Gravimetric Analysis

Gravimetric Analysis

quantitative determination of analyte concentration through a process of analyte precipitation, isolation of the precipitate, and weighing the isolated product

Uses of Gravimetric Analysis

• chemical analysis of ores and industrial materials

• calibration of instrumentation

• elemental analysis of inorganic compounds

Criteria for Gravimetric Analysis

• the analyte must be soluble and must form an insoluble precipitate from the solution

• the weighed form of the product should be of known composition

• the product should be “pure” and easily filtered (it is usually difficult to obtain a product that is ‘pure’ and completely free from impurities; careful precipitation and sufficient washing may reduce the level of impurities)

Features of Gravimetric Analysis

• time-consuming and labor intensive (procedure is difficult to automate, takes time)

• a “traditional” method (cheap, easily available apparatus, simple to carry out)

• wide concentration range (ng to kg levels)

• accurate (better than +-0.01% atomic weight)

• precise (0.1 - 0.3%)

Methods of Gravimetric Analysis

• Volatilization (Combustion) Method or Thermogravimetry

• Precipitation Method

• Electrogravimetry Method

Volatilization / Combustion Methods

• "thermogravimetry” is used to determine volatile components (e.g. H2O and CO2 in a sample)

• the sample is warmed or ignited

• the loss in mass of the sample, or

• the volatile components absorbed by a suitable adsorbent (components are determined from the increase in the weight of the adsorbent)

How to determine the amount of the component in volatilization / combustion method of gravimetry?

Precipitation Method

determination of an analyte precipitated by a precipitating agent

Precipitate

a slightly soluble substance with a known composition or a converted compound of known composition

• solubility of the precipitate

• temperature

• pH

• precipitating agents (precipitants)

What are the factors that influence the properties of the precipitate?

Precipitants

reagents that have the ability to be selective in forming highly insoluble precipitates and have reproducible stoichiometry

Solubility Product (Ksp)

The solubility of a sparingly soluble salt or electrolyte is characterized by its?

TRUE

the smaller the Ksp is, the less soluble the compound

TRUE

We want a less soluble compound in order to achieve insoluble precipitates.

Ksp

gives you a numerical value that helps you understand the number of ions in the solution once a compound is fully dissolved.

Molar Solubility

a measure of how many moles of a substance can dissolve in one liter of water before the solution becomes saturated

TRUE

The solubility of the precipitate increases with temperature increase

TRUE

Still in some cases, the solubility of precipitate decreases with rise of temperature

• increase pH

• shifts equilibrium to the left

• increases the precipitation (decreasing the solubility of Mg(OH)2)

What does adding OH- ions do in the pH of solution?

• decrease pH

• shifts the equilibrium from left to right

• increases the solubility of Mg(OH)2, decreases the precipitation

What does adding H+ ions do in the pH of solution?

TRUE

Precipitation can also be affected by adding more of any of the similar precipitating ions

The reaction will shift to the left, meaning more precipitate forms; also called the common ion effect

What would happen if you add more of any of the similar ions into the solution?

Steps in Gravimetric Analysis

1. Sample preparation

2. Precipitation

3. Digestion

4. Filtration

5. Washing the precipitate

6. Drying/Ignition and Weighing to Constant Weight

7. Data Calculation

Sample Preparation

In this step, solids are dried, weighed and dissolved in a suitable solvent. This is where preliminary elimination of interfering materials is done.

Precipitation

This is the step where precipitant or precipitating agent is added to form the insoluble compound called precipitate.

• should consist of perfect crystals large enough to be easily washed and filtered

• perfect crystal should be free from impurities and be large enough so that it presents a minimum surface area onto which foreign ions could be absorbed

• should be insoluble such that loses from dissolution would be minimal

What should the characteristics of the precipitate be?

Criteria for Choosing Precipitant

• to be selective towards the analyte

• to be easily filtered

• to have reproducible reactions

Types of Precipitants

• Inorganic Precipitant

• Organic Precipitant

Inorganic Precipitant

• forms slightly soluble salts or hydrous oxides with the analyte

• most are not very selective

• most common are: AgNO3 which precipitate halide ions and BaCl2 which precipitates sulfate ions

Organic Precipitants

• useful precipitants for metals

• examples are 8-hydroxyquinoline (oxine) and dimethylglyoxime (DMG)

Advantages of Organic Precipitants

• forms chelate compounds with cations (very insoluble in water) so that metal ions may be quantitatively precipitated

• has a larger molecular weight, so that small amount of metal may yield a large weight of precipitate

• some are fairly selective, yielding precipitates with only a limited number of cations

• precipitates obtained with organic precipitants are often coarse and bulky which is easy to handle

TRUE

By controlling pH and the concentration of masking reagents, the selectivity of an organic reagent can be enhanced

Chelate compounds

complex compound consisting of a central metal atom attached to a larger molecule

Oxine

It can precipitate many elements and can be used for group separation by controlling pH. Aluminum ion can be precipitated at pH 4 and a higher pH is required to precipitate magnesium.

DMG

• principally used for determination of nickel

• the precipitate is so bulky that only small amount of nickel can be handled conveniently

By comparing solubility product constant Ksp with ion product constant Q.

How do you predicate the saturated condition of a solution?

Q < Ksp

the solution is unsaturated (more solute can be dissolved)

Q = Ksp

the solution is saturated, the system is at equilibrium (no more solute dissolves)

Q > Ksp

the solution is supersaturated, MX will precipitate until the product of the ionic concentrations is equal to Ksp (crystals may grow)

Steps in Precipitate Formation

1. Analyte + Precipitant

2. Supersaturation

3. Primary Nucleation

4. Secondary Nucleation

Supersaturation

• achieved when precipitant is added

• this is when the solution contains more of the dissolved salt than occurs at equilibrium

• achieved by dissolving the solute in water at an elevated temperature using enough to give a concentration just under its solubility at that temperature, as it cools down the maximum solubility will also decrease causing the formation of crystals

Primary Nucleation

• a process where molecules in solution join randomly to form a small aggregate called nucleus

Secondary Nucleation

• particle growth or crystal growth

• more particles are added to the nucleus to form larger particles, diameter 1-100 nm (colloids 1-100 nm - fine crystals - coarse crystals with size more than 10 micrometers)

• adding digestive agents

• controlling pH to get proper size of nuclei

What other steps can we do to promote crystal growth?

Induction Period

the time before nucleation occurs after the addition of the precipitating agent to the solution (ranges from milliseconds to several mins)

Nucleation

• formation of small, stable aggregates or nuclei of precipitate

• nuclei have sizes down to ~1 nm, composed of a few atoms, and may be up to 10^10 nuclei per mole of analyte

• excess ions from solution collect around the nuclei

Particle/Crystal Growth

particles grow with the addition of ions of the precipitate until system comes to equilibrium

Von Weimarn

introduced the concept of relative supersaturation in which he stated that the particle size of precipitates is inversely proportional to the relative supersaturation of the solution during precipitation

The Von Weimarn Ratio of Relative Supersaturation

Q-S/S

Q = concentration of mixed reagents before any precipitation

S = solubility of the product (ppt) when the system has reached equilibrium

Q-S = degree of supersaturation

TRUE

lower value for Q-S (degree of supersaturation) means that the nucleation rate is lower than the particle growth rate resulting in a coarser and larger crystal.

TRUE

To get coarse crystals, we want (Q-S) to be small.

If (Q-S) is large, it means that:

• the relative supersaturation is high

• nucleation is favored

• more particles formed, but are smaller in size

• colloidal precipitates form

If (Q-S) is small, it means that:

• the relative supersaturation is low

• the particle growth is predominant over nucleation

• this way, fewer particles are formed but are larger in size

• crystalline precipitates form

To get small (Q-S):

• Q should be as low as possible and S should be high

• dilute solution so that molar concentration Q is low

• make hot solutions so that solubility S will increase

• add dilute precipitant slowly with effective stirring as stirring prevents local excesses of the reagent which lowers Q

• precipitating at acidic (low pH) to maintain quantitative precipitation

• digesting the precipitate to get larger crystals

• precipitating from homogenous solution

Homogenous Precipitations

• a technique where a precipitating agent is not added but slowly generated throughout the solution

Ex:

1. Slow increase of pH

2. Cation release

3. Precipitation from mixed solvents (change the pH or polarity of solvents)

4. Anion release

5. Valency change - change in oxidation state

To prevent the formation of hydroxides, because they tend to participate in further unintended reactions.

Why are solutions kept acidic?

Electric Double Layer

primary layer of positively charged cation plus secondary layer of negatively charged anion

Adsorption

a process in which a substance (gas, liquid, or solid) condenses onto the surface of a solid

Heat

What do you use to coagulate or increase size of crystal?

Digestion

process of coagulating using heat; heating the precipitate within the mother liquor for a certain period of time to encourage densification of nuclei; helps produce larger crystals that are more easily filtered from solution

As the precipitate is heated, water will be expelled from the solid giving a denser mass. Moreover, during digestion, smaller particles dissolve and larger ones grow as stated by the principle of Ostwald ripening.

How does coagulation work?

• Co-precipitation

• Post Precipitation

How do impurities in precipitates form?

Co - Precipitation

soluble compounds are carried out of solution onto the precipitate

• the precipitation of unwanted species along with the analyte of interest

• occurs to some degree in every gravimetric analysis

• cannot be avoided but may be minimized by careful precipitation and thorough washing of the precipitate

Types of Co-Precipitation:

• Surface Adsorption

• Inclusion (mixed-crystal formation)

• Occlusion (mechanical entrapment)

Post Precipitation

a foreign compound precipitates on top of the desired precipitate

Surface Adsorption

unwanted material is adsorbed onto the surface of the ppt

Through digestion, there will be reduced relative surface area where impurities can be adsorbed. Washing also helps remove impurities bound to the surface of precipitates.

How do you overcome the problem of surface adsorption?

Inclusion (Mixed-crystal formation)

• interferences are incorporated into the precipitating crystal either as substitute elements on a lattice site, or extra elements between sites (interstitial)

• can occur if two ions/cpds have the same charge, type of formula or sufficiently close ionic diameters to fit into the same crystal lattice

• one compound replaces part of another in a crystal, forming mixed crystals

• inclusions occur throughout the crystal (not only on the surface), thus, the changes in particle size will not affect the extent of inclusion

Occlusion

occurs if crystal growth is too rapid, some counter ions do not have time to escape from the surface, so it becomes trapped (occluded) within a growing crystal

Mechanical Entrapment

occurs if two crystals grow together and trap a species in the space between them (these crystals lie close together during growth)

Filtering Precipitates

• the step after digestion

• filtration helps separate the precipitate from impurities and the mother liquor

3 types of filter media:

1. Ashless Filter Paper - suitable for crystalline precipitates, very little ash from the paper is left behind after it is burnt, leaving mostly dried precipitate

2. Sintered Glass Filter Crucibles - suitable for curdy precipitates, the glass filter able to withstand very high temperatures (up to 500 deg C)

3. Sintered Porcelain Filter Crucibles - the filter able to withstand temperatures up to 1000 deg C)

TRUE

the choice of a medium depends on the type of precipitate and on the temperature at which the precipitate is to be heated

Washing Precipitates (Peptization)

WASHING removes adsorbed impurities and the mother liquor

small amounts of pure water

What is used to wash crystalline precipitates?

Volatile electrolyte solution

What is used to wash colloids?

TRUE

Washing colloids with pure water will dilute and remove foreign ions, and the counter ions will occupy a larger volume

Peptization

a process whereby the charge of the primary ions repel each other and revert back to the colloidal form (this results in the loss/removal of colloids through the filter)

• Test the filtrate for excess Ag+ from the precipitant by adding NaCl or dilute HCl

• If the solution becomes cloudy it means the washing process is not yet complete because AgCl still forms

• Washing is continued until the test is negative (absence of ppt)

How to know when the washing step is finished?

Drying/Ignition and Weighing

• filtered precipitates are heated (ignited) to produce a constant weight of precipitate

• drying removes the solvent and any volatile species carried down with the precipitate

• done by heating at 110 - 120 deg C for 1-2 hours

• After, the precipitate is cooled to room temperature in a DESICCATOR prior to weighing

• Heating, cooling and weighing is repeated until a constant weight is achieved (difference between two consecutive weighing is less than 0.0002 g)

to prevent moisture absorption because as heated samples cool, they can attract moisture from the air, which may alter their mass and chemical properties

Why place the ppt in a desiccator?

Calculating Results

• the last step in gravimetric analysis

• the result of a gravimetric determination is reported as percentage analyte, %A

Gravimetric Factor

molar ratio of analyte and precipitate multiplied to the the molecular weight of analyte and precipitate

Percentage Analyte Calculation

%A = (weight of ppt x GF / weight of sample) x 100%