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Gravimetric Analysis - Theory and Applications

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Marco Filho

on 6 August 2014

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Transcript of Gravimetric Analysis - Theory and Applications

Precipitation occurs when the value of Q = [Ag ][Cl¯] exceeds the solubility product
K
sp of AgCl (1.8 x 10¯¹⁰).

Solution:
Since the Ni content is about 3%, 1.0 g of steel will contain about 0.03 g of Ni, which corresponds to:



this much Ni² requires the following molar equivalent of DMG (RFM = 116.12 g/mol plus an additional 50% excess to ensure that the precipitation is quantitative:

this corresponds to a volume of:
In addition to the
primary adsorbed silver ions
, some nitrate ions form an electrostatic layer around the nucleus.
These
counter ions
tend to aggregate around the [AgCl:Ag] center because these centers have a net positive charge (excess Ag ) and additional negative charge is required to maintain electrical neutrality.

Primary adsorbed Ag

Example:
When Ag is precipitated from solution through the addition of Cl¯ :



the (low) solubility of AgCl(
s
) is further reduced by the excess of Cl¯ that is added, pushing the equilibrium to the right (
Le Chatelier’s
Principle).


Spreadsheet Problem

Additional Problems

Steps in the Procedure:

A weighed sample (containing the
analyte
) is dissolved
An excess of a precipitating agent is added to this solution
The resulting precipitate (
precipitated form
) is filtered, dried (and/or
ignited
) and weighed
From the mass and known composition of the precipitate, the amount of the original ion can be determined (from the
Gravimetric Factor
).
In general the sought form of the analyte is different from the weighed precipitate (
weighing form
). The stoichiometry is important (always write down the chemical equations!)

Gravimetric Analysis

Criteria for Gravimetric Analysis

Harris 8th ed., pg 681.

To measure Ni in steel, the alloy is dissolved in 12 M HCl and neutralised in the presence of citrate ion, which maintains iron in solution.

The slightly basic solution is warmed and dimethylglyoxime (DMG) is added to precipitate the strawberry-red DMG-nickel complex quantitatively.

The product is filtered, washed with cold water, and dried at 110 °C.

Example: Ni in Steel

Some current uses of gravimetric analysis…
Chemical analysis of ores and industrial materials
Calibration of instrumentation
Elemental analysis of inorganic compounds

CVB210 Chemical Measurement Science
Definition:

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


Gravimetric Analysis with Solubility Equilibria
Harris 8th Ed., QCA, pg 674

Precipitation of Cl¯ as AgCl(
s
)
Gravimetric Analysis

Module 1:
Learning Objectives:
Main topics:
tick where you stand?

Solubility Equilibria
(solubilty product, common-ion effect,
calculations)

Gravimetric Analysis
(understanding the steps: precipitation,
digestion, filtration, drying, performing
the calculations)

Quality Issues (ISO/IEC 17025)
(theory, practice, basic statistics)
□ Good
□ Progressing
□ Needs effort
□ Good
□ Progressing
□ Needs effort
Self-assessment : where am I?
□ Good
□ Progressing
□ Needs effort
Gravimetry is among the most accurate and precise methods of analytical techniques (but it is tedious!). T. W. Richards used it to determine atomic weights. He received the Nobel Prize in 1914 for his work (Christian
et al.
p342).
see http://nobelprizes.com
Criterion 1

1. The desired substance must completely precipitate from solution.

In most determinations the precipitate must be of such low solubility that dissolution of the analyte is negligible.

An additional factor is to make use of the
"common ion"
effect, further reducing the solubility of the precipitate.

+
+
Practical Gravimetric Analysis

Criterion 2
2. The weighed form of the product should be of known composition.

The Table below (from
Harris QCA
) lists the substance sought, the precipitate
formed, the form in which it is weighed, and the common elements that will interfere and must be absent.
Criterion 3

+
Precipitation video
Harris 8th Ed., Table 26-1

Note:
that the precipitated form is not always the form weighed.

3. The product should be "pure" and easily filtered.
It is usually difficult to obtain a product that is "pure“ (
i.e.
, free from impurities)
Careful precipitation and sufficient washing may reduce the level of impurities


Criterion 2
Harris 8th Ed., Table 26-2, pg 676.

There are also a large number of organic compounds that are very useful precipitating agents for metals. Organic precipitating agents have the advantages of giving precipitates with very low solubility in water and a favorable gravimetric factor.
IMPURITIES IN PRECIPITATES

Precipitates tend to carry down from the solution other constituents that are normally soluble, causing the precipitate to become contaminated. This process is called coprecipitation. The process may be equilibrium based or kinetically controlled. There are a number of ways in which a foreign material may be coprecipitated:

Occlusion and Inclusion
Surface Adsorption
Isomorphous Replacement
Postprecipitation

Note:
In order to understand how impurities are introduced the fundamentals of precipitation needs to be understood.

Crystallization occurs in phases:

Induction period
The time before nucleation occurs after the addition of the precipitating agent to the solution
May range from milliseconds to several minutes
Nucleation
Formation of small, stable aggregates or nuclei of precipitate
Nuclei have sizes down to ~1 nm, composed of a few atoms, and there may be up to 10¹⁰ nuclei per mole of analyte
Excess ions from solution collect around the nuclei.
Particle Growth
Nucleation proceeds faster than particle growth in a highly supersaturated solution to produce tiny particles
In a less supersaturated solution, nucleation is slower, and nuclei have a chance to grow into larger, more tractable particles.
Mechanism of Precipitation

Crystal Growth
Ideal Analytical Precipitation
+
Colloidal Paricles
Electrical Double Layer
+
+
Ionic compounds are usually precipitated in the presence of an electrolyte. To understand why, consider how tiny colloidal crystallites coagulate (come together) into larger crystals.

Example:
Silver nitrate is added very slowly to an acidic solution containing chloride. Silver chloride nuclei form with a surface layer of ions. The “charged” AgCl particles (or colloidal particles) repel each other.

Loosely associated
counter ion

Nucleus of AgCl(s) colloid

Harris 8th Ed., Figure 26-2, pg 678.

Homogeneous solution (charges balanced)

Illustration of an Electrical
Double Layer
Adsorption
is a process in which a substance (gas, liquid, or solid) condenses onto the surface of a solid.

The
electric double layer
of a colloid consists of a layer of charge associated with the surface of the particles and a layer with a net opposite charge in the solution surrounding the particles.

A
colloid
is a finely divided particle (typically with diameters from 10 nm to 1 mm) that forms a stable dispersion within a medium (air or liquid).
Terminology:
Counter ions are less tightly held than the primary adsorbed ions and the counter ion layer is somewhat diffuse and contains ions other than those of the counter ions.
These layers of charged ions associated with the surface of the nuclei are known as the electric double layer.
The degree to which attractive or repulsive forces in the electrical double layers dominate, determines the stability, or coagulation, of the colloid.
In an ideal world, an analytical precipitate for gravimetric analysis should consist of perfect crystals large enough to be easily washed and filtered.

The perfect crystal would be free from impurities and be large enough so that it presented a minimum surface area onto which foreign ions could be adsorbed.

The precipitate should also be "
insoluble
" (
i.e
., low solubility such that loses from dissolution would be neglible).
Digestion
Coprecipitation…

…is the precipitation of an unwanted species along with your analyte of interest;
… occurs to some degree in every gravimetric analysis;
A major factor for precipitations of barium sulfate and those involving hydrous oxides
… and cannot be avoided, but can be minimized by careful precipitation and a thorough washing of the precipitate.
Impurities in Precipitates
Conditions for Precipitation
Precipitation from hot solution
The molar solubility (
S
) of precipitates increases with an increase in temperature
An increase in S decreases the supersaturation and increases the size of the particle.

Precipitation from dilute solution
This keeps the molar concentration of the mixed reagents low. Slow addition of precipitating reagent and thorough stirring keeps
Q
low. (Uniform stirring prevents high local concentrations of the precipitating agent.)

Digestion
Heating the precipitate within the mother liquor (or solution from which it precipitated) for a certain period of time to encourage densification of nuclei.

During digestion, small particles dissolve and larger ones grow (Ostwald ripening). This process helps produce larger crystals that are more easily filtered from solution

Conditions for Precipitation
Precipitation at a pH near the acidic end of the pH range in which the precipitate is quantitative.
Many precipitates are more soluble at the lower (more acidic) pH values and so the rate of precipitation is slower.
Digestion of the precipitate.
The digestion period can lead to improvements in the organization of atoms within the crystalline nuclei, such as expulsion of foreign atoms (or other impurities).

Surface adsorption
Unwanted material is adsorbed onto the surface of the precipitate

Impurities in Precipitates
Impurities in Precipitates
Impurities in Precipitates
Precipitation in the presence of electrolyte
Coulombic repulsion is diminished in the presence of electrolyte because of a compression of the volume of the ionic atmosphere
Impurities in Precipitates
Digestion of a precipitate reduces the relative surface area and, therefore, the area available for adsorption of impurities
Washing can remove impurities bound to the surface
Occlusion
A type of coprecipitation in which impurities are trapped within the growing crystal

Post-precipitation
Sometimes a precipitate in contact with the mother liquor is contaminated by the precipitation of an impurity

Insect trapped in amber
Inclusion
A type of coprecipitation in which the impurities occupy the crystal lattice sites (also known as
isomorphous replacement
)

Peptidization
A procedure where the precipitate is washed and filtered, but part of the precipitate reverts to the colloidal form because supporting electrolyte is gone.
Cooling the system with an ice-water bath minimizes loss of precipitate due to dissolution
Digestion
Raising the temperature will increase the collision energy for colloidal particles and overcome Coulombic repulsion, leading to formation of larger particles (coalescence)

Re-precipitation
a procedure including washing away the mother liquor, redissolving the precipitate, and precipitating the product again

Drying the solid
Generally the solids are dried at  ~120 °C, but conditions for  drying can vary considerably.
Impurities in Precipitates
Problem:
If the nickel content is known to be near 3 wt% and you wish to analyze 1.0 g of steel, what volume of 1.0 wt% alcoholic DMG solution should be used to give a 50% excess of DMG for the analysis? Assume that the density of the alcoholic solution is 0.79 g/mL.
How Much Precipitant to Use?
+
Chloride can be determined by adding excess silver nitrate to a sample containing chloride by the formation of AgCl(
s
):
Precipitation of Cl¯ as AgCl(
s
)
A 10.0 mL solution containing Cl¯was treated with excess AgNO₃ to precipitate 0.4368 g of AgCl. What was the concentration of Cl¯ in the unknown? (RFM for AgCl = 143.321 g/mol)

Number of moles of Cl¯ = number of moles of AgCl
Harris 8th Ed., QCA, pg 681.

Don't forget to try Problems 1.7 to 1.9 from the Lecture 1 notes
The iron content in a 2.287-g sample of iron ore is determined by precipitating as Fe(OH)₃, igniting to Fe₂O₃, and weighing. The result is a net weight of 0.8792 g. Prepare a spreadsheet to calculate the wt% Fe in the ore.
Christian
et al
., pg 362-3.
Examples
and
Problems
Full transcript