Determination of purity

The freezing point of several samples of benzene was determined as a function of the time (Francis, Reading University). Some of the results obtained are given in table 1, where t denotes time and E the e.m.f. obtained with a copper-constantan single junction thermocouple for which 1 degree interval gave 39.0 pV. 

The time scale has been slightly corrected so that the rate of extraction of heat is the same in the three cases. The values at i = 0 have been obtained by extrapolation to avoid the error due to supercooling. 

Column (a) relates to Analar benzene as supplied. Column (b) relates to this benzene after being washed with sulphuric acid and fractionally distilled. Column (c) rdates to the same benzene after seven recrystallizations. 

It will be assumed that equal quantities of benzene crystallize in equal times and that the oystalline phase is pure. 

Then from the theimodynamics of extremely dilute solutions 

---

Determination of purity. The ultraviolet and visible absorption is often a fairly intensive property thus e values of high intensity bands may be of the order of 10 -10 . In infrared spectra e values rarely exceed 10 . It is therefore often easy to pick out a characteristic band of a substance present in small concentration in admixture with other materials. Thus small amounts of aromatic compounds can be detected in hexane or in cyclohexane. 

Pressures Helium—60 psig, Hydrogen—10 psig Quantitative determinations of purity were made at the Analytical Laboratory of Dow Chemical Co. by GLC using flame ionization and electron capture detectors. Unknown samples were compared with prepared standards containing the compounds in question. 

Some GLP professionals consider positive control substances (i.e., those materials intended to validate responsiveness of the test system to a chemical or biological challenge) to also be reference substances, as opposed to control substances however, the point is somewhat moot as the FIFRA GLP Standards characterization and handling requirements are exactly the same for both control and reference substances. As with test and control substances, reference substances must be characterized (usually determination of purity and stability) according to the abbreviated GLP Standards (see next section). 

Liquids can be sampled as either the neat liquid (pure) or mixed with a solvent (solution). Neat liquids are tested when the purpose of the experiment is either identification or the determination of purity. Identification is possible because the spectrum is a fingerprint when no solvent or contaminant is present. Impurities are found when extraneous absorption bands or distortions in analyte absorption bands appear. 

Although gradient elution is generally required for RPLC separations of proteins, isocratic elution can be successful in some instances. For example, isocratic elution has been used for the determination of purity of production batches of biosynthetic human growth hormone (HGH).42 The method was used to... 

Kasicka, V, and Prusik, Z. (1989). Isotachophoretic analysis of peptides — selection of electrolyte systems and determination of purity.. Chromatogr. 470, 209—221. 

To further confirm the ability of the reduced CE-SDS method to be used for determination of purity, percent corrected peak area (%HC, %LC, and %non-main) was calculated for each component with respect to the total corrected peak area. Eigure 8 presents the relationship between %HC, %LC, %non-main species, and injection time. 

S) S.M. Kaye, PATR 1937(1953) (Determination of purity of EDNA by titration in non aqueous medium)... 

Note N.M-Liszt suggested the use of Fe for determination of purity of EGDN, but he did not describe the procedure... 

Then, % NG = 2.523VN/0.1W, where V is the vol of Ti(III) corrected for a blank, N is the normality of the Ti(Ilt), and W is the weight of the NG sample. Variations of this redox titration may be found in Refs 3, 4 8 Determination of Purity of NG by the Dewarda Method. This method involves the saponification of NG to glycerol and nitrate ion. The latter is.then reduced with Dewarda s alloy, a mixture of Al, Zn and Gi. The resultant ammonia is thin distd out and titrated (Refs 2, 5, 10 St 17)... 

Determination of purity. The purity shall be detd by either the nitrometer method or the uit rtiutive chrc iOus chloride tsethed... 

Measurements are made of the simple physical properties of the freezing point (this property will usually already have been measured in the determination of purity), boiling point, density, and refractive index. The foregoing properties are necessary. The following properties may also be determined if equipment is available viscosity, solubility in a proper solvent, and critical solution temperature in a proper solvent. 

The optical properties of the components of petroleum have been of major importance in connection with their identification and in the determination of purity. The primary effort has been directed to the study of pure hydrocarbons and only limited work has been concerned with the prediction of the index of refraction and the specific rotation of hydrocarbon mixtures. Table V summarizes the optical properties of a number of the principal components of petroleum. Only a few references to the optical properties of pure hydrocarbons of primary interest to the analyst have been included. Developments (9) in refractometers have materially increased the potentialities of the index of refraction measurements at atmospheric pressure as an analytical method. Consideration of the pertinent data in this field is beyond the scope of the present discussion. Reviews of developments in infrared (24, 26) and mass spectrometry (68) are available. 

Determination of purity of inorganic compds of ordnance interest by titration in inorganic medium 3 D102... 

Determination of Purity of Inorganic Compounds of Ordnance Interest by Titration in Inorganic Medium. Title of PATR 1947 (1953) by Seymour Kaye... 

Amino acid analysis can be used for the partial assessment of the primary structure of a polypeptide. With small peptides, such as relaxin, it is a useful technique for the determination of purity (11). At the very least,... 

Problem 6.1 Determining of Purity Within a Two-component Cluster Derivatives, Correlation Coefficients and PC plots... 

Statements relating to solubility given in the Description, however, are intended as information regarding approximate solubilities only and are not to be considered as Codex-quality requirements. Such statements are considered to be of minor significance as a means of identification or determination of purity. For those purposes, dependence must be placed upon other specifications. 

---