Gravimetric Methods of Analysis: A Comprehensive Guide
Gravimetric analysis is a highly precise quantitative technique in analytical chemistry that measures an analyte based on its mass. It’s commonly used to determine sample concentration or analyte quantity.
Types of Gravimetric Analysis
There are two main types:
Precipitation Gravimetry: The analyte is precipitated from a solution, and the resulting solid is weighed.
- Involves converting analyte into a sparingly soluble precipitate
- Precipitate is filtered, washed, and converted to a product of known composition for weighing
- Example: Determining calcium in water using oxalic acid and ammonia to form calcium oxalate
Volatilization Gravimetry: Volatile compounds are removed from the sample, and the change in mass is measured.
- Commonly used for determining water and carbon dioxide
- Water is distilled by heating and collected using solid desiccants
- Direct method measures mass gain of the desiccant
- Indirect method measures loss of mass of the sample during heating (less reliable)
- Widely used for determining moisture in commercial products, like cereal grains
- Semi-automated instrument can directly measure percentage of moisture in a heated sample using an infrared lamp
| Features | Precipitation Gravimetry | Volatilization Gravimetry |
|---|---|---|
| Method | Converts analyte into a sparingly soluble precipitate | Distills water or other volatile components by heating |
| Process | Filter, wash, convert to known composition, and weigh | Collect on desiccants or measure loss of mass during heating |
| Applications | Determining calcium in water, for example | Determining water and carbon dioxide in samples |
| Reliability | Highly reliable, based on known composition of precipitate | Less reliable for indirect method, as it assumes only water is volatilized |
Factors Affecting in Gravimetric Analysis
A. Particle Size
The particle size of precipitates can range from tiny colloidal particles to larger particles in crystalline suspensions. Factors influencing precipitate size include solubility, temperature, reactant concentrations, and mixing rate. These factors can be accounted for using relative supersaturation:
Relative Supersaturation = (Q – S) / S
- Here, Q represents the solute concentration at any instant, and
- S is its equilibrium solubility.
Since precipitation reactions are slow, some supersaturation is likely even with careful reagent introduction.
Particle size is inversely related to average relative supersaturation, with large supersaturation leading to colloidal precipitates and small supersaturation resulting in crystalline solids.
- Colloidal suspensions consist of tiny particles that don’t settle and are hard to filter. They scatter light, causing the Tyndall effect.
- Crystalline suspensions have larger particles (tenths of a millimeter or greater) that settle spontaneously and are easily filtered. The temporary dispersion of these particles is characteristic of crystalline suspensions.
B. Crystalline Precipitates
Crystalline Precipitates are more easily filtered and purified than coagulated colloids, and their particle size and filterability can be controlled to some extent.
Methods to Improve Particle Size and Filterability:
- Minimize Q or maximize S in the Relative Supersaturation equation by using dilute solutions, slow addition of the precipitating reagent, and good mixing.
- Increase S by precipitating from hot solutions or adjusting the pH.
- Digestion of crystalline precipitates (without stirring) after formation improves purity and filterability due to dissolution, recrystallization, and bridging between particles.
Coprecipitation is the removal of otherwise soluble compounds during precipitate formation. It’s not the same as contamination by a second substance whose solubility product has been exceeded. Four types of coprecipitation exist:
- Surface adsorption
- Mixed-crystal formation
- Occlusion
- Mechanical entrapment
Surface adsorption and mixed-crystal formation are equilibrium processes, while occlusion and mechanical entrapment arise from the kinetics of crystal growth.
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C. In Homogeneous Solutions
Precipitation from Homogeneous Solution is a technique where a precipitating agent is generated in the analyte solution through a slow chemical reaction. This method prevents local reagent excesses and keeps relative supersaturation low.
Homogeneously formed precipitates, both colloidal and crystalline, are better suited for analysis than those formed by direct addition of a precipitating reagent.
Urea is often used for the homogeneous generation of hydroxide ions. This hydrolysis is slow at temperatures just below 100°C.
Urea is valuable for precipitating hydrous oxides or basic salts, resulting in dense, easily filtered, and higher purity products. Homogeneous precipitation also increases crystal size and improves purity.
Gravimetric Analysis Steps
Performing a gravimetric analysis involves the following steps:
- Sample preparation: Dissolve the sample in a suitable solvent and filter if needed.
- Precipitation: Add a reagent to precipitate the analyte as a solid.
- Filtration: Separate the precipitate from the remaining solution.
- Drying or ignition: Dry or heat the precipitate to remove volatile substances.
- Weighing: Determine the mass of the dried or ignited precipitate.
- Calculations: Calculate the analyte amount or concentration based on the measured mass and precipitation reaction stoichiometry.
Accuracy and precision depend on careful execution of these steps.
When Gravimetric Method is Used?
Applications of Gravimetric Methods are widespread in determining various inorganic anions, cations, neutral species, and organic substances. Gravimetric methods are among the most widely applicable analytical procedures.
Inorganic Precipitating Agents: Common inorganic precipitating agents, typically form slightly soluble salts or hydrous oxides with the analyte. These reagents are not very selective.
Reducing Agents: several reagents that convert an analyte to its elemental form for weighing.
| Reducing Agent | Analyte |
|---|---|
| SO2 | Se, Au |
| SO2 + H2NOH | Te |
| H2NOH | Se |
| H2C2O4 | Au |
| H2 | Re, Ir |
| HCOOH | Pt |
| NaNO2 | Au |
| SnCl2 | Hg |
| Electrolytic reduction | Co, Ni, Cu, Zn, Ag, In, Sn, Sb, Cd, Re, Bi |
Organic Precipitating Agents: Many organic reagents have been developed for gravimetric determination of inorganic species, offering higher selectivity compared to inorganic reagents. There are two types of organic reagents:
- Those that form slightly soluble nonionic products called coordination compounds.
- Those that form products with bonding between the inorganic species and the reagent being largely ionic.
Common Examples
Gravimetric methods have been widely employed across various industries due to their accuracy and precision. Some applications include:
- Environmental Analysis: Assess air quality by monitoring particulate matter (PM) concentrations.
- Pharmaceutical Industry: Quality control and purity assessment of raw materials and finished products.
- Metallurgical Applications: Assessment of metal purity and alloy composition.
Advantages and Disadvantages
Advantages
- Accuracy and Precision: Gravimetric analysis provides highly accurate results when followed carefully.
- No Calibration Curves: Solid-phase nature eliminates the need for calibration curves, increasing reliability.
Disadvantages
- Time-consuming: Gravimetric analysis can be slow compared to other techniques like spectrophotometry.
- Limited to Single Elements: Gravimetric analysis may not be suitable for multi-element analysis, as techniques are often intricate.
In conclusion, gravimetric analysis is a valuable analytical technique that offers accuracy and precision for specific determinations.
However, its limitations and challenges should be considered when deciding on the most suitable method for a particular analysis.
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