GRAVIMETRIC (LABORATORY)

• Classical methods/ wet chemical methods/ earliest methods of analysis

  • Relied mainly on chemical properties of analytes.
• Analytes are treated with reagents to form products that could be identified.
• Gravimetric and titrimetric methods were used for quantitative analysis.
• Classical Methods of Analysis

  Examples:

  1.Formation of precipitate and measurement of mass.

  2.Oxidation of analyte and detection of end via the change of color of the analyte.

  3.Neutralization of analyte and detection of end point using acid-base indicator.

  4.Complexation of analyte and use of metallochromic indicators to detect end point.

  Separation of analyte from matrix was achieved using precipitation, extraction and distillation.
• Gravimetric analysis

  • it describes as a set of methods used in analytical chemistry for the quantitative determination of an analyte (the ion being analyzed) based on its mass.
• Gravimetric Analysis

  • The principle behind this type of analysis is that once an ion's mass has been determined as a unique compound, that known measurement can then be used to determine the same analyte's mass in a mixture, as long as the relative quantities of the other constituents are known.
• Step in Gravimetric Method:

  Step 1: Dissolve

  Step 2: Add solution containing So4^2-

  Step 3: Filter and weigh the solid PbSO4
• ADVANTAGES:

  • Provides for exceedingly precise analysis
  • Gravimetric analysis was used to determine the atomic masses of many elements to six figure accuracy.
  • Gravimetry provides very little room for instrumental error.
  • It does not require a series of standards for calculation of an unknown.
  • Also, methods often do not require expensive equipment.
  • Gravimetric analysis, due to its high degree of accuracy, when performed correctly, can also be used to calibrate other instruments in lieu of reference standards.
• DISADVANTAGES:

  • Gravimetric analysis usually only provides for the analysis of a single element, or a limited group of elements, at a time.
  • Comparing modern dynamic flash combustion coupled with gas chromatography with traditional combustion analysis will show that the former is both faster and allows for simultaneous determination of multiple elements while traditional determination allowed only for the determination of carbon and hydrogen.
• DISADVANTAGES:

  • Methods are often convoluted and a slight mis-step in a procedure can often mean disaster for the analysis (colloid formation in precipitation gravimetry, for example).
• FOUR MAIN TYPES OF GRAVIMETRIC METHOD:

  • Precipitation
  • Volatilization
  • Electro-analytical
  • Miscellaneous physical method
• Miscellaneous physical method:

  • Gravimetric titrimetry
  • Atomic Mass Spectroscopy
• Precipitation Gravimetry

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

  • The analyte used in precipitation gravimetry is converted into a sparingly soluble precipitate. The precipitate is filtered, washed free of impurities, converted to a product of known composition by suitable heat treatment and weighed.
• Precipitation Gravimetry (Procedure)

  • Precipitation and Digesting
  • Filtering the Precipitate
  • Rinsing the Precipitate
  • Drying and weighing the final Precipitate
• Precipitating and Digesting:

  Properties of Precipitates and Precipitating Reagents:

  1. Easily filtered and washed free contaminants
  2. Sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing.
  3. Unreactive with constituents of the atmosphere.
  4. Known chemical composition after it is dried or, if necessary, ignited.
• Precipitation Gravimetry (Avoiding Impurities)

  • In addition to having a low solubility, the precipitate must be free from impurities. Because precipitation usually occurs in a solution that is rich in dissolved solids, the initial precipitate is often impure. We must remove these impurities before determining the precipitate’s mass.
  • The greatest source of impurities is the result of chemical and physical interactions occurring at the precipitate’s surface. A precipitate is generally crystalline—even if only on a microscopic scale—with a well-defined lattice of cations and anions.
• Precipitation Gravimetry (Controlling the particle size)

  • A homogeneous precipitation produces large particles of precipitate that are relatively free from impurities. These advantages, however, are offset by requiring more time to produce the precipitate and a tendency for the precipitate to deposit as a thin film on the container’s walls. The latter problem is particularly severe for hydroxide precipitates generated using urea.
• Precipitation Gravimetry (Controlling the particle size)

  • A visible precipitate takes longer to form when RSS is small both because there is a slow rate of nucleation and because there is a steady decrease in RSS as the precipitate forms. One solution to the latter problem is to generate the precipitant in situ as the product of a slow chemical reaction. This maintains the RSS at an effectively constant level.
• Precipitation Gravimetry (Controlling the particle size)

  • Because the precipitate forms under conditions of low RSS, initial nucleation produces a small number of particles. As additional precipitant forms, particle growth supersedes nucleation, resulting in larger precipitate particles. This process is called homogeneous precipitation.
• Precipitation Gravimetry (Controlling the particle size)

  • Two precipitates of PbCrO4. In Beaker A, combining 0.1 M Pb(NO3)2 and 0.1 M K2CrO4 forms the precipitate under conditions of high RSS. The precipitate forms rapidly and consists of very small particles.
• Precipitation Gravimetry (Controlling the particle size)

  • In Beaker B, heating a solution of 0.1 M Pb(NO3)2, 0.1 M Cr(NO3)3, and 0.1 M KBrO3 slowly oxidizes Cr3+ to CrO42–, precipitating PbCrO4 under conditions of low RSS. The precipitate forms slowly and consists of much larger particles.
• Filtering the Precipitate:

  • After precipitating and digesting the precipitate, we separate it from solution by filtering. The most common filtration method uses filter paper, which is classified according to its speed, its size, and its ash content on ignition. Speed, or how quickly the supernatant passes through the filter paper, is a function of the paper’s pore size. A larger pore allows the supernatant to pass more quickly through the filter paper, but does not retain small particles of precipitate.
• Filtering the Precipitate:

  • Proper procedure for transferring the supernatant to the filter paper cone.
• Filtering the Precipitate:

  • An alternative method for filtering a precipitate is a filtering crucible.
  • The trap prevents water from an aspirator from back-washing into the suction flask.
• Rinsing the Precipitate:

  • Because the supernatant is rich with dissolved inert ions, we must remove any residual traces of supernatant to avoid a positive determinate error without incurring solubility losses.
  • In many cases this simply involves the use of cold solvents or rinse solutions containing organic solvents such as ethanol. The pH of the rinse solution is critical if the precipitate contains an acidic or basic ion.
• Rinsing the Precipitate:

  • Adding a volatile inert electrolyte to the rinse solution prevents the precipitate from reverting into smaller particles that might pass through the filter. This process of reverting to smaller particles is called peptization. The volatile electrolyte is removed when drying the precipitate.
• Drying the Precipitate:

  • After separating the precipitate from its supernatant solution, the precipitate is dried to remove residual traces of rinse solution and any volatile impurities.
  • The temperature and method of drying depend on the method of filtration and the precipitate’s desired chemical form. Placing the precipitate in a laboratory oven and heating to a temperature of 110°C is sufficient when removing water and other easily volatilized impurities.
• Drying the Precipitate:

  • Higher temperatures require a muffle furnace, a Bunsen burner, or a Meker burner, and are necessary if we need to thermally decompose the precipitate before weighing.
• Volatilization Gravimetry

  • the analyte is separated from other constituents of a sample by converting it to a gas known chemical composition. The mas of the gas then serves as a measure of the analyte concentration.
• Volatilization Gravimetry

  • In volatilization methods, removal of the analyte involves separation by heating or chemically decomposing a volatile sample at a suitable temperature In other words, thermal or chemical energy is used to precipitate a volatile species For example, the water content of a compound can be determined by vaporizing the water using thermal energy (heat). Heat can also be used, if oxygen is present, for combustion to isolate the suspect species and obtain the desired results.
• Volatilization Gravimetry:

  • One method for determining the products of a thermal decomposition is to monitor the sample’s mass as a function of temperature, a process called thermogravimetry.
• Volatilization Gravimetry:

  • Depending on the method of analysis, the equipment for volatilization gravimetry may be simple or complex. In the simplest experimental design, we place the sample in a crucible and decompose it at a fixed temperature using a Bunsen burner, a Meker burner, a laboratory oven, or a muffle furnace. The sample’s mass and the mass of the residue are measured using an analytical balance.
• Volatilization Gravimetry:

  • Trapping and weighing the volatile products of a thermal decomposition requires specialized equipment. The sample is placed in a closed container and heated. As decomposition occurs, a stream of an inert purge-gas sweeps the volatile products through one or more selective absorbent traps.
• Thermogravimetric Analysis

  • the sample is placed on a small balance pan attached to one arm of an electromagnetic balance. The sample is lowered into an electric furnace and the furnace’s temperature is increased at a fixed rate of few degrees per minute while continuously monitoring the sample’s weight. The instrument usually includes a gas line for purging the volatile decomposition products out of the furnace, and a heat exchanger to dissipate the heat emitted by the furnace.
• Volatilization Gravimetry:

  • In a thermogravimetric analysis, the sample is placed on a small balance pan attached to one arm of an electromagnetic balance. The sample is lowered into an electric furnace and the furnace’s temperature is increased at a fixed rate of few degrees per minute while continuously monitoring the sample’s weight. The instrument usually includes a gas line for purging the volatile decomposition products out of the furnace, and a heat exchanger to dissipate the heat emitted by the furnace.
• Volatilization Gravimetry:

  • Volatilization methods can be either direct or indirect. Water eliminated in a quantitative manner from many inorganic substances by ignition is an example of a direct determination. It is collected on a solid desiccant and its mass determined by the gain in mass of the desiccant.
• Volatilization Gravimetry:

  • Another direct volatilization method involves carbonates which generally decompose to release carbon dioxide when acids are used. Because carbon dioxide is easily evolved when heat is applied, its mass is directly established by the measured increase in the mass of the absorbent solid used.
• Volatilization Gravimetry:

  • Determination of the amount of water by measuring the loss in mass of the sample during heating is an example of an indirect method. It is well known that changes in mass occur due to decomposition of many substances when heat is applied, regardless of the presence or absence of water. Because one must make the assumption that water was the only component lost, this method is less satisfactory than direct methods.
• Electrogravimetry

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