Chapter 8 Gravimetric Methods

Question 1

coprecipitation

Answer 1

adsorption, occlusion or inclusion of impurities along with the desired analyte in a precipitate

Question 2

adsorption

Answer 2

binding of impurities to the surface of a crystal

Question 3

absorption

Answer 3

inclusions or occlusion of impurities within a crystal

Question 4

digestion

Answer 4

letting a precipitate stand with the mother liquor to promote slow recrystallization

Question 5

peptization

Answer 5

breaking up of a precipitate upon washing with water in the absence of an electrolyte

Question 6

ionic strength

Answer 6

sum of product of concentration of different types of ions and electric charges in the solution

Question 7

solubility

Answer 7

maximum amount of solute dissolved in a liter of solution

Question 8

common ion effect

Answer 8

solubility of a partially soluble salt that decreases by the addition of a soluble salt that has an ion in common

Question 9

Solubility of an ionic compound increase as ionic strength of solution

Answer 9

increases

Question 10

ionic strength of solution increases when salt is

Answer 10

added to the solution, solubility of ionic compound also increases

Question 11

increase in ionic strength will ___ ionic atmosphere around individual ion

Answer 11

increasing, shields from other ions and decreases the tendency for ions to bind together

Question 12

gravimetry

Answer 12

QUANTITATIVE method based on the mass measurement of a pure compound chemically related to ANALYTE

Question 13

crystalline suspension

Answer 13

0.1mm or bigger, easy to filter

Question 14

colloidal suspensions

Answer 14

10^-7 -10^-4

difficult to filter

Question 15

size of particles depends on

Answer 15

1) precipitate SOLUBILITY
2) Temperature
3) Reactants concentrations
4) Mixing rate of reactants

Question 16

Supersaturation

Answer 16

Q-S/S
Q>S NO prec
LARGE RSS=Colloidal

Question 17

nucleation

Answer 17

formation of new solid particle

Question 18

particle growth

Answer 18

increase in size of particles which already exist

Question 19

type of precipitate depends onw hich process dominates

Answer 19

nucleation: many small particles formed (colloidal) RN increases strongly with increased supersaturation

particle growth: fewer and bigger particles are formed, rate increases slightly with increased supersaturation

Question 20

Minimiza Supersaturation

Answer 20

increased temperature=increased solubility

dilute solution=minimize instant concentration Q

slow addition

pH control IF s depends on pH

Question 21

make colloids to coagulate=agglomerate

Answer 21

increased temp, stirring, external electrolyte

Question 22

Why colloidal precipitates are stable?

Answer 22

particles are either + or -

Question 23

peptization

Answer 23

during washing ,coagulated colloid goes back to dispersed form

Question 24

coprecipitation

Answer 24

normally soluble compounds end up in precipitate causing contamination,

1) surface adsorption
2) mixed crystal
3) occlusion

Question 25

Colloidal vs Crystalline Precipitate

Answer 25

a) size: less than 10^-4 vs more than b) suitability: colloidal are not suitable c) requirement of pretreatment: colloidal have no tendency of settling so coagulation needs to occur

Question 26

Gravimetric Precipitation vs Volatization

Answer 26

Grav: conversion of analyte to sparingle soluble precipitate, filtered and washed free from impurities. Then converted to a product of known composition by suitable heat Vol: analyte volatilized at a suitable temp, collected, weighed or loss of mass

Question 27

precipitation vs coprecipitation

Answer 27

pre: solid phases are obtained and seperated out of solution co: get precipitated in presence of analyte

Question 28

coagulation vs peptization

Answer 28

co: stick colloidal particles together, loss of double layer from electrolye peptization: reversal coagulation

Question 29

occlusion vs mixed

Answer 29

occ: coprecipitation where foreign ion may entrap during crystal growth in the counter ion layer, controlled by digestion mixed: ion in the lattice of a crystal is replaced by contaminant ion

Question 30

nucleation vs particle size

Answer 30

further precipitation or deposition of solid phase occurs and stable solid is formed precipitation involves nucleationand particle growth, nucleation and particle both compete IF NUCLEATION DOMINATES: COLLOIDAL

Question 31

digestion

Answer 31

WAY TO REMOVE WATER, bonded weakly to solution

Question 32

adsorption

Answer 32

small ions are attracted and held on surface of precipitates, helps crytal growth

Question 33

reprecipitation

Answer 33

minimizing adsorbents but removes impurity, inclusion

Question 34

inclusion

Answer 34

similar sized charged particle substitutes the ion or particle of precipitates lattice

Question 35

precipitation from homogeneous

Answer 35

way to get filterable and less contaminated precipitates

Question 36

counter ion layer

Answer 36

crystallizing agent that leads to crystal formation, charge on surface develops due to adsorbed charged ions that are in excess First layer of adsorbed ions on surface of the solid is primary adsorption layer, surrounding, counter ion layer that balances the charge on the surface of particles, Both form double layer which is responsible for stability of colloidal suspensions

Question 37

Stability of Colloidal Suspensions

Answer 37

electric double layer 1) adsorbed ions helps in developing charge 2) counter ion layer that balances the charge

Question 38

mother liquor

Answer 38

solution that remains after crystallization

Question 39

supersaturation

Answer 39

solution contains more solute concentration than a saturated solution

Question 40

process of supersturation

Answer 40

involves increase in solubility, make solution at high temperature very unstable and more liable to crystallization

Question 41

How can relative supersaturation be varied during precipitate formation?

Answer 41

slow addition, gentle heating, gentle stirring, dilute solution relationship between particle size and process Q large= less precipitate= colloidal to get crystalline pre, low supersaturation