Chemical Analysis & EvaluationFlame Atomic Spectroscopy (Experiment 4)Dr Jorge Chacon

AimThe aim of this experiment is to demonstrate the application of atomic spectroscopy. In addition to this, to show that atomic emission spectroscopy can be used for analytical determination for certain elements. Furthermore, to illustrate and identify the problems of a sample matrix and effective solution to the problems identified.

IntroductionAtomic spectroscopy is a widely used and sensitive technique to determine metallic elements that may exist in a particular sample. Atomic spectroscopy is originated upon measuring the absorption of electromagnetic radiation by atoms in their elementary state (e.g. K+). In this particular experiment, the analytical technique used is flame atomic spectroscopy. This consists of a lamp made of sodium (cathode) with a supply of 8 milliamps. In addition to this, a burner supplying acetylene and oxygen (air) burns at 2300oC, above the atomisation of the element (sodium). It is vital the temperature is above the atomisation of the respective element, otherwise the element will not atomise. Furthermore, a beam of light is fired across, which is in line with the burner. The sample gets drawn up and sprayed into the burner. Any water that exists boils away, and the resultant element, in this case sodium is left. Finally, the light then hits the sodium and the absorption is determined by the detector. Equipment Various sizes of volumetric flasks and pipettes Flame atomic spectrophotometer

MethodPart 1 250cm3 of 10ppm sodium solution is prepared from a 1000ppm solution of sodium. The resulting solution is diluted to 250 cm3 in a volumetric flask. Using the 10ppm sodium solution made above, 100 cm3 of the following solutions are made: (in ppm) 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.6, 2.0 and 3.0. These solutions made are analysed by an atomic absorption spectrophotometer. To set the instrument up, in order to zero the instrument, it is to be sprayed with demineralised water. Once the instrument is set up, the absorbance of tap water is determined. Then the absorbance of each solution made up in the second bullet point is analysed 3 times whilst spraying demineralised water between each solution. The absorbance of tap water is determined a further two times. The appropriate data i.e. the absorbance readings, is written on the lab proforma.Part 2 The instrument used is adjusted so that it is able to read the solutions in the atomic emission mode. The ppm standards made up in in part 1 are to be used again for part 2. Similarly from part 1, the emissions of tap water is determined. Then the emissions of each solution made up in part 1 are to be analysed 3 times whilst spraying demineralised water between each solution. The appropriate data i.e. the absorbance readings, is written on the lab proforma.Part 3 100 cm3 of 50ppm iron solution is made by dilution from 500ppm iron solution. Two solutions of iron of 2 and 4ppm are prepared by dilution with deionised water of the 50ppm iron stock solution into a 25 cm3 volumetric flask. Three 25 cm3 volumetric flasks, labelled A, B and C, are to contain 10 cm3 of the given irn bru solution. Into flask A, 1 cm3 of the 50ppm of the iron stock solution is added. Into flask B, 2 cm3 of the 50ppm of the iron stock solution is added. Nothing is added in flask C. All three flasks are made up to the 25cm3 mark with deionised water. The instrument is set up to read the iron absorption line. The instrument is zeroed using deionised water, as previously. The absorbance for all five standards are analysed. Flask A, B and C are repeated 3 times. The appropriate data i.e. the absorbance readings, is written on the lab proforma.

CalculationsFor part 1, when preparing the 250cm3 of 10ppm sodium solution from a 1000ppm solution of sodium, the equation M1V1=M2V2 was used.V2 = = 2.5cm3; of 1000ppm sodium is required to make a 10 ppm sodium solution up to 250cm3. (The rest of which is made up with deionised water)The same approach is used when preparing 100cm3 solutions of 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.6, 2.0 and 3.0 (in ppm). As previously, the equation M1V1=M2V2 is used. For part 3, a similar method as described above is used to prepare 100 cm3 of 50ppm iron solution from 500ppm iron solution. Again, the equation M1V1=M2V2 is used.V2 = = 10cm3; of 500ppm iron solution is required to make a 50ppm iron solution up to 100cm3. (The rest of which is made up with deionised water)For the atomic absorbance unknown. To determine the concentration, the absorbance is analysed from the spectrophotometer. The reading is the absorbance. Once a plot of the absorbance against concentration (ppm) is obtained, the equation of a line y = mx + c can be used. This gives a value for m and c, the absorbance reading is y. Rearranging this equation to x gives; where x = concentration. The dilution factor is 10 so the whole equation is to be multiplied by 10. From the plot, y = 0.163x + 0.015. For reading 1; x = x 10 = 4.049 ppmThe same equation is used for the rest of the 4 readings. In addition to this, the same method is also used for the atomic emission unknown. Where the equation of the line is; y = 0.558x + 0.004. To calculate the iron content in irn bru, a plot of absorbance against concentration is to be obtained. The equation of the line is; y = 0.006x + 0.008. The Iron content in Irn Bru can be determined in two ways:

By extrapolating the slope of the graph back to the x axis.

Or alternatively, the equation; y = mx + c, can be used. Where x is the iron content in irn bru, and y= 0.

Y = 0.006x + 0.008Y = 0, so x = = = 1.33 ppm. However, the dilution factor is 2.5. So, the iron content in irn bru = 1.33 x 2.5 `= 3.325 ppm.

Discussion When determining the absorbance of the various range of solutions, the method was repeated three times and an average of the three results is taken. The reason for this is to make sure the absorbance reading from the instrument is accurate and precise.

The difference between accuracy and precision is; Accuracy means getting a result that is close to the real result. Precision means getting the same result every time you try.

For the limits of calibration, the main cause of the loss of linearity at a high concentration is because, for sodium. The atom is hugely excited, so much that it causes it to self-absorb, lowering the content of Na+ atoms.

The lower limits of the techniques used are determined by the sensitivity of the instruments.

Out of the two spectroscopic techniques used (Atomic Absorption and Atomic Emission) In terms of precision; Atomic Emission is more precise and is better suited for an easily exited atom such as sodium. ConclusionTo conclude this experiment, the metallic content of a range of samples were analytically determined successfully. This illustrates the application of atomic spectroscopy and why one may use such a technique. In this particular experiment, the iron content in irn bru was determined which may be of significance to a health and safety consultant. In addition, to an environmentalist or farmer, this technique can be used to detect the richness of soil by the detection of metallic minerals. Furthermore, in the same way, a chemical engineer may want to detect the metallic content of a solution that is fed into a reactor. Finally, with support from the calculations, this experiment is successful and fulfilling to the original aim.

Please turn overFigures & GraphsFLAME ATOMIC ABSORPTION

PART 1

SODIUM IN POTABLE WATER

SODIUM CONCENTRATION (PPM)ABSORBANCE

00

0.20.053

0.40.091

0.60.121

0.80.15

10.174

1.20.205

1.60.276

20.337

30.508

PART 2

SODIUM - FLAME EMISSION

SODIUM CONCENTRATION (PPM)EMISSION

00

0.20.107

0.40.216

0.60.325

0.80.438

10.562

PART 3

IRON IN IRN BRU

IRON CONCENTRATION (PPM)ABSORBANCE

20.026

40.027

ATOMIC ABSORPTION UNKNOWNABSORBANCE

10.081

z20.081

30.082

40.082

50.08

MEAN ABSORBANCE0.0812

STD. DEV0.00083666

ATOMIC EMISSION UNKNOWNABSORBANCE

10.316

20.314

30.311

40.32

50.318

MEAN ABSORBANCE0.3158

STD. DEV0.00349285

IRN BRUABSORBANCE

00.008

20.022

40.035