Normal chromium

Let s try an example. You know that chromium is a micronutrient that is an essential part of our diets. You might also remember that chromium pico-linate got a lot of attention several years ago as a magic weight-loss supplement. The Centers for Disease Control and Prevention lists normal chromium levels in blood as around 2.5 pg per 100 mL of whole blood. What is the concentration of chromium in parts per million You will need to know that the density of whole blood is 1.06 g/mL. 

Normal chromium levels in human fluid and tissues should be interpreted with caution. The low sensitivity of the most commonly used detection methods and the ubiquitous presence of chromium in laboratories make detection of low levels of chromium in blood and urine difficult. Everyone is exposed to chromium in the diet, estimated to range from 25 to 224 pg/day with an average of 76 pg/day (Kumpulainen et al. 1979). Only a small amount of dietary chromium is absorbed ( 3%). Normal endogenous chromium levels for the general population (exposed only via the diet) have been reported as 0.01-0.17 pg/L (median 0.06 pg/L) in serum (Sunderman et al. 1989), 0.24-1.8 pg/L (median 0.4 pg/L) in urine (Iyengar and Woittiez 1988), and 0.234 mg/kg in hair (Takagi et al. 1986). 

From the isotopic decomposition of normal chromium one finds that the mass-50 isotope, 5°Cr, is the second least abundant of all the Cr isotopes 4.35% of all Cr. Using the total abundance of elemental Cr = 1.35 x lo4 per million silicon atoms in solar-system matter, this isotope has... 

Figure 7 Normalized chromium La.a level AEAPS spectra for the CrN films and elemental chromium. The emission current was 2 mA and the modulation voltage was 1 Vp p. The strength of the signal (i.e., the density of unoccupied states at the Fermi level) is more for the films. (Reproduced with permission from Chourasia AR and Hood SJ (2001) Auger electron appearance potential spectroscopy. Surface and Interface Analysis 31 291-296 John Wiley and Sons Ltd.)...

The martensitic steels are normally chromium steels with 12-18% Cr, carbon contents of 0.1-1.2% and in some cases alloyed with molybdenum (up to 1.5%) or nickel (up to 3%). A subgroup is formed by steels with only about 0.05% C and a higher nickel content (4—5%), the soft or nickel martensitic steels. The mechanical properties of martensitic steels can be changed within a broad range by heat treatment (hardening or tempering) depending on the carbon content. Martensitic steels also include hardenable steel with about 0.04% C, 16% Cr, 4% Ni, 4% Cu. 

Carbon content is usually about 0.15% but may be higher in bolting steels and hot-work die steels. Molybdenum content is usually between 0.5 and 1.5% it increases creep—mpture strength and prevents temper embrittlement at the higher chromium contents. In the modified steels, siUcon is added to improve oxidation resistance, titanium and vanadium to stabilize the carbides to higher temperatures, and nickel to reduce notch sensitivity. Most of the chromium—molybdenum steels are used in the aimealed or in the normalized and tempered condition some of the modified grades have better properties in the quench and tempered condition. 

Cobalt—chromium films (20 at. % Cr) exhibiting strong perpendicular anisotropy, ie, hexagonal i -axis normal to the substrate surface, have been studied (53). Fifty nanometer films are composed of columnar crystaUites and the domain size was found to be a few stmctural columns in diameter. Magnetization reversal was shown to occur by domain rotation in thick films. Thinner (ca 10-nm thick) films do not show the columnar crystaUite... 

Over time a large variety of materials have been used, including ivory, stainless steel, chromium—cobalt, and ceramics for the acetabular component. None proved sufficient. The implant material composition must provide a smooth surface for joint articulation, withstand hip joint stresses from normal loads, and the substance must disperse stress evenly to the cement and surrounding bone. 

Carbon disulfide is normally stored and handled in mild steel equipment. Tanks and pipes are usually made from steel. Valves are typically cast-steel bodies with chrome steel trim. Lead is sometimes used, particularly for pressure reUef disks. Copper and copper alloys are attacked by carbon disulfide and must be avoided. Carbon disulfide Hquid and vapor become very corrosive to iron and steel at temperatures above about 250°C. High chromium stainless steels, glass, and ceramics maybe suitable at elevated temperatures. 

Russia and the RepubHc of South Africa account for more than half the world s chromite ore production. Almost all of the world s known reserves of chromium are located in the southeastern region of the continent of Africa. South Africa has 84% and Zimbabwe 11% of these reserves. The United States is completely dependent on imports for all of its chromium (4). The chromite s constitution varies with the source of the ore, and this variance can be important to processing. Typical ores are from 20 to 26 wt % Cr, from 10 to 25 wt % Fe, from 5 to 15 wt % Mg, from 2 to 10 wt % Al, and between 0.5 and 5 wt % Si Other elements that may be present are Mn, Ca, Ti, Ni, and V. AH of these elements are normally reported as oxides iron is present as both Fe(II) andFe(III) (5,6). 

Low Oxidation State Chromium Compounds. Cr(0) compounds are TT-bonded complexes that require electron-rich donor species such as CO and C H to stabilize the low oxidation state. A direct synthesis of Cr(CO)g, from the metal and CO, is not possible. Normally, the preparation requires an anhydrous Cr(III) salt, a reducing agent, an arene compound, carbon monoxide that may or may not be under high pressure, and an inert atmosphere (see Carbonyls). 

When Cr202 is introduced as an impurity into the a-Al202 lattice, as occurs in the semiprecious mineral mby, the color is red rather than the normal green. This color anomaly is the result of ligand field splitting of the Cr(III) ion (51,52). Chromium (ITT) also colors other minerals (53). 

Acute and Chronic Toxicity. Although chromium displays nine oxidation states, the low oxidation state compounds, -II to I, all require Special conditions for existence and have very short lifetimes in a normal environment. This is also tme for most organ ochromium compounds, ie, compounds containing Cr—C bonds. Chromium compounds that exhibit stabiUty under the usual ambient conditions are limited to oxidation states II, III, IV, V, and VI. Only Cr(III) and Cr(VI) compounds are produced in large quantities and are accessible to most of the population. Therefore, the toxicology of chromium compounds has been historically limited to these two states, and virtually all of the available information is about compounds of Cr(III) and/or Cr(VI) (59,104). However, there is some indication that Cr(V) may play a role in chromium toxicity (59,105—107). Reference 104 provides an overview and summary of the environmental, biological, and medical effects of chromium and chromium compounds as of the late 1980s. 

This conversion is normally accompHshed by immersion, but spraying, swabbing, bmshing, and electrolytic methods are also employed (178) (see Metal SURFACE treatments). The metals that benefit from chromium surface conversion are aluminum, cadmium, copper, magnesium, silver, and 2inc. Zinc is the largest consumer of chromium conversion baths, and more formulations are developed for 2inc than for any other metal. 

In some parts of the world, as in Russia, fermented alcohol can serve as a cheap source for hutadiene. The reaction occurs in the vapor phase under normal or reduced pressures over a zinc oxide/alumina or magnesia catalyst promoted with chromium or cohalt. Acetaldehyde has been suggested as an intermediate two moles of acetaldehyde condense and form crotonaldehyde, which reacts with ethyl alcohol to give butadiene and acetaldehyde. 

---