AIM:
To use atomic absorption spectrometry and a standard addition calibration to determine the concentration of lead and zinc in a solid bronze sample.
INTRODUCTION:
Atomic Absorption Spectroscopy is generally performed with external standardisation using a calibration curve of known concentration standards and using the equation of the linear trendline to determine the concentration of the unknown by rearranging the linear equation. However, the efficienchy of this method is reduced and can be afflicted with errors by the presence of other components in the experimental sample. These matrix interferences are overcome using the standard-addition method by measuring and varying a single physical parameter of the mixed analyte, establishing a relationship between additioin of the unknown with the change in physical parameter that allows an in-situ method of calibration.
RESULTS:
0.2ppm x 100 = 20mg.L-1
2 =
20mg.L-1 * 100/3 = 666mg.L-1
500ml (ie ½ L);
33.3mg/500ml = 0.333g/500ml
Real mass used = 0.3292g
Part A: External Standardisation
Zinc 10ppm from solution
V2 = C1V1/C2 = (100*100)/100 = 10ml
0.5ppm from 10ppm = 5ml
1ppm = 10ml
1.5ppm = 15ml
2ppm = 20ml
Lead 2.5ppm = 25ml
Figure 1: Concentration of Zinc in ppm and Absorbance using the GBC 901 AAS instrument
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... calibrated using standard solutions prior to passing samples through. The results are interpreted using a calibration curve. This method is valued ... and 69.6% correspondingly. The expected concentration of zinc in the unknown was 0.00121 M. The concentration of zinc was determined to be ... were recorded in Table 4. Three standard solutions of 1.2, 2.4, and 3.6 ppm of zinc were run through the AAS ...
Concentration of Zinc (ppm) | Absorbance |
0 | -0.004 |
0.5 | 0.2873 |
1 | 0.5031 |
1.5 | 0.6427 |
2 | 0.7519 |
2.5 | 0.8247 |
Figure 2: Absorbance readings of unknown Zinc concentration and their mean, standard deviation, and standard error of their mean
Unknkown Absorbances (ppm) | Mean unknown absorbance (ppm) | Standard Deviation | Standard Error of Mean |
0.7992 | 0.801567 | 0.002122 | 0.001225 |
0.8033 | | | |
0.8022 | | | |
Figure 3: Absorbance with respect to concentration of zinc in ppm via external standardisation
Part B: Method of Standard Addition
Figure 4: Table of standard addition of unknown lead sample to 25ml of known lead standard, and absorbances
ml of known added | absorbance |
Blank | 0.0002 |
0 | 0.2171 |
5 | 0.4308 |
10 | 0.6075 |
15 | 0.7495 |
20 | 0.8436 |
Figure 5: Absorbance with respect to addition of unknown lead sample (ml) to 25ml of known lead standard
Figure 6: Table of [Pb](ppm) and absorbances recorded
[Pb](ppm) | Absorbance |
0 | 0.0004 |
0.5 | 0.2645 |
1 | 0.4871 |
1.5 | 0.6692 |
2 | 0.7887 |
2.5 | 0.884 |
Figure 7: Absorbance readings of unknown Lead concentration and their mean, standard deviation, and standard error of their mean
Unknown Absorbances (ppm) | Mean unknown absorbance (ppm) | Standard Deviation | Standard Error of Mean |
0.6703 | 0.674167 | 0.006018 | 0.003474 |
0.6711 | | | |
0.6811 | | | |
Figure 8: Absorbance with respect to concentration (ppm) of lead
DISCUSSION:
The AAS was used to create a Zn calibration curve, and then the unknown sample was measured three times for accuracy. The absorbances may be averaged to determine the mean absorbance.
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Excel may determine the standard deviation of the absorbances, which is used to calculate the standard error of the mean.
The mean absorbance may be substituted into the equation of the line in order to determine the concentration of the unknown.
Figure 9: Concentration readings and mean concentration of the unknown zinc sample
Unknown Concentration Readings (ppm) | Mean unknown concentration (ppm) |
2.267 | 2.281 |
2.291 | |
2.285 | |
The Pb sample may similarly be averaged to determine the mean absorbance.
(0.6703+0.6711+0.6811)/3 = 0.67416
Excel may determine the standard deviation of the absorbances, which is used to calculate the standard error of the mean and the mean absorbance may be substituted into the equation of the line in order to determine the concentration of the unknown (as per Zn example) – see figure 7.
Rearranging the equation of the line:
X = (y-c)/m = (0.674167 – 0.0747)/0.3527 = 1.69965
However, the analytical equipment yeilds the following:
Figure 10: Concentration readings and mean concentration of the unknown lead sample
Mechanically-determined concentrations | Mean Concentration |
15.673 | 15.804 |
15.7 | |
16.039 | |
Whereas the rearranging of the zinc linear equation yielded a calculate dvalue very close to the measured value, the measurement of the lead is plagued with the aforementioned issues due to the sample matrix, creating a very large error.
The standard addition method to compensate for the matrix issues yields a much more accurate result, whereas the external standardisation method is sufficient for the zinc-only analyte.
CONCLUSION:
The concentrations of zinc and lead in the unknown bronze allow were calculated manually to be 2.177ppm and 1.69965ppm respectively,and mechanically to be 2.281ppm and 15.804ppm respectively. Whilst external calibration was found to be accurate in the case of a single-ingredient analyte, the zinc/lead analyte yielded large inaccuracies if external calibration calculations are applied to it, and the standard addition method is required to compensate for the matrix factors.
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