Dichromate, adsorption

Equation (3) also shows that the Cr(III)/ Cr(VI) equilibrium is pH-dependent in a manner that favors Cr(VI) uptake during coating formation, and Cr(VI) release during service. Dichromate adsorption is favored at low pH - typical of coating baths. Dichromate release is favored under higher pH conditions more typical of service conditions in which release and self-healing are necessary. 

Glass slides with the dimensions of 76 x 26 x 1 mm were used as supports for the multilayers. The slides were thoroughly cleaned by a mixture of sulfuric acid and potassium dichromate at 80 °C for about 2 hours in an ultrasonic bath prior to the films deposition. The initial concentration co of polyelectrolytes in the solution was 1 x 10 2 mole of the repeating units per liter. The pH value of the solution was about 6. Adsorption was carried out at room temperature in open glass beakers of 100 mL without stirring for 20 min. After every deposition step, the glass slides were rinsed three times for 1 min with Millipore Milli-Q water. The substrate was not dried between the adsorption steps. 

For example, the corrosion rate of steel in 0.01 % Na2Cr207 is <0.0001-in. penetration per year (i.p.y.), but on addition of NaCl to make a 3.5% solution, the rate increases to 0.0017 i.p.y., which is still a low rate. In absence of dichromate, the rate is 0.024 i.p.y. Halide ions catalyze reduction of dichromates probably by introducing imperfections in the adsorbed film (through competitive adsorption), at which areas metal ions not only enter solution but also HjO+ can discharge and hydrogen atoms can adsorb. It is probably the adsorbed hydrogen atoms that reduce adsorbed chromate, or dichromate. The situation is similar to that described by Langmuir... 

Water solutions of mercury in the fig/L concentration range have been found to be unstable due to absorption to the storage containers and/or evaporation after spontaneous reduction to Hg(0). Pretreatment of glass vessels with an acid-dichromate solution has been reported to considerably reduce loss of mercury from dilute (0.3 fig/L) standards, probably due to saturation of the available adsorption sites with chromium (Litman et al., 1975). Numerous other container treatment procedures and preserving additives have been suggested to avoid loss of mercury from aqueous samples (see section aqueous samples ). 

On the other hand, aluminum metal also has an oxide coating. Smith (29) found that the layer is approximately 5 nm thick on ethanol-HC104-electropolished metal and about 20 nm thick on acid-dichromate-etched metal (also determined by ellipsometry). However, adsorption of virus on aluminum is considerably stronger than on AI2O3 and is quite characteristic of metals as predicted by the Lifshitz theory. 

In the following sections it will be shown that several valence states of chromium are present on chromia-alumina catalysts. These are associated with the adsorption of oxygen, and the formation of chemical bonds between the chromium and oxygen ions. Therefore it is of interest to discuss intermediate chromium oxides (49,50) which are analogs of chromates, dichromates, peroxychromates, etc. Hydrated oxides such as CrOOH (51) will not be treated. 

Speciation and solubility of chromium in wetlands and aquatic systems is governed by the competition among chromium oxidation states, adsorption/desorption mechanism, and soil/sediment redox-pH conditions. Chromium (VI) is reduced to chromium (HI) at approximately +350 mV in soils and sediment. Reduced Cr(III) can be rapidly oxidized to the tetravalent chromate and dichromate forms by manganese compounds. Cr(III) is much less soluble in natural system than the hexavalent form and has a much lower toxicity. Chromium is less likely to be a problem in wetlands than in nonwetlands because the reducing conditions cause its reduction or conversion to the more insoluble Cr(III) form. This is depicted in Figure 12.15, which shows changes in water-soluble chromium as affected by the soil redox potential. 

Distorted bands are the result of bad sample appbcation, adsorption of protein to the column material, or a balance of protein between different polymerization states. A prebminary run with BSA and/or adding TRITON-X-100 or salt to the column buffer helps against the adsorption to the column material. Ideally, you apply colored markers (e.g., a mixture of cytochrome c, dextran blue, potassium dichromate) to the column before the run to check for run properties and band distortion. 

Equilibrium between the Cr(III)/Cr(VI) mixed oxide of the CCC and dichromate in solution, hence the extent of dichromate release, is proposed to be governed by Langmuir-like adsorption behavior that depends on solution ionic Strength, pH, and the ratio of CCC surface area to solution volume. Therefore, the extent of Cr(VI) release is not simply governed by solubility of dichromate in solution, or total amount of Cr(VI) in the mixed oxide, as is more closely the case for SrCr04 pigments in primer coatings. Kinetically,... 

Methods—Chemical. Fabre and Br5mond (1935) compared pycnometric, hydrometric, ebulliometric, and chemical methods for determination of alcohol and for the most accurate results preferred the chemical over the pycnometric. The former is based on oxidation of the alcohol with dichromate. Prior to chemical determination of alcohol in sweet wines and vermouth, Paronetto (1938) used a double distillation, the second one with alkali. A special adaptation of the chemical method for the determination of alcohol was proposed by Schulek and Rdzsa (1939), in which the distilled alcohol was purified by adsorption on charcoal. 

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