Frequent Impurities

Oxalate is found to be a frequent impurity in pharmaceutical substances belonging to the category of either organic acids e.g.. anhydrous citric acid, tartaric acid or salts of organic acids e.g., ferrous gluconate, sodium citrate, potassium citrate and sodium cromoglycate. The presence of this impurity is due to the following two prime factors, namely ... 

Frequent Impurities Table A.4. Recognition of frequent impurities ... 

The data base of some 27,000 powder diffraction patterns that is used in the CIS (5) is in fact a direct descendant of that with which Hanawalt carried out his pioneering work. A problem that arises in connection with this particular component stems from the fact that powders, as opposed to crystals, are frequently impure and so the patterns that are obtained experimentally are often combinations of one or more file entries. A reverse searching program, that examines the experimental data to see if each entry from the file is contained in it, has been written after the general approach of Abramson (23), and seems to cope with this particular difficulty. It is currently running in test on the NIH PDP-10 and will be made available to the scientific community during the latter part of 1978. 

The most frequent impurities of commercial a-titanium trichloride are generally other chlorides (TiCU, TiCU), metallic titanium, titanium nitride, and the products resulting from oxydation or hydrolysis of the titanium chlorides, the latter being unstable at air and moisture. 

In order to overcome some of these shortcomings, a number of HPLC methods have been introduced. These methods usually offer good resolution of the most important fatty acids, but the detection of underivatized fatty acids is neither sensitive nor selective, because these compounds generally do not contain suitable chromophores. Absorption of underivatized fatty acids near 200 nm cannot be recommended, because it is adversely affected by the properties of solvents and frequent impurities in organic solvents, which is specifically undesirable in gradient elution. 

In the case of impure starting compounds (hydroxide and oxide layers are frequent impurities), corrosion problems appear leading, for example, to the formation of phosphide oxides. Such corrosion problems may be prevented by the use of iimer containers made from Mo, Nb, or Ta. Silica as a crucible material is also problematic, since silicon might replace some of the phosphorus atoms, and this is hardly detectable by X rays. 

Talc occurs naturally and is mined, therefore the mineral composition may vary depending on the source [IS]. The most frequent impurities are alumina, calcium oxide, and iron oxide [4], The mined material is pulverized, and the talc is separated from impurities using a floatation process [16]. The fine talc is boiled in dilute hydrochloric acid to remove iron and other soluble impurities. It is then washed with water and dried. 

Amyl nitrite is frequently impure its boiling-point should not vary more than two or three degrees from that given al ve. 

Apart from water, the most frequent impurity is acetaldehyde. The water content is eliminated by reaction with acetic anhydride. Acetaldehyde can be oxidized to acetic add with chromium(VI) oxide or potassium permanganate. The simplest purification procedure consists of refluxing glacial acetic add containing acetic anhydride for 1 h with chromium(VI) oxide, followed by fractional distillation. 

Frequent impurities in ben2 ne are thiophene, cyclohexane and sometimes toluene. Removal of the thiophene content is of particular importance and can be effected by shaking with concentrated sulphuric acid. Benzene is then freed from sulphuric acid by washing with dilute sodium hydroxide solution, and the residual alkali is removed by washing with water. The solvent is dried by treatment on a molecular sieve, and the purification process is completed by fractional distillation. 

Apart from water, the most frequent impurity in this solvent is the peroxide, but it may also contain other organic impurities. Refluxing with lithium aluminium hydride and subsequent distillation may be employed for simultaneous dehydration and purification in one step. 

Dibutyl phthalate [25] (frequent impurity due to its use as polymer plasticizer)... 

Compounds for isotopic analysis must be rigorously purified, since any oxygen containing impurities will give spurious results. The two most frequent impurities are water, which can usually be removed by prolonged vacuum drying, and carbonate (particularly in phosphates and other salts), which can only be removed by chemical methods, such as treatment with dilute acid. Other impurities can be removed by the usual methods of purification summarized in Section IV. 

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