Why is a sodium carbonate extract necessary

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Soda extract

The soda extract is used to bring the anions of poorly soluble salts into solution and to separate off troublesome cations.

execution

In an Erlenmeyer flask, the solid analysis substance (approx. 0.5) is mixed with three to five times the amount of anhydrous soda (Na2CO3) and distilled water (approx. 30) are added and approx. 10 are heated to the boil.

warning
Attention! - Alkaline solutions tend to delay boiling, so it is advisable to place a boiling stick in the Erlenmeyer flask.

If the solution is colored by chromate,, or permanganate,, yellow or purple, these anions must be reduced by adding 1 ethanol. The hot soda extract is centrifuged. Almost all anion detection reactions are carried out with the clear, colorless solution. The substance to be analyzed must be tested separately for carbonate, borate, sulfide and fluoride.

Explanation

According to the principle of the reciprocal salt pair, the metal cations are converted into their carbonates in a soda solution in the heat, because all heavy metal carbonates except thallium carbonate are sparingly soluble. The solution then contains the anions in the form of their sodium salts. Some example reactions for the soda extract of poorly soluble compounds are:

Very poorly soluble compounds such as BaSO4 often dissolve very slowly. Anions such as chromate, permanganate and arsenate can also be found in the solution of the soda extract. Since these anions can interfere with other detection reactions due to their color or their chemical properties, chromate and permanganate are converted to Cr by adding ethanol3+ or Mn2+ reduced. Arsenate can interfere with the detection of phosphate.