Question

a) Explain bragg's spectrometer method.
b) Write about the preparation of colloids by chemical methods.

Answer 1

a) Bragg's spectrometer method is used for studying crystals using X-rays.
The apparatus consists of a X-ray tube from which a narrow beam of X-rays is allowed to fall on the crystal mounted on a rotating table. The rotating table is provided with scale and vernier from which the angle of incidence $\theta$ can be measured.
An arm which is rotating about the same axis as the crystal table, carries an ionization chamber. The rays reflected from crystal enter into the ionization chamber and ionize the gas present inside. Due to the ionization, current produced is measured by electrometer. The current of ionization is a direct measure of intensity of reflected beam from the crystal. For different angles of incidence, the corresponding ionization current is measured from the electrometer. These values are plotted in the form of graph.
For sodium chloride, the maximum reflection for peaks for 100 plane occurred at $\theta ={5.9}^0,{11.85}^0$ and ${18.15}^0$. The sines of these angles are $0.103,0.205$ and $0.312$ which are in the ratio $1:2:3$. These peaks represent the first, second and third order reflection. The ratio confirms the correctness of Bragg's equation.
b) Preparation of colloids by chemical methods:
The chemical methods involve chemical reaction in a medium in which the dispersed phase is sparingly soluble. Some of the methods are

i) Double decomposition
An arsenic sulphide sol is prepared by passing a slow stream of hydrogen sulphide gas through a cold solution of arsenous oxide. This is continued till the yellow colour of the sol attains maximum intensity.
$A{s}_2{O}_3+3H_{2}S\to A{s}_2{S}_3+3H_{2}O$
$($Yellow$)$
Excess $H_{2}S$ is removed by passing in a stream of hydrogen.
ii) Oxidation: A colloidal solution of sulphur is obtained by passing
$H_{2}S$ Into a solution of $S{O}_2$
$2H_{2}S+S{O}_2\to 2H_{2}O+3S$
iii) Reduction: Silver sol and gold sol can be obtained by treating dilute solution, of silver nitrate or gold chloride with organic reducing agents like tannic acid or formaldehyde.
$AgN{O}^3+$Tannic Acid $\to AgSol$
$AuC{l}^3+$Tannic Acid $\to AuSol$
iv) Hydrolysis: Colloidal solutions of the hydroxides of $Fe,Cr,Al$ etc. can be prepared by hydrolysis of their salts. A colloidal solution of ferric hydroxide is obtained by boiling a dilute solution of ferric chloride.
$FeC{l}_3+3H_{2}O\to Fe{\left(OH\right)}_3+3HCl$