Colorimetric ferric thiocyanate method

Several methods have been introduced which express the degree of oxidation deterioration in terms of hydroperoxides per unit weight of fat. The modified Stamm method (Hamm et at 1965), the most sensitive of the peroxide determinations, is based on the reaction of oxidized fat and 1,5-diphenyl-carbohydrazide to yield a red color. The Lea method (American Oil Chemists Society 1971) depends on the liberation of iodine from potassium iodide, wherein the amount of iodine liberated by the hydroperoxides is used as the measure of the extent of oxidative deterioration. The colorimetric ferric thiocyanate procedure adapted to dairy products by Loftus Hills and Thiel (1946), with modifications by various workers (Pont 1955 Stine et at 1954), involves conversion of the ferrous ion to the ferric state in the presence of ammonium thiocyanate, presumably by the hydroperoxides present, to yield the red pigment ferric thiocyanate. Newstead and Headifen (1981), who reexamined this method, recommend that the extraction of the fat from whole milk powder be carried out in complete darkness to avoid elevated peroxide values. Hamm and Hammond (1967) have shown that the results of these three methods can be interrelated by the use of the proper correction factors. However, those methods based on the direct or indirect determination of hydroperoxides which do not consider previous dismutations of these primary reaction products are not necessarily indicative of the extent of the reaction, nor do they correlate well with the degree of off-flavors in the product (Kliman et at. 1962). 

Peroxide value (P V) is the most commonly used measurement of lipid oxidation. The standard iodometric method requires a relatively large sample (5 g) when the lipid is only slightly oxidized. The ferric thiocyanate method, based on the oxidation of ferrous to ferric ion, involves colorimetric measurement of ferric thiocyanate. This method is more sensitive than the iodometric method and requires a relatively small sample (0.1 g). The PV is a useful measure for samples with low levels of oxidation and when the hydroperoxides are not decomposed. During prolonged oxidation, a maximum PV is reached and the value then begins to decrease due to peroxide degradation. This maximum value occurs early for soybean and rapeseed oil, due to the more rapid decomposition of the hydroperoxides of the polyunsaturated fatty acids. 

The ferric thiocyanate method for peroxide value is based on the oxidation of ferrous to ferric ions, which are determined colorimetrically as ferric thiocyanate. This method is more sensitive and requires a smaller sample (about 0.1 g) than does the iodometric method (Table 5.3). However, the values obtained by the ferric thiocyanate method are higher by a factor of 1.5 to 2 relative to those of the iodometric method. The peroxide values obtained by both methods are of only relative significance. The ferric thiocyanate method is commonly applied to dairy products, which undergo oxidative deterioration at relatively low peroxide values. Other colorimetric methods for peroxide values include the determination of the blue starch-iodine complex in the iodometric method, the red color developed with 1,5-diphenyl-carbohydrazide, the color developed with ferric ions and xylenol orange, ferrous chloride and 2,6-dichlorophenolindophenol, titanium dichloride, and xylenol orange. 

Chloride. Chloride is common in freshwater because almost all chloride salts are very soluble in water. Its concentration is generally lO " to 10 M. Chloride can be titrated with mercuric nitrate. Diphenylcarbazone, which forms a purple complex with the excess mercuric ions at pH 2.3—2.8, is used as the indicator. The pH should be controlled to 0.1 pH unit. Bromide and iodide are the principal interferences, whereas chromate, ferric, and sulfite ions interfere at levels greater than 10 mg/L. Chloride can also be deterrnined by a colorimetric method based on the displacement of thiocyanate ion from mercuric thiocyanate by chloride ion. The Hberated SCN reacts with ferric ion to form the colored complex of ferric thiocyanate. The method is suitable for chloride concentrations from 10 to 10 M. 

Water and waste water (APHA Method 4500-CN M) Filtration of sample optional treatment with resin treatment with ferric nitrate solution Colorimetric detection (thiocyanate) No data 71-99, 0.07-1.42 mg/L APHA 1992... 

Colorimetric Methods have frequently been suggested,1 but of these, that originated by Skey and studied by several others 2 appears to be the most useful. It hinges on the fact that potassium thiocyanate yields a blue colour with solutions of cobalt salts, due to the formation of cobalt thiocyanate. On adding alcohol and ether to the liquid, a blue layer is produced. This is destroyed by mercuric chloride, sodium acetate, or sodium thiosulphate, and is masked by the presence of iron salts in consequence of the intense red colour of ferric thiocyanate consequently these substances should not be present when the colorimetric test is applied. 

In addition to the DCP determinations, other analytical techniques were employed as necessary. One of these was the colorimetric determination of ferric ion by the thiocyanate method (2). This was used to calculate the ferrous iron content which is the difference between the total iron, determined by DCP, and the trivalent iron obtained by the thiocyanate reaction. 

Colorimetric method. Mercuric thiocyanate (Hg(SCN)2) is added to the specimen, forming HgCl2 and releasing thiocyanate ions which react with the Fe " of ferric nitrate solution, to produce the red coloured ferric thiocyanate. This method is used in continuous flow instruments. 

The application of colorimetric methods for the analysis of hydroperoxides in polymers requires the sample to be dissolved, or at least swollen, in a solvent (the reactants have to diffuse into the amorphous regions in the polymer matrix). The polymer sample should be in contact with the reagent for a minimum of 30 min (in the dark). A mixture of benzene-methane (95 4 or 92 8) can be used as a solvent for polyolefins or poly(vinyl chloride). The colorimetric measurements are carried out at A = 512.5 nm (A, of the ferric thiocyanate complex). The concentration of hydroperoxide in the polymer sample is calculated according to the expression ... 

Traces of iron may be estimated colorimetrically with considerable accuracy. The formation of ferric thiocyanate, which was the basis of the B.P. 1932 method, is no longer used because of the many interferences which are possible. Strafford preferred the use of thioglycollic acid which produces a purple colour with traces of ferrous iron in ammoniacal solution (ferric salts also respond because the reagent has strongly reducing properties). Quantitative use of this reagent was described by Swank and... 

Iron. Excess iron in wines causes cloudiness, interferes with the color, and can impair flavor. The mechanism of ferric phosphate precipitation has been intensively studied, and numerous colorimetric methods have been developed. For routine purposes the color developed with thiocyanate is adequate (6,9), but many enologists prefer the orthophenanthro-line procedures (3, 4, 6, 22). Meredith et al. (Ill) obtained essentially the same results for iron using 2,4,6-tripyridyl-s-triazine (TPTZ) to develop the color. Atomic absorption spectrophotometry can be used but, as with copper, corrections for reducing sugar and ethanol are necessary (51). 

It has been known for a long time that thiosulfate is present in normal human urine, but reliable methods for the determination of fairly low concentrations have not been available. A method based on the precipitation of the nickel-ethylenediamine complex of thiosulfate followed by iodometric determination was reported many years ago, but gives according to our experience unreliable results. We have earlier described a simple colorimetric method for determination of thiosulfate, based on the cyanolysis of thiosulfate to thiocyanate by the action of cyanide and cupric ions followed by determination of thiocyanate as its ferric ion complex. Unfortunately, this method is not sensitive enough for direct application to urine and, furthermore, other urinary compounds interfere in the cyanolysis reaction. It may, however, be used for assay of the very... 

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