Gravimetric Analysis Flashcards by 횐호 강

are quantitative methods that are based
on determining the mass of a pure compound to which the
analyte is chemically related

Gravimetric Methods

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the analyte is separated from a
solution of the sample as a precipitate and is then converted
to a compound of known composition that can be weighed.

Precipitation Gravimetry

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the analyte is separated from
other constituents of a sample by conversion to a gas of
known chemical composition. The weight of this gas then
serves as a measure of the analyte concentration

Volatilization Gravimetry

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the analyte is separated by deposition of
an electrode by an electrical current. The mass of this product
then provides a measure of the analyte concentration

Electrogravimetry

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the mass of a reagent of known concentration required to
react completely with the analyte provides the information
needed to determine the analyte concentration

Gravimetric Titrimetry

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uses a mass spectrometer to separate the gaseous ions formed
from the elements making up a sample of matter. The
concentration of the resulting ions is then determined by
measuring the electrical current produced when they fall on
the surface of an ion detector

Atomic Mass Spectrometry

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The process of precipitation involves three steps which are

supersaturation, nucleation, and particle growth

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________ occurs upon addition of the first drops of the precipitating agent

supersaturation

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then starts to occur
wherein a minimum number of atoms, ions, or molecules
aggregate together to form a stable solid.

Nucleation

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then starts to occur
wherein a minimum number of atoms, ions, or molecules
aggregate together to form a stable solid.

Nucleation

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Further
precipitation then involves a competition between additional
nucleation and growth on existing nuclei

particle growth

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[Factors That Determine the Particle Size of Precipitates]

are invisible to
the naked eye (10^-7 to 10^-4 cm in diameter).

show no tendency to settle from
solution and are difficult to filter

Colloidal suspensions

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[Factors That Determine the Particle Size of Precipitates] The temporary dispersion of particles in the liquid
phase

tend to settle
spontaneously and are easily filtered

Crystalline suspension

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The net effect of these variables can be accounted for, at least
qualitatively, by assuming that the particle size is related to a
single property of the system called

states that the particle size of
precipitates is inversely proportional to
the relative saturation of the solution
during the precipitation process

Relative Supersaturation / Von Weirman Equation

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a minimum
number of atoms, ions, or molecules aggregate together to form a
stable solid.

nucleation

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Often, these nuclei form on the surface of suspended
solid contaminants, such as dust particles. Further precipitation then
is governed by the competition between additional nucleation and
growth of existing nuclei

particle growth

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Control of Particle size

Increasing the solubility of the precipitate S
Precipitation using dilute solutions to minimize Q
Slow addition of the precipitating agent with good stirring to
keep the concentration of solute Q low

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Brownian motion prevents their settling out of solution under the influence of gravity. However, we can coagulate, or agglomerate, the individual particles of most colloids to give a filterable, amorphous mass that will settle out of solution

Colloidal Precipates

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Coagulation can be hastened by:

Heating
Stirring
Adding an electrolyte to the medium

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Coagulation can be hastened by:

Heating
Stirring
Adding an electrolyte to the medium

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a substance (gas, liquid, or solid) is held on the
surface of a solid.

Absorption

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is the process by which a coagulated colloid reverts to its original dispersed state

Peptization

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is the process by which a coagulated colloid reverts to its original dispersed state

Peptization

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are more easily filtered and purified than
coagulated colloids.

Crystalline precipitates

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The particle size of crystalline solids can often be improved significantly by

minimizing Q or maximizing S, or both.

of crystalline precipitates (without stirring) for some time after formation often yields a purer, more filterable product.

Digestion

The amount of impurities depends on nature of precipitate and condition of precipitation It may be due to

-coprecipitation -post-precipitation

is the process in which normally soluble compounds are carried out of the solution. It leads to an increase in the mass of precipitate

Coprecipitation

four types of coprecipitation

surface absorption mixed-crystal formation occlusion mechanical entrapment

Is likely to cause significant contamination of precipitates with large specific surface areas, that is, coagulated colloids.

Surface Absorption

How to minimize absorbed impurities on colloids

Washing of colloidal particles

is a drastic but an effective way to minimize the effects of absorption.

Reprecipitation

one of the ions in the crystal lattice of a solid is replaced by an ion of another element

Mixed Crystal Formation

is a type of coprecipitation in which a compound is trapped within a pocket formed during rapid crystal growth

Occlusion

occurs when crystals lie close together during growth. Several crystals grow together and in so doing trap a portion of the solution in a tiny pocket

Mechanical entrapment

________may occur in both colloidal and crystalline precipitates, but _________ are confined to crystalline precipitates.

Mixed-crystal formation; occlusion and mechanical entrapment

may cause either negative or positive errors in an analysis. If the contaminant is not a compound of the ion being determined, a positive error will always result. In contrast, when the contaminant does contain the ion being determined, either positive or negative errors may occur

Coprecipated Impurities

is a process in which a precipitate is formed by slow generation of a precipitating reagent homogeneously throughout a solution

Homogeneous precipitation

Solids formed by homogeneous precipitation are?

generally purer and more easily filtered

Steps in gravimetric analysis

Preparation of the Solution Precipitation Digestion Filtration Washing Drying or Ignition Weighing Calculation

Preliminary separation of potential interferences before precipitating the analyte Adjustment of solution condition (pH / temperature / volume / concentration of test substance) to maintain low solubility of precipitate and maximum precipitate formation

Preparation of the Solution

The precipitating reagent is added at a concentration that favors the formation of a “good” precipitate This may require low concentration, extensive heating (often described as“digestion”), or careful control of the pH

Precipitation

Also known as Ostwald ripening, the small particles tend to dissolve and precipitate on the surfaces of the larger crystals

Digestion

Precipitate is separated from mother liquor (the solution from which a precipitate was formed. Choice depends on the nature of precipitate, cost of media, and heating temperature required for drying

Filtration

Coprecipitated impurities, especially those on surface, are removed by

Washing

its purpose is to remove the remaining moisture It is done by heating about 120-150℃ for 1-2 hours

Drying

Its purpose in a muffle furnace at temperatures ranging from 600-1200℃ is to get the material with exactly known chemical structure so that the amount of analyte can be determined accurately. The precipitate is converted to a more chemically stable form

Ignition

After the precipitate is allowed to cool (preferably in a desiccator to keep it from absorbing moisture), it is

Weighed (Process: Weighing)

The combined constant factors in a gravimetric calculation are referred to as the

Gravimetric Factor

Volumetric Analysis is also known as

Titration

Involve measuring the volume of a solution of known concentration that is needed to react completely with the analyte

Volumetric Analysis / Titration

A reagent of known concentration that is used to carry out volumetric titration

Titrants or titrators (Standard solution/ Standard Titrant)

A theoretical point reached when the amount of added titrant is chemically equivalent to the amount of analyte in the sample

Equivalence point

Is a point in titration that signifies the completion of the titration by a change in the colour or intensity of the solution

End point

Are often added to the analyte solution to produce observable physical change (which signals end point) at or near the equivalence point

Indicators

Refers to the mass difference in volume or mass between the equivalence point and the end point

Titration error

Highly purified compound that serves as a reference material in titrations

Primary Standards

[Primary Standards] Acid-base reactions

sodium carbonate Na2CO3 - sodium tetraborate Na2B407 - potassium hydrogenphthalate KH(C8H404) - constant boiling point hydrochloric acid - potassium hydrogeniodate KH(IO3)2 - benzoic acid (C6H5COOH).

[Primary Standards] Complex formation reactions

silver, silver nitrate, sodium chloride, various metals and salts, depending upon the reaction used.

[Primary Standards] Precipitation reactions

silver, silver nitrate, sodium chloride, potassium chloride, and potassium bromide (prepared from potassium bromate).

[Primary Standards] Oxidation-reduction reactions

- potassium dichromate K2Cr2O7 - potassium bromate KBrO3 - potassium iodate KIO3 - potassium hydrogeniodate KH(IO3)2 - sodium oxalate Na2C204 - arsenic(II) oxide As2O3 - pure iron.

are widely used to determine the amounts of acids and bases and to monitor the progress of reactions that produce or consume hydrogen (H+) ions.

Neutralization titrations

Standard reagents used in acid-base titrations are always strong acids or string bases such as:

* Hydrochloric (HCL) acid; * Perchloric acid (HCLO4) acid; * Sulfuric (H2SO4) acid; * Sodium hydroxide (NaOH); and * Potassium hydroxide (KOH)

There are 3 types of indicators of Acids and Bases

1. Natural 2. Synthetic 3. Olfactory

- these are indicators that can be organically found in nature

Natural indicators

the most common and well-known acid/base indicator

Litmus paper

naturally yellow in color and turns red when it comes into contact with a basic solution

Turmeric

indicators which are synthesized and created by the chemical process.

Synthetic indicators

Examples of synthetic indicators

Methyl Orange Phenolphthalein

these are substances whose smell changes in acidic or basic solutions

Olfactory indicator

have a characteristic sharp smell. In a basic medium, the smell cannot be detected, but in an acidic medium it retains its strong smell

Onions

this occurs which the indicator changes color differs from the pH at the equivalence point - usually minimized by choosing the indicator carefully or by making a blank correction

Determinate error

originates from the limited ability of the human eye to distinguish reproducibly the intermediate color of the indicator

Indeterminate error

The pH interval over which a given indicator exhibits a color change is influenced by:

a. Temperature b. Ionic strength of the medium c. Presence of organic solvents and colloidal particles

Two Sources of Hydronium (OH-) ions in an aqueous solution of strong acids:

1. Reaction of the acid with water; and 2. The dissociation of water itself

at this stage we compute the concentration of the acid from its starting concentration and the amount of base that has been added.

Pre-equivalence

hydronium and hydroxide ions are present in equal concentrations, and the hydronium ion concentration is derived directly from the ion-product constant for water.

Equivalence

analytical concentration of the excess base is computed, and the hydroxide ion concentration is assumed to be equal to or a multiple of the analytical concentration.

Post equivalence

it is the measurements of physical properties of analytes, such as current, electrode potential, the absorption or emission of light, and mass-to-charge ratios, and fluorescence, used for quantitative analysis of a variety of inorganic, organic, and biochemical analyte.

Instrumental methods of analysis

quantitative method used to measure the potential of an electrochemical cell under static condition.

Potentiometry

in the Potentiometry is when the pair of electrodes is placed in the sample solution, it shows the potential difference by the addition of the titrant or by the change in the concentration of the ions.

Principle

a device for measuring the potential of an electrochemical cell without drawing a current or altering the cell’s composition.

Potentiometer

the electrode which contains its own potential value and it is stable when dipped into sample solution.

Reference electrode

commonly used to establish a reference potential for measuring other electrode's potentials. It is dependable but large, bulky, and affected by temperature.

Calomel Reference Electrodes or mercury sulfate electrode and mercury oxide electrode

The most widely marketed reference electrode system consists of a silver electrode immersed in a solution of potassium chloride that has been saturated with silver chloride. - Widely used because it is simple, inexpensive, very stable and non- toxic. - More compact– overall better and faster

Silver- silver Chloride Reference Electrodes

consists of a platinized platinum electrode in HCl solution with hydrogen at atmospheric pressure bubbled over the platinum surface.

Normal Hydrogen Electrode

the electrode which responds to change in the potential of analyte solution.

Indicator electrode

It is convenient to classify metallic indicator electrodes as electrodes of the first kind, electrodes of the second kind, and inert redox electrodes.

Metallic Indicator Electrodes

is a pure metal electrode that is in direct equilibrium with its cation in the solution. A single reaction is involved.

Electrodes of the First Kind

Metals not only serve as indicator electrodes for their own cations but also respond to the activities of anions that form sparingly soluble precipitates or stable complexes with such cations.

Electrodes of the second Kind

Several relatively inert conductors respond to redox systems. Such materials as platinum, gold, palladium, and carbon can be used to monitor redox systems.

Inert Metallic Electrodes for Redox Systems

the most convenient method for determining pH has involved measurement of the potential that appears across a thin glass membrane that separates two solutions with different hydrogen ion concentrations.

Membrane Indicator Electrodes

a type of ion-selective electrode made of a doped glass membrane that is sensitive to a specific ion.

The Glass Electrode for measuring pH

Also known as the liquid junction, used to prevent the mixing or interference of the analyte solution with that of the reference solution.

Salt bridge

are potentiometric sensors that include a selective membrane to minimize matrix interferences.

Ion – selective Electrode (ISE) or specific ion electrode (SIE)

Most common ISE:

pH electrode, which contains a thin glass membrane that responds to the H+ concentration in a solution.

TYPES OF ION SELECTIVE ELECTRODE (ISE): Any electrode that preferentially responds to one ion species.

1. Glass Membrane Electrode: H+ 2. Solid State Electrode 3. Liquid Membrane Electrode 4. Gas Sensing Electrode 5. Enzyme electrodes

Gravimetric Analysis Flashcards

(97 cards)