Gravimetry Methods and Properties of Analysis

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Gravimetric Methods of Analysis

Gravimetric methods of analysis are based on the measurement of mass.

There are three types of gravimetric methods :

• Precipitation methods : In this method the analyte is converted to a sparingly soluble precipitate. This precipitate is then filtered, washed free of impurities, and converted to a product of known composition by suitable heat treatment, and the product is weighed.

• Volatilization methods : In this the analyte or its decomposition products are volatilized at a suitable temperature. The volatile product is then collected and weighed, or, alternatively, the mass of the product is determined indirectly from the loss in mass of the sample.

• Electrolysis methods : Electrolysis method is done by reducing the dissolved metal ions into metal precipitate. Metal ions are in the form of cations when fed with certain large listrik dengan flow in a certain time there will be a reduction reaction to the metal oxidation numbers 0. Electrolysis can be applied to samples suspected to contain levels of dissolved metals is quite large as wastewater

Properties of Precipitates and Precipitating Reagents

A gravimetric precipitating agent should react specifically, and selectively with the analyte.

The ideal precipitating reagent would react with the analyte to give a product that is :

1. Readily filtered and washed free of contaminants
2. Of sufficiently low solubility so that no significant loss of the solid occurs during filtration and washing
3. Unreactive with constituents of the atmosphere
4. Of known composition after it is dried or, if necessary, ignited

Particle Size and Filterability of Precipitates

Precipitates made up of large particles are generally desirable in gravimetric work because large particles are easy to filter and wash free of impurities.

In addition, such precipitates are usually purer than are precipitates made up of fine particles. What Factors Determine Particle Size ?

The particle size of solids formed by precipitation varies enormously.

At one extreme are colloidal suspension, whose tiny particles are invisible to the naked eye (10-7 to 10-4 cm in diameter).

Colloidal particles show no tendency to settle from solution, nor are they easily filtered. At the other extreme are particles with dimensions on the order of tenths of millimeter or greater.

The temporary dispersion of such particles in the liquid phase is called a crystalline suspension. The particles of a crystalline suspension tend to settle spontaneously and are readily filtered.

The particle size of a precipitate is influenced by experimental variables as precipitate solubility, temperature, reactant concentrations, and the rate at which reactants are mixed.

The particle size is related to a single property of the system called its relative supersaturation, where relative supersaturation = (Q – S)/S

In this equation, Q is the concentration of the solute at any instant and S is its equilibrium solubility.

When (Q – S)/S is large, the precipitate tends to be colloidal. When (Q – S)/S is small, a crystalline solid is more likely.

How do Precipitates Form ?

Precipitates form in two ways, by nucleation and by particle growth. The particle size of a freshly formed precipitate is determined by which way is faster.

In nucleation, a few ions, atoms, or molecules (perhaps as few as four or five) come together to form a stable solid.

Often, these nuclei form on the surface of suspended solid contaminants, such as dust particles.

Further precipitation then involves a competition between additional nucleation and growth on existing nuclei (particle growth).

If nucleation predominates, a precipitate containing a large number of small particles results. If growth predominates, a smaller number of larger particles is produced.

Peptization of Colloids

Peptization refers to the process by which a coagulated colloid reverts to its original dispersed state.

When a coagulated colloid is washed, some of the electrolyte responsible for its coagulation is leached from the internal liquid in contact with the solid particles.

Removal of this electrolyte has the effect of increasing the volume of the counter-ion layer.

The repulsive forces responsible for the original colloidal state are then reestablished, and particles detach themselves from the coagulated mass.

The washings become cloudy as the freshly dispersed particles pass through the filter.

Crystalline Precipitates

Crystalline precipitates are generally more easily filtered and purified than coagulated colloids. In addition, the size of individual crystalline particles, and thus their filter ability, can be controlled to a degree.

The particle size of crystalline solids can often be improved significantly by minimizing Q, maximizing S, or both in Equation.

Minimization of Q is generally accomplished by using dilute solution and adding the precipitating from hot solution or by adjusting the pH of the precipitation medium.

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

The improvement  in filterability results from the dissolution and recrystallization.

Minimizing Adsorbed Impurities on Colloids

The purity of many coagulated colloids is improved by digestion.

During this process, water is expelled from the solid to give a denser mass that has a smaller specific surface area for adsorption.

Washing a coagulate colloid with a solution containing a volatile electrolyte may also be helpful because any nonvolatile electrolyte added earlier to cause coagulation is displace by the volatile species.

Washing generally does not remove much of the primarily adsorbed ions because the attraction between these ions and the surface of the solid is too strong.

Exchange occurs, however between existing counter ions and ions in the wash liquid.

Reprecipitation

A drastic but effective way to minimize the effects of adsorption is reprecipitation, or double precipitation.

Here, the filtered solid is redissolved and reprecipitated. The first precipitate ordinarily carries down only a fraction of the contaminant present in the original solvent.

Thus, the solution containing the redissolved precipitate has a significantly lower contaminant concentration than the original, and even less adsorption occurs during the second precipitation.

Reprecipitation adds substantially to the time required for an analysis.

Determination of Calcium in the limestone

Limestone is a sedimentary rock, composed mainly of skeletal fragments of marine organisms such as coral, forams and molluscs.

Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO₃).

Procedures are:

CaCO₃:

-  CaCO₃ is weighed that has been refined as 0,2000 gr

-  Added diluted HCl to dissolve the samples

Reaction: CaCO₃ + HCl | CaCl₂ + CO₂ + H₂O (Brown)

• The water is heated in the temperature of 700 – 800⁰C (bubble)
• Added Ammonium Oxalate as forming the precipitate
• Ca is precipitate (white precipitate)
• Heated about 30 minutes (brown precipitate) it happen This happens because in the solution contained impurity substances bound by NH₄ (COOH)₂ were contaminated with an excessive solvent 

Reaction: Ca²⁺ + (NH₄)₂C₂O₄→ CaC₂O₄ + NH₄⁺+

Filter with filter paper washed with distilled water repeatedly until free of chlorine and sulfate (qualitative test), the washing water is tested qualitatively by adding a precipitating reagent AgNO₃ and HNO₃.

Testing Cl added a solution of AgNO₃ and added a solution of HNO₃ still contains Cl it indicates that the sediment still contains impurities such as Cl.

These deposits occur due to the reaction of Cl with Ag + ion to form a precipitate AgCl according to the reaction:

Ag+ + Cl- → AgCl

Then the next step is to do the heating and drying in the oven, the precipitate remaining on the filter paper dried in an oven 100-110 derajat Celcius for ± 30 minutes.

Sludge drying to remove water and volatile substances. with the following equation :

CaCO₂O₄ ↓ ↓ chocolate brown CaO + CO₂ (g) + CO (g)

After heated then cooled at eksikator for ± 5 minutes Eksikator added to dipijarkan for ± 5 minutes.

The precipitate weighed:

1. Principles

• Solution reaction between analytes and reagents to give sparingly soluble products.
• Drying or ignition of precipitates.
• Weighing

2. Apparatus

• Flasks, beakers, pipettes, crucibles and filter papers.
• Oven or furnace and a dessicator.
• Analytical quality balance.

3. Applications

• Extensive numbers of inorganic ions are determined with excellent precision and accuracy.
• Routine assays of metallurgical samples.
• Relative precision 0,1 to 1%.
• Good accuracy

4. Disadvantages

• Careful and time consuming.
• Scrupulously clean glassware.
• Very accurate weighing.
• Coprecipitation.

*Sumber: Putri W, Poppy D, Shulhan Zalil A, Nadya R

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