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principios de analise instrumental 6ed skoog resolucoes

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Fe
−× 1 mol Fe 
55.85 g Fe
× 2.50 mL ×
50 mL
6= 7.073 10 M−× 
A = εbc = 7.00 × 103 L mol–1 cm–1 × 2.50 cm × 7.073 × 10–6 M = 0.124 
13-10. cZnL = 1.59 × 10–4 M A = 0.352 
(a) %T = 10–0.352 × 100% = 44.5% 
(b) A = 2.50 × 0.352 = 0.880 %T = 10–0.880 × 100% = 13.2% 
(c) ε = A/(bc) = 0.352/(1.00 × 1.59 × 10–4) = 2.21 × 103 L mol–1 cm–1 
13-11. [H3O+][In–]/[HIn] = 8.00 × 10–5 [H3O+] = [In–] [Hin] = cHin – [In–] 
(a) At cHIn = 3.00 × 10–4 
2
5
4
[In ] = 8.00 10
(3.00 10 ) [In ]
−
−
− − ×× − 
2 5[In ] + 8.00 10 [In ] 2.40 10 = 0− − −× − × 8− 
[In–] = 1.20 × 10–4 M [HIn] = 3.00 × 10–4 – 1.20 × 10–4 = 1.80 × 10–4 M 
A430 = 1.20 × 10–4 × 0.775 × 103 + 1.80 × 10–4 × 8.04 × 103 = 1.54 
 
 
Principles of Instrumental Analysis, 6th ed. Chapter 13
 
 3
A600 = 1.20 × 10–4 × 6.96 × 103 + 1.80 × 10–4 × 1.23 × 103 = 1.06 
The remaining calculations are done the same way with the following results. 
cind, M [In–] [HIn] A430 A600
3.00 × 10–4 1.20 × 10–4 1.80 × 10–4 1.54 1.06 
2.00 × 10–4 9.27 × 10–5 1.07 × 10–4 0.935 0.777 
1.00 × 10–4 5.80 × 10–5 4.20 × 10–5 0.383 0.455 
0.500 × 10–4 3.48 × 10–5 1.52 × 10–5 0.149 0.261 
0.200 × 10–4 2.00 × 10–5 5.00 × 10–6 0.056 0.145 
 
13-12. 
2
142 7
2 2 2
4
[Cr O ] = 4.2 10
[CrO ] [H ]
−
− + × 
[H+] = 10–5.60 = 2.512 × 10–6 
2 2 7
2
2 4
2 7 K Cr O
[CrO ][Cr O ] = 
2
c
−
− − 
2 2 7
2
4
K Cr O
14
2 2 6 2
4
[CrO ] 
2 = 4.2 10
[CrO ] (2.512 10 )
c
−
− −
−
×× × 
 
 
Principles of Instrumental Analysis, 6th ed. Chapter 13
 
 4
2 2
K Cr O
−
4−
2 2 7
2 3
K Cr O 4 4 0.500[CrO ] = 2.65 10 [CrO ]c
− −− × 
2 2 7
2 2 4 2 4
4 4[CrO ] + 1.887 10 [CrO ] 3.774 10 = 0c
− − − −× − × 
When 
2 2 7
4
K Cr O = 4.00 10c
−×
2 2 4 2 7
4 4[CrO ] + 1.887 10 [CrO ] 1.510 10 = 0
− − −× − × 
2 4
4
2 4 4
2 7
[CrO ] = 3.055 10 M
[Cr O ] = 4.00 10 3.055 10 /2 = 2.473 10 M
− −
− − −
×
× − × ×
 
A345 = 1.84 × 103 × 3.055 × 10–4 + 10.7 × 102 × 2.473 × 10–4 = 0.827 
A370 = 4.81 × 103 × 3.055 × 10–4 + 7.28 × 102 × 2.473 × 10–4 = 1.649 
A400 = 1.88 × 103 × 3.055 × 10–4 +1.89 × 102 × 2.473 × 10–4 = 0.621 
The following results were obtained in the same way 
2 2 7K Cr O
c 24[CrO ]
− 22 7[Cr O ]
− A345 A370 A400
4.00 × 10–4 3.055 × 10–4 2.473 × 10–4 0.827 1.649 0.621 
3.00 × 10–4 2.551 × 10–4 1.725 × 10–4 0.654 1.353 0.512 
2.00 × 10–4 1.961 × 10–4 1.019 × 10–4 0.470 1.018 0.388 
1.00 × 10–4 1.216 × 10–4 3.920 × 10–5 0.266 0.613 0.236 
 
 
Principles of Instrumental Analysis, 6th ed. Chapter 13
 
 5
 
13-13. (a) Hydrogen and deuterium lamps differ only in the gases that are used in the discharge. 
Deuterium lamps generally produce higher intensity radiation. 
(b) Filters provide low resolution wavelength selection often suitable for quantitative 
work, but not for qualitative analysis or structural studies. Monochromators produce 
high resolution (narrow bandwidths) for both qualitative and quantitative work. 
(c) A phototube is a vacuum tube equipped with a photoemissive cathode and a 
collection anode. The photo electrons emitted as a result of photon bombardment are 
attracted to the positively charged anode to produce a small photocurrent proportional to 
the photon flux. A photovoltaic cell consists of a photosensitive semiconductor 
sandwiched between two electrodes. An incident beam of photons causes production of 
electron-hole pairs which when separated produce a voltage related to the photon flux. 
 
 
Principles of Instrumental Analysis, 6th ed. Chapter 13
 
 6
Phototubes are generally more sensitive and have a greater wavelength range. Photocells 
are in general simpler, cheaper and more rugged. Photocells do not require external 
power supplies. 
(d) A photodiode consists of a photo-sensitive pn-junction diode that is normally 
reverse-biased. An incident beam of photons causes a photocurrent proportional to the 
photon flux. A photomultiplier tube is a vacuum tube consisting of a photoemissive 
cathode, a series of intermediate electrodes called dynodes, and a collection anode. Each 
photoelectron emitted by the photocathode is accelerated in the electric field to the first 
positively charged dynode where it can produce several secondary electrons. These are, 
in turn, attracted to the next positively charge dynode to give rise to multiple electrons. 
The result is a cascade multiplication of 106 or more electrons per emitted photoelectron. 
Photomultipliers are more sensitive than photodiodes, but require a high voltage power 
supply compared to the low voltage supplies required by photodiodes. Photomultipliers 
are larger and require extensive shielding. Photodiodes are better suited for small, 
portable instruments because of their size and ruggedness. 
(e) Both types of spectrophotometers split the beam into two portions. One travels 
through the reference cell and one through the sample cell. With the double-beam-in-
space arrangement, both beams travel at the same time through the two cells. They then 
strike two separate photodetectors where the signals are processed to produce the 
absorbance. With the double-beam-in-time arrangement, the two beams travel at 
different times through the cells. They are later recombined to strike one photodetector at 
different times. The double-beam-in-time arrangement is a little more complicated 
 
 
Principles of Instrumental Analysis, 6th ed. Chapter 13
 
 7
mechanically and electronically, but uses one photodetector. The double-beam-in-space 
arrangement is simpler, but requires two matched photodetectors. 
(f) Spectrophotometers have monochromators or spectrographs for wavelength selection. 
Photometers generally have filters are use an LED source for wavelength selection. The 
spectrophotometer can be used for wavelength scanning or for multiple wavelength 
selection. The photometer is restricted to one or a few wavelengths. 
(g) A single-beam spectrophotometer employs one beam of radiation that irradiates one 
cell. To obtain the absorbance, the reference cell is replaced with the sample cell 
containing the analyte. With a double-beam instrument, the reference cell and sample 
cell are irradiated simultaneously or nearly so. Double-beam instruments have the 
advantages that fluctuations in source intensity are cancelled as is drift in electronic 
components. The double-beam instrument is readily adapted for spectral scanning. 
Single-beam instruments have the advantages of simplicity and lower cost. 
Computerized versions are useful for spectral scanning. 
(h) Multichannel spectrophotometers detect the entire spectral range essentially 
simultaneously and can produce an entire spectrum in one second or less. They do not 
use mechanical means to obtain a spectrum. Conventional spectrophotometers use 
mechanical methods (rotation of a grating) to scan the spectrum. An entire spectrum 
requires several minutes to procure. Multichannel instruments have the advantage of 
speed and long-term reliability. Conventional spectrophotometers can be of higher 
resolution and have lower stray light characteristics. 
13-14. (a) %T = P/P0 × 100% = I/I0 × 100% = 41.6 μA/63.8 μA = 65.2% 
(b) A = – log T = 2 – log %T = 2 – log(65.2) = 0.186 
 
 
Principles of Instrumental Analysis, 6th ed. Chapter 13
 
 8
(c) A = 0.186/3 = 0.062; T = 10–A = 10–0.062 = 0.867 
(d) A = 2 × 0.186 = 0.372 T = 10–0.372 = 0.425 
13-15. (a) %T = P/P0 × 100% = I/I0 × 100% = 256 mV/498 mV × 100% = 51.4% 
A = 2 – log %T = 2 – log(51.4)