Volumetric and Gravimetric Analytical Methods of Organic Compounds

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Review of Fundamental Developments in Analysis

Volumetric and Gravimetric Analytical Methods for Organic Compounds W a l t e r T. Smith, Jr., William F. W a g n e r , and John

M.

Patterson

University o f Kentucky, lexington, Ky.

T

HE ANALYTICAL METHODS discussed in this review have been selected from the literature which has become available t o the reviewers from November 1957 to November 1959.

DETERMINATION OF ELEMENTS

Carbon and Hydrogen. Several modifications of combustion procedures for special samples have been reported. The combustion of easily volatile substances was achieved by neighing the sample in a sealed ampoule which was then placed in a special copper casing in which the ampoule could be broken easily in the 'combustion tube (44). A simple technique was described for the rapid determination of carbon and hydrogen in highly volatile combustible organic liquids by weighing the sample in a special tube and transferring it to the combustion zone in an atmosphere of nitrogen (136). A procedure was developed for handling samples of pyrophoric and hygroscopic materials by using polyethylene bags (147). The Korshun method (97) was modified to carry out semimicro elementary analyses in macroadalytical apparatus (3). In an automatic macrocombustion apparatus for rapid and precise analyses, the a m p l e is alternately heated with a bare resistance wire heater and cooled with an air blast to maintain the desired vaporization rate which is controlled by a mercury manometer. The combustion cycle is completed in about 20 minutes (222). Korbl has extended the use of the thermal decompositim product (74) of silver permanganate as a combustion catalyst in the carbon hydrogen determination to compounds containing fluorine by absorbing HF and SiF, in a layer of Pb304 on pumice in the combustion tube (73, 166). The silver permanganate catalyst has been used to determine carbon and hydrogen in compoucds containing phosphorus and sulfur (105). The product prepared by heating silver dichromate a t 400' for a half hour and at 950' for 2 hours was reported to be a 262 R

ANALYTICAL CHEMISTRY

good catalyst to replace copper oxide (161). Terent'ev and Luskina described a special apparatus in which the sample is oxidized with a mixture of sulfuric and chromic acid a t 150' to 160" C. Procedures for the determination of carbon, hydrogen, halides, nitrogen, and metals in the compounds are given (201).

Macdonald has described procedures for the determination of sulfur, chlorine, bromine, phosphorus, and carbon-14 for analysis in industry by the combustion of organic compounds in a closed flask with oxygen (106). Allam and Agiza developed an apparatus for the wet combustion of samples for the determination of carbon. The reagents were modified to avoid the liberation of iodine from potassium iodate (6). Halogens. Fedoseev and Sobko in a serips of publications have described a method with improvements for the The determination of halogens. method involves the combustion of the sample through a series of furnaces and adsorption tubes. The combustion products are passed first through a tube charged with KBr, next, one charged with KI, and finally through a 10% KI solution. I n the first tube, chlorine oxidizes bromide, in the second the iodide is oxidized by bromine, and in the third, the iodine is trapped. The amounts of the three halogens present in the sample may be calculated from the change in weights in the tubes and titration of the iodine. Sulfur interferes (46-48). A slight modification of the double combustion method for the determination of sulfur permits the determination of the halogens. The halogens are reduced to their respective acids which are absorbed in either 15 ml. of a 4% aqueous solution of KOH to which 3 to 5 ml. of H202is added or in a solution of hydrazine. The latter is better for iodine (217). Chiang determined halogens by using the Volhard titration after the sample was fused with calcium hydroxide in a sealed tube (25). Halogens were determined rapidly

by refluxing the sample in water, ethyl alcohol, or propyl alcohol in an alkali solution with Raney nickel. The halides formed were titrated by the Volhard method (204). The determination of chlorine and bromine by a high-temperature combustion method similar to that used for sulfur in petroleum was achieved by absorbing the halides formed in a 5% aqueous solution of sodium bicarbonate and titrating the chloride and bromide with silver nitrate (129). A fusion of a sample containing chlorine and bromine with magnesium nitride will release the halides which may be dissolved and determined in the filtrate (209). The semimicrodetermination of chlorine in poly(viny1 chloride) and related polymers was accomplished by a fusion with starch and sodium peroxide in a specially designed bomb, followed by a titration with silver nitrate (67'). Fiijisaki recommends that in the Stepanow method for chlorine 180 W ml. of ethyl alcohol and 25 W grams of sodium should be used where W is the weight of the sample in grams (56). Otter described a new apparatus for the rapid combustion of compounds for the determination of chlorine (1%). A direct titration method for determining chlorine after a Carius combustion n a s based on the observation that freshly precipitated silver chloride reacts nith Amberlite IR-120 (H) to liberate KCl which may be titrated with Hg(NO& with diphenylcarbazone indicator or with AgN03 conductometrically (107). The decomposition of fluoro- and chlorofluoro- compounds by forming a complex with biphenylsodium-dimethoxymethane was used to determine the two halogens. The sodium salts were extracted with water and separated by passage through a cation exchange column (Amberlite IR-120 (H)]. The eluate was titrated with sodium hydroxide (83). The determination of fluorine by a chelatometric titration was reported. After a peroxide fusion in a nickel bomb, the fluoride is allowed to react with a known amount of calcium chlo-

ride solution to precipitate calcium fluoride. The excess calcium chloride is titrated with a standard solution of disodium magnesium ethylenediaminetetraacetate (68). The separation and semimicrodetermination of iodine in compounds containing other halogens were achieved by decomposing the sample with sodium peroxide in a bomb. The filtrate from the acidified washings from the bomb is treated with hydrazine and PdC12. After 2 days the iodine is completely precipitated as PdIz which is determined gravimetrically (93). A mixture of concentrated "03, HClO4, and H2S04 was employed to oxidize organic compounds containing iodine. The iodine is absorbed in a tube containing NaCl buffered to p H 4.5. The liberated iodine is titrated with sodium thiosulfate (90). Konovalov made an experimental criticism of the different methods for the determination of organic iodine. A modification of the combustion method of Grote-Krekeler (62) was given which requires a constant flow of oxygen and a platinum catalyst (95). Metals. Boron n-as determined by ignition to H3B03 in a high-pressure oxygen bomb followed by titration of the boric acid with alkali after the addition of mannitol (99). Germanium, tin, and silicon may be determined in their alkyls, chloroalkysilanes, and siloxanes by dry combustion. The sample is carried by a stream of oxygen into a silica tube where the sample is ignited to the metal oxide and weighed (19). The mercury in organic compounds R-as determined by complexonietry emrloying disodium (ethylenedinitri1o)tetraacetate, as titrant with Xylenol Orange indicator (149). The titration of nickel in nickel naphthenate and of lead in tetraethyllead was made with sodium ethylenediaminetetraacetate with murexide indicator (163).

Procedures for the determination of tin in organotim and inorganic compounds were described by Farnsworth and Pakola (41). Nitrogen. Several modifications and applications of the Kjeldahl procedure have been reported since the previous revier7. Low results in the Kjeldahl-Ronchese nipthod which are cauwd by the formation of secondary and tertiary amines niay be eliminated by using a mercury catalyst, continuing the heating after decolorization, and adding excess KaBr to avoid the mercury interference (216). The Kjeldahl method was extended t o nitro and nitrogen-nitrogen single bond compounds by a preliminary reduction with zinc dust to amino compounds in such nonoxidizing media as formic acid, acetic, acid, and phos-

phoric acid containing hydrochloric acid. Only pyrazoline nitrogen defies this method (30). Work was done to demonstrate the use of the semimicro-Kjeldahl method to analyze a wide variety of pyridinium halide and oxyhalide salts (55). Morris has patented an antifoam material for the Kjeldahl digestion. The catalyst is packaged in a container of polyethylene which serves as a defoaming agent. Vinylidene chloride is also effective (129). A modification of the Kjeldahl method in which the ammoniadistillation is eliminated was reported (224). Yellow HgO was found to be superior to selenium metal, copper sulfate, and mixed catalyst. Optimum concentrawas 7.5 grams per 20 ml. tion of of H 8 0 4 with 1 gram of HgO per flask (160). Certain fluorinated compounds containing 2.5 t o 12.5y0 nitrogen were analyzed by the Kjeldahl method without any dficulty when the fluorine content was 9 to 50% (49). Perrin's modification (141) of the Kjeldahl method mas adapted for the rapid mineralization on a semimicro scale (loci). Fedoseev and coworkers have extended and revised their method for halogens and sulfur to the determination of nitrogen. The sample is mixed with magnesium powder and ignited a t 560' to 650' to obtain magnesium nitride, sulfide, and halides. Hydrogen sulfide is liberated in an acid solution, ammonia is then distilled out of an alkaline solution, and the halides are determined argentometrically (@). Powdered oalcium may replace the magnesium powder (200). Hydrous MnOz prepared from KMn04 and MnS04 liberates large amounts of pure oxygen by pyrolysis. A sample containing nitrogen is placed in a platinum boat, covered with MnOz, and ignited in a combustion tube in the absence of air. The liberated nitrogen is mpasured in azotometers (126). -4. method was described for the determination of nitrogen based on the combustion of the substance in vacuo in the presence of copper oxide, followed by a gasometric determination of the nitrogen (43). A method for the rapid determination of oxidation values of nonvolatile compounds by an iodic acid combustion was applied to the determination of nitrogen by collecting the nitrogen in an azotometer. Volatile compounds, nitro-, nitroso-, and nitrogen heterocyclic compounds cannot be analyzed (132). Compounds when mixed with KCIOs or Coz03 and analyzed by a method adapted from Pregl give good results for nitrogen in several compounds. Results

were compared with the micro-Dmm method (6). Oxygen. A review with 23 references for the direct determination of oxygen has been compiled (114). The direct determination of o v g e n by the Unterzaucher method for cornpounds containing sulfur was improved by including copper in the with HIOa, and train, replacing I& determining both carbon dioxide and iodine, thus leading to simultaneoua equations to correct for the sulfur (162) . Phosphorus. Phosphorus has been determined after fusion of tbe compound with sodium peroxide by precipitating and weighing as quinoline phosphomolybdate (60). The relative efficiencies of wet oxidation by HC10, and Ha04 and of ignition in air for the determination of phosphorus by three different methods were compared and were found to be equivalent statistically. For routine work the ignition method was more rapid (11). The Schoniger method for halogens and sulfur was modified t o dete&ie phosphorus. After combustion of the sample on filter paper in a flask of oxygen, the P20sformed is converted to orthophosphate and precipitated as MgNHz04. This precipitate is filtered, dissolved, treated with an excess of ethylenediaminetetraacetic acid, the excess of Khich is titrated with a standard magnesium solution (54). Selenium. After a peroxide bomb fusion of the sample, the acid washings were treated with hydrazine after which the selenium was filtered and weighed. Nitrogen, sulfur, and halogens do not interfere (94). Silicon. Silicon was determined by wet combustion of the sample in a mixture of perchloric and sulfuric acids, to form SiOz which was weighed. The method is not applicable to compounds with boiling points below 60' (92).

Sulfur. The determination of sulfur

Kas accomplished by fusing the sample

with KK03 and KOH in a nickel crucible. The sulfate formed was precipitated as benzidine sclfate which was filtered and titrated with standard 0.1N sodium hydroxide (19s). The wet oxidation of compounds by KClOs and KNOI to form sulfate was studied. The sulfate was determined by the benzidine titration (196). In a semimicromethod for sulfur, the sample is burned in oxygen a t 750" in an empty tube. The SOs reacts with silver gauze to form AgSO,. The amount of sulfur may be calculated from the increase in weight; if halogens are present, the A G O , is dissolved in hot water and the loss in weight may be used to calculate the sulfur content (38)* VOL. 32, NO. 5, APRIL 1960

263 R

Sulfur in liquid fuels was determined by burning the oil in a stream of oxygen, and collecting the sulfur oxides in hydrogen peroxide and titrating with standard alkali (108). Sulfur in compounds, plants, minerals, and soil may be converted by decomposition with KClOs to sulfate which is determined gravimetrically (4). Schoniger’s method for halogens was adapted to the determination of sulfur (104).

Compounds containing thiol, sulfide, disulfide, and thiophenic sulfur were heated with Raney nickel to produce XiS from which H2S is liberated, absorbed, and the sulfide is titrated with mercuric acetate using dithizone indicator (61). -4 total sulfur determination is used to arrive a t the combining weight of sulfonates. The sulfonate is converted to the sodium salt which is burned in a stream of oxygen to SOz which is absorbed and titrated iodometrically (57). FUNCTIONAL GROUPS

A review consisting of 36 references describes the application of functional group determinations to organic analysis (211).

Acids. Methods available for the characterization and the determination of acids have been reviewed by Veibel (212, 918’). The equivalent weights of acids have been determined with an error of 0.5 unit or less (173) by decomposition of the 8-benzylthiouronium salts with potassium hydroxide and potassium mercuric iodide. Acid Anhydrides. Veibel (212,213) has revieEed the procedures for the determination of acid anhydrides. Acetic anhydride may be determined by hydrolysis, catalyzed by sulfuric acid or pyridine, in a known volume of water. The excess water is titrated with the Karl Fischer reagent. Acid Halides. A volumetric estimation of the chlorides of oxalic monoesters (69) involves saponification followed by precipitation of the oxalic acid with excess 0.2M calcium chloride. The excess calcium chloride is determined by chelatometry. Active Hydrogen. The Zerewitinoff determination for active hydrogen has been modified to include highly insoluble organic compounds (140) through the use of a l-methyl-1,2,3,4tetrahydroquinoline solvent and extended to the analysis of the silanol group (64). Application of the Zerewitinoff method to several quinol acetates (86) gives unexpected results. Methyl lithium possesses advantages over the Grignard reagent while lithium aluminum hydride gives different results. Alcohols. Analytical applications

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ANALYTICAL CHEMISTRY

of the acetylation of hydroxyl groups have been reviewed (117). Esters other than a c e t a b are r e ported (119)to cause only slight interference when acetic acid and sodium acetate are used as the acetylating agent. The oxidation of alcohols and glycols continues t o receive attention as an analytical method. The use of hypobromite in the determination of isopropyl alcohol has been described (63). The products resulting from the olddation of various alcohols with sulfuric acid and potassium dichromate are discussed by Jaulmes and Mestres (89). The use of a mixture of ethyl acetate and ethyl alcohol (168’)permits the determination of water-insoluble glycols by the periodate method. Diomn and methyl formate are less satisfactory solvents. A dehydration procedure has been adapted for the determination of tertiary alcohols as well as of easily dehydratable primary and secondary alcohols (146). Dehydration catalysts include toluenesulfonic acid and sodium hydrogen sulfate (186). A direct titration of ethanolamine and substituted ethanolamines may be accomplished (18.2) by the use of lithium aluminum dibutylamide as titrant, diethoxyethane or tetrahydrofuran as solvent, and 4phenylazodiphenylamine as indicator. An approximate analysis of tertiary butyl alcohol ( 8 ) involves treatment of the alcohol with an excess of concentrated hydrochloric acid followed by titration of the excess acid. The conversion of alcohol to chloride is 94 to

98%.

Aldehydes and Ketones. Indirect titration methods for the determination of aldehydes and ketones involving reaction with hydrazine derivatives, hydroxylamine and bisulfite, have been reviewed (12). The hydroxylamine procedure continues to receive considerable attention. Applications of the method include the determination of ketones in lacquer solvents ($?’),the determination of acetone in acetone cyanohydrin (166),the determination of crossed unsaturated ketones (I.+$), and the determination of acetone and methyl ethyl ketone in the pre nce of aldehydes (146). In the last determination, the aldehydes are converted quantitatively to S c M bases with aniline and the ketone is removed by distillation, The acetone produced in the Meerwein-Ponndorf reduction of ketones (177) is the basis of a method proposed for the determination of oxo groups. The acetone is estimated by the hydroxylamine procedure. In a modification of the hydroxylamine method, Pesez (142) suggests the use of hydroxylammonium formate in methanol, followed by titration of

the excess hydroxylamine with perchloric acid in dioxan in the presence of a thymol blue indicator. Acetals and organic acids do not interfere. Methyl ketones may be determined by measuring the pH of the solution (66) after treatment with hydroxylammonium chloride and comparing the pH with a calibration curve. The method is reported to be more reliable than the one in which the liberated hydrochloric acid is titrated. Carbonyl compounds have been determined (9) by treatment with excess 2,4dinitrophenylhydrazine. After the removal of the substituted hydrazone by filtration, the excess hydrazine is titrated with potassium iodate in the presence of hydrochloric acid. The hypoiodite method of Romijn (156) for the determination of formaldehyde was modified and extended to the determination of small quantities of acetaldehyde (16). A correction factor of 2% is required since results are consistently 98% of the theoretical. Nitroacetophenone has been quantitatively estimated in the presence of nitroethylbenzene (128) by the iodoform reaction. The oxidation of acetone to acetic acid and carbon dioxide by hppobromite (63)has been used to analyze, correctly, aqueous solutions of acetone. Cyclohexanone oxime, alone or in the presence of caprolactam (59), may be determined gravimetrically as the pnitrophenylhydrazone or as the 2,4-dinitrophenylhydrazone. An alternate procedure (60) involves hydrolysis of the oxime, steam distillation, and determination of the cyclohexanone in the distillate as the 2,4dinitrophenylhydrazone. A Fimilar procedure is used fo’ the estimation of 0-and N-bema1 groups (84). Hydrolysis produces benzaldehyde which is steam distilled into a 2,khitrophenylhydrazine reagent, the precipitate is removed and the excess hydrazine is determined by titration with ferrous ammonium sulfate in a carbon dioxide atmosphere. Amides. Methods for the determination of the carboxylic acid amides involve saponification followed by titration of the volatile amine which bas been absorbed in hydrochloric acid (159)or by titrating the excess base used in the saponification (78). A similar procedure, involving a modification of the method of Waltz and Taylor (918), uses benzyl alcohol as the solvent (207) for the determination of polyamide end groups. Salts of dithiocarbamic acids’(1) or thiuram disulfides (11)

S

4

S

I

IS

RZNC4H wCS--S--CNR, I I1

are determined by hydrolysis with pyridine and phosphoric acid to yield carbon disulfide (168). The carbon disulfide is estimated as the xanthate. An indirect titration procedure has been wed for the analysis of urea (13). Add 2.5 t o 3.0 grams of boric acid, 0.1 ta 0.2 gram of potassium bromide, and 10 to 20 ml. of water to the solution t o be tested. Heat the solution to between 50° and 60" and add an accurately measured excess of 0.1N sodium hypochlorite to the sample. Keep the solution on a water bath for 2 minutes, add 10 ml. of 30% sodium hydroxide solution, and an amount of 0.1N sodium thiosulfate which is equivalent to the amount of sodium hyFochlorite. Cool, add 1 drop of indicator (lye brasilin solution in ethyl alcohol) and 1 drop of 5% potassium iodide solution, and titrate slowly with 0.1N sodium hypochlorite to a color change from red to yellow-green. The direct titration of urea was unsuccessful. Amine Oxides. The N-oxide functional group in pyridine N-oxide and related compounds may be determined by reduction with titanium trichloride (18). Reaction periods of 5 minutes are required for most compounds. The addition of citrate or thiocyanate to the reaction mixture shortens the reaction period for thcw substances requiring longer reaction times. Amines. Nonaqueous titrations continue to be the most frequently used method for the determination of amines and organic bases. With a perchloric acid titrant, bases such as cdeine in a chlorobenzene solvent (139), benzimidazole and d e rivatives in a propionic acid solvent (72), and high molecular weight amines in acetic acid solvent (118) have been determined. Diphenylguanidine has been determined by titration in alcohol solution with sulfuric acid using a bromophenol blue or fluorescein indicator (88). The titration of ethyl paminobenzoate in glacial acetic acid (28) with perchloric acid has been compared with an iodometric titration and with a photocolorimetric method and was found to produce results in agreement with these procedures. Acetyl chloride is used as the solvent (138) in the titration of quinoline, e picoline, and dimethylaniline with stannic chloride or titanium tetrachloride using crystal yiolet as indicator. Aromatic amines and nitrogen hetemcyclic compounds can be determined quantitatively with sodium tetraphenylborate (80) in the presence of 0.1N sodium hydroxide. The Van Slyke method for the determination of aliphatic amino p u p s has been modified (I@) to enable more rapid and accurate determinations. Modifications in the nitrosation pro-

cedure for the determination of c a r bazole (26)have been described (1%). The reaction of primary, secondary, and tertiary amines with sodium hypobromite to form bromamines is the basis of an aualysis of these compounds (164). Treatment of the bromamine with potassium iodide liberates an equivalent of iodine whkh is determined by titration with 0.1N sodium thiosulfate. The error is 5 to 6%. Tertiary amine salts or quaternary ammonium salts can be titrated qurtntitatively with perchloric acid in anhydrous propionic acid containing mercuric propionate (71) or in acetic acidacetic anhydride solvents containing mercuric acetate (6s). Crystal violet or cresol red and metanil yellow were the indicators used, respectively. Amino Acids. The determination of amino acids according to the method of Slavickova (181) involves the transfer of copper from a suspension of copper phosphate, removal of the copper phosphate by filtration, acidification of the a t r a t e followed by iodometric titration of copper ion. Vavrinecz (210) recommends the use of hydrochloric acid in the acidification step and discusses the sources of error in the determination. Carbon-Methyl Groups. An improved apparatus and procedure for the determination of acetic acid (192) resulting in carbon-methyl analysis has been described. Diazo Compounds. The methods currently used in the analysis of stable diazo compounds have been reviewed (91).

Enols. The use of Grignard reagent for the determination of enol content of hydroxymethylene ketones (14 ) gave only approximate results due to the difliculty involved in determining the optimum reaction time. Esters. In a review of analytical methods, Veibel (212) indicates that esters are best determined by saponification. Methods usually m e r in the procedure employed for analysis of one of the products or unused base. The method has been applied to the determination of esters of acetj-lenic alcohols (134). Esters of oxalic acid are estimated by treatment with calcium chloride after saponification (69). The excess unprecipitated calcium ion is measured by chelatometry. Monoglycerides may be determined by an oxidative procedure ($06) using sodium iodate, NaJtJOk or HJOI instead of periodic acid as oxidizing agent. Ester groupings in polyesters are determined by saponification in mixtures under bemendcoho1 pressure (31) or in aqueous alkali (61). The exalkali is titrated with hydrochloric acid.

Ethers. Alkyl iodides, obtained in the Zeisel alkoxy analysis, have been identified and determined (98) with an accuracy of *l% by gas chromatogrsPhY. A detailed procedure (93)using phenol, 5% cadmium sulfate, and 5% 80dium thiosuIfate has been described for the determination of alkoxyl values for tert-butyl substituted phenols. Anomalous alkoxyl values are obtained in the procedure usually employed. Hydrazine Derivatives. Hydrazine and hydrazine derivatives, including phenylhydrazine, azines, semicarbazide, and semicarbazones, have been analyzed by oxidizing the hydrazine nitrogen to elemental nitrogen. Direct titrations may be accomplished with sodium hypochlorite (162, 178), and

potassium persull'ste (180) if the titrations are camed out in the presence of an iodine monochloride catalyst. Other oxidizing agents used as titrants include iodine monochloride in the presence of mercuric chloride and hydrochloric acid (179),bromine monochloride (169), and potsSeium iodate in the presence of hydrochloric acid (9).

In another procedure (116) the hydrazine derivatives are determined gasometrically. The volume of nitrogen produced on oxidation with potasmum iodate is m& in a Warburg apparatus. The hydrazide of isonicotinic acid is quantitatively oxidized in the presence of potassium iodide with chloramine T (189) or with potassium iodate (188). The iodine is titrated with sodium thiosulfate. The Dewar method (99) for the analysis of hydrazo compounds involves the use of Bindschedler's Green as oxidizing agent; the excess dye is titrated to a colorless end point with aqueous titanous chloride. An improved method of synthesis of Bindschedler's Green and a discussion of its structure have been reported (176). Nitriles. The oxidative hydrolysis of nitriles with potassium hydroxide and hydrogen peroxide is the basis of an analysis of simple aliphatic nitriles (293). An equivalent of potassium hydroxide is consumed per mole of nitrile, the excess baae being titrated with sulfuric acid. Substances oxiditable to acids interfere. Nitro Compounds. Mono-, di-, and trinitro- compounds have been determined by reduction with an excess of titanium(I1I) and. polyphosphate (194). The excess is titrated with potsssium dichromate, ceric sulfate, or sodium vanadate using methylene blue, diphenylamine, or indigo Carmine a~ indicator. VOL 32, NO. 5, APWL 1960

0

265 R

Chromous sulfate solutions (199) may be used for direct titrations of 0-nitroaniline; o-nitrophenol; pnitm phenol; 2,4-dinitrophenol: pnitrotoluene: m-nitrobemaldehyde: pnitrobenzaldehyde; and picric acid in acidic solution under carbon dioxide. An indirect method using an excess of chromous sulfate and titrating the exceM with ferric ion is also possible. The reduction of nitrocyclohexane with hydrogen iodide produces an equivalent of iodine (122). The nitro compound is estimated quantitatively by titration of the liberated iodine with sodium thiosulfate. Nitrocylohexane in alkaline solutions wm determined (87) by titration with hydrochloric acid to a phenolphthalein or a thymolphthalein end

poht. Nitroso Compounds.

Aromatic nitroso compounds are reduced with cadmium and hydrochloric acid (W). The dinsolved cadmium is proportional to the nitroso compound and is determined by titration with ethylene diaminetetraacetic acid using Eriochrome Black T as indicator. The procedure is not applicable to aliphatic nitroso compounds. Oxiranes. The following procedure has been described for the determination of the epoxide group (70). Excess 0.W hydrochloric acid in a mixture of isopropyl ether and carbon tetrachloride is added to the sample and after 6 hours the sample is titrated with 0.W sodium acetate in acetic acid to a methyl violet end point. A blank is required. Acids do not interfere.

Oxirane oxygen in salts of epoxy acids and in the presence of amines in epoxy compounds (38) is determined by an argentimetric method utilizing hydrobromic acid in acetic acid and silver nitrate or a completely nonaqueous titration with hydrobromic acid and perchloric acid in acetic acid. Titrations are rapid and accurate within 1%. Ethylene oxide in glycols, polyglycols, and their ethers and esters reacts with hydriodic acid in the presence of magnesium oxide (59) to produce ethylene. The ethylene is absorbed in 0.2N iodine monochloride in glacial acetic acid. Sodium tetraphenylborate has been suggested as a reagent (1%') for the determination of the epoxide group in polyalkene oxides. Peroxides. The Wheeler method (891) for the iodometric analysis of peroxides has been improved by shortening the reaction time to 5 to 10 minutes and by extending the procedure to both aliphatic and aromatic tettbutyl pemters (176). The use of a ferric chloride catalyst and an acetic acid solvent was responsible for these improvements. Factors contributing to analytical

266 R

m m c u CHEMISTRY

errors in the iodometric determination of peroxides have been investigated (172).

The following procedure has been used for the determination of peroxyacetic acid in hydrogen peroxide (27). Acidify the solution (-0.1N) with 5

ml. of 20% sulfuric acid and add 0.1N

arsenious oxide. Dilute to about 50 ml., add 1 drop of ferroin indicator, and titrate the hydrogen peroxide cerimetrically. Add 1 drop of osmium tetroxide catalyst (decomposes the peracetic acid) and determine the newly formed hydrogen peroxide cerimetrically. Homer and Jurgens (75) present detailed directions for the determination of dialkyl peroxides, alkyl hydroperoxides, acyl hydroperoxides, and diacyl peroxides in mixtures by an iodometric procedure. Phenols. The methods used in the determination of the phenol functional group in both control and research laboratories have been discussed by Gaupel and Mangenery (58). A method of analysis applicable to phenol, dihydroxydiphenylmethanw, multi-ring phenols, bis-(o-hydroxybenzy1)amine and phenols in the presence of hexamethylenetetramine, ethers, and hydroxymethyl groups (77) involves a bromination by bromine in acetic acid catalyzed by pyridine. The reaction is generally completed within 2 to 20 minutes at room temperature. At least 40 phenols have been directly titrated within 0.3% accuracy (185) by adding a slight excess ,of bromate-bromide in acetic acid and immediately back-titrating with thiosulfate. Chloro-, nitro-, carboxy-, acyl-, and alkoxyphenols could not be determined by this procedure. Modifications necessary for the determination of these compounds were presented. In a similar procedure (do), monohydric phenols are separated from polyhydric phenols by steam distillation from a copper sulfate solution and then determined by the bromide-bromate method. An iodometric procedure has been reported (167) for the determination of phenol in the presence of reducing substances. The phenol is converted to tribromophenyl hypobromite upon reaction with excess bromine. After removal of the excess bromine, the hypobromite convertg iodide to iodine when acidified potassium iodide is added. The use of bromine monochloride as a brominating agent has been investigated (168). The rate of reaction is increased appreciably by the use of this reagent. Hydroquinone has been determined by oxidation with iodine monochloride (13'7). The liberated iodine is titrated with standard sodium chlorite in a saturated potassium bromide solution. Sulfides and Disulfides. The forma-

tion of sulfonium salts from organic sulfides has been applied to the determination of the sulfide group. In one method (214), dialkyl sulfides are reacted with pbromophenacyl bromide and the resulting sulfonium salt is isolated as the picrate. The picrates are determined by titration with perchloric acid in glacial acetic acid. Aralkyl and diary1 sulfides do not react. In another method (1) the sulfonium salt is formed by reaction of the sulfide with methyl iodide in a sealed tube. After removal of excess methyl iodide, the iodide in the sulfonium salt is determined by the Volhard method. Organic disulfides are reduced qumtitatively to the mercaptan with sodium borohydride and aluminum chloride (190). The mercaptans are determined by potentiometric titration with silver nitrate. Thiols. A method of analysis of thioalcohols and thiophenols is based (1%) on the determination of escess acrylonitrile. CH,=CH-CN

+ RSH

-P

RS----CHa-CHZ-CN

Sodium sulfite is added to the reaction mixture and the alkali formed is titrated with hydrochloric acid to a thymolphthalein-alizarin yellow end point (yellow). Unsaturation. The determination of ethylenic double bonds by quantitative hydrogenation has been revierred by Tiong and Waterman (203). In a modification of the catalytic hydrogenation method (170) the hydrogen is generated from a standard lithium aluminum hydride solution, the excess hydrogen reacting with oxygen to form water. The water is determined by the Karl Fischer method. Good reproducibility was obtained but the standard dewation varied from 3 to 13%. The bromometric method of Rosenmund (167) has been applied to the determination of unsaturation in mixtures of isopentane, isoprene, and isoamylene. The accuracy of the method lies between 1 and 3% but may be improved to 1% by the use of empirically determined correction coe5cients. Methyl vinyl ketone has been determined by the bromate-bromide method ($1) in which the e x c w reagent is determined iodometrically. Methyl vinyl ether in gas samples has been determined (191) by treating the sample with 40 to 60% sulfuric acid and measuring the loss in weight of the sample. Compounds with terminal triple bonds may be determined by nonaqueous titration procedures. In one method (180) the liberated nitric acid from a silver nitratepyri-

dine reagent is titrated with methanolic sodium hydroxide to a thymolphthalein end point. The procedure is satisfactory for acetylenic compounds including alcohols, esters, aliphatic amines, halogenated compounds, ethers, and heterocyclic bases but fails with propargylic alcohol, propargylic bromide, and phenylethynylcarbinol. In another procedure (10) the perchloric acid liberated from a methanolic silver perchlorate reagent is titrahed with 0.1N tris(hydroxymethy1) aminomet hane. MISCELLANEOUS METHODS

Mixtures. 2,2-Dichloropropioi~ic acid, 2-chloropropionic acid, and 2,2,3trichloropropionic acid may be determined in their mixtures by use of mercuric nitrate and mercuric propionate (111). 2,2-Dichloropropionic acid is hydrolyzed to pyruvic acid which is treated with mercuric nitrate to g v e the anhydride of 3,3-bis(hydroxymercuric)-3-nitratomercuric pyruvic acid. This mercurial is decomposed by pot:tssium iodide to give potassium hydroxide which can be titrated. 2-Chloropi-opionic acid is hydrolyzed to lactic acid, The lactic acid is oxidized to pyruvic acid and determined as above. 2,2,3Trichloropropionic acid reduces mercuric propionate in boiling aquecus solution. The mercurous salts formed are converted to mercurous oxide, dissolved in acid, oxidized, and titrated s i t h ammonium thiocyanate. Addition of bromine to 8,y-unsaturated acids is quantitative in 5 to 10 minutes with 0.1N bromine in methanol saturated with sodium bromide. Siniilar reaction with a,p-unsaturated acids requires 24 hours with 0.2N bromine in methanol saturated with sodium brcmide. This difference in reactivity is the basis for a determination of one type of acid in the presence of the other (12) hfixtures of maleic acid and maleic anhydride may be titrated with aqueous sodium hydroxide and methanolic sodium methoxide to determine the amount of anhydride present. The first titration gives sodium maleate and the second converts anhydride to sodium methyl maleate. The dzerence between the two titrations is proportional to the amount of maleic anhydride present (76). Cerate oxidimetry permits the estimation of formic and oxalic acids. Oxalic acid is oxidized by ceric sulfate in boiling sulfuric acid solution, while formic acid is not. Formic acid is then oxidized slowly by cerate (174) in the presence of chromium(II1) ion. A procedure for the determination of methyl phenylmalonate in the presence of methyl phenylethylmalonate is based on the difference in the rate of saponification of the two compounds

and is reported to be accurate to =k2% (79)* A determination of methanol in mixtures with ethyl alcohol and water is based on the more rapid oxidation of ethyl alcohol by permanganate in sulfuric acid (110). The oxidation is stopped a t a suitable point and the excess permanganate is titrated. It is necessary to refer to a special diagram made under standardized operating conditions to calculate the amount of methanol present. Conditions for the quantitative separations of various organic bases as reineckates have been described (101). Bases studied include piperidine, pyridine, codeine, caffeine, and strychnine. Various procedures for determining xylene isomers and ethylbenzene mixtures are based on selective nitration techniques, coupled with distillations, fractional crystallizations, and melting point curves (103, 121). Mixtures of thiols and alkyl sulfides may be determined by using two aliquots. One aliquot is titrated in the usual way with iodine to determine thiol. The second aliquot is treated with acrylonitrile and then titrated vith bromate-bromide solution. The sulfide formed by addition of the thiol to acrylonitrile is oxidized readily by the bromate-bromide solution. The original alkyl sulfide in the mixture is determined by difference (81). Mixtures of thiol and disulfide are determined by iodometric titration of thiol before and after reduction of the disulf;,de with zinc in acetic acid-hydrochloric acidalcohol. A simple method for determining melamine in mixtures involves precipitation by cyanuric acid (86). Acetylene in acetaldehyde has been determined by titrating the nitric acid liberated when the sample is treated with silver nitrate (55). Earlier methods (148, 208) for determining anthracene by reaction with maleic anhydride have been modified to give a rapid method in which extraction of unreacted maleic anhydride is avoided by titrating directly the upper aqueous layer of a two-phase system in which the lower xylene-carbon tetrachloride layer retains the adduct and the upper aqueous layer contains the unreacted maleic anhydride (197). A novel method for the determination of iodoform (and hence indirectly of methyl ketones) is based on its decomposition by light to form free iodine (15). The method is limited to small samples (0.1 gram or less) of iodoform. Primary allyl-type chlorides can be determined in mixtures with isomeric tertiary chlorides by reaction with potassium iodide and urotropine. Unreacted urotropine is titrated with hydrochloric acid in the presence of formalin (102).

Hexamethyleneimine may be separated from hexamethylenediamine by virtue of the solubility of its nickel dithiocarbamate in organic solvents. The corresponding dithiocarbamate of hexamethylenediamine is not soluble in organic solvents (186). Sugars and Related Substances. A new indirect titration method for reducing sugars takes advantage of certain properties of sodium cuprosulfosalicylate (160, 161). Aqueous solutions of this reagent deposit a precipitate of cupric oxide when heated nith sodium hydroxide, but in the presence of reducing substances quprous oxide is formed in amounts proportional to the amount of reducing substance. The amount of cupric oxide formed can be determined iodometrically and from this value the cuprous oxide and reducing sugar can be calculated. A calibration curve is used. The method is reported to be rapid, and can be used with samples containing 2 to 10 mg. of glucose, or with mannose, galactose maltose, and lactose. Copper trihydroxyglutarate has been used for the determination of g l u e , fructose, and maltose ( 2 ) . Fehling’s solution is recommended by Takahashi (196) for the determination of reducing sugars. An excess of Fehling’s solution is added to the sugar sample and the excess Fehling’s solution is titrated with a standard sugar solution to a methylene blue end point. The apparatus of McCready (116) for uronic acid determinationv has been modified for semimicro applications (7). The carbon dioxide liberated by decarboxylation is determined by titration in an absorption trap of special design. In the determination of ascorbic acid by several oxidizing agents (Chloramine-?‘, KIs, FeCl,) Variamine Blue is a suitable indicator (96). Sant has described a ferricyanometric method for ascorbic acid (164). Zinc acetate is added to precipitate the ferrocyanide ion as it is formed and to maintain the reaction medium at pH 6. A starch-iodide internal indicator is used. A 2% solution of metaphosphoric acid (HPOJ appears to be the choice reagent for extracting ascorbic acid from plant tissues and processed foods, whether the subsequent determination is with 2,4dinitrophenylhydrazine(198) or with 2,6-dichloroindophenol (130). Water. The Van der Meulen reagent (909) has been compared with a modified Fischer reagent (171) for the determination of traces of water in ethyl alcohol, isopropyl alcohol, isobutyl alcohol, polyoxymethylene, ethylene glycol, diethylene glycol, triethylene glycol, benzene, toluene, and chloroform. The two methods were found to give similar resulte. VOL. 32, NO. 5, APRIL 1960

* 267 R

Methylene blue has been previously suggested as an indicator in the Fischer titration but a new mixed indicator i s reportedly easier to use, giving a color change from green to brown (119). The indicator consists of 10 mg. of quinalizarin, 15 mg. of Naphthol Green, 20 mg. of methylene blue, and 0.1 ml. of acetic anhydride in 20 ml. of a 1:l mixture of anhydrous pyridine and methanol. Unclassified. A study of the oxidation of various organic compounds with quinquevalent vanadium solutions suggests that this reagent may have a number of uses in organic analysis (210). The oxidation procedures are simple and exact control of temperature and acid concentration is not critical. a-Hydroxy acids undergo oxidative decarboxylation in what appears to be a stoichiometric reaction. Ethylene glycol, glycerol, and 1.3-propanediol are oxidized to formic acid. The stoichiometry of the oxidation of l,%propanediol and malonic acid is not so simple. Optimum conditions for the determination of choline by potassium chromate oxidation have been worked out and described ( I 86). The following reaction of acetic anhydride is quantitative, and is the basis of a method for measuring the rate of acetylation by acetic anhydride. (CH3CO)zO

-

pyridine + (COzH)? CO + COz + 2 CH3COzH

Samples are removed from the acetylation reaction and decomposed according to the above reaction in a simple apparatus designed to measure the gas evolved (89). A gravimetric procedure in which morphine is precipitated by ammonium vanadate and ammonium molybdate is reported to give accurate results (96).

For determining furfural from peat the bromide-bromate method (34) either with or without an ammonium molybdate catalyst is preferable t o the method based on reaction with hydroxylamine hydrochloride (35). A rapid titration procedure for 4,4'dihydroxybiphenyl is based on oxidation to the corresponding 4,4'-diphenoquinone by a trivalent copper complex such as potassium diperiodate-cuprate (113).

The number of bromine atoms taken up by cinnamic acid and some of ita derivatives on treatment with bromidebromate solution has been studied under a variety of conditions (216). Pyridine in denatured alcohol may be determined by precipitation with CdC12 followed by titration of the cadmium in the precipitate with 0.1M disodium ethylenediaminetetraacetate

(24. 268 R

0

ANALYTICAL CHEMISTRY

Of several methods studied for r e moval of chlorine from 1,Michloroethane, the use of sodium and alcohol in the presence of Raney nickel catslyst appears t o be the most rapid (2006). Even by. this method only 95% of the chlorine is removed in 20 minutes and increased reaction time does not result in increased removal of chlorine. Dichloroacetaldehyde is converted t o potassium glycolate by hydrolysis with alcoholic potassium hydroxide a t 100" in a stoppered bottle. The chloride liberated can be titrated with silver nitrate solution. An alternative procedure involves hypoiodite oxidation of the aldehyde to dichloroacetic acid (109). In the determination of sodium carboxymethylcellulose, the acid may be precipitated and weighed as the aluminum salt, or preferably by p r e cipitation as the copper salt by a known excess of copper sulfate. The excess copper in the filtrate is then determined iodometrically (62). Solutions of acetaldehyde react with sodium h-ypoiodite containing excess alkali in two ways: oxidation to acetic acid and formation of iodoform. If an insufficient amount of alkali is used, the simple oxidation reaction can be made quantitative, thus providing a suitable method for determining acetaldehyde (17). Hydrazobenzene i s osidized quantitatively to azobenzene by alkaline permanganate. Aniline and phenylhydroxylamine interfere; nitrobenzene, nitrosobenzene, and azosybenzene do not (165). The procedure calls for the addition of a known excess of d.02-V potassium permanganate. After the oxidation the excess is determined by addition of potassium iodide and acid, followed by titration of the iodine with thiosulfate solution. Aryloxysilanes have been determined by making use of the reaction of the aryloxy groups with bromide-bromate solutions ( I 84). A new gravimetric method for picric acid utilizes the complex formed when the sample is treated m-ith silver nitrate and thiourea. Composition of the complex corresponds to [Sg(CSr\'A)] [CaHzX;307] (187). N-alkylamino derivatives of phenothiazine such as chloropromazine can be titrated readily with ceric sulfate solutions (33). The presence of other nitrogen heterocycles does not interfere. A careful study of various methods for the determination of total nitrogen and of nitro nitrogen in nitroguanidine has been reported (4%'). West (220) has reviewed recent developments in inorganic and organic analytical chemistry.

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