Dichromate method

The methods of analysis of the American Oil Chemists Society (AOCS) are the principal procedures followed in the United States and Canada and are official in commercial transactions. When the material is for human consumption or dmg use, it must meet the specifications of the USP (12). Commercial distilled grades of glycerol do not requite purification before analysis by the usual methods. The deterrnination of glycerol content by the periodate method (13), which replaced the acetin and dichromate methods previously used, is more accurate and more specific as well as simpler and more rapid. 

Soil organic carbon concentration was determined by the sulfuric acid-potassium dichromate method (Kalembasa and Jenkinson 1973) at the Soil Chemistry Laboratory of South China Institute of Botany, CAS. Results... 

It is interesting to observe that the solutions of ammonium ceric sulphate possess a number of advantages over permanganate and dichromate methods discussed earlier in this chapter, viz.,... 

Discuss direct titration method using dichromate method in the assay of Iron Ore with reference to the following aspects ... 

Colour Tests. Potassium Dichromate (Method 2)—green SchifTs Reagent—violet Sodium Nitroprusside (Method 2)— red. 

Colour Tests. Ammoniacal Silver Nitrate—red-brown/red-brown Ferric Chloride—green Folin-Ciocalteu Reagent— blue Marquis Test—orange—> violet Methanolic Potassium Hydroxide—pink-orange—> brown Nessler s Reagent—black Palladium Chloride—orange —> brown Potassium Dichromate (Method 1)—green— brown. 

Colour Test. Potassium Dichromate (Method 2)—green. 

Reagent—black Palladium Chloride—orange - brown Potassium Dichromate (Method 1)—green - brown (30 s). 

Colour Tests. Ammoniacal Silver Nitrate—red-brown/black Ferric Chloride—green Folin-Ciocalteu Reagent—blue Mandelin s Test—orange Marquis Test—yellow- violet Methanolic Potassium Hydroxide—yellow- orange Nessler s Reagent—black Potassium Dichromate (Method 1)— green - brown (30s). 

Colour Tests. p-Dimethylaminobenzaldehyde—orange/violet Folin-Ciocalteu Reagent—blue Marquis Test—yellow Potassium Dichromate (Method 1)—brown (slow). 

For all types of waters (drinking, surface, waste) oxidation with dichromate in a strongly acid environment of H2SO4 is used (2 hours boiling) the dichromate method. The following reaction takes place ... 

A disadvantage of the dichromate method for the determination of COD(Cr) in very clean waters, i.e. in drinking and unpolluted natural waters is the low reproducibility of the determinations. Therefore, the less eificient permanganate Kubel s method is still used for these waters [15, 16, 19]. However, in the USA only dichromate method has been recommended for these waters since 1955 [13]. For the determination of COD of wastewaters only the dichromate method COD(Cr) is used throughout the world [13, 14]. 

Fessler (1941) gave a practical procedure for the dichromate method. Later modifications of the Fessler procedure were outlined by Zimmermann (1949). Dubrowskaja (1946) diluted the wine 1 to 60 and used only 8 ml. of the diluted wine plus 5 or 6 ml. of water. This is made slightly alkaline, and 8 ml. are then distilled into 10 ml. of standardized potassium dichromate (plus 5 ml. of concentrated sulfuric acid). Then 10 to 15 ml. of 20% potassium iodide are added after 5 minutes 200 to 250 ml. of water are added and the iodine released is titrated with thiosulfate to a starch end point. A modification of the chemical oxidation procedure to eliminate calculations was developed by Barini-Banchi (1948). 

Fessler, J. H. 1941. Alcohol determination by dichromate method. Wines db Vines 22(4), 17-18. 

Zimmermann, H. W. 1949. Determination of alcohol in wines by the dichromate method. Proc. Wine Tech. Conf., Davis 1949, 107-109. 

M. Novic, B. Pihlar, and M. Dular. Use of flow-injection analysis based on iodometry for automation of dissolved-oxygen (Winkler method) and chemical oxygen demand (dichromate method) determinations. Fresen. Z. Anal. Chem., 332 750-755, 1988. 

---