Birnessite, [Na, K] [Mn4+ Mn3+] O4- ?H2O, is a mineral with a layered structure
ID: 855089 • Letter: B
Question
Birnessite, [Na, K] [Mn4+ Mn3+] O4- ?H2O, is a mineral with a layered structure of the same type as CdI2, comprizing octahedral MnO6 units, Na+ and K+ ions and H2O molecules are sited between the layers. The Na/K coposition is variable. Analysis for Na is carried out using AAS. A 20 mg sample of brinessite is dissolved in concentrated HCl/HNO3 and the solution made up to 200cm3 in a volumetric flask. Why is brinessite treated with the acid? Detail how you would proceed with the sodium analysis. How woulkd you determine the water content in the sample of brinessite?
Explanation / Answer
Titration Mohr method
The Mohr method uses chromate ions as an indicator in the titration of chloride ions with a silver nitrate standard solution. Following the precipitation of the whole amount of chloride (usually as white silver chloride) the first excess of titrant results in the production of a silver chromate precipitate that indicates the end point.
The reactions are summarised as below:
Before end point: Ag+ + Cl- --> AgCl
At end point: 2Ag+ + CrO42- --> Ag2CrO4
The salt is initially required to be extracted from the food sample by means of accurate ashing at 500
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