Chromates, estimation

Addition of silver nitrate to a solution of a chloride in dilute nitric acid gives a white precipitate of silver chloride, AgCl, soluble in ammonia solution. This test may be used for gravimetric or volumetric estimation of chloride the silver chloride can be filtered off, dried and weighed, or the chloride titrated with standard silver nitrate using potassium chromate(VI) or fluorescein as indicator. 

Silver nitrate is used volumetrically to estimate chloride, bromide, cyanide and thiocyanate ions. Potassium chromate or fluorescein is used as an indicator. 

Owing to environmental regulations, the consumption of lead-based pigments, lead chromate [7738-97-6], in particular, has been decreasing steadily at an average aimual rate of about 5% since the early 1980s. In 1993 production was estimated to be around 20,000 t (see Lead compounds). 

The prices of some important chromium chemicals are given ia Table 4, and production and shipment data for sodium chromate and dichromate are given ia Table 5. Data for the productioa and shipment of chromic acid have not been available siace 1972. However, traditionally CrO has held at about 30—35% of sodium dichromate production. The estimated capacity for domestic production of sodium dichromate is 150,000 to 200,000 t/yr. 

Sodium Chloride [25]. Sodium chloride estimation is based on sodium titration. To 20 ml of a 1 1 mixture of toluene (xylene) isopropyl alcohol, add a 1-ml sample of oil-base mud, stirring constantly and 75 to 100 ml of distilled water. Add 8-10 drops of phenolphthalein indicator solution and titrate the mixture with H SO (N/10) until the red (pink) color, if any, disappears. Add 1 ml of potassium chromate to the mixture and titrate with 0.282N AgNO (silver nitrate, 1 ml = 0.001 g chloride ions) until the water portion color changes from yellow to orange. Then... 

Strongly basic antibiotics may be precipitated by formation of the coloured reineckate salt which may then be determined spectrophotometri-cally 65. Bickfordl66 dissolved the precipitated neomycin reineckate in acetone and has successfully used this procedure to assay neomycin extracted from topical formulations. Roushdi et al 73 preferred to oxidise the precipitate with potassium permanganate and then colorimetrically estimate the chromate produced with diphenylcarbazide. 

I he precipitation of lead(II) chromate is used to estimate lead gravimetrically the yellow precipitate of lead(II) chromate is filtered off. dried and weighed. Lead(II) chromate is used as a pigment under the name chrome yellow . 

Chromium is a common anthropogenic contaminant in surface waters, therefore Cr isotope fractionations are of potential interest in tracking Cr + pollution in groundwaters. Ellis et al. (2002, 2004)) and Izbicki et al. (2008) analyzed ground-water samples from contaminated sites and observed an increase in Cr/ Cr ratios up to 6%c during the reduction of chromate. Equilibrium fractionations between Cr(VI) and Cr (III) have been estimated by Schauble et al. (2002), who predicted Cr isotope fractionations >l%c between Cr species with different oxidation states. 

Phosphate must be applied as fertilizer to the soil. Ideally it is added in quantities sufficient to guarantee optimal yields, but not in excess in order to avoid P transportation into other compartments of the ecosystem. The amount added should be based on an accurate estimation of the plant-available fraction of P already present in a soil.This is an old and difficult task and a large number of extraction methods have been used since intensive land use was practised. Recently methods have been worked out in which a strip of filter paper impregnated with an Fe oxide (2-line ferri-hydrite) is dipped into a soil suspension and the amount of P adsorbed by the paper is taken as being plant-available (Sissingh,1988 Van der Zee et ah, 1987 Sharpley, 1993 Sharpley et ah,1994 Kuo and Jellum, 1994 Myers et ah 1997). Anion and cation resins extracted more P from four heavily fertilized soils than from goethite (Delgado Torrent, 2000). Other oxyanions adsorbed by soil Fe oxides are silicate, arsenate, chromate, selenite ( ) and sulphate. Adsorption of sulphate led to a release of OH ions and was substantially lowered once the Fe oxides were selectively removed (Fig.16.17). 

Other reagents which may be applied to the volumetric determination of sulphuric acid and sulphates are barium chromate and benzidine. In the case of the former, the solution of sulphate is precipitated by a solution of barium chromate in hydrochloric acid on subsequent neutralisation of the filtrate, a quantity of chromic acid, equivalent to the barium sulphate which has been precipitated, remains in solution and may be estimated iodometrically.6 Benzidine, on the other hand, is an organic base which forms a very sparingly soluble sulphate the solution of mineral sulphate is treated with a solution of benzidine hydrochloride and the precipitated benzidine sulphate removed by filtration when subsequently suspended in pure water the benzidine sulphate undergoes hydrolysis to a sufficient extent to permit titration of the sulphuric acid with standard alkali.7 Lead nitrate may also be... 

A laser lens for electrons has spherical aberration, which has a negligible effect on estimate (6), and also chromatic aberration, which is a more serious matter. For the particular numerical example we are discussing here, the electrons would have to be monochromatic within something on the order of 0.01%. 

Silver chromate-based cells were manufactured in button and rectangular (prismatic) form in a number of sizes. The energy density of such complete systems is estimated as 200 Wh/kg or 575 Wh/dm3, to a 2.5 V cut-off. A schematic cross-section of a typical button cell of this type is shown in Fig. 

Excess of Lead Oxide in Chrome Orange and Chrome Red.— 1 gram of the substance is treated in the cold with dilute acetic add, which dissolves only the lead oxide not combined as normal chromate in the filtered solution the lead is estimated by the ordinary methods. When no other substances soluble in acetic acid are present, the insoluble part (normal chromate and any extraneous insoluble substances present) may be collected, washed, dried and weighed the lead oxide is then given by difference. 

Funt et al. (1991, 1992) use a finite dimensional linear model to recover ambient illumination and the surface reflectance by examining mutual reflection between surfaces. Ho et al. (1992) show how a color signal spectrum can be separated into reflectance and illumination components. They compute the coefficients of the basis functions by finding a least squares solution, which best fits the given color signal. However, in order to do this, they require that the entire color spectrum and not only the measurements from the sensors is available. Ho et al. suggest to obtain the measured color spectrum from chromatic aberration. Novak and Shafer (1992) suggest to introduce a color chart with known spectral characteristics to estimate the spectral power distribution of an unknown illuminant. 

Figure 8.4 Funt et al. (1996) transform the colors of the input image to rg-chromaticity space. The input layer of the neural network samples the triangular shaped region of the chromaticity space. The network consists of an input layer, a hidden layer, and an output layer. The two output neurons estimate the chromaticity of the illuminant.

Cardei and Funt (1999) suggested to combine the output from multiple color constancy algorithms that estimate the chromaticity of the illuminant. Their approach is called committee-based color constancy. By combining multiple estimates into one, the root mean squared error between the estimated chromaticity and the actual chromaticity is reduced. Cardei and Funt experimented with committees formed using the gray world assumption,... 

Figure 12.17 The images in (a) and (b) show the chromaticities of the actual color of the illuminant. The images in (c) and (d) show the estimated chromaticities. The two graphs in (e) and (f) show the estimated and the actual color of the illuminant for a horizontal line of the image.

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