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MINERALS. Ashing A. Dry Ashing 1. 550oC until light-gray ash or to constant wt. 2. Cool in dessicator 3. Weigh soon after reaching room temperature. Advantages: 1. Safe 2. Multiple samples 3. Only oven and dishes needed Disadvantages: Contamination. Wet Ashing

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MINERALS

Ashing

A. Dry Ashing

1. 550oC until light-gray ash or to constant wt.

2. Cool in dessicator

3. Weigh soon after reaching room temperature


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Advantages:

1. Safe

2. Multiple samples

3. Only oven and dishes needed

Disadvantages:

Contamination


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  • Wet Ashing

  • Oxidation of organic compounds

  • Time: Completely oxidized

  • Advantages: Complete digestion

  • Disadvantages:

  • 1. Explosion or corrosion

  • 2. Requires time


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Elemental Analysis

Step 1: Ashing

Step 2: Solublize ash in Conc. HCl - boil and evaporate solution to dryness.

Step 3: Re-dissolve residue in 0.5 N HCl.

Step 4: Conc. or dilute as desired

Step 5: Determination of individual components.


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METHODS FOR DETERMINING MINERAL CONSTITUENTS

1. Spectrometric Methods

2. Emission Spectroscopy - Flame Photometry Method

3. Atomic Absorption Spectrometry


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1. Spectrophotometric Method

Formation of colored complex with some ligand.

Example: Fe++ (Ferrous Ion) with phenanthroline (orthophenanthrolines).


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A pair of unshared electrons can coordinate certain metallic ions to give complexes.

In the case of ferrous ion, the orthophenanthroline complex is quite stable and is intensely red in color.

The complex is sometimes called Ferroin.


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2. Flame Emission Spectroscopy ions to give complexes.

Measure the intensity of emitted radiation


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Flame Emission Spectroscopy ions to give complexes.

hu -hu

M+ -> M0 -> M* -> M0


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3. Atomic Absorption Spectrometry ions to give complexes.

Hollow

Graphite

Monochromator

Cathode

Furnace

Lamp

Detector

Slit

Slit

Atomic Absorption spectroscopy involves the study of the absorption of radiant energy by neutral atoms in the gaseous state.


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1) The elements must be reduced to the elemental, neutral ground state by the flame.

2) The elements must be in vaporized state.

3) The elements must be imposed in the beam of radiation from the source.

Advantages: Very sensitive.

Fast.

Disadvantages: Hollow cathode lamp for each element.

Expensive element.


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Relationship Between Atomic Absorption and Flame Emission Spectroscopy

  • Flame Emission -> it measures the radiation emitted by the excited atoms that is related to concentration.

  •  Atomic Absorption -> it measures the radiation absorbed by the unexcited atoms that are determined.

  • Atomic absorption depends only upon the number of unexcited atoms, the absorption intensity is not directly affected by the temperature of the flame.

  • The flame emission intensity in contrast, being dependent upon the number of excited atoms, is greatly influenced by temperature variations.


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Flame Emission Spectroscopy Spectroscopy

Flame Emission Spectroscopy is based upon those particles that are electronically excited in the medium.

The Functions of Flame

1. To convert the constituents of liquid sample into the vapor state.

2. To decompose the constituents into atoms or simple molecules:

M+ + e- (from flame) -> M + hn

3. To electronically excite a fraction of the resulting atomic or molecular species

M -> M*


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