Ferric thiocyanate

Add a known volume ofo oaM.AgNOj solution (in excess) and boil the solution until the silver chloride has coagulated. Filter through a conical 5 cm. funnel, ensuring that the filter-paper does not protrude above the r m of the funnel. Wash the silver chloride and the filter-paper several times with a fine jet of distilled water. To the united filtrate and washings add i ml. of saturated ferric alum solution. The solution should be almost colourless if it is more than faintly coloured, add a few drops of concentrated nitric acid. Then titrate with 0 02M-ammonium thiocyanate solution until the permanent colour of ferric thiocyanate is just perceptible. (Alternatively the chloride may be determined potentiometrically.)... 

Quantitative. Classically, silver concentration ia solution has been determined by titration with a standard solution of thiocyanate. Ferric ion is the iadicator. The deep red ferric thiocyanate color appears only when the silver is completely titrated. GravimetricaHy, silver is determined by precipitation with chloride, sulfide, or 1,2,3-benzotriazole. Silver can be precipitated as the metal by electro deposition or chemical reduciag agents. A colored silver diethjldithiocarbamate complex, extractable by organic solvents, is used for the spectrophotometric determination of silver complexes. 

Ana.lytica.1 Methods. Thiocyanate is quantitatively precipitated as silver thiocyanate, and thus can be conveniendy titrated with silver nitrate. In the presence of a ferric salt, a red-brown color, produced by the ferric thiocyanate compex, indicates the end point. 

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. 

Eisen-reihe, /. iron aeries, -refin, -resinit, m. (Min.) humboldtine. -rhodanid, n. ferric thiocyanate, iron(III) thiocyanate, -rho-daniir, n. ferrous thiocyanate. iron(II) thiocyanate. -rogenstein, m. oolitic iron ore. -rohr, n., -rohre, /. iron pipe or tube, -rost, m. iron rust, -rostwasser, n. iron liquor, iron mordant, -rot, n. colcothar. -safraQt m. saffron (or crocus) of Mars, -salmiak, m. (Pharm.) ammoniated iron, iron and ammonium chloride, -salz, n. iron salt, -sand, m. ferruginous sand, -sau, /. iron sow. 

Rhodan-baryum, n. barium thiocyanate, -eisen, n. iron thiocyanate, esp. ferric thiocyanate, iron(lll) thiocyanate, -eisenrot, n. ferric thiocyanate, iron (III) thiocyanate. 

A number of methods have been proposed for the detection of rancidity. The determination of active oxygen consists of dissolving the fat in a suitable medium such as chloroform and acetic acid, adding potassium iodide, and titrating the liberated iodine with a standard thiosulfate solution (16, 20). This is perhaps the most widely used method at the present time. Another procedure which has been proposed for the detection of peroxides employs ferrous ammonium sulfate and ammonium thiocyanate in acetone. The resulting red color of ferric thiocyanate is measured spectrophotometrically, and is said by the authors to yield more reproducible results than do the usual titration methods (21). 

Figure 14 also demonstrates the principle that sometimes extraction into an organic solvent increases the intensity of the color. This is true, for example, for ferric thiocyanate, where extraction into an organic solvent will increase the sensitivity of iron determination to that obtainable with the phenanthrolines. The advantage of the iron ferric thiocyanate method in organic solvents is the fact that it is insensitive to pH changes, which is not true for the phenanthroline procedures (19). 

Another method is that based on the ferrous/ferric thiocyanate determination of hydroperoxides [1]. This exploits the ion-catalysed decomposition as shown by... 

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

However, in actual practice the thiocyanate solution is always taken in the burette and is run directly into the silver nitrate solution in the flask that has been duly acidified with nitric acid. Ferric ammonium sulphate is the choicest indicator since the end point is visibly detected by a deep red colour (ferric thiocyanate) due to the interaction of Fe2+ ions with a trace of SCN ion. 

Temperature of the solution should be maintained below 25°C since at an elevated temperature the red colour of the ferric thiocyanate complex fades away rapidly. Therefore, we have ... 

NH4SCN is weakly acidic reacts with caustic soda or caustic potash to form sodium thiocyanate (NaSCN) or potassium thiocyanate (KSCN). It reacts with ferric salts to form a deep-red ferric thiocyanate complex ... 

The reactions of potassium thiocyanate in aqueous solution are essentially those of the thiocyanate anion. Its reaction with ferric ammonium sulfate, apphed in Volhard titration, results in the formation of ferric thiocyanate, Fe(SCN)3. Similarly, in titration against shver nitrate, it forms insoluble silver thiocyanate, AgSCN. 

Elemental composition Na 28.36%, S 39.54%, C 14.81%, N 17.28%. The aqueous solution may he analyzed for sodium. Thiocyanate may he measured hy gravimetry by reacting with ferric ion to form red ferric thiocyanate, Fe(SCN)3, which may he filtered, washed, dried, and weighed. 

Chlorides, bromides, and iodides can be quantitatively determined by treatment with silver nitrate, and, with suitable precautions, the precipitated halide is washed, dried, and weighed. Chlorides in neutral soln. can be determined by F. Mohr s volumetric process 27 by titration with a standard soln. of silver nitrate with a little potassium chromate or sodium phosphate as indicator. When all the chloride has reacted with the silver nitrate, any further addition of this salt gives a yellow coloration with the phosphate, and a red coloration with the chromate. In J. Volhard s volumetric process, the chloride is treated with an excess of an acidified soln. of silver nitrate of known concentration. The excess of silver nitrate is filtered from the precipitated chloride, and titrated with a standard soln. of ammonium thiocyanate, NH4CN8—a little ferric alum is used as indicator. When the silver nitrate is all converted into thiocyanate AgN03-fNH4CNS=AgCNS +NH4NOS, the blood-red coloration of ferric thiocyanate appears. 

If iron is present, a ferric thiocyanate complex will form and the soln will change from light blue to deep red. Back-titrate the ferric thiocyanate complex with 0.2N TiCla soln from the same buret untii the light blue color reappears. Determine the mis of Ti Cl3 soln... 

Various microchemical tests are available for the detection of minute quantities of sulphur, both free and combined. The substance under examination may be treated with a little sodium hydroxide solution, the extract evaporated just to dryness, a few drops of aqueous sodium cyanide (0-1 per cent.) added and the evaporation repeated. The residue, moistened with dilute sulphuric acid and a drop of ferric chloride, gives the characteristic ferric thiocyanate colour if sulphur is present.6 In the ease of minerals, traces of sulphur dioxide produced on heating may be detected 6 by the colour change of an alkaline solution of Bromocrcsol Green or by the deeolorisation of starch-iodine solution. 

Sulphur Thiocyanate, S(SCN)2, is formed as colourless, pearly crystals, when a solution of thiocyanogen in ether reacts with dry hydrogen sulphide. It decomposes at atmospheric temperatures. Heated in an open tube on a water-bath it darkens rapidly and then suddenly decomposes, evolving orange fumes. A solution of sulphur thiocyanate in a mixture of ether and benzene does not react with powdered iron, but on the addition of a drop of water the characteristic red colour of ferric thiocyanate develops immediately. By this reaction sulphur thiocyanate can be distinguished from free thiocyanogen.2... 

The discharging of the colour by oxalates, tartrates, etc., appears to be caused by the formation of complex ions with the ferric ions of the ionised ferric thiocyanate, which causes further dissociation of the red non-ionised salt and consequent loss of colour. 

Ferric thiocyanate is readily soluble in aqueous ether, and the extract possesses a deep violet colour which can be completely discharged by the addition of ferric chloride. The explanation of this effect put forward by Clarens3 is that an excess of thiocyanate is necessary for the formation of ferric thiocyanate when a ferric salt is added this excess of thiocyanate is removed and a salt of dithiocyanic acid, insoluble in ether, is formed.4... 

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). 

Loftus Hills, G. and Thiel, C. C. 1946. The ferric thiocyanate method of estimating peroxide in fat or butter, milk and dried milk. J. Dairy Res. 14, 340-353. 

With very few exceptions, quantitative epoxide assay techniques currently in use are derived from the reeotion of ethylene oxides with halogen adds, notably hydrochloric acid and hydrobromio add, in a variety of solvents. Acid uptake may be determined by any of several reliable procedures. These include titration with standard base8 nr back-titration with standard acid.744 The end-point may be detected visually in the presence of suitable acid-base indicators, or by the more precise technique of potontionaetry.447.4 -470 A useful alternative, applicable in the presence of easily hydrolysed substances or of amines that buffer the end-point, is the technique of argentiometry. In this procedure excess of halide ion is titrated with silver nitrate in tV presence of ferric thiocyanate indicator,470 1884 or potentiometri-cally.188 ... 

A ferric thiocyanate test paper has boon developed by Dockort422 for the detection of ethylene oxide in the atmosphere or in other gas mixtures. A positive tost is given, however, by any basic gas, .< . ammonia. 

Ferric thiocyanate -as flocculating agent [FLOCCULATING AGENTS] (Vol 11) -in mercury analysis [MERCURY] (Vol 16)... 

The ferric salts of benzoic, succinic, hydroferrocyanic, gallic and tannic acids are sparingly soluble in cold water, while basic ferric salts of formic and acetic acids formed on boiling are also sparingly soluble. Solutions of ferric thiocyanate and salicylate exhibit characteristic colorations. 

Michael BD, Adams GE, Hewitt HB, Jones WBG, Watts ME (1973) A posteffect of oxygen in irradiated bacteria A submillisecond fast mixing study. Radiat Res 54 239-251 Mihaljevic B, Katusin-Razem B, Razem D (1996) The reevaluation of the ferric thiocyanate assay for lipid hydroperoxides with special considerations of the mechanistic aspects of the response. Free Rad Biol Med 21 53-63... 

Chloride ion reacts with mercuric thiocyanate forming unionized mercuric chloride, liberating thiocyanate ion (SCN ). The liberated thiocyanate ion reacts with Fe3+ to form a highly colored ferric thiocyanate. These reactions are shown below ... 

Thiocyanate reacts with ferric ion under acidic conditions to form ferric thiocyanate, which has an intense red color. 

The intensity of color of the ferric thiocyanate formed is proportional to the concentration of thiocyanate ion in the sample. The absorbance or transmittance is measured at 460 nm using a spectrophotometer or a filter photometer. The concentration of SCN- in the sample is determined from a standard calibration curve. The detection range of this method is 0.1 to 2.0 mg SCN-/L. Dilute the samples if the concentration exceeds this range. 

Chloride ions react with mercury (II) thiocyanate to form a sparingly dissociating mercuric chloride complex and liberate a stoichiometrically equivalent amount of thiocyanate ions (2CT + Hg(SCN)2 - Hgd2 + 2SCN) die thiocyanate reacts with iron (III) ions, yielding die intensely red ferric thiocyanate complex (SCN + Fe3+ -> Fe(SCN)2+), which is determined at 460 nm. 

Chloride. The pattern of flow for this method is depicted in Fig. 19. The method is based on a modification proposed by Zall et al. (Z1). In this method, chloride ion in acid solution containing ferric ions displaces thiocyanate ion from mercuric thiocyanate. The thiocyanate liberated reacts with excess ferric ions to form the red ferric thiocyanate (FeSCN) + + complex. The reaction is sensitive to the nature and amount of acid present. The presence of bromide would interfere with the results. 

Aldol condensation of the zinc enolate of resin-bound alkyl ester 29 with aromatic aldehyde or ketone forms a P-hydroxy ester, which upon treatment with DIBAL-H leads to simultaneous reduction and cleavage of the ester moiety from the resin to give a soluble 1,3-diol 31 [31], Parallel synthesis utilizing three ester and nine carbonyl building blocks afforded a library of 27 analogs which was screened for antioxidative efficiency using a ferric thiocyanate assay. 

Measurement of hydroperoxides is the classical method for quantifying lipid oxidation and a variety of assay procedures are available. The oxidation of ferrous to ferric iron by hydroperoxides in the presence of ammonium thiocyanate to produce ferric thiocyanate, which can be quantified spectrophotometrically at 505 nm, has been used extensively to study lipid oxidation in milk (Loftus-Hills and Thiel, 1946). Newstead and Head-ifen (1981) recommend that extraction of fat from whole milk powder be carried out in the dark when using this procedure to avoid artefactually high... 

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