Ferric alums acid salts

Foreign Bodies which Reduce Permanganate when Zinc Oxide is Dissolved in Dilute Sulphuric Acid. — Very carefully Iril unite 3 gm. of zinc oxide, in a mortar with 20 cc. of water conluining in solution 0.2 gin. of pure ferric alum free from ferrous salt. Then add to the mixture. 25 cc. of diluted sulphuric acid, and effect complete solution by gently heating. 

This energetic reducing agent can be maintained at constant strength in aqueous hydrochloric acid solution for a reasonable period. It is advisable, however, to re-standardise it after 24 hours standing. It serves for the reduction of aromatic nitro compounds, some nitroso bodies, many azo dyes, and of nearly all the dyes which yield leuco-compounds. It is easily standardised against a ferric salt—say ferric alum—using potassium thiocyanate as indicator. From the equations —... 

L. P. de St. Gilles, and I. M. Kolthoff, found that potassium permanganate oxidizes hypophosphorous acid completely to phosphoric acid. L. Amat found that the oxidation proceeds more quickly the more cone, the soln., the more acidic the soln., and the higher the temp. at ordinary temp., and in dil. soln., the oxidation is incomplete. If the soln. be too hot, some permanganate may be decomposed without interaction with the hypophosphorous acid. The reaction was studied by I. M. Kolthoff. M. Major, and A. Sieverts found that reduced iron readily dissolves in a hot soln. of sodium hypophosphite ferric salts are reduced to the ferrous state and ferric alum reacts at the temp, of the water-bath, while phosphorous acid is not attacked after several hours. 

After adding (NH4)2S04 to the solution so obtained, the double salt, ammonium ferric sulphate, or ferric alum, may be crystallized out. In order to obtain a satisfactory product it is very important to adjust the amount of add very precisely. Too little acid will allow a brown basic salt to form (mFe(0H)3-nFe2(S04)3), which will discolor the preparation. Too much acid has the obvious disadvantage that it will cling to the crystals of the product from which it cannot be removed by evaporation or by washing (because of the great solubility of the product). 

The hydrolysis of ferric salts is so common that the color of ferric ion, Fe(H20)g+ + , is usually masked by that of the hydroxide complexes. Ferric ion is nearly colorless it eems to have a very pale violet color, seen in crystals of ferric alum, KFe(SO )o I2H.2O, and ferric nitrate, Fe(N03)3 9H20, and in ferric solutions strongly acidified with nitric or perchloric acid. Solutions of ferric salts ordinarily have the characteristic yellow to brown color of the hydroxide coniplexe Fe(H20)50H + + and Fe(H20)4(OH)o+, or even the red-brown color of colloidal particles of hydrated ferric hydroxide. 

The hexaquo ion exists in very strongly acid solutions of ferric salts, in the several ferric alums, MIFe(S04)2- 12H20, and presumably also in the highly hydrated crystalline salts. 

The most commonly used coagulating agents are ferric and aluminium salts (e.g. ferric chloride, FeCla, ferric sulfate, Fe2(S04)3 and alum, Al2(S04)3 I4H2O), which are precipitated as the hydroxides, (e.g., equations 28.1, 28.2). Both reactions generate mineral acids which may need to be neutralised by the addition of an alkali to produce the required pH. 

To about 0-5 g add 25 ml of O IN silver nitrate. After the addition of 10 ml of dilute nitric acid and heating on a water-bath for thirty minutes the salt is completely decomposed and the silver iodide precipitated. Titrate the excess of silver nitrate with 0 1 N potassium thiocyanate solution, using ferric alum as indicator. 1 ml O IN = 0 01269 g I. 

As a preliminary, ferric sulfate is made by the oxidation of ferrous sulfate. Dissolve 100 g. of ferrous sulfate in 100 cc. of boiling water, to which has been added before heating 10 cc. of sulfuric acid. Add concentrated nitric acid portionwise to the hot solution, until a diluted sample gives a reddish-brown (not black) precipitate with ammonia. This will require about 25 cc. Boil the solution down to a viscous liquid to get rid of excess nitric acid, dilute to about 400 cc., and add the calculated weight of ammonium sulfate. The crystallization is conducted as in the former exercise, preferably under 20°. By the addition of potassium sulfate, the corresponding potassium iron alum may be secured. In this case, it is necessary to concentrate the solution until there is about four parts of water to one of the hydrated alum and cool to about zero to secure crystallization. Both of these alums are amethyst in color, the potassium salt being much less stable and having a rather low transition point. 

Abundant yellow or white salt crusts are present on waste rock and at the surface of the soil. The crusts comprise alum-like sulfate minerals containing variable amounts of sodium, potassium, iron and aluminium, such as the mineral jarosite. They are often very soluble in water, releasing acid and precipitating ferric hydroxides. 

On addition of concentrated sulphuric acid to a solution of the alum a white precipitate is obtained which contains ferric and ammonium sulphates in some uncertain state of combination. The precipitate when dry is stable in air, slowly soluble in cold water, but readily soluble in hot hydrochloric acid, and is possibly an ammonium salt of ferri-sulphuric acid (see p. 162). 

The chemicals required are alum, lime, and the ferric salts FeCl3 and Fe2(S04)3. These are in addition to the acid or bases needed to adjust the pH. The formulas used to calculate the amounts of these chemicals for pH adjustment were already derived in Chapters 11, 12, and 13. The chemical requirements to be discussed here will only be for alum, lime, and the ferric salts. 

About 2 gm. benzyl iodide are weighed into a flask and then 50 ml. 20% alcoholic potash solution are added and the mixture refluxed for about an hour. At the completion of the saponification the contents of the flask are allowed to cool and then transferred to a 500-ml. flask and made up to volume with water. 100 ml. of the resulting solution are placed in a distillation flask and distilled in steam after adding 10 gm. ferric ammonium alum and acidifying with sulphuric acid. By this treatment, the ferric salt is converted to the ferrous condition, liberating iodine which is distilled over into 5% potassium iodide solution. At the end of the distillation, the free iodine in the potassium iodide solution is titrated with a decinormal solution of sodium thiosulphate. From this, the amount of iodine and so the quantity of benzyl iodide in the sample may be calculated. 

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