Views: 64

FLAME PHOTOMETRY

1. Explain why alkali metals are analyzed by flame photometry

Flame photometry is usually  useful for analysis  of elements , which are easily, excited  aseptically the alkali and alkaline earth metals  ie Na, K, Ca, Ba, Li.. This  elements  are easily excited to higher energy levels at flame temperature.

2. Explain the origin of color in flames of alkali metals

 The origin of color in flame of alkali metals are : When an element eg Na or K or Ca,is  in its atomic state  and is placed in a flame , its atoms increases in energy ( i.e. they become excited ) and hence move  to the next higher energy level .These acquired energy makes these atoms to become less stable and they would then attempt to fall back or come back to their original energy level by losing  or emitting these same amount of energy  , These amount of energy is emitted in form of  light . The light emitted is  specific  in wavelength for each  element  

2. Outline the principle on which flame photometry is based

The basis of flame photometric working is that, the species of alkali metals (Group 1) and alkaline earth metals (Group II) metals are dissociated due to the thermal energy provided by the flame source.

Due to this thermal excitation, some of the atoms are excited to a higher energy level where they are not stable. When sample interacts with light, absorption process occurs.

Ground state electrons of the sample atom tend to move to the excited states with the energy of absorbed light. This process can also be called excitation. Apart from light, heat can cause excitation. Since excited state is unstable, electrons want to return back to the ground state. When an excited electron returns back to its ground state, a radiation is emitted that is equal to the energy difference between excited and ground states.

The absorbance of light due to the electrons excitation can be measured by using the direct absorption techniques. The subsequent loss of energy will result in the movement of excited atoms to the low energy ground state with emission of some radiations, which can be visualized in the visible region of the spectrum.

The absorbance of light due to the electrons excitation can be measured by using the direct absorption techniques while the emitting radiation intensity is measured using the emission techniques. The wavelength of emitted light is specific for specific elements.

When an element eg Na or K or Ca,is  in its atomic state  and is placed in a flame , its atoms increases in energy ( i.e. they become excited ) and hence move  to the next higher energy level .these acquired energy makes these atoms to become less stable and they would then attempt to fall back or come back to their original energy level by losing  or emitting these same amount of energy  ,these amount of energy is emitted in form of  light . The light emitted is  specific  in wavelength for each  element  e.g.

                            Na+ emits 589nm

                         k+ emits 404 and 767nm

The intensity of light emitted is proportional  to the concentration of atoms present in the solution. The emitted wavelength is normally a measure from which  the elements can be determined

3. Name two types of  burners used in flame photometry
4. State two disadvantages of flame photometry

5. State and explain components of a flame photometer
6. Source of flame: 

A burner that provides flame and can be maintained in a constant form and at a constant temperature.

2. Nebuliser and mixing chamber: 

Helps to transport the homogeneous solution of the substance into the flame at a steady rate.

3. Optical system (optical filter):

The optical system comprises three parts: convex mirror, lens and filter. The convex mirror helps to transmit light emitted from the atoms and focus the emissions to the lens. The convex lens help to focus the light on a point called slit. The reflections from the mirror pass through the slit and reach the filters. This will isolate the wavelength to be measured from that of any other extraneous emissions. Hence it acts as interference type color filters.

4. Photo detector:

Detect the emitted light and measure the intensity of radiation emitted by the flame. That is, the emitted radiation is converted to an electrical signal with the help of photo detector. The produced electrical signals are directly proportional to the intensity of light.

6. List the steps followed when analyzing a sample for a  given component using a flame photometer

• Both the standard stock solution and sample solution are prepared in fresh distilled water.
• The flame of the photometer is calibrated by adjusting the air and gas. Then the flame is allowed to stabilize for about 5 min.
• Now the instrument is switched on and the lids of the filter chamber are opened to insert appropriate colour filters.
• The readings of the galvanometer are adjusted to zero by spraying distilled water into the flame.
• The sensitivity is adjusted by spraying the most concentrated standard working solution into the flame. Now the full scale deflection of the galvanometer is recorded.
• Again distilled water is sprayed into the flame to attain constant readings of galvanometer. Then the galvanometer is readjusted to zero.
• Now each of the standard working solutions is sprayed into the flame for three times and the readings of galvanometer are recorded. After each spray, the apparatus must be thoroughly washed.
• Finally sample solution is sprayed into the flame for three times and the readings of galvanometer are recorded. After each spray, the apparatus must be thoroughly washed.
• Calculate the mean of the galvanometer reading.
• Plot the graph of concentration against the galvanometer reading to find out the concentration of the element in the sample.
• Flame Photometer graph, flame photometer readings, galvanometer readings, sample concentration
  7. Outline the process  involved in flame photometry

In this technique, first aerosols are formed from sample solution by a jet of compressed gas. This process is called nebulization. Then the flow of the gas carries the aerosols into a flame where atomization takes place. A tomization is the conversion of sample aerosols into an atomic vapor by flame. When a sample is atomized, a substantial fraction of the metallic constituents are reduced to gaseous atoms and also depending on the temperature of the flame a certain fraction of these atoms are ionized. Then, electrons of the formed atoms are excited to upper state. Light is emitted at characteristic wavelengths for each metal as the electron returns to the ground state.

5. Summerize flame photometry  process
6.  The solvent is first evaporated leaving fine divided solid particles.
7. This solid particles move towards the flame, where the gaseous atoms and ions are produced.

3  The ions absorb the energy from the flame and excited to high energy levels.

4. When the atoms return to the ground state radiation of the characteristic element is emitted.
5.  The intensity of emitted light is related to the concentration of the element.

6.Describe the process taking place in the flame during flame photometry

The various processes in the flame are discussed below:

1. Desolvation: The metal particles in the flame are dehydrated by the flame and hence the solvent is evaporated.
2. Vapourisation: The metal particles in the sample are dehydrated. This also led to the evaporation of the solvent.
3. Atomization: Reduction of metal ions in the solvent to metal atoms by the flame heat.
4. Excitation: The electrostatic force of attraction between the electrons and nucleus of the atom helps them to absorb a particular amount of energy. The atoms then jump to the exited energy state.
5. Emission process: Since the higher energy state is unstable the atoms jump back to the stable low energy state with the emission of energy in the form of radiation of characteristic wavelength, which is measured by the photo detector.

7.State any three fuels and  gases  used in flame photometry

Fuels

1. Natural Gas
2. Hydrogen
3. Acetylene

Gases : Air, Oxygen or nitrous oxide (N2O) as oxidant

8.Explain self absorption as used in flame photography

Self absorption occurs when the radiation  emitted from one  atom is just  the right energy  to be absorbed  by another atom . The effect is that  several atoms  may be involved  but the detector  may see only one  or few atoms in the final process. These results in a lower reading than that actually  present

9. State and explain the advantages and disadvantages of flame photometry

Advantages

• Simple quantitative analytical test based on the flame analysis.
• The determination of elements such as alkali and alkaline earth metals is performed easily with most reliable and convenient methods.
• Quite quick, convenient, and selective and sensitive to even parts per million (ppm) to parts per billion (ppb) range.

Disadvantages:

Moreover the flame photometer has a wide range of applications in the analytical chemistry, it possess many disadvantages which are explained below:

• The concentration of the metal ion in the solution cannot be measured accurately..
• A standard solution with known molarities is required for determining the concentration of the ions which will corresponds to the emission spectra.
• It is difficult to obtain the accurate results of ions with higher concentration.
• The information about the molecular structure of the compound present in the sample solution cannot be determined.
• The elements such as carbon, hydrogen and halides cannot be detected due to its non radiating nature.

10.State and explain the application of flame photometry

Flame photometry can be applied in the  following various fields

• In analysis  that are difficult or impossible to perform  using other techniques  or where speed is important than accuracy
• For analysis of alkali metals
• In biomedical research
• In clinical chemistry– Na+ and K+ ions in body fluids, muscles and heart can be determined by diluting the blood serum and aspiration into the flame.
• In sample chemistry–  in the Analysis of soft drinks, fruit juices and alcoholic beverages can also be analyzed by using flame photometry
• In water and sewage treatment
• In agriculture – The fertilizer requirement of the soil is analyzed by flame test analysis of the soil. it also help to determine the availability of alkali and alkaline earth metals which are critical for soil cultivation