In Calibrating A 10

 

  1.  In calibrating a 10-mL pipet, a measured volume of water was transferred to a tared flask and weighed, yielding a mass of 9.9814 g. (a) Calculate, with and without correcting for buoyancy, the volume of water delivered by the pipet. Assume that the density of water is 0.99707 g/cm3 and that the density of the weights is 8.40 g/cm3. (b) What are the absolute and relative errors introduced by failing to account for the effect of buoyancy? Is this a significant source of 2. 

3. 

4. 

5. 

determinate error for the calibration of a pipet? Explain. A 10.00-g sample containing an analyte was transferred to a 250-mL volumetric flask and diluted to volume. When a 10.00-mL aliquot of the resulting solution was diluted to 25.00 mL iitt was found to give a signal of 0.235 (arbitrary units). A second 10.00-mL aliquot was spiked with 10.00 mL of a 1.00- ppm standard solution of the analyte and diluted to 25.00 mL. The signal for the spiked sample was found to be 0.502. Calculate the weight percent of analyte in the original sample. A standard sample was prepared containing 10.0 ppm of o f an analyte and 15.0 ppm of o f an internal standard. Analysis of the sample gave signals for the analyte and internal standard of 0.155 and 0.233 (arbitrary units), respectively. Sufficient internal standard was added to a sample to make it 15.0 ppm in the internal i nternal standard. Analysis of the sample yielded signals for the analyte and internal standard of 0.274 and 0.198, respectively. Report the concentration of analyte in i n the sample. The following standardization data were provided for a series of external standards of Cd2+ that had been buffered to a pH of o f 4.6.14 2+ [Cd ] (nM) 15.4 30.4 44.9 59.0 72.7 86.0 Smeas (nA) 4.8 11.4 11.4 18.2 18.2 26.6 26.6 32.3 32.3 37.7 (a) Determine the standardization relationship by a linear regression analysis, and report the confidence intervals for the slope and y -intercept. -intercept. (b) Construct a plot of the residuals, and comment on their significance. At a pH of 3.7 the following data were recorded [Cd2+] (nM) 15.4 30.4 44.9 59.0 72.7 86.0 Smeas (nA) 15.0 42.7 58.5 77.0 101 118 (c) How much more or less sensitive is this method at the lower pH? (d) A single sample is buffered to a pH of 3.7 and analyzed for cadmium, yielding a signal of 66.3. Report the concentration of Cd2+ in the sample and its 95% confidence interval. To determine the concentration of analyte in a sample, a standard additions was performed. A 5.00-mL portion of the sample was analyzed and then successive succ essive 0.10-mL spikes of a 600.0-ppb standard of the analyte were added, analyzing after each spike. The following results were obtained Volume of Spike(mL) Signal (arbitrary units) 0.00 0.119 0.10 0.231 0.20 0.339 0.30 0.442 Construct an appropriate standard additions calibration c alibration curve, and use a linear regression analysis to determine the concentration of analyte in the original sample and its 95% confidence interval.

 

 

x

Σ= Σ=  

y

xi²

xiyi

(yi-ŷi)²   (yi-ŷi)²

15.4

4.8

237.16

30.4

11.4

924.16

346.56 11.81424 0.171595

44.9

18.2

2016.01

817.18 18.73219 0.283226

59

26.6

3481

72.7

32.3

86 308.4

73.92

ŷi  ŷi 

1569.4

4.65774 0.020238

25.4593 1.301196

5285.29 2348.21 31.99557 0.092678

37.7 7396 3242.2 131 19339.62 8397.47

38.341 0.410881 2.279814

 

pH 4,6 140 118 120

101

y = 0.4771 [Cd2+] - 2.6 2.6896 8960 0

100

77

    s 80     a     e     m 60      S

58.5 42.7

40 15

20 0 0

10

20

30

40

50

60

70

80

90

100

konsentrasi

 

x 15.4 30.4 44.9 59 72.7 86 308.4

Σ= Σ=  

y

xi²

xiyi

ŷi

(yi-ŷi)² (yi-ŷ

15

237.16

231

17.06788

4.276128

42.7

924.16

1298.08

38.58088

16.96715

58.5

2016.01

2626.65

59.37678

0.768743

77

3481

4543

79.599

6.754801

101

5285.29

7342.7

99.24754

3.071116

118

7396

10148

118.3224

0.103942

412.2

19339.62

26189.43

31.94188

pH 3,7 140 120

y = 1.4342x 1.4342x - 5.018 5.0188 8

100

    s     a 80     e     m 60      S

40 20 0 0

20

40

60

konsentrasi

80

100

 

 

(Vs)

ŷi ŷi  

(yi-ŷi)²   (yi-ŷi)²

(Vf)

(Signal)y

5

0.119

0.1191

1E-08

0

0.1

5.1

0.231

0.230865

1.83045E-08

11.7647059

0.2

5.2

0.339

0.338331

4.4787E-07 4. 4787E-07

23.0769231

0.3

5.3

0.442

0.441742

6.68174E-08

33.9622642

1.130037

5.42992E-07

68.8038931

0

Σ= Σ=  

0.6

1.131

0.5

Cs(Vs/Vf)

0.442

0.45

y = 0.0095x + 0.1191

0.4

0.339

0.35 0.3

     l     a     n 0.25     g      i      S

0.231

0.2

0.150.119 0.1 0.05 0 0

5

10

15

20

Cs(Vs/Vf 

25

30

35

40