Ferric alums solubility

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). 

We detect the end point by adding iron(III) as a ferric alum (ferric ammonium sulfate), which forms a soluble red complex with the first excess of titrant ... 

For back-titration the method of Volhard is used and is probably the most widely employed technique for determination of halide ions. The halide is precipitated from a nitric acid solution by addition of excess silver nitrate and the excess is back-titrated with ammonium thiocyanate using ferric alum as indicator at the end-point the blood-red ferric thiocyanate is formed. Since silver thiocyanate is much less soluble than silver chloride, ammonium thiocyanate will attack the silver chloride precipitate and so cause erroneous results. For this reason the silver chloride must be removed or protected before titration of the excess silver nitrate is begun. This is best obtained by filtering off the precipitate before back-titration, but this step is time-consuming. As an alternative, some organic solvent may be added which will form a protective coating around the silver chloride and for this purpose nitrobenzene appears to be the most satisfactory. It should be remembered, however, that nitrobenzene is a powerful... 

Alum dosages between 200 and 500 mg/1 were not significantly different. Table II also Illustrates that sedimentation Is effective In lowering the concentration of the pesticides to the solubility limit. Metrlbuzln Is soluble In water to a concentration of about 1200 mg/1 and the sedimentation step reduced the concentration from 2000 and 3000 mg/1 to about 1000 mg/1. At lower concentrations sedimentation was not effective. Other coagulants and flocculant aids such as hydroxide and ferric chloride, were tested but alum with an anionic polymer (Watcon 1255) was the most effective. 

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. 

Potassium ferri-sulphite, KFe(S03)S04, is prepared by the action of potassium hydrogen sulphite upon ferric ammonium alum. It crystallises in slender needles, which are sparingly soluble in cold water. With hot water, ferrous and potassium sulphates pass into solution, leaving an insoluble yellow residue. 

Ammonium ferri-sulphite, NH4.Fe(S03)S04.H20, may be obtained by treating ferric ammonium alum with ammonium hydrogen sulphite. It crystallises in slender, yellow needles which are but slightly soluble in cold and hot water. Prolonged boiling decomposes the salt. 

The high solubility of the caesium salt is interesting in view of the low solubility of ferric caesium alum (see p. 164). 

Ammonium ferri-disulphate, NH4[Fe(S04)2], is prepared by heating a solution of ferric ammonium alum with sulphuric acid for some hours.3 It is a white crystalline powder, sparingly soluble in water. 

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). 

Coagulation and Flocculation The coagulants most frequently used for membrane plant source water conditioning are ferric salts (ferric sulfate and ferric chloride). Aluminum salts (such as alum or polyaluminum chloride) are rarely used because it is difficult to maintain aluminum concentrations at low levels in the filtered water and in the dissolved form because aluminum solubility is very pH dependent. 

---