Diphenylcarbazide method

BQuantitative Tests a)Small amts of chromates can be detd by diphenylcarbazide method as described in Ref l,p 290 or in Ref 3,p 791 or by colorimetric method developed by Dittrich and described in Ref l,pp 289 90 b)Large amts of chromates may be detd... 

Chromium is usually determined by the diphenylcarbazide method. This method is particularly useful for determining traces of chromium. Larger amounts of chromium can be determined either by the chromate method, or by the method based on the Cr(IlI)-EDTA complex. 

The diphenylcarbazide method is almost specific for chromium(Vl). Interferences result only from Fe, V, Mo, Cu, and Hg(II) present at much higher concentrations than the chromium. Iron(lll) can be masked by phosphoric acid or EDTA. Iron(III) can also be separated as Fe(OH>3, after chromium has been oxidized to Cr(VI), or by extraction. Vanadium can be separated from Cr(VI) by extraction as its oxinate at pH -4. Molybdenum is masked with oxalic acid, and Hg(II) is converted into the chloride complex. 

Oxidize the chromium to Cr(VI) in a solution containing not more than 3 mg of Cr, as described in the diphenylcarbazide method. [More KMn04 will be needed, in this case, if the Cr(III) content is greater]. 

The classical diphenylcarbazide method and a new, sensitive flotation method, based on the anionic complex of Os with SnCh, associated with Rhodamine B, are presented in detail below. 

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

Chromium (hexavalent) in chromium plating mists Colorimetric field method using 1,5-diphenylcarbazide 52/3... 

In the second method diphenylcarbazide is employed as an adsorption indicator. The end-point is marked by the pink colour becoming pale violet (almost colourless) on the colloidal precipitate in dilute solution (ca 0.01 M) before the opalescence is visible. In 0.1M solutions, the colour change is observed on the precipitated particles of silver cyanoargentate. 

Method B. Prepare the solution and transfer 25 mL of it to a 250 mL conical flask as detailed under Method A. Add two to three drops of diphenylcarbazide indicator and titrate with standard 0.1M silver nitrate solution until a permanent violet colour is just produced. 

A more sensitive method is to employ 1,5-diphenylcarbazide CO(NHNHC6H5)2 in acid solution (ca 0.2M), chromates give a soluble violet compound with this reagent. 

Aliphatic amines have been determined by a number of methods. Batley et al. [290] extracted the amines into chloroform as ion-association complexes with chromate, then determined the chromium in the complex colorimetri-cally with diphenylcarbazide. The chromium might also be determined, with fewer steps, by atomic absorption. With the colorimetric method, the limit of detection of a commercial tertiary amine mixture was 15ppb. The sensitivity was extended to 0.2 ppb by extracting into organic solvent the complex formed by the amine and Eosin Yellow. The concentration of the complex was measured fluorometrically. Gas chromatography, with the separations taking place on a modified carbon black column, was used by Di Corcia and Samperi [291] to measure aliphatic amines. 

Fodor and Fischer [84] have investigated problems of chromium speciation in soils. When employing spectrophotometric detection, only a method based on the diphenylcarbazide reaction was found suitable for chromium speciation analysis. 

Total chromium and Cr in samples of stainless steel welding dusts were determined after extraction of the samples with a pH 4 buffer solution (Girard and Hubert, 1996). The analytical method involved the use of an FI system equipped with two detectors. The first detector (spectrophotometer) measured Crvl as the diphenylcarbazide complex. The second detector (AA spectrometer) measured total chromium. The recovery of chromium was 96%, and the lower detection limit was 0.005 mg ml-1. 

International Standard Organization. 1994. Water quality. Determination of chromium(VI). Spectrometric method using 1,5-diphenylcarbazide. ISO 11083. International Organization for Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland. 

Welding fumes (total chromium(VI)) Air particulate collected on PVC filter, extracted with H2S04and complexed with diphenylcarbazide Chromatography at 540 nm Spectrophotometry at 540 nm 3.5 pg/sample No data NIOSH 1994f (Method 7604)... 

Colorimetric methods have been developed for the detection of phosgene in trichloromethane or tetrachloromethane [33,52,425,1360b,1504,1815]. A sensitive visual test involves the addition of an excess of phenylhydrazine (usually as its tra/w-3-phenylpropenoate salt), which reacts with phosgene according to equation (3.4) [52,425]. The diphenylcarbazide... 

Diphenylcarbazide reacts in acid medium with chromium(Vl) ions to give a violet solution which is the basis of this sensitive method. It has been shown that the cationic Cr(lll)-diphenylcarbazone complex is formed by oxidation of diphenylcarbazide with chromium(Vl). It has been assumed that the reaction proceeds with nascent, not yet hydrated Cr(lll) ions, obtained in the reduction of Cr(Vl) with diphenylcarbazide which is oxidized to diphenylcarbazone ... 

Zinc metal can be determined in zinc oxide by means of indirect methods. Zinc is oxidized with dichromate (in an H2SO4 or H3PO4 medium), and then the unreacted part of Cr(VI) is determined with 1,5-diphenylcarbazide [106]. In another version, the sample is dissolved in dil. sulphuric acid in the presence of Fe(III) and 1,10-phenanthroline. The red Fe(n)-phen complex is equivalent to the content of Zn in the zinc oxide studied [107]. 

A volumetric method is based on the fact that ammonium molybdate precipitates thorium as the normal molybdate but has no action on the rare earth elements.3 The mixed nitrates are dissolved in 1 15 acetic acid to which a little sodium acetate has been added. This solution is titrated cold with ammonium molybdate, using diphenylcarbazide as an outside indicator. The end point is the appearance of a deep rose color which fades quickly. Another volumetric method 4 precipitates thorium from a mixed nitrate solution with hot oxalic acid let stand, filter, wash, and add the precipitate to hot water, then add 1 1 sulfuric acid and titrate with decinormal permanganate. 

Figure 5.16 A colorimetric method, based on diphenylcarbazide, used for the analysis of hexavalent chromium[24].

There exist several analytical methods for chromium VI determination, often very complex [37]. One of the most simple and widespread is the diphenylcarbazide colorimetric method. Its detection limit is about 1 p.g/L, but the procedure is time consuming. 

A comparison between the diphenylcarbazide and UV-visible method for Cr (VI) determination has been carried out for more than 50 samples of different origins (44 industrial wastewaters, 8 urban wastewaters and 4 natural waters), the concentration of which... 

The reason why Fe3+ interferes is shown in Fig. 24. Even for a concentration of 1 mg/L, Fe3+, which is the most probable form of dissolved iron in water, absorbs around 300 nm in acidic medium. In this case, the quadratic error of the deconvolution is too high. An increase of the pH value up to 9 (with some drops of NaOH 1M, for example), leads to the precipitation of the hydroxide form, the spectrum of which is very close to the reference of suspended solids. With this simple pretreatment, the error in Cr (VI) determination between the diphenylcarbazide and the UV-visible methods becomes lower than 2.5%. 

For the determination of hexavalent chromium (at concentrations > 0.01 mg 1 ) a photometric method with diphenylcarbazide (CgHg-NH-NH-CO-NH-NH-CgHj) is used (red-violet colour). Absorbance is measured directly or after extraction into amyl alcohol at a wavelength of 540 nm (green filter) [19, 41]. 

Maximum Cr(VI) removal has been observed at pH 1.0 and Cr(Vl) removal by CM-g-PMMA decreases with the variation of pH 1.0-10.0 at 100 mg/L initial Cr(VI) concentration. Total chromium in the equilibrium solution (after the adsorption was detected by the Diphenylcarbazide (DPC) method) indicates that some Cr(Vl) converts to Cr(III) during the adsorption and remains in the equilibrium solution along with the remaining Cr(Vl). Krishnani et al. [96] have made a similar observation for the adsorption of chromium on Ugnocellulosic substrates. At a low pH, lignin reportedly reduces hexavalent chromium into Cr(III), which is subsequently adsorbed contrary to the adsorption by CM-g-PMMA where the copolymer adsorbed chromium mainly as Cr(VI), while Cr(IlI), that is generated from the reduction of the Cr(Vl) at acidic pH still remains in solution. However, at pH 10 a negligible conversion of Cr (VI) to Cr(III) is reported. 

A recent modification to EPA Method 7196 has been published and will be implemented in this lab exercise (4). The method uses a hot alkaline solution (pH 12) to solubilize chromates that are to be found in soils obtained from hazardous waste sites. One portion of the aqueous sample would then be aspirated into the FIAA for a determination of total Cr, whereas diphenylcarbazide dissolved in acetone will be added to another portion, and the absorbance of the red violet complex will be measured at 540 nm using a visible spectrophotometer. In this manner, both total Cr and Cr(VI) can be determined on the same sample. Thus, the ratio of the concentration of Cr(III) to the concentration of Cr(VI) in a soil sample can be calculated from the data generated in this experiment. 

Several sensitive and selective methods have been proposed recently for vanadium determination based on different chromogenic reagents, such as 1,5-dip-henylcarbohydrazine (1,5-diphenylcarbazide) in a slightly acid medium (531 nm), 2-(5-nitro-2-pyridy-lazo-)-5-(N-propyl-N-sulfopropylamino)phenol in a weakly acid medium (592 mn), 2-(2-thiazolylazo)-p-cresol in the presence of ascorbic acid (525 mn), and 6-chloro-3-hydroxy-7-methyl l-2-(2thienyl)-4H-chr-omen-4-one in a weakly acidic medium (425 mn). Also, the utilization of 4-(2-pyridylazo)-resorcinol (PAR) in the presence of hydrogen peroxide (542 mn, LOD 0.0028 p-gml"" ) and in the presence of cetyl-pyridinium chloride and ethylenediaminetetraacetic acid (600mn, LOD 3.0p.gml ) has been proposed. 

Besides the common inorganic anions (F , CU, Br, NOa , P04 , and S04 ) and cations (Na, K, NH4", Mg " ", and Ca " "), chromate and arsenite are of primary concern because of their greater toxicides as compared to chromium(III) and arsenate, respectively. Hexavalent chromium is a toxic form of chromium that must be monitored in manufacturing wastes. Ion chromatography with postcolumn addition of diphenylcarbazide is probably the most specific and sensitive method available for the determination of hexavalent chromium. 

A microchemical procedure was devised by Prange (1941). Only 1 ml. is required and the dichromate-oxidation procedure was employed. If the wine was made alkaline prior to distilling, high results were obtained. A similar micro procedure was used by Tomaghelli (1942). An error of only 0.1% was claimed. A colorimetric modification of the dichromate oxidation method was developed by Williams and Reese (1950). They used dilute solutions and measured the violet color of the residual dichromate ion with s-diphenylcarbazide at 540 m/i. Chromic ion does not interfere, and Beer s law is followed. Other alcohols interfere but no more so than in other similar pro-... 

A method has been described [17] for the determination of chlorine in polymers containing chlorine, fluorine, phosphorus and sulfur, which involves oxygen flask combustion over water, addition of ethanol, and titration to the diphenylcarbazide indicator end point with 0.005 M mercuric nitrate ... 

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