Chromate solutions

Dissolve 15-0 g. of A.R. barium nitrate and 130 g. of A.R. cupric nitrate trihydrate in 450 ml. of water at 80°. Prepare a solution of sodium chromate by dissolving 89 g. of recrystallised sodium dichromate dihydrate in 200 ml. of water and adding 112 5 ml. of cone, ammonia solution (sp. gr. 0-90). Add the warm solution (80°) of nitrates in a thin stream, with stirring, to the sodium chromate solution (at 25°). Collect the orange precipitate by suction Bltration, wash it with two 50 ml. portions of 5fiter, drain well, and dry at 75-80° for 12 hours powder finely. 

Strontium Chromate. Strontium chromate [7789-06-2] SrCrO, is made by precipitation of a water-soluble chromate solution using a strontium salt or of chromic acid using a strontium hydroxide solution. It has a specific gravity of 3.84 and is used as alow toxicity, yellow pigment and as an anticorrosive primer for zinc, magnesium, alurninum, and alloys used in aircraft manufacture (8) (see Corrosion and corrosion control). 

The neutralized, alumina-free sodium chromate solution may be marketed as a solution of 40° Bh (specific gravity = 1.38), evaporated to dryness, or crystallized to give a technical grade of sodium chromate or sodium chromate tetrahydrate [1003-82-9] Na2Cr04 4H2O. If the fuel for the kilns contains sulfur, the product contains sodium sulfate as an impurity. This compound is isomorphous with sodium chromate and hence difficult to separate. High purity sodium chromate must be made from purified sodium dichromate. 

Steel immersed in dilute chromate solutions does not mst. The exact mechanism of the inhibition is not known, although it is agreed that... 

A dichromate or chromate solution is reduced under pressure to produce a hydrous oxide, which is filtered, washed, and calcined at 1000°C. The calcined oxide is washed to remove sodium chromate, dried, and ground. Sulfur, glucose, sulfite, and reducing gases may be used as reducing agent, and temperatures may reach 210°C and pressures 4—5 MPa (600—700 psi). 

Nagayama, M. and Okamoto, G., The Anodic Behaviour of Passive Iron in Chromic Acid-Chromate Solutions , Corros. Sci., 2, 203 (1962)... 

Phosphate solutions containing fluorides are used for processing steel, zinc and aluminium when assembled together, but chromate solutions are generally preferred when aluminium is treated alone. The increasing use of cathodic electrophoretic painting on steel, however, has led to a reassessment of the basic processes and formulations that might be most effective. 

The protective value of a phosphate coating is enhanced by a dip or rinse in an acid chromate solution. Joint Service Specification DEF-29 makes such a rinse mandatory for steel parts treated by an accelerated process, and optional after treatment by a non-accelerated process. Details of rinses are given in Section 15.2 (Table 15.10, p. 15 30). 

Magnesium is a relatively reactive metal, and can be chromated in nearly neutral solutions as well as in acid solutions. The range of treatments possible illustrates well the role of pH, activating anion, temperature and duration of treatment in promoting the breakdown of passivity in the chromate solution and the consequent formation of a chromate him. 

Another treatment consists of making the silver parts cathode in an alkaline chromate solution. 

Magnesium alloys Dip for approximately 1 min in boiling 15% chromic acid to which has been added with agitation 1 % silver chromate solution. 

Preparing one liter of 0.100 Of potassium chromate. A 0.100 M potassium chromate solution is made by adding enough water to 19.4 g of K2C1O4 to make one liter of solution. 

Weigh out accurately about 0.10 g of analytical grade sodium chloride and about 0.20 g of potassium bromide, dissolve the mixture in about 2.0 mL of water and transfer quantitatively to the top of the column with the aid of 0.3 M sodium nitrate. Pass 0.3 M sodium nitrate through the column at a flow rate of about 1 mL per minute and collect the effluent in 10 mL fractions. Transfer each fraction in turn to a conical flask, dilute with an equal volume of water, add 2 drops of 0.2M potassium chromate solution and titrate with standard 0.02M silver nitrate. 

The theory of the process is as follows. This is a case of fractional precipitation (Section 2.8), the two sparingly soluble salts being silver chloride (Xsol 1.2 x 10 10) and silver chromate (Kso] 1.7 x 10 12). It is best studied by considering an actual example encountered in practice, viz. the titration of, say, 0.1M sodium chloride with 0.1M silver nitrate in the presence of a few millilitres of dilute potassium chromate solution. Silver chloride is the less soluble salt and the initial chloride concentration is high hence silver chloride will be precipitated. At the first point where red silver chromate is just precipitated both salts will be in equilibrium with the solution. Hence ... 

FIGURE 16.8 The chromate ion, Cr04 . is yellow. When acid is added to a chromate solution the ions form orange dichromate ions. 

Wastewaters from the coil coating on aluminum subcategory contain higher levels of cyanide and fluorides than the other subcategories as a result of chromating solutions containing cyanide ions... 

Figure 7 Regeneration of ODA-clinoptilolite columns loaded with chromate by means of 2% NaCI and 2% Na2S04 aqueous solutions and breakthrough curves for ODA- clinoptilolite in 0.5 mM/L chromate solution by 30 BV/hr and 15 BV/hr in downflow mode (from the left)...

Chromate solution should be chilled before addition of H2S04,... 

Materials Required Potassium chloride 0.25 g potassium chromate solution (5% w/v in water) 10 ml 0.1 N silver nitrate solution. 

Procedure Weigh accurately about 0.25 g of potassium chloride in a conical flask and dissolve it in 50 ml of DW and titrate with 0.1 N silver nitrate solution, using 2-3 drops of potassium chromate solution as indicator till precipitation of red chromate is indicated. Each ml of 0.1 N silver nitrate solution is equivalent to 0.007455 g of KC1. 

Materials Required Chloral hydrate 4.0 g sodium hydroxide (N) 30 ml sulphuric acid (N) phenolphthalein solution (1.0% w/v in 50% v/v alcohol) 0.1 N silver nitrate solution potassium chromate solution (5% w/v in water). 

A great research interest concerns zinc corrosion inhibition. Chromate solutions were used [289-293] for the inhibition of zinc corrosion. The surface film contained a mixture of zinc oxide and hydroxide of chromium(III). Different behaviors of the passivated zinc electrode was observed depending on the cation of the used chromate [293]. 

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