Hydrochloric acid, impurities purification

Analytical and Laboratory Operations. Sulfamic acid has been recommended as a reference standard in acidimetry (55). It can be purified by recrystaUization to give a stable product that is 99.95 wt % pure. The reaction with nitrite as used in the sulfamic acid analytical method has also been adapted for determination of nitrites with the acid as the reagent. This reaction is used commercially in other systems for removal of nitrous acid impurities, eg, in sulfuric and hydrochloric acid purification operations. 

Other uses of HCI are legion and range from the purification of fine silica for the ceramics industry, and the refining of oils, fats and waxes, to the manufacture of chloroprene mbbers, PVC plastics, industrial solvents and organic intermediates, the production of viscose rayon yam and staple fibre, and the wet processing of textiles (where hydrochloric acid is used as a sour to neutralize residual alkali and remove metallic and other impurities). 

Electronic Grade Silicon (EGS). As the first step in the production of electronic grade silicon (EGS), an impure grade of silicon is pulverized and reacted with anhydrous hydrochloric acid, to yield primarily tricholorosilane, HSiClg. This reaction is carried out in afluidizedbed at approximately 300°C in the presence of a catalyst. At the same time, the impurities in the starter impure silicon reactto form their respective chlorides. These chlorides are liquid at room temperature with the exception of vanadium dichloride and iron dichloride, which are soluble in HSiCl3 at the low concentration prevailing. Purification is accomplished by fractional distillation. 

Developments of recent years include plants designed to precipitate the calcium sulfate in the form of the hemihydrate instead of gypsum, hi special cases, hydrochloric acid is used instead of sulfuric acid for rock digestion, the phosphoric acid being recovered in quite pure form by solvent extraction. Solvent-extraction methods have also been developed for the purification of merchant-grade acid, which normally contains impurities amounting to 12 18% of the phosphoric acid content. Processes for recovering part of the fluorine in the phosphate rock are in commercial use. 

The crude fumaric acid is from 74-78 per cent pure as found by titration with standard alkali. The only impurity present besides inorganic salts consists of small quantities of sodium hydrogen maleate which is decomposed by the hydrochloric acid during the purification process. 

The purification of uranium by precipitation may involve the formation of insoluble oxalate or peroxide complexes.50 In the oxalate method the ore concentrate is dissolved in nitric acid to give a uranyl nitrate solution from which uranyl oxalate is precipitated according to equation (101), leaving the bulk of the impurities in solution. This approach is favoured over dissolution in hydrochloric acid and reduction to UIV prior to U(C204)2 precipitation since it is simpler and... 

On partly saturating the melt with concentrated hydrochloric acid, the hydrochloride of triphenylrosaniline separates out almost in a state of chemical purity, while impurities remain dissolved in the concentrated solution of aniline in aniline hydrochloride. This latter is separated and saturated with dilute acid, and the precipitate is worked up for inferior blue. This method of purification has completely replaced the older proeess with alcohol. 

We then attempted purification of impure (.S )-144 by enantioselective HPLC. Fortunately, TBS derivative G was found to be separable by preparative HPLC on Chiralcel OD to give pure (S)-G. Deprotection of the TBS group of (S )-G under conventional conditions with TBAF caused partial racemization of (S)-144. However, treatment of (S )-G with dilute ethanolic hydrochloric acid at room temperature caused no appreciable racemization to give (.S )-144 (98.4% ee), [ah24 = -90.7 (acetone), in 40% yield. Similarly, (R)-144 was also synthesized by employing AD-mix-a instead of AD-mix-f) . (ft)-Cytosporone E (144 ), [a]o25 = +91.3 (acetone) could be obtained pure (>99% ee). As shown in the present case, use of preparative HPLC is becoming more and more important in the preparation of pure enantiomers. 

Curphey and Daniel (1978) found the catalytic effect of this acid by serendipity. They observed that crude samples of 2.26 reacted with aqueous LiN02 much more rapidly than the recrystallized samples. TLC tests indicated iV-(trifluoroacetyl)serine as a likely catalytic impurity, and chloroacetic acid was selected as an acid of similar strength. Neither acetic nor hydrochloric acid was as effective. The authors chose LiN02 rather than NaN02, because inorganic Li salts are soluble in the ethanol used for purification of azaserine (Curphey, 1989). 

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