Chromium solution

Concern over the health hazards of the hexavalent chromium solutions used to form the top coat of conventional nickel plus chromium coatings have encouraged research into trivalent chromium plating solutions. A process with better throwing power and improved covering power than those of hexavalent chromium has been described by Smart etal". A process for depositing a chromium-iron, or chromium-nickel-iron alloy, has been outlined by Law. ... 

Self regulating chromium The self-regulating chromium solutions were introduced to eliminate the need for maintaining the correct catalyst concentration by periodic analysis they depend on the addition of a sparingly soluble sulphate to the bath which supplies the correct amount of SO 4 automatically. Initially strontium sulphate (solubility approx. l-75g/l at 30°C and 21 g/1 at 40°C) was employed for this purpose. The strontium sulphate forms a layer on the bottom of the bath, which must be stirred from, time to time. A bath with a CrO, concentration of 250 g/1 would have a catalyst content of l 52g/l SrS04 and 4-35 g/1 of KjSiF. Potassium dichromate and strontium chromate have also found application as additives for the control of the saturation solubility of the catalyst. 

The typical amber color of the hexavalent chromium solution will turn to a pale green once the chromium has been reduced to the trivalent state. Although this color change is a good indicator, redox control is usually employed. 

Replacement of hexavalent chromium with trivalent chromium offers important environmental advantages. Trivalent chromium is considerably less toxic than hexavalent. Trivalent systems use chromium concentrations that are typically two orders of magnitude less than in hexavalent systems. Thus, far less chromium enters the waste stream. Trivalent systems also generate few toxic air emissions, while hexavalent systems involve a reaction that produces hydrogen bubbles which entrain chromium compounds and carry them out of the baths. Trivalent chromium is readily precipitated from wastewater, while hexavalent chromium solutions must go through an additional step in a treatment system in which the chromium is reduced to its trivalent form before precipitation. It has been shown that trivalent chromium systems can successfully replace hexavalent ones for decorative chrome applications. Trivalent chromium systems are not suitable for hard chrome applications. More information regarding trivalent chromium plating can be obtained from Roy (1984), Robison (1978), Chementator (1982), and Smart (1983). 

In aqueous solution, manganous salts are oxidised to manganese dioxide,6 and if silver nitrate is present as catalyst, to permanganate 0 the latter change constitutes Marshall s reaction. Chromium solutions in a similar manner give rise to chromate,7 even without a catalyst. Ferrous and cerous salts are converted into ferric and ceric salts, respectively, and phosphites are oxidised to phosphates. 

Any anhydrous chromium solution will work. Cr03 in acetonitrile or chromate esters in hexane are good candidates. Lower valent compounds, like diarenechromium in hexane, can also be used because they are oxidized to the same Cr(VI) surface species during the second calcining in air. Even chromyl chloride vapor can be used if enough surface hydroxyls are left to... 

Typical coatings of chromium applied over nickel are about 0.3 pm thick and are under high stress—which gives rise to cracking. Cracks in the chromium are initiated by corrosion of the underlying nickel and (ultimately) of the substrate however, development in the sixties indicated that an increase in the extent of cracking would spread corrosion more widely over the surface of the nickel and so reduce the speed of penetration—a conclusion that led in turn to chromium solutions giving micro-cracked deposits and to overall enhanced resistance to corrosion. 

Standard Chromium Solution (1000 mg/kg) Transfer 2.829 g of l C Oy, accurately weighed (National Institute of Standards and Technology No. 136) into a 1-L volumetric flask dissolve in and dilute to volume with water. 

Procedure Set the instrument at the optimum conditions for measuring chromium as directed by the manufacturer s instructions. Prepare a series of seven standard chromium solutions containing Cr at approximately 5, 10, 15, 20, 40, 50, and 60 mg/kg by appropriate dilutions of the Standard Chromium Solution into 100-mL volumetric flasks add 80 mL of the Standard Colorant Solution, and dilute each flask to volume with water. 

Chromium 520.8 nitrous oxide/ acetylene Chromium is subject to interference from nickel. It is advisable to adjust the fuel flow until the absorbance obtained from a 1500 Mg ml-1 chromium solution and a 1500 Mg ml-1 chromium plus nickel at the sample level produce the same absorbance... 

The application of microanalytical techniques, such as flow injection in ICP-MS (FI-ICP-SFMS), is also of special interest in medical research where very small sample volumes have to be characterized, e.g. for Cr determination in DNA by sector-field ICP-MS.For the separation of isobaric interferences of Cr and Ar C the measurements were performed at a mass resolution of m/Am = 3000. Transient signals of Cr and Cr analyzed by FI-ICP-SFMS of a I0p,gl chromium solution (sample loop 20p,l) are illustrated in Figure 9.31a. For quantification by the isotope dilution technique, the small volume of DNA available (diluted 1 10) was injected into a continuous flow of 2 % HNO3, which is spiked with high-enriched Cr solution (the isotope abundance of Cr was 83.4%). The application of the isotope dilution technique in flow injection ICP-SFMS is shown in Fig. 9.31 b. ... 

Note. In an emergency, the sample of urine may be aspirated directly into a conventional air-acetylene flame and its absorbance at 357.9 nm compared with urine from an unexposed subject. Standard Chromium Solutions. Dissolve 7.6958 g of chromic nihate [Cr(NO ), 9H O] in sufficient M nitric acid to produce 1000 ml. This solution contains 1 mg of Cr in 1 ml. Serially dilute the solution with 0.01 M nihic acid to produce solutions containing 0.01,0.02,... 

When the apparatus has been flushed, a boiling solution of 16 g. (0.20 mol) of anhydrous sodium acetate dissolved in 35 ml. of water is added through E A slow nitrogen flow is maintained, and a solution of 9 g. (0.034 mol) of chromium(III) chloride 6-hydrate in 15 ml. of 0.4 N sulfuric acid is poured into the top of the redactor. The rate of flow of the chromium solution can be controlled by stopcock B. If too fast a rate is used, there is a possibility of incomplete reduction. Distilled water is poured after the chromium chloride until the effluent is only slightly colored by chromium. This requires approximately 125 ml. 

Derivation From chromite by direct reduction (fer-rochrome), by reducing the oxide with finely divided aluminum or carbon, and by electrolysis of chromium solutions. 

Two commercial size plants for groundwater treatment based on liquid membrane technology in general, and the supported liquid membrane using hollow fibers in particular, were built and operated in Baltimore, U.S.A. Specifically, the purpose of the two plants is for hexavalent chromium cleanup. One plant went into commercial operation in March 1999 and the other approximately about a year later. The liquid membrane system in these two plants is able to reduce metal-ion concentration from 100-1000 ppm range to approximately 0.05 ppm and, meanwhile, produce a concentrated chromium solution, which is the spent strip solution, at approximately 20% Cr (VI). This concentration is suitable for sale for reuse. 

Prepare the standard chromium solutions for calibration. Pipette respectively, for example, 0.1 ml, 0.2 ml, 0.4 ml, 0.5 ml and 0.7 ml of a concentrated standard chromium solution (such as 1000 ppm, commercially available) into 100 ml volumetric flasks. Dilute them to the mark with deionized water. The concentrations of the diluted standard solutions are 1 ppm, 2 ppm, 4 ppm, 5 ppm and 7 ppm, respectively. 

Turn on the instruments and select the element to be tested. Enter the values of the concentrations of the diluted standard chromium solutions. 

FIGURE 129 LCB contents of polymers made with three types of catalysts. LCB is highest in polymers when the catalyst is activated by the two-step process with anhydrous chromium impregnation. In the bottom line, an aqueous chromium solution rehydrated the catalyst surface, which leads to low LCB again. Polymerization tests were conducted at 100 °C, 3.8 MPa, and 0.24 mol 1-hexene L... 

We observed that by using the same Zr02 solution and the same copper-chromium solution we could prepare successive honeycomb catalysts with which the temperature for 50% conversion of CO and HC varied less than 10°C. 

Figure 3.13. Illustration of the determination of concentration by the method of standard additions. Curve A Polarogram of chromate ion reduction from the chromium in an aliquot of a digested steel sanv-pie. Curve B A polarogram of a similar aliquot to which has been added a known volume of a standard chromium solution.

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