Chem 267 ASV Lab report PDF

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Lab report for anodic stripping voltammetry...

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Experiment 4. Determination of Lead and Cadmium by Anodic Stripping Voltammetry Nov 12th, 2018 In fulfillment of CHEM 267 – Introduction to Analytical Chemistry Results. Table 1. Conversion of volume of 20ppm Cd or Pb added to cell Cd or Pb concentration in parts per billion. For quantitative analysis, 10mL of 5.5pH 0.2M sodium acetate buffer and 40µL of Hg plating solution were added initially, explaining the initial total volume of 10.04mL. Each time the solution was spiked, both metals were added so total volume increases by 100µL, not 50µL. Volume of metal Mass of pure Total volume (mL) Added cell metal added (µL) metal (µg) concentration (ppb) 0 0 10.04 0 50 1 10.14 98.61932939 100 2 10.24 195.3125 150

3

10.34

290.1353965

Table 2. Cd concentration in cell (ppb) and peak current height (µA). Values for Cd concentrations are from Table 1. Added Cd Peak current height (µA) concentration Run 1 Run 2 Mean Standard deviation (ppb) 0 10.08 10.32 10.2 0.169705627 98.61932939 17.23 16.38 16.805 0.601040764 195.3125 19.77 24.01 21.89 2.998132752 290.1353965 26.47 28.92 27.695 1.732411614

Table 3. Pb concentration in cell (ppb) and peak current height (µA). Values for Pb concentrations are from Table 1. Added Pb Peak current height (µA) Run 1 Run 2 Mean Standard deviation concentration (ppb) 0 8.136 8.014 8.075 0.086267027 98.61932939 11.71 10.93 11.32 0.551543289 195.3125 12.93 15.6 14.265 1.887975106 290.1353965 16.8 18.51 17.655 1.209152596

Figure 1. Standard additions calibration curve for Cadmium concentration and peak height current. The errors bar shown are one standard deviation. Table 3. Regression statistics for cadmium calibration curve. Regression Statistics Predicted Y Multiple R 0.998998756 10.4536142 R Square 0.997998515 16.32543956 Adjusted R Square 0.996997772 22.08258083 Standard Error 0.40773705 27.72836541 Observations 4 Slope 0.05954

Residuals -0.253614201 0.479560439 -0.192580832 -0.033365405

By the method of standard additions, the absolute value of the x-intercept is the concentration of cadmium before any standard additions. Through interpolating the calibration curve, the concentration of cadmium in Unknown “G” is 175.5 ppb.

Figure 2. Standard additions calibration curve for lead concentration and peak height current. The error bars shown are one standard deviation. Table 4. Regression statistics for lead calibration curve. Regression Statistics Predicted Y Multiple R 0.999544438 8.045604667 R Square 0.999089083 11.27612721 Adjusted R Square 0.998633624 14.44355361 Standard Error 0.151253651 17.54971451 Observations 4 Slope 0.03276

Residuals 0.029395333 0.043872789 -0.178553611 0.105285489

By the method of standard additions, the absolute value of the x-intercept is the concentration of lead before any standard additions. Through interpolating the calibration curve, the concentration of lead in Unknown “G” is 245.6 ppb.

LOD: According to Stone & Ellis (2011), the limit of detection can be determined in two ways: y LOD = y blank +3 sblank , C LOD =C sample × y LOD ÷ y sample 1. ÷m, 2. C LOD =3 s xy where mis the slope of thelinear regression∧s y is the standard error of regression x

The former is not often used because “at least 20 readings of the blank” (Stone & Ellis, 2011) are necessary which is tedious. For LOD of Cadmium, using values from Table 3, C LOD =3 ×0.40773705 ÷ 0.05954=20.54 ppb For LOD of Lead, using values from Table 4,

C LOD =3 ×0.15125365 ÷ 0.03276=13.85 ppb

For Standard deviation of the blank, Table 5. Readings of the blank. Run 1 (µA) 6.073 4.963

Pb Cd

Run 2 (µA)

Mean (µA)

6.384 5.057

6.2285 5.01

Standard Deviation (µA) 0.219910209 0.066468037

Note that, using method 1 for LOD determination, with the standard deviation of the blank, the LOD for Cd and Pb are 3.349 ppb and 20.13 ppb respectively

Discussion.

1. Describe why the method of external standards would be difficult to use for the quantitation of a metal ion using ASV. Using external standards would be difficult because the concentrations are so tiny that external standards would introduce uncertainty comparable or greater in magnitude than the actual concentration of the unknown. Even when disregarding uncertainty, the nature of the method of ASV favours internal standards because one can simply add discrete amounts of Pb/Cd to the initial cell and be able to perform a chemical analysis, rather than use a completely different cell for each external standard and having to prepare far more samples and rinse more glassware. For example, the concentrations of Cd and Pb are on the order of magnitude of 100 ppb or 100µg/L. In the context of CHEM 267, standards are prepared in 100mL volumetric flasks (larger ones would waste too much water), meaning 10µg of pure metal would have to be dissolved. The analytical balances in OM 100 have an uncertainty of 0.1mg (100µg), which is greater than 10µg. Alternatively, if a solution of 20ppm Cd or Pb were diluted using a micropipette and a volumetric flask, to achieve a standard at 100 ppb / 0.1ppm, one would require a dilution factor of 200X and involve uncertainty of both the glassware and micropipette, whereas internal standards require no dilution and only involve uncertainty of the micropipette. Furthermore, ASV allows pre-concentration of Pb and Cd in the original matrix (the cell); thus, it makes a lot more sense to use internal standards and keep the matrix effects constant, rather than trying to mimic the matrix effects in every external standard. 2. Are the detection limits of your set-up different between Pb and Cd? Briefly explain what calibration parameter can be used to determine this. Yes, the detection limits for Pb and Cd are different (14 ppb and 21 ppb or 3 ppb and 20ppb). By method 1 (Stone & Ellis, 2011) detection limits are determined by using the standard deviation of the blank. Therefore, the calibration parameter that can be used to determine detection limits is the time of pre-concentration, as that is the only independent variable which affects the blank signal. The blank signals are measurements of the pre-concentration of Pb, Cd on the Hg film. If the pre-concentration stirring time of 50s is controlled exactly for every ASV run, if matrix effects did not matter and everything were performed perfectly, the standard deviation would be zero and the detection limits would be the same. References. David C. Stone & Jon Ellis. (2011). Stats Tutorial - Instrumental Analysis and Calibration. Retrieved from https://www.chem.utoronto.ca/coursenotes/analsci/stats/LimDetect.html Appendix.

Appendix 1. Sample calculations for Table 1. Volume of metal added (µL) 100

Mass of pure metal added (µg) 2

Total volume (mL) 10.24

Volume of metal added=2 spikes × 50 µL per spike=100 µL

Added cell metal concentration (ppb) 195.3125 µg ; 1 ppm=1 mL

µg =2 µg mL Total volume=10 mLunknown+40 µL Hg+2× 100 µL metal added=10.24 mL mass of pure metal 1000 ppb × Added cell metal concentration∈ppb= 1 ppm total volume 1000 ppb 2 µg =195 ppb =0.195 ppm;0.195 ppm × 1 ppm 10.24 mL Mass of pure metal added=100 µL × 20 ppm=0.1 mL× 20

Appendix 2. Sample calculations for Table 2 or 3. Added Cd concentration (ppb) 98.61932939 Mean=

Peak current height (µA) Run 1 Run 2

Mean

Standard deviation

17.23

16.805

0.601040764

16.38

Run 1+ Run 2 17.23 + 16.38 =16.805 = 2 2

√

2

2

√

2

2

( Run 1−m ean) +( Run 2− mean ) ( 17.23−16.805) + (16.38−16.805 ) = =0.6010 Standard deviation= n−1 1 Appendix 3. Sample calculations for Cd/Pb concentration interpolation. −10.45 =−175.5 ppb From Figure 1, y=0.05954 x +10.45 ; y=0 ; x= 0.05954

[ Cd ] =175.5 ppb...