Read an authoritative source for a discussion of spectrophotometry The two compo
ID: 530897 • Letter: R
Question
Read an authoritative source for a discussion of spectrophotometry The two components of the solution you will be making in this experiment must be used with great care. List these two substances and describe the problems associated with each. 1.10-Phenanthroline forms a bright red complex with ironman. The complex has an analytical wavelength of 510 nm and has a high molar absorptivity. Hydroxylamine hydrochloride is added to keep the iron in a reduced +2 state, and the pH is controlled using an acetate buffer. A series of iron ID-phenanthroline solutions were made by adding various amounts of 4.300 times 10 M to 5 mL each of hydroxylamine hydrochloride, acetate buffer, and 1, 10-phenanthroline. Distilled water was added to make the total volume of each solution 50.00 mL. The percent transmittance of each solution was read, using a blank containing everything but the iron (II) solution. Data for the determination are 1) Calculate the molar concentration of [Fe(phenanthroline)_3^2+ (2) Convert percent transmittances to equivalent absorbances, using Equation 4 Prepare a Beer's law plot for [Fe(phenanthroline)_3]^2+, using the data obtained in (1) and (2). Draw a best straight line through the data points. A solution containing an unknown amount of iron(II) was treated with hydroxylamine hydrochloride, acetate buffer, and 1, 10-phenanthroline as described above. The percent transmittance of the sample at lambda = 510 nm was 52.4 when read against a blank containing all but the iron(II) solution. Determine the molar concentration of iron(II) in the unknown. Calculate the molar absorptivity for [Fe(phenanthroline)_3]^2+. Assume a 1.00-cm path-length.Explanation / Answer
3) (1) Concentration of stock Fe2+ = 4.300*10-4 M.
Take the first entry as an example.
Volume of stock solution taken = 1.00 mL; volume of final solution = 50.00 mL.
Use the dilution equation.
M1*V1 = M2*V2 where M1 = concentration of Fe2+ in the stock solution; V1 = volume of stock Fe2+ taken; V2 = final volume of prepared solution and M2 = concentration of iron (II)-phenanthrolin solution.
Plug in values:
(4.300*10-4 M)*(1.00 mL) = (50.00 mL)*M2
===> M2 = (4.300*10-4*1.00)/(50.00) M = 8.600*10-6 M
Fill in the table as below:
Trial
Volume of stock 4.300*10-4 M Fe2+ (mL)
Final volume of iron (II)-phenanthrolin solution (mL)
Concentration of iron (II)-phenanthrolin solution (M)
1
1.00
50.00
8.600*10-4
2
2.00
50.00
1.720*10-5
3
5.00
50.00
4.300*10-5
4
7.00
50.00
6.020*10-5
5
9.00
50.00
7.740*10-5
6
10.00
50.00
8.600*10-5
(2) Use the relation between absorbance (A) and percent transmittance (%T) as
A = 2 – log (%T)
Plug in values:
Trial
%T
A = 2 – log (%T)
1
80.35
0.0950
2
64.71
0.1890
3
33.65
0.4730
4
21.78
0.6619
5
14.09
0.8511
6
11.32
0.9461
(3) Prepare a plot of absorbance (A) vs concentration of iron(II)-phenenthrolin solution as below:
Plot of absorbance vs concentration of iron (II) phenanthrolin complex.
(4) Use the regression equation from above.
Given %T = 52.4 , we have A = 2 – log (%T) = 0.2807
Put y = 0.2807 and obtain x.
0.2807 = 10996x + 0.0001
===> 0.2806 = 10996x
===> x = 0.2806/10996 = 2.55*10-5
The concentration of iron (II) phenanthrolin in the unknown sample is 2.55*10-5 M (ans).
(5) Beer’s law is
A = *c*l where = molar absorptivity of the complex.
The regression equation is
y = 10996x + 0.0001 where 10996 M-1 is the slope of the plot.
Comparing the two equations, we must have
10996 M-1 = *(1.00 cm)
===> = (10996 M-1)/(1.00 cm) = 10996 M-1 cm-1 (ans).
Trial
Volume of stock 4.300*10-4 M Fe2+ (mL)
Final volume of iron (II)-phenanthrolin solution (mL)
Concentration of iron (II)-phenanthrolin solution (M)
1
1.00
50.00
8.600*10-4
2
2.00
50.00
1.720*10-5
3
5.00
50.00
4.300*10-5
4
7.00
50.00
6.020*10-5
5
9.00
50.00
7.740*10-5
6
10.00
50.00
8.600*10-5
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