What is the formula of alum
Alums. As Alums denotes a series of double salts of the general formula:
R. can include potassium, sodium, ammonium, cesium, rubidium, etc. or be an organic base, R.1 besides aluminum also iron and chromium. The actual alumwho gave the name to this group of salts is the Potassium alum, a potassium aluminum sulfate from the formulaKAl(SO4)2 + 12H2O or K2SO4 ∙ Al2(SO4)3 + 24H2O.
It comes as a weathering product of marl slate containing sulphurous gravel (Alum slate) of course. This mineral, as well as the Alum stone or Alunit, a basic aluminum sulphate with varying amounts of potassium sulphate, is used as a raw material for alum production. After roasting in a pile, if necessary, the minerals are removed with water. The solution, which predominantly contains aluminum sulfate, is purified by evaporation and crystallization of any iron salts which are also present. The alum, which is relatively sparingly soluble in water, is then precipitated from this lye as a white crystalline powder by adding potassium sulfate or potassium chloride (Alum flour) out. The washed flour is dissolved in hot water until the solution has a spec. Shows weight from 1.5-50 ° Bé. Except for a small amount of mother liquor, such a solution solidifies in separable crystallization vessels (Alum merchandise, wax alum). Not enough pure salt has to undergo another crystallization (refined alumn). The commodity forms colorless crystals combined to form thick crusts or, as alum flour, a white crystal powder. The alum is soluble in water (cold 1: 8, hot 1: 3). Application: in dyeing, paint and paper production, white tannery, as a clarifying agent for  some liquids, for hardening plaster, as a flame-proof coating for wood, as a refining agent when melting sebum, in medicine. The active ingredient of potash alum for the various types of use is aluminum sulphate. That is why the potassium sulphate of common alum has been replaced by ammonium sulphate where the price conditions dictated it Ammonium alum or Ammonia alum obtained according to the formula (NH4)So4)2 + 12H2O. However, both products have lost much of their importance since a cheap and pure raw material was replaced by a pure one Aluminum sulfate disposes. Since this salt, or rather the aluminum oxide it contains, is, as already mentioned, the active component of alums, it is understandable that that of the formulaAl2(SO4)3 + 18H2O corresponding, under the name concentrated alum commercial sulfate offers significant packaging, freight and consumption advantages. One simply dissolves commercially available alumina hydrate (s. Alumina hydrate) in warm sulfuric acid diluted with a little water. The termination of the reaction: 2Al(OH)3 + 3H2SO4 = Al2(SO4)3 + 6H2O can be seen by examining a sample taken from the mixture with bluewood extract. The ratios between alumina hydrate, sulfuric acid and water are calculated on the basis of the composition of the raw materials tested beforehand in such a way that after the solution has been made, every molecule is usedAl2(SO4)3 as precisely as possible 18H2O available. A solution obtained in this way, after it has been drained from the mixing vessel (wooden tubs lined with lead) into flat lead boxes, very soon solidifies to a white crystalline mass which is marketed in the form of plates or coarse chunks. The concentrated alum is easily soluble in water and has the same uses as potash alum. - In the dyeing and garment printing works, the so-called “common” alum is sometimes used in cases in which the acidic reacting common alum would attack the colors neutral alumK2SO4 ∙ Al2(SO4)3 ∙ 2Al2(OH)3. The same can be obtained if small amounts of potassium or sodium carbonate are added to an alum solution for as long as the resulting precipitate dissolves again on shaking. When this alum evaporates voluntarily, cube-shaped crystals separate out (more cubic or Cube alum). From the solution of the neutral, more correct basicAlum falls at 40 ° C. an even more basic compound, the so-called insoluble alumK2SO4 ∙ Al2(SO4)3 ∙ 4Al(OH)3. The chemical nature of alum was not discovered until 1797 by Chaptal and Vauquelin recognized. At the beginning of the 19th century, the very pure Liège or Lücker alum completely dominated the market. The German alum industry was from Stay faithful who set up an alum factory on the Haardt in 1806. The procurement of pure and, above all, cheap alum from the Rhenish industry (Elberfeld, Barmen) gave rise to significant advantages over French and Belgian manufacturers. S.a. Chrome alum and Iron ammonium alum.
Literature: Seger, About the technical use of pebbles of slate and clays from the lignite formation, Neuwied 1869.
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