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Chapter 18: Q10 P (page 456)

18-10. Color and absorption spectra. Color Plate 17 shows coalored solutions and their spectra. From Table 18-1, predict the color of each solution from the wavelength of maximum absorption. Do observed colors agree with predicted colors?

Short Answer

Expert verified

In Compound B and F the observed colors does not agree with the predicted colors. In Compound A,C,D and E the observed colors agree with the predicted colors.

Step by step solution

01

Definition

Spectrophotometry is a standard and inexpensive technique to measure light absorption or the amount of chemicals in a solution. It uses a light beam which passes through the sample, and each compound in the solution absorbs or transmits light over a certain wavelength.

02

Absorbance and the observed color in all compounds

The compound A has maximum absorbance at 760 nm. The predicted color from table 18-1 is green, and the observed color is green.

The compound B has maximum absorbance at 700 nm. The predicted color from table 18-1 is green, and the observed color is blue- green.

The compound C has maximum absorbance at 600 nm. The predicted color from table 18-1 is blue, and the observed color is blue.

The compound D has maximum absorbance at 530 nm. The predicted color from table 18-1 is violet, and the observed color is violet.

The compound E has maximum absorbance at 500 nm. The predicted color from table 18-1 is red or purple red, and the observed color is red.

The compound F has maximum absorbance at 410 nm. The predicted color from table 18-1 is green-yellow, and the observed color is yellow.

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Most popular questions from this chapter

18-29. Biotin-streptavidin fluorescence titration. Biotin is a cofactor in enzymatic carboxylation reactions. Biotin activatesCO2for biosynthetic reactions.


Streptavidin is a protein isolated from the bacterium Streptomyces avidinii that binds biotin with a formation constant of ~1014M-1. The biotin-streptavidin complex is widely used in biotechnology because the noncovalent complex is stable in the presence of detergents, protein denaturants, and organic solvents, and at extremes of pH and temperature.

The stoichiometry of the biotin-streptavidin complex was measured by a fluorescence titration. Fluorescein (page 453 ) covalently attached to biotin via the biotin carboxyl group fluoresces at 520 nm when irradiated at 493 nm. When biotin-fluorescein (BF) binds to streptavidin (SA), fluorescence decreases. The table gives emission intensity for addition of BF to SA and also for addition of SA to BF. Data are already corrected for dilution.

(a) Make a graph of fluorescence versus mole ratio for each titration and state the stoichiometry of binding of biotin to streptavidin.

(b) Explain the shape of each titration curve.

18-8: What is an absorption spectrum?

Consider a molecule that can fluoresce from the S1 state and phosphoresce from the T1 state. Which is emitted at longer wavelength, fluorescence or phosphorescence? Make a sketch showing absorption, fluorescence, and phosphorescence on a single spectrum.

Nitrite ionNO-2, is a preservative for bacon and other foods, but it is potentially carcinogenic. A spectrophotometric determination ofNO-2makes use of the following reactions

Here is an abbreviated procedure for the

determination:

1. To 50.0ml of unknown solution containing nitrite is added 1.00mL of sulfanilic acid solution.

2. After 10min , 2.00mL of 1 鈥揳 minonaphthalene solution and 1.00 mL of buffer are added.

3. After 15 min, the absorbance is read at 520 nm in a 5.00-cm cell.

The following solutions were analyzed:

A. 50.0 mL of food extract known to contain no nitrite (that is, a negligible amount); final absorbance =0.153.

B. 50.0mL of food extract suspected of containing nitrite; final absorbance \(=0.622\).

C. Same as B, but with 10.0渭Lof7.50脳10鈭�3MNaNO2added to the 50.0-mL sample; final absorbance =0.967.

(a) Calculate the molar absorptivity, of the colored product. Remember that a \(5.00\)-cm cell was used.

(b) How many micrograms ofNO-2were present in 50.0mL of food extract?

Ammonia can be determined spectrophotometrically by reaction with phenol in the presence of hypochlorite(OCl-)

A 4.37 - mgsample of protein was chemically digested to convert its nitrogen into ammonia and then diluted to 100.0mL. Thenof the solution were placed in a 50 - mLvolumetric flask and treated with 5mLof phenol solution plus 2mLof sodium hypochlorite solution. The sample was diluted to 50.0mLand the absorbance at 625nmwas measured in a 1.00 - cmcuvet after 30min. For reference, a standard solution was prepared from 0.0100g of NH4Cldissolved in 1.00Lof water. Then 10.0mLof this standard were placed in a50 - mL volumetric flask and analyzed in the same manner as the unknown. A reagent blank was prepared by using distilled water in place of unknown.

(a)Calculate the molar absorptivity of the blue product.

(b)Calculate the weight percent of nitrogen in the protein.
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