Alkaline fall

The alkaline fall is a precipitation reaction that is used as part of the cation separation process in the ammonium sulfide group. It is used to precipitate manganese and iron salts.

Reactions

A strongly basic hydrogen peroxide solution is added to the centrifugate of the ammonium sulphide precipitation, in which nickel and cobalt salts are precipitated . Sodium or potassium hydroxide can be used as bases . Due to the presence of hydrogen peroxide, the solution has a strong oxidizing effect .

Manganese ions in solution are precipitated as the hydrate of manganese dioxide (MnO (OH) ₂ ):

${\displaystyle \mathrm {Mn^{2+}\ +\ H_{2}O_{2}\ +\ 2\ OH^{-}\longrightarrow \ MnO(OH)_{2}\downarrow \ +\ H_{2}O} }$

Iron ions are precipitated in a basic environment as iron (III) hydroxide (Fe (OH) ₃ ):

${\displaystyle \mathrm {Fe^{3+}\ +\ 3\ OH^{-}\longrightarrow \ Fe(OH)_{3}\downarrow } }$

As a result of the oxidizing conditions, Fe (II) in solution is oxidized to iron (III). The precipitated connections can then be examined individually.

Chromium , zinc , aluminum , tungsten , vanadium and uranium compounds remain in the solution and can be individually detected in the further separation process. Instead of caustic soda / hydrogen peroxide, sodium peroxide can also be used.

In the context of urotropine precipitation, the alkaline fall can also be used to precipitate titanium and iron salts . Titanium is oxidized to Ti (IV) in the oxidizing environment, which precipitates as titanium dioxide .

${\displaystyle \mathrm {2\ Ti^{3+}\ +\ H_{2}O_{2}\ +\ 6\ OH^{-}\longrightarrow \ 2\ TiO_{2}\downarrow \ +\ 4\ H_{2}O} }$

source

• Gerhart Jander, Ewald Blasius, Joachim Straehle, Eberhard Schweda, Rolando Rossi: Textbook of analytical and preparative inorganic chemistry. 16th edition, S. Hirzel Verlag, Stuttgart 2006, ISBN 3-7776-1388-6 , pp. 557-558.

Web links

Wikibooks: Practical Inorganic Chemistry / Ammonium Sulphide Group # Alkaline Fall  - Learning and Teaching Materials