Answer to Question #207561 in General Chemistry for Khorshed

Question

• Describe in details the quantitative spectrophotometric determination of a metal ion with a suitable example.

Answer 1

Solution

Spectrophotometric is concerned with the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength.

Absorbance spectrometry- involves transfer o energy from a photon of light to an analyte to produce an excited species

A=-radiation as shown below:

A = -log T = ëbc

Where A is absorbance, T is transmittance, ë is a constant called the molar absorptivity, b is the

path length through the sample, c is analyte concentration.

REAGENTS

1,10-phenanthroline (0.1 g of 1,10-phenanthroline monohydrate in 100 mL of distilled

water, warming to effect solution if necessary).

Hydroxylamine hydrochloride (10 g in 100 mL of distilled water).

Sodium acetate (10 g in 100 mL of distilled water).

Ferrous ammonium sulfate hexahydrate.

SAMPLE PREPARATION

1. Weigh accurately about 0.07 g of pure ferrous ammonium sulfate hexahydrate, dissolve it in

water, and transfer the solution to a 1-liter volumetric flask. Add 2.5 mL of concentrated

sulfuric acid and dilute the solution to the mark. Calculate the concentration of the solution in

mg of iron per mL. (Remember, your solution was prepared using Fe(NH4)2(SO4)26H2O).

2. Prepare the unknown sample as follows. Add about 0.12 g of the solid unknown and

approximately 0.25 mL concentrated sulfuric acid into a 100 mL volumetric flask and dilute

to the mark. Now transfer a 1 mL aliquot of this solution to another 100 mL volumetric flask

Determination of Iron with 1,10 phenanthroline

In this portion of the experiment, you will record the absorbance spectrum of the iron-

phenanthroline complex by measuring the absorbance as a function of wavelength, and use this

data to select a wavelength at which to measure the rest of your standards and unknowns. Since

you want your absorbance values to be meaningful, you will need to set zero transmittance to account for dark current and 100% transmittance with a blank each time you change wavelength.

PROCEDURE

1. Using the blank as a reference and any one of the iron solutions prepared above, measure the

absorbance at 20-nm intervals in the range from 400 to 600 nm. (Note that it is necessary to re-adjust the 0% T and 100%T settings whenever the wavelength is changed).

2. Once you have reached 600 nm, return to the region where absorbance was maximum and

collect absorbance readings at 5 nm intervals in this region.

3. Plot the absorbance vs. wavelength and connect the points to from a smooth curve. Select the

proper wavelength to use for the determination of iron with 1,10-phenanthroline and

calculate the molar absorption coefficient, at the wavelength of maximum absorption

(max) on the absorption curve (assume b = 1 cm).

4. Measure the absorbance of each of the standard solutions and the unknown at the selected

wavelength. Plot the absorbance vs. the concentration of the standards. Note whether Beer's law is obeyed. Using the absorbance of the unknown solution calculate the % (w/w) iron in

your original solid sample and its 95% confidence interval, remember to correct for dilutions.