Acidity Determination by Iodometric Method

Acid-Insoluble Ash, 748 Acidity Determination by Iodometric Method (Flavor Chemicals), 565 Acid-Modified Starch, 159 Acid Number (Rosins and Related Substances), 832... 

Kupec et al. [180] determined total sulphur in sludge by a method involving magnesium reduction in which the sample is heated with magnesium powder to convert all sulphur compounds into magnesium sulphide. The magnesium sulphide is treated with sulphuric acid and the evolved hydrogen sulphide determined by iodometric titration. 

Vapor phase chromatography was used to analyze allylic oxidation products. The column, inch X 35 feet, was packed with 20% UCON 5100 on Celite (50-70). NMR analyses were performed using a Varian A-60 instrument. Molecular weights were determined by cryoscopic methods (benzene and acetic acid solvents). Organoselenium compounds were analyzed for selenium by the method of Gould (2), which involves wet digestion of the sample, followed by iodometric titration of the selenium dioxide produced. 

Such methods are wet oxidation of pulp followed by estimation of sulfate by precipitation of barium sulfate (Canadian Pulp and Paper Association Standard G28 1970), X-ray fluorescence spectroscopy (Rivington 1988, Kibblewhite et al. 1987), and combustion of pulp followed by analysis of sulfur as sulfur dioxide or as sulfate. The sulfur dioxide evolved is determined by iodometric titration (Canadian Pulp and Paper Association useful method G.7U, March 1959). Sulfate can be determined by titration with barium chloride (Ora 1960), back-titration with sulfuric acid after addition of barium perchlorate (Aldrich 1974), potentiometric titration with lead perchlorate using an ion-selective electrode (Ross and Frant 1969), or ion chromatography (Douek and Ing 1989). 

The addition of excess calcium acetate to purified cellulosic material suspended in water, followed by titration of the liberated acetic acid, was suggested by L(idtke. From the 0.1% carboxyl content obtained, a D.P. value of 277 was calculated for the cellulosic material employed. This method has been studied further and comparative data with the earlier conductometric titration procedure have been presented by Heymann and Rabinov, by Kenyon and coworkers, and by Sookne and Harris. The latter workers report that electrodialyzed depectin-ized cotton has a carboxyl content of 0.045% or a D.P, of 616. An iodometric titration procedure for estimating carboxyl groups in cellulosic materials has also been described by Ltidtke. The procedure involves a thiosulfate titration of the iodine liberated from a mixture of potassium iodate and potassium iodide by the carboxyl groups of the cellulosic material. The carboxyl content determined by this method agreed well with that obtained by the calcium acetate method. 

Another method is the iodometric procedure described by the reviewer (G6, G7), where dissolved mucin was precipitated with 1.5 volumes of acetone at 40°G from trichloroacetic acid filtrate of the gastric juice. Mucin, in the precipitate, was quantitated by iodometric titration with the use of a standardization curve of electrodialyzed mucin solution and methylene blue as indicator. The mucin values of gastric juice determined by this method were 20-210 mg%, those of saliva 50-300 mg %. The method was subsequently used by many investigators, yielding satisfactory results (K6, K20, M25, M52, T33, W20, W21) it was considered to yield accurate results (Przylecki, cited in K20) with 6% error (Labby et al, cited in T33). 

Volumetrically tellurium may be determined by several methods (1) Tellurous acid is oxidized to telluric by permanganate either in acid solution 3 or alkaline solution.4 (2) Tellurous acid in hydrochloric acid solution is reduced by an excess of standard stannous chloride, according to the reaction TeCl< + 2 SnCU — Te + 2 SnCL. The excess stannous chloride is determined iodometrically.5 (3) Telluric acid may be reduced to tellurous by the use of potassium bromide in sulfuric acid solution, thus H2TeO< + 2 HBr = H2TeO + H20 + Br2. The bromine is distilled into a solution of potassium iodide, the liberated iodine being determined by standard thiosulfate.6... 

Chemical methods also exist for the various organic wine acids. For tartaric acid, the OIV describes a method involving sample cleanup on an ion-exchange column followed by reaction between tartaric acid and vanadic acid to give a red complex that is measured spectrophotometrically. This reaction may be incorporated into a flow injection system, which eliminates the need for the ion-exchange sample cleanup step. Lactic acid is first oxidized to ethanol and measured by colorimetry following reaction with ni-troprusside and piperidine for citric acid the chemical method requires controlled oxidation to form acetone which is subsequently separated by distillation and determined by iodometric titration. 

Unique methods based on new principles have been developed within the past 10 years. Threonine (27,28,249) is oxidized by lead tetraacetate or periodic acid to acetaldehyde, which is determined by photometric analysis of its p-hydroxydiphenyl complex or iodometric titration of its combined bisulfite. Serine is oxidized similarly to formaldehyde, which is determined gravimetrically (207) as its dimedon (5,5-dimethyldihydro-resorcinol) derivative or photometric analysis (31) of the complex formed with Eegriwe s reagent (l,8-dihydroxynaphthalene-3,5-disulfonic acid). It appears that the data obtained for threonine and serine in various proteins by these oxidation procedures are reasonably accurate. [Block and Bolling (26) have given data on the threonine and serine content of various proteins. ]... 

Satisfactory 40% peracetic acid is obtainable from Buffalo Electrochemical Corporation, Food Machinery and Chemical Corporation, Buffalo, New York. The specifications given by the manufacturer for its composition are peracetic acid, 40% hydrogen peroxide, 5% acetic acid, 39% sulfuric acid, 1% water, 15%. Its density is 1.15 g./ml. The peracetic acid concentration should be determined by titration. A method for the analysis of peracid solutions is based on the use of ceric sulfate as a titrant for the hydrogen peroxide present, followed by an iodometric determination of the peracid present.3 The checkers found that peracetic acid of a lower concentration (27.5%) may also be used without a decrease in yield. The product was found to be sufficiently pure, after only one recrystallization from 60 ml. of petroleum ether (b.p. 40-60°) and cooling overnight to —18°, to be used in the next step. 

Elemental composition Na 58.93%, S 41.07%. An aqueous solution is analyzed to determine sodium content. Also, an aqueous solution may be analyzed for sulfide by methylene blue colorimetric test or by iodometric titration (APHA, AWWA, and WEF. 1999. Standard methods for the Examination of Water and Wastewater, 20 ed. Washington, DC American Pubhc Health Association). The methylene blue test is based on reacting sulfide, ferric chloride and dimethyl-p-phenylenediamine to produce methylene blue. Also, sulfide can be measured by using a sdver-sdver sulfide electrode. Quahtatively, sulfide may be identified from the hberation of H2S on treatment with acid. The H2S turns the color of paper soaked with lead acetate black (See Hydrogen Sulfide). 

Hydroperoxides may be determined by measuring at 290 nm (e = 44100 M cm ) or 360 nm (e = 28000 cm ) the concentration of 13 formed in the presence of a large excess of ions. The reaction may be too slow for practical purposes, unless a catalyst is present. For example, an assay for lipid hydroperoxides conducted without a catalyst may require several measurements every 6 min until the absorbance reaches a maximum. Exclusion of air from the sample cuvette is important. The method is about 1000-fold more sensitive than thiosulfate titration The iodometric method with UVD at 360 was adopted for detecting the presence of hydroperoxides derived from protein, peptide or amino acid substrates subjected to y-radiation, after destroying the generated H2O2 with catalase. ... 

The kinetics of the noncatalytic reaction were studied by the method of Bawn and Williamson (4). They found 3.3-3.7 moles/liter for the equilibrium constant for the formation of the intermediate acetaldehyde monoperacetate (AMP) and a first-order rate constant for the decomposition of this intermediate to acetic acid of 0.015 min."1 at 25°C. We found difficulty in reproducing our results probably caused mainly by the high values of the blanks in the iodometric methods used. However, as an average of four determinations we obtained 0.03 min."1 at 30°C. 

Estimation of Selenium in Sulphide Minerals.s—In various sulphite-cellulose manufactories difficulties have occurred which have been traced to the presence of selenium in the pyrites used for burning. Part of the selenium remains in the burnt pyrites and part volatilises with the sulphur dioxide. 20 to 30 grams of pyrites are dissolved in hydrochloric acid (dens.=1-19) and potassium chlorate. Zinc is added to reduce the iron to the ferrous condition more hydrochloric acid is then added, the solution boiled and stannous chloride added to precipitate selenium. Since the selenium may contain arsenic, it is collected on an asbestos filter, dissolved in potassium cyanide and reprecipitated using hydrogen chloride and sulphur dioxide. The element may then be estimated by the iodometric method described below. In order to determine the relative proportion of volatile to non-volatile selenium, the pyrites may be roasted in a current of oxygen. After this treatment the contents of the tube are dissolved in warm potassium cyanide and the selenium reprecipitated and estimated in the ordinary way. 

The iodometric method depends on the fact that chromates interact with hydrochloric acid, liberating chlorine, which in its turn liberates an equivalent amount of iodine from potassium iodide. The iodine may then be determined by titration with standard sodium thiosulphate. This method is largely used for chromium in whatever state of oxidation it may be, a chromium salt being first converted to chromate either by fusion v th sodium peroxide or by boiling its solution wdth nitric acid and lead dioxide. - ... 

The iodometric method was then modified (G23, G24) by changing the final concentration of trichloroacetic acid from 8% to 3.3%, leaving more proteose-like substances in the filtrate, and quantitating the mucosubstances in the filtrate colorimetrically by the Folin-Ciocalteu reaction with the phenol reagent, instead of iodometric titration. This colorimetric reaction determines the tyrosine and tryptophan content after alkaline hydrolysis of the total dissolved mucin. 

In another popular chemical method, hexacyano-ferrate(ni) is reduced to hexacyanoferrate(II), and the unchanged Fe(III) can be quantitatively determined, e.g., by iodometric back-titration with sodium thiosulfate. Modifications have been proposed to allow direct photometric quantification. Other titri-metric methods have also been proposed, for example, using an iodometric method with vanadium(V) in perchloric acid as reagents. The simple reduction methods are regarded as nonspecific due to the interference of other similar compounds with reducing properties. 

The total hypophosphorous acid may be determined by the iodometric method. 

Lauric diethanolamide is a fairly unreactive substance. A likely approach to the problem of determining low concentrations of this substance involved hydrolysing it to DBA and fatty acid and determining the former by a conventional iodometric method. The lauric diethanolamide content of the extractant could then be calculated from the amount of DBA produced upon complete hydrolysis. It is more appropriate to determine DBA rather than the fatty acid, as it was possible that the extractant could contain fatty acids originating from sources other than hydrolysis of the dialkanolamide, (e.g., free fatty acids or their metal salts or fatty acid esters, all of which might be present in the original polymer). 

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