Calculation of Results from Gravimetric Data

The results of a gravimetric analysis are generally computed from two experimental measurements: the mass of sample and the mass of aproduct of known composition.

The precipitate we weigh is usually in a different form than the analyte whose weight we wish to report. The principles of converting the weight of one substance to that of another depend on using the stoichiometric mole relationships. We introduced the gravimetric factor (GF), which represents the weight of analyte per unit weight of precipitate. It is obtained from the ratio of the formula weight of the analyte to that of the precipitate, multiplied by the moles of analyte per mole of precipitate obtained from each mole of analyte, that is,

We obtain the weight of substance sought from the weight of the precipitate and the corresponding weight/mole relationship

where g_A represents the grams of analyte (the desired test substance)

and g _sample represents the grams of sample taken for analysis. We can write a general formula for calculating the percentage composition of the substance sought:

29)Extractionin chemistry is aseparation processconsisting in the separation of asubstance from a matrix. It may refer to Liquid-liquid extraction, and Solid phaseextraction.

Other techniques include Supercritical carbon dioxide extraction, ultrasonic extraction, heat reflux extraction, microwave-assisted extraction, DIC (instant controlled pressuredrop).

Extractions use two immiscible phases to separate a solute from one phase into the other. The distribution of a solute between two phases is an equilibrium condition described by partition theory. Boiling tea leaves in water extracts the tannins, theobromine, and caffeine out of the leaves and into the water. More typical lab extractions are of organic compounds out of an aqueous phase and into an organic phase.

Common extractants are arranged from ethyl acetate to water (ethyl acetate < acetone < ethanol < methanol < acetone:water (7:3) < ethanol:water (8:2) < methanol:water (8:2) < water) in increasing order of polarity according to the Hildebrand solubility parameter.

The extract can be put back to dried form using a centrifugal evaporator or a freeze-drier.

Chemical extraction does not destroy wastes but is a means of separating hazardous contaminants from soils, sludges, and sediments, thereby reducing the volume of the hazardous waste that must be treated. The technology uses an extracting chemical and differs from soil washing, which generally uses water or water with wash-improving additives. Commercial-scale units are in operation. They vary in regard to the chemical employed, type of equipment used, and mode of operation.

Liquid-Liquid Systems

Liquid-liquid extraction is another type of heterogeneous equilibrium. The phase boundary is between two liquids, and most often this is an aqueous and an organic layer. The substrate/solute is distributed between the two layers. For practical purposes, organic products should ideally be transferred almost completely into the organic layer. Although liquid-liquid extraction has lost some importance in analytical chemistry due to the time factor in attaining equilibrium, it is still very valuable. In quantitative laboratory work, in particular, it is used for the separation of metal ions, because it does not require special tools.

Technically, liquid-liquid extraction is used, e.g., for the recovery of metals from scrap or waste, and for the extraction or purification of valuable metals, such as copper, cobalt, nickel, and zinc, from ores. High purity phosphorous acid is separated from sulfuric or hydrochloric acids for food purposes.

This chapter covers the topics of distribution theory and metal extraction.

Procedures that involve the application of the phase distribution law to the distribution of a solute between immiscible liquid phases are used extensively in analytical, preparative industrial, and laboratory processes.

In a two-phase system consisting of two immiscible liquid phases, a chemical compound is distributed according to its relative solubility in the individual phases and eventually reaches equilibrium. The distribution of this compound between the two phases is often used to enrich or separate a chemical species or a group of species from each other or from the matrix. This process is called solvent-solvent or liquid-liquid extraction.

The preparative mode of solvent-solvent extractions is still very popular in organic synthesis laboratories. For efficient, industrial applications continuous extraction processes and instrumentation have been developed. The use of dynamic rather than static principles has led to the development of countercurrent extraction systems.

During and after World War II. special solvent extraction procedures were formulated to separate rare earth metal and transuranium compounds.

Solvent-solvent extraction was the method of choice before more effective separation techniques, such as chromatography became popular in the 1950s and 1960s.

In the Manhattan Project, a giant scientific research project to produce fissionable material used in the development of the atomic bomb, solvent-solvent extraction played an important role. At that time only one method was available for the production of Pu²³⁹ in a breeder reactor. Quantity production of this fissionable metal made it necessary to develop chemical extraction procedures which work under extreme conditions.

Today, solvent-solvent extractions play a less important analytical role, because they are time consuming. They have been replaced by more modern and efficient separation techniques.

However, in technical processes, such as the plutonium/ uranium extraction (PUREX) process, it is of commercial interest. The PUREX process is used for the reprocessing of spent nuclear fuel. Uranium and plutonium are separated from the fission products and from one another after dissolution of the irradiated fuel in nitric acid. Uranium and plutonium are extracted into an organic phase of tributyl phosphate (TBP) in kerosene. The fission products remain in the aqueous nitric acid phase.

Distribution Constant: Partition Coefficient

The distribution of a solute S, equilibrated between an aqueous phase and an organic solvent may be described by an equilibrium equation:

There is no need to choose an aqueous phase as one of the two phases, and we can describe distributions between any pair of immiscible solvents, liquid 1 and lquid 2, with the appropriate solutes, S₁ and S₂. respectively:

Such systems are described by an equilibrium constant:

Where K_d is called the partition coefficient.

The determination of the distribution constant К¹_d as a thermodynamic quantity requires knowledge of the corresponding activity coefficients. However, these are generally not known, and as a first approximation we use the concentrations. Therefore, the derived quantities depend on the chosen conditions and, strictly speaking, are not constant quantities. The partition is related to the thermodynamic distribution constant according to:

where y_org and y_aq are the activity coefficients in the organic and aqueous phase species, respectively.

Distribution ratio.

In the context of practical chemical procedures, we have to go a step further beyond ideal thermodynamic behavior. For example, in the extraction of an acid HA from an aqueous into an organic phase, we are dealing with more than one chemical entity, namely the dissociated and undissociated acid forms. We can define a partition coefficient relating only to the ratio of the undissociated acid forms, according to:

which does not tell the whole story, because, in the aqueous phase the acid may coexist in the dissociated form, and m the organic phase, higher ion pair products may be formed. In general, total solute concentrations are determined by analytical investigations, and the information gained is used to explain the distribution of the species between the two phases, including speciation. The ratio of the total concentrations of the solute is a practical means of deahng with distrib ution equilibrium situations, and is called the distribution ratio D_c:

Owing to the differences in speciation in the organic and aqueous phases, D_c is concentration dependent.

Let us pursue the problem of the distribution of an acid between an aqueous and organic phase in more detail, and let us assume that the acid is a moderately strong acid in the aqueous phase and predominantly undissociated in the organic phase. Therefore, we can express our distribution ratio according to Eq. 2.

In the aqueous phase, we can apply the acid dissociation constant K_a: