Chromium salts, standardization

Workplace. The Occupational Safety and Health Administration (OHSA) has estabUshed workplace permissible exposure limits (PEL) for chromium metal and three forms of chromium compounds. OSHA s PEL for chromic acid and chromates is 0.1 mg/m 3 both a ceiling, ie, no exposure above this concentration is allowed, and an 8-h time-weighted average (TWA). Chromium metal and insoluble chromium salts have an 8-h TWA PEL of 1.0 mg/m Cr, and the same standard is 0.5 mg/m Cr for soluble Cr(III) and Cr(II) compounds (144). 

Chloro-5-thiophenethiol, 50,106 3a-Cholestanol, 50, IS Chromium(II)-en perchlorate, 52, 62 Chromium(II) salts, standardization procedure for solutions, 52, 64 Chromium trioxide-pyridine complex, 52,5... 

The first polymerizations were free radical reactions. In 1933 researchers at ICI discovered that ethene polymerizes into a branched structure that is now known as low density polyethene (LDPE). In the mid- 50s a series of patents were issued for new processes in which solid catalysts were used to produce polyethene at relatively low pressures. The first was granted to scientists at Standard Oil (Indiana) who applied nickel oxide on activated carbon and molybdenum oxide on alumina. Their research did not lead to commercial processes. In the late 40s Hogan and Banks of Phillips were assigned to study the di- and trimerization of lower olefins. The objective was to produce high octane motor fuels. When they tried a chromium salt as promoter of a certain catalyst (Cr was a known reforming... 

The iodometric method depends on the fact that chromates interact with hydrochloric acid, liberating chlorine, which in its turn liberates an equivalent amount of iodine from potassium iodide. The iodine may then be determined by titration with standard sodium thiosulphate. This method is largely used for chromium in whatever state of oxidation it may be, a chromium salt being first converted to chromate either by fusion v th sodium peroxide or by boiling its solution wdth nitric acid and lead dioxide. - ... 

The corrosion products, that are produced in corrosion of higher chromium-alloyed steels, contain chromium salts that lower pH levels considerably in the process of hydrolysis. To investigate the potential influence of this more pronounced pH level reduction, steels with different chromium contents were tested [66]. Table 75 lists the alloying contents of the steels used in order of increasing chromium content. These are four standard commercial types and four test melts. Bending samples were used as described in DIN 50915 [64]. The sample material was adjusted to different hardness values by means of simulated heat treatments. 

BackTitrations. In the performance of aback titration, a known, but excess quantity of EDTA or other chelon is added, the pH is now properly adjusted, and the excess of the chelon is titrated with a suitable standard metal salt solution. Back titration procedures are especially useful when the metal ion to be determined cannot be kept in solution under the titration conditions or where the reaction of the metal ion with the chelon occurs too slowly to permit a direct titration, as in the titration of chromium(III) with EDTA. Back titration procedures sometimes permit a metal ion to be determined by the use of a metal indicator that is blocked by that ion in a direct titration. Eor example, nickel, cobalt, or aluminum form such stable complexes with Eriochrome Black T that the direct titration would fail. However, if an excess of EDTA is added before the indicator, no blocking occurs in the back titration with a magnesium or zinc salt solution. These metal ion titrants are chosen because they form EDTA complexes of relatively low stability, thereby avoiding the possible titration of EDTA bound by the sample metal ion. 

Primary and secondary amines, double bonds, aldehydes, sulfides and certain aromatic and dihydroaroraatic systems are also oxidized by chromium VI reagents under standard hydroxyl oxidizing conditions. Amines are commonly protected by salt formation or by conversion to amides. Aldehydes and... 

Discussion. Chromium (III) salts are oxidised to dichromate by boiling with excess of a persulphate solution in the presence of a little silver nitrate (catalyst). The excess of persulphate remaining after the oxidation is complete is destroyed by boiling the solution for a short time. The dichromate content of the resultant solution is determined by the addition of excess of a standard iron(II) solution and titration of the excess of the latter with standard 0.02 M potassium dichromate. 

Procedure. Weigh out accurately an amount of the salt which will contain about 0.25 g of chromium, and dissolve it in 50 mL distilled water. Add 20 mL of ca 0.1 M silver nitrate solution, followed by 50 mL of a 10 per cent solution of ammonium or potassium persulphate. Boil the liquid gently for 20 minutes. Cool, and dilute to 250 mL in a graduated flask. Remove 50 mL of the solution with a pipette, add 50 mL of a 0.1 M ammonium iron(II) sulphate solution (Section 10.97, Procedure A), 200 mL of 1M sulphuric acid, and 0.5 mL of /V-phenylanthranilic acid indicator. Titrate the excess of the iron(II) salt with standard 0.02M potassium dichromate until the colour changes from green to violet-red. 

Potassium trisoxalatochromate(III) was prepared by Graham s method (75), as described by Palmer (32). Solutions of the recrystallized salt, used for studies on the trisoxalatochromate(III) ion, were standardized by analysis for chromium as before (7, 20). 

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