with water, into the less soluble silver chromate. 

Heat with sodium carbonate and potassium nitrate This test may be carried out in a loop of platinum wire or upon platinum foil or upon a piece of broken porcelain. If chromium is present, a yellow melt is produced. This should be dissolved in water, acidified with dilute acetic acid, and (a) silver nitrate solution added, when brownish-red silver chromate is precipitated, (6) lead acetate solution added, when yellow lead chromate is precipitated, or (c) 1-2 ml diphenylcarbazide reagent added, when a deep-red colouration is produced. 

Precipitation of silver chromate by adding potassium chromate to a solution of silver nitrate. 

Silver Chromate, Ag2Cr04, is obtained as. a reddish-brown precipitate on mixing solutions of soluble silve r salts with those of chromate or dichroraate of potassi)Hjnf preferp.bly the former. Wh n prepared... 

Various theories have been advanced in explanation. Ostwald suggested that supersaturation took place, followed by precipitation, which cleared the immediate neighbourhood of the reactants, and it was therefore necessary for the silver nitrate to diffuse further before supersaturation was again reached. Hatsehek, however, shows that the periodic precipitation takes place in conditions which render supersaturation impossible. Williams and Mackenzie maintain that the silver chromate is precipitated according to the usual rules of the solubility product, and does not behave in any way as a protected colloid but as a crystalloid. More recent w ork suggests that whenever precipitation takes place, the precipitate first passes through the colloidal... 

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