A System of Characterization of Common Organic Acids RAY T. WENDLAND North Dakota State College, Fargo,
N. D.
DONALD H. WHEELER Research Laboratories, General Mills, Inc., Minneapolis, M i n n .
The problem of identifying organic compounds ought to be examined periodically to determine whether a reorganization of existing information may simplify the experimental solution. Much information on organic acids is dispersed in the literature, but a general treatment aimed at many varieties of acids is missing. Acids may be considered as derivatives of many other types of compounds convertible to acids. Organic acids containing carbon, hjdrogen, and oxygen only can be divided into four well-defined classes based on common physical properties and simple chemical tests. Identification of the drjing oil acids calls for ultraviolet absorption studies, or chemical processes of bromination, hydroxylation, and isomerization. Practical significance of the work lies in the increased ease of identification of a large number of compounds encountered frequently in research and industrial operations.
A
LTHOUGH several systems for identification of organic compounds have come into common use-e.g., those of Kamm ( 9 ) , Shriner and Fuson ( 9 4 ) , and Huntress and Mulliken ( 7 ) improvements seem desirable. The system offered here aims a t the identification of organic acids containing carbon, hydrogen, and oxygen (Order I acids of Huntress and Mulliken). Such a system possesqes considerable generality because the acids frequently serve as suitable derivatives of many other types of compounds. Thus, easy identification of acids may mean rather easy identification of esters, aldehydes, primary alcohols, methyl ketones, nitriles, and amides, in addition to several types of hydrocarbons. The alkenes, cyclenes, and alkyl benzenes are easily characterized ( 3 6 ) ; permanganate or nitric acid ovidation gencrally forms an acid identifiable by the present system. Despite structural variations that may defy general treatment in any given series of compounds, the individual carboxylic acids fall into ~~-ell-dcfined groups. For instance, a liquid acid having a sour odor, partially miscible with water, with density less than 1.0, whose alkaline solution is not soapy, would necessarily be an acyclic mono acid with 5 to 10 carbon atoms; a second acid, differing from the first only by displaying a density greater than 1.0 would have to be a cycloalkane mono acid, such as cyclohexylacetic acid. Ultimate identification of the unknown characterized in this manner would then hinge on ascertaining two or three additional properties, density, refractive index, etc., or melting points of one or two derivatives. I n order t o employ properties and characteristics most easily observed in the laboratory, the authors have set up a classification based on the following considerations: Class I includes liquid acids melting at 30" C. or lower. Classes 11, 111, and IT- include solid acids melting above 30" C. Class I is subdivided according t o odor, miscibility with water, density, character of alkaline solutions (soapy or not), reaction n ith aqueous permanganate, and determination of neutral equivalent. Classes I1 and I11 are subdivided according to limits imposed on melting points and neutral equivalents, reactivity t o permanganate and bromine, and for low-melting solids, water miscibility, and nature of alkaline solutions. Class IV (phenol acids) is distinguished from the others on the basis of deep coloration with aqueous ferric chloride coupled with the fact that most phenol acids are high melting (generally above 150" C.).
T o establish that an unknown is an acid (Order I assumed), the generic test of Huntress and Mulliken was used-Le., in a prescribed titration the compound consumes sufficient alkali to give a neutral equivalent 400 or less, and the end point is sharp. I n most cases a simple test with saturated sodium bicarbonate is sufficient; otherwise, quantitative titration is required. Metallic salts can be included if the organic acid can be isolated after liberation with a mineral acid. If the unknown is an aldehydic acid, it must be treated as a n aldehyde, or oxidized t o a new acid, which, if stable, can be handled by the present system. Before the properties of the unknova are measured, the ordinary criteria of purity should have been satisfied. Except for liquids completely miscible with water and difficult to separate from it, the others should have been fractionally distilled or crystallized, or otherwise separated from contaminants. With few exceptions, notably for the drying oil acids, identification by derivatives has not been specified, because the amides, anilides, phenacyl estek, etc., are well enough established. T h e principal classification is given in Table I, properties and data for identification of the drying oil acids are given in Table 11. IDEhTIFlCATIOY OF UYSiTURATED ( C d DRYING OIL ACIDS
Procedures for identification and estimation of the drying oil acids (Table I, Class I ) are necessarily based upon a thorough evaluation of their properties as pure compounds. Phvsical property data and particularly those concerning characteristic absorption in the ultraviolet and infrared regions are widely scattered through the literature and frequently discordant. Therefore, best values are summarized in Table 11, in which most values have been rechecked or evaluated originally in the General Mills Laboratories. Some acids in Table I1 occur naturally as mixtures of the mixed glycerides in drying and semidrying oils (identified in the table by footnote b ) . The most common are oleic, linoleic, and linolenic, occurring in linseed, perilla, soybean, corn, safflon-er, and cottonseed oils. These liquid acids are accompanied by saturated acids (higher mrlting), which can be removed by low temperature crystallization from acetone. Ricinoleic, eleostearic, and licanic acids occur abundantly in castor, tung, and oiticica oils, respectively, which, therefore, are preferred source materials for their preparation. Eleostearic and licanic acids are high melting and may be isolated with relative ease from the saponification mixture (Procedures B and C). The liquid acids are separated from each other x i t h considerable difficulty; hence analytical methods for detection and estimation involve direct study of the mixed acids. The various analytical procedures are based on the following principles. Quantitative determination of linoleic and linolenic acids depends on isomerizing them t o the conjugated forms by heating with alkali in glycerol or glycol a t 180" C. The amount of linolenic acid is calculated from the ultraviolet absorption a t 2680 A. (characteristic of conjugated triene) and linoleic from the absorption a t 2330 A. (conjugated diene) after correcting for the conjugated diene, formed partially from the linolenic acid. After the original total unsaturation by the iodine number is determined and the amounts of linoleic and linolenic acids are calculated, the amount of oleic acid is computed from the residual unsaturation ( 2 7 ) . This conjugation method is limited t o the naturally occurring cis- acids, because acids containing trans double bonds conjugate with alkali much more slowly (8). Acids occurring as conjugated dienes and trienes may be determined directly by ultraviolet absorption (dienes a t 2310 to 1469
ANALYTICAL CHEMISTRY
1470
-
______
Table I.
System of Characterization of Common Organic Acids Containing Carbon, Hydrogen, and Oxygen Only
(Halogens absent, copper wire flame test negative) These acids are presumed to be colorless, or capable of being rennot colored by ferric chloride. Aliphatic dioarhoxy acids dered so by simple treatment. Excluded are the aldehyde acids with 8 to 10 carbon atoms, suberic, azelaic, sebacic, et?. which color the fuchsin reagent. Phenylsuccinic. l,l-, 1,2-trans-(dextro and levo), 1.3-cisC L 4 S S I. LIQUIDk I D S and trans-, and 1,4-cis-cyclohexane-dicarboxylicacids. Other isomers are high melting and fall into Class 11. Group C 1 ; Assumed temperature defining the liquid state = 30' C. also homophthalic and &camphoric acids. Colored by ferric chlorideh. Miscellaneous nonphenolic Group A. With sharp or sour odors, miscible with w*ater. distillable hydroxy or keto arids of high molecular weight. Cwith steam, hygroscopic. Neutral equivalents 105 to 120 ( + 2 ) . Naphthalene-i,2-&carboxylic acid. Other naphthalene dicarboxylic acids melt at 200' to 300' C . and fall into Class 11, Group C 2 .
1. Stable to permanganatea. Acetic, propionic, and butyric
acidsb. 2.
Oxidized by permanganate". Formic, acrylic. methacrylic, vinylacetic, and pyruvic acidsc.
Neutral equivalents 120 to 200 ( + 2 ) , not colored by ferric chloride. -4romatic monocarboxy acids, benzoic, toluic. B-benzoylpropionic and butyric acids. etc. Aryloxyacetic acids. Ethoxy- and methoxyhenzoic acids, naphthoic and naphthyl acetic acids .iryloxyacetic and alkoxybenzoic acids caii umally be hydrolyzed with iodic acid or hydrochloric acid to give the phenolic components which then react with ferric chloride). Colored by ferric chloride. Aryl a-hydroxy acids, such as mandelic.
Group B. With sharp or sour odors, limited miscibility with water. Can generally be rendered anhydrous by disti!lation (reduced pressure advantageous). Distillable with steamb. Alkaline solutions are not soapy. Neutral equivalents 88 to 172(&3). 1.
Densities less than l.Od. Aliphatic monocarboxy acids, isobutyric, and others with 5 t o 10 carbon atomsb~e~f.
2.
Densities more than l.Od. Cycloalkane monocarboxy acids (cyclopropane, -butane, -pentane, -hexane, and -heptane carboxylic acids) : Cyclohexyl formic, and acetic acids, also oethoxybenzoic acid (slight odor pleasant)
Neutral equivalents over 200 (not colored by ferric chloride). hroylhenzoic acids (CsHKOCsHaCOOH). diphenylated aliphatic acids [ (CaHj!zCHCOOH]. (For detection of o-aroylbenzoic acids i.) Colored by ferric chloride. Diary: a-hydroxy acid-e.g.. benzilic acid.
Group C. Odors slight or absent, insoluble in water, alkaline solutions turning soapy 1.
2.
Neutral equivalents less than 190 (densities less than 1.0). A few aliphatic monocarboxy acids with more than ten carbon atoms-e.g., undecylic and undecylenic acidse. Densities greater than 1.0. Alkyl cyclohexylacetic and -propionic acids. Alkyl cyclohexane carboxylic acids. Neutral equivalents over 190 but difficult t o determine because alkaline solutions are very soapy?: Soapiness is discharged by addition of 1% calcium chloride solution. Unsaturated long-chain aliphatic acids, oleic. ricinoleir. linoleic, linolenic, eleostearic, etc. (Table 11).
Group D. Odors slight or absent, miscible with water. 1. Stable to permanganate.
,%Ketobutyric (acetoacetic, decomposes on heating) niethoxy- and ethoxyacetic acids, lerulinic arid' ( 7 , p. 91).
2. Oxidized by permanganate. Lactic, acids, liquid a-hydroxy acidso.
a-
and @-hydroxybutyric
CLASS11. SOLID SATURATED ACIDS
Melting points over 30' C. Cnreactive t o aqueous permanganate, no addition of bromine from carbon tetrachloride solutiona. Group A. Melting points less than 80' C. 1. Insoluble in water. Densities less than l.Od.
Alkaline solutions are soapy but soapiness is discharged by addition of 1% calcium chloride solutionf. Aliphatic monocarboxy (fatty) acids with 12 or more carbon atoms, lauric, stearic, etc. Capric is border line case-melting point = 31 C. Alkaline solutions are not soapy. A few highly branched aliphatic monocarboxy acids-e.g., trimethylacetic and 2.2,3trimethylbutanoic acids. 2.
Insoluble in water. Densities more than l . O d . . I variety of phenylated aliphatic acids-e.g., phenyl acetic, phenylpropionic (hydrocinnamic), w-phenylbutyric and -valeric acids. Also some benzoylated acids-e.g.. w-benzoyl valeric and wbenzoyl pelargonic acids.
Group B. Melting points 90' to 189' C. Neutral equivalents can be sharply determined. Alkaline solutions not soapy unless neutral equivalents are very high. Neutral equivalents 45 to 81 (+2) (fbirly soluble in water). Not colored by ferric chlorideh. Aliphatic dicarboxy acids, oxalic. malonic. etc., u p t o HOOC(CH?)&OOH; also tricarballylic acid. Colored by ferric chloride*. Citric, malic, tartronic, and tartaric acids; acetone dicarboxylic acid (violet with ferric chloride). Neutral equivalents 82 to 101 ( + 2 ) (slight solubility in water);
Group C . Melting points 190' C . or more. 1.
Neutral equivalents below 80. Polycarboxy aclds froin benzene-e.g., triniellitic triinesic, and mellitic acids. Polycarboxy acids from cyclohexane and naphthalene-e.g., naphthalene-1,4.5.8 acid.
2.
Keutral equivalmts 82 to 121 (*2j. Aromatic dicarboxy arids-e.g., o-. m-, and p-phthalic acids, diphenir. 1,8-naphthalic and other naDhthalene dicarboxvlic acids. cis 1.2-. dltrans-1,2- and trans-1 ,-l-cyclohexane dicarboxylic acids (Class 11, Group B2.j
3. Neutral equivalents above 130. Miscellaneous nonphenolio aromatic acids, such a3 m-hydroxy benzoic (no color with ferric chloride), piperonylic. syringic, p-ethoxybeneoic. anthracene and phenanthrene carboxylic acids, mucic and other sugar acids. (Last named frequently give neutral equivalents greater than calculated due t o lactone formation.) CL.AsS 111. S O i . I D I-NShTTRATED .icIDS Melting points more than 80' C
Group A. React with aqueous permanganate and add bromine readily froni CClr solution Ferric chloride negative. 1. Melting points less than 80" C. Insoluble in water, alkaline solutions are soapy but soapines- is discharged by addition of 1% calcium chloride solution'. Higher aliphatic unsaturated acids-e.g., erucic, elaidic, (trar~soleic), hrasdic. d-hydnocarpic, d-chaulmoogric acids. Eleostearic and licanic acids may fall herek. Moderately soluble in water. alkaline solutions not soapv. .. Lower aliphatic unsaturated acids-e.g.. trans-a-methyl crotonic (angelic) acid. cis-a-methylcrotonic (tiglic) acid, crotonic acid.
2.
Meltink pointa above 90' C . Seutral equivalents less than 115. Polyunsaturated aliphatic acids. e.g.. sorbic. and HOOC(CH=CH),COOH--, n = 2 and 3; where n = 4 , arid is colored yellow (16): acetylene dicarboxylic and inethylene succinic acids (itaconic), Neutral equivalents above 115. Various aromatic unsaturated acids, cinnamic, phenyl propiolic, beta fury1 acrylic. and also benzoyl acetic acid (very easily brominated.)
Group B. Acids exhibiting partial unsaturation; reactive t o permanganate (perhaps slorrly) but only slow-ly reactive or not itt all t o bromine in carbon tetrachloride. l a . Melting points less than 80' C.. colored by ferric chloride. Enolic keto acids and a-hydroxy aliphatic acids-e.g.. glycolic acid, a-hydroxy isobutyric acid. Enols give deep color with ferric chloride. a-hydroxy acids yellow color.
1471
V O L U M E 26, NO. 9, S E P T E M B E R 1 9 5 4 l b . Not colored by ferric chloride, phenylglyoxylic acid. 2.
Melting points above 80' C.; no color with ferric chloride. Unsaturated acids of the type R'-C-COOH cis or trans.
I/
(.
R"-C-COOH Methylnialeic (citracoriic), maleic, methyl fumaric (mesaconic), aconitic, fumaric, and others similarly constituted. Most of these react with dienes-e.g., chloroprene to form Diels-Adler adducts (also furoic acid). 3.
Melting points above 80' C., colored by ferric chloride. of the hydroxy acids (also keto) of Class 11, Group B1.
pound among the saturated acids of Class 11, Group A ; both should be positive for Class 111, Group A. * Volatile monocarboxy acids usually characterized by Ducleaux distillation ( 3 4 ) . SDecial distinguishing tests and suitable derivatives for identification are described by Huntress and Mulliken ( 7 ) . For compounds of slight solubility the test for density relative t o water is to stir up 0.1 gram of compound in a few Inl. of water; if it floats, it may be buoyed up by air. In this case. heat to melt the compound, after which its density is properly revealed. Heating the mixture gives information on approximate melting point of compound i n range 30' to 100" C. e rnsaturated acids in Class I, Groups B and C, can he detected by permanganate and bromine in carbon tetrachloride teats. Refractive index measurement$ identify individual fatty acids C6 to C18 ( 3 ) . a-Hydroxy acids (nonphenolic) give yellow coloration* with ferric chloride, further distingiiishing them from nonhydroxylated and keto acids. Keto acids if enolized give blue or violet colors with ferric chloride, resembling phenols: acetone dicarboxylic acid gives a violet color. A few common hydroxy acids may fall into Class 11. Group Bl b. Although generally negative to bromine in carbon tetrachloride. they may react slowly with permanganate, For further differentiation carry out the permanganate test on original (acidic) sclution and that neutralized by sodium carbonate. The ferric chloride yellow coloration produced by the acids easily differentiates a-hydroxy acids from others. To detect o-aroylbenzoic acids (reaction products from phthalic anhydride plus benzene-type hydrocarhons in Friedel-Crafts synthesis) heat with excess concentrated sulfuric acid. The resulting ring closure forms anthraquinone derivatives which by reaction with alkaline sodium hydrosulfite give deep red vat colors which fade by air oxidation. j For this group of acids the alkali titration requires alrohol, such that at the end point the alcohol concentration is at least 607,. Otherwise hydrolysis of the soap causes neutral equivalent to he too high. Eleostearic acid mixed with other liquid acids of this group will appear as a liquid, although several ironleric forins are solids (Tahle
Some
CL.SSSIT;. SOLIDPHENOLIC ACIDS Melting above 100' C. Give deep coloration nith aqueous ferric chloride. Generally susceptible to permanganate oxidation. Evolve bromic acid on treatment with bromine in carbon tetrachloride. Further differentiation is not clear cut.
Very easily oxidized, likely to be colored in aqueous solution, particularly at pH above 7.0. Reduce Tollens silver nitrate solution. 0- and p-di (or tri) hydroxybenzoic acids-e.g., protocatechuic, gentisic, and gallic acids. The latter two acids also reduce Fehling's solution. Tannic acid forms yellow or brown aqueous solutions. Acids generally stable in aqueous solution (no oxidation by air 0- and p-hydroxybenzoic acids (salicylic) (the meta acid is in Class 11, Group C 3 ) . p-hydroxyphenylacetic acid, trans-cournaric acid, hydroxynaphthoic acids, resorcylic, etc. or by Tollens reagent).
Acids colored by ferric chloride only after standing for some time. *Mostly esters of salicrlic acid, such as aspirin, which are notably subject to hydrolysis. especially at pH above 7.0. Permanganate test made on an aqueous solution prevloualy neutralized with sodium carbonate. For acids very insoluble In water test reagent should be an acetone solution of permanganate. Both permanganate and bromine te,ts should be negative to place a com-
2380 A. and trienes a t 2700 -4.).Of the naturally occurring conjugates only eleostearic and licanic acids are common. aEleostearic acid isolated from mixed t u n g oil acids b y cryst'allization from methanol is easily converted t o t h e beta isomer (melting point 71" C.) b y treating its solution with a trace of iodine. Licanic acid from oiticica oil also isomerizes with iodine t o the higher melting beta acid melting at 101 O C. These beta isomers constitute excellent derivatives for the original acids. T h e ultraviolet absorption d a t a are given in Table 11. N o simple qualitative test is available for characterizing the conjugated diene acids except ultraviolet absorption but this will not distinguish specific isomers. A qualitative test for d e h y d r a k d castor oil is based on the fact t h a t its nonconjugat,ed dienic acids contain the g-cis, 12-trans isomer (not present in other oils) which, after alkali conjugation, produces the high melting 10-trans-12-trans-octadecadienoic acid (Table 11). This is easily separated from t h e cis, trans conjugated acids formed from the cis, cis-normal nonconjugated acids found in natural oils ( 2 3 ) . I n mixtures containing appreciable amounts of the unsaturated acids, oleic and linoleic acids are characterized by hydroxylation of the double bonds with permanganate and isolation of the diand tetrahydroxy acids (di- is insoluble, tetra- is crystallizable from hot, water; hexahydroxy from a n y triene present remains dissolved in t'he aqueous mixture). Linolenic and linoleic acids are characterized by bromine addition and isolation of the corresponding hexa- and tetrabromides: the hexabromo is insoluble in ether, the tetrabromo is soluble (Table 11). Conjugated acids may be determined b y the amount of maleic anhydride added in the Diels-Alder reaction. Eleostearic acid adds one mole of maleic anhydride (6, 13). Conjugated diene acids also add one mole of anhydride, but a trace of iodine is necessary t o convert cis, trans t o trans, trans isomers for a quantitative reaction ( 2 1 ) . ISOLATION OF UNSATURATED ACIDS FROM THE DRYING OILS
A. Saponification a n d Separation of Mixed Acids. I n a 500ml. flask attached to a reflux condenser, mix 50 grams of vegetable oil with 125 ml. of ethyl alcohol, and warm on the
11).
steam bath. [Best sources of various acids are oleic acid-olive oil; linoleic-safflower, cottonseed, and corn oils; linoleic and linolenic (mixed)-linseed, soybean, perilla; eleostearic-tung oil; licanic-oiticica; and ricinoleic-castor oil. ] Dissolve 10 granis sodium hydroxide in 13 ml. of water, and add slowly t,o the alcohol solution. Boil gently for 1 hour (sealed stirring system recommended). T o recover the unsaturated acids, pour the saponification inist,ure while still hot into 700 ml. of freshlyboiled distilled water acidified with a t least 10 ml. of concentrated sulfuric acid. (This operation is best carried out' in a liter separatory funnel filled with nitrogen or carbon dioxide gas t o prevent oxidation.) Draw off the aqueous layer, a n d wash the free acids 1 t,imes wit,h hot water. Drain off t h e washings, while keeping t,he mixture warm t o prevent, solidification of the acids. (Chilling t h e wash water t o 0" C . precipitat>esadditional grain of acid.) D r y by chilling t o 0 " C., and use of a porous plate for solids, otherwise with anhydrous sodium or calcium sulfates. Preserve in a dark colored bott,le under nitrogen gas. B. Preparation of Eleostearic Acid. ALPHA ISOMER. Saponify tung oil and recover the crude acids. They need not be complet,ely dehydrated for the present purpose. I>issolve the acids in 75 ml. of x a r m methanol. Add 2 t o 5 ml. of water t o decrease t h e solubility and cool with good stirring to prevent oil formation. Once begun, crystallization proceeds rapidly and the mixture may set to a stiff solid. Break up the solid, chill t o 0' C., filter, wash with ice cold methanol. (The decrease in solubility of eleostearic acid with falling temperature is very great, so t h a t a good recovery is possible without very low temperahres. j D r y only b y evacuation because exclusion of oxygen is essential. Yield is 10 grams, melting point 49 O C. Preservation is successful only if air in container is displaced b y inert gas, and temperature is kept low. Unless this isomer is to be specially investigated, it should be converted t o t h e more stable betaisomer (56). BET.^ IsonxER. Isomerization b y Action of Iodine. 1. Dissolve t h e crude acids from Procedure A in 4 times their %eight of 85% methanol. For each gram of original tung oil used, add 8 mg. of iodine (dissolved in a little methanol), warm t o about 50" C., and allow t o cool t o room temperature. Aft,er 2 t o 3 hours chill in salt-ice mixture, collect crystals by suction, and wash with a little cold 85% methanol. Crystallize again from 85% methanol, and finally from absolute methanol.
ANALYTICAL CHEMISTRY
1472 D r y in vacuum desiccator over sodium hydroxide. Melting point should be 71-72' C. and the amount should be about 4501, of the original tung oil used. 2. Use the purified alpha eleostearic acid obtained from Procedure B. Dissolve 10 grams in 40 ml. of methanol plus 3 to 4 ml. of water. Add 0.10 gram of iodine dissolved in a few milliliters of methanol, warm to about 50" C., and allow to cool slowly. TThen crystallization of the beta acid is plentiful, chill in salt-ice mixture, filter, \)-ash with 85% methanol, and dry in vacuum desiccator. Yield is 'JOToof the alpha acid, melting point 71-72" C. This compound is much less sensitive to oxidation than the alpha acid, but one is r i s e to preserve it in a dark bottle under nitrogen. C. Preparation of Licanic Acid. ALPHAIsoMER. Repeat Procedure -4using oiticic oil v hich contains about i 4 7 0 licanic acid. K a s h the crude acids with hot water as before and dehydrate in a vacuum desiccator. T h e crude product is very sticky. Crystallize the mixed acids from petroleum ether (five times the weight of original oil), and recover the solid alpha acid a t low temperature (-10" to -25' C.). BETAISOMER. Dissolve the alpha acid in 4 times its u-eight of 85% methanol and add iodine (0.01 gram of iodine per gram of acid dissolved in methanol). Allow to, stand 2 hours at room temperature, chill a t 0 C. for 1 hour or more, and collect crystals. Recrystallize in 10 times their xeight of wwm absolute methanol. Dry in vacuum over a t 40" to 50' C. Yield is about 70% of the alpha acid, melting point 101" C. (36).
d
Y
A h A L Y S I S OF ACID MIXTURES
The identification of any pure drying oil acid follows readily from Table I and the classified data of T a b k 11. The realistic problem, horn-ever, is to encounter a mixture of acids conforming roughly to the description of Class I (Group C2) or Class I11 (Group -4,l a ) of Table I. A mixture that is semisolid a t room temperature may contain the following: Saturated F a t t y Acids. These can be separated from the liquid unsaturated acids by crystallization from acetone a t -40" C. ( 5 ) . Eleostearic or Licanic Acids. These dissolve readily in warm methanol and crystallize on cooling: further identification is by iodine isomerization (Procedures B and C ) and by ultraviolet spectral measurements (Table 11). Elaidic or Isomeric Linoleic Acids. These \%-auld appear only if the mixture had arisen from some special isomerization process. A mixture that is persistently liquid even on chilling t o 0" C. may contain oleic, linoleic, and linolenic acids. For this common problem, steps in the estimation of composition are the following (the order is not imperative and one or more steps may be omitted, depending on complexity of the mixture). 1. Determination of iodine number. 2. Examination of ultraviolet absorption; negligible absorption above 2200 A. indicates absence of conjugated unsaturation. Absorption a t 2300 and/or 2700 -4.indicates diene or triene conjugation. 3. Qualitative identification of individual acids by preparation of bromo and hydroxyl derivatives. 4. Alkali isomerization of nonconjugated acids to conjugated forms and quantitative determination by ultraviolet absorption.
D. D e t e r m i n a t i o n of Iodine Numbers of Unsaturated Acids. Several modifications of techniques for quantitative halogen absorption
.
b!
E
c rn
1 %
m
Y
Y
B
8 L
.-ex d
d
9
s
4
V O L U M E 26, NO. 9, S E P T E M B E R 1 9 5 4 are in current use. That of von hlikusch (26) using a modified Hanus reagent is recommended, as it was devised t o give correct values for conjugated acids such as those from dehydrated castor oil and tung oil, as well as for nonconjugated acids. The Wijs met'hod ( 1 ) most commonly used in industry, does not give correct values for conjugated compounds; however, for acids and oils presumably free of conjugated structures the K j s method has the virtue of simplicity and speed. E. Qualitative Detection of Oleic and Linoleic Acids by Hydroxylation with Alkaline Permanganate. (This process is on a macro scale; liquid quant'ities are large because the reactions require dilute solutions.) Dissolve 5.0 grams of acid in 400 ml. of 1.0% potassium hydroxide, cool to 0' C. in an ice bath, and add 700 ml. of 1 % potassium permanganate solution which has been previously cooled to 0" C. Keep the mixture cooled below 5" C. during 30 minutes. Pass in sulfur dioxide (or add saturated sulfurous acid solution) to dissolve the manganese dioxide formed, acidify with sulfuric acid, chill, and filter the crude hydroxy acids. D r y and extract repeatedly with warm petroleum ether (boiling point = 60' to 80" C.) to remove saturated acids and unoxidized material. Extract the residue (hydroxylated acids) with boiling water and filter hot to remove tetra- and hexahydroxy acids. The residue will be tmns-9,lO dihydrosy stearic acid ( 1 6 ) , melting point 132" C., derived from oleic acid. [The yield, according to Kunn ( S O ) , is 96% of the oleic acid present.] The hot water filtrate on chilling to 0" C. will deposit 9-, lo-, 12-, and 13-tetrahydrosp stearic acids, melting points 156' to 165" C. derived from linoleic acid. [The yield is low (50) and better quantitative results are obtained by bromination (Procedure F).] If t'he process is carried out on a large enough scale so as to isolate a significant quantity of tetrahydroxy acids, the mixture melting at 156' to 165" C. suffices to characterize linoleic acid. TWOstereo isomers are present ( 1 6 ) , melting points 163.5' and 174" C.; the higher melting one has bgen isolated by repeated extraction with boiling acetone followed by crystallization of the residue from 50% ethyl alcohol (35). The watersoluble hexahydroxy acids from linolenic and eleostearic acids will not crystallize under the conditions.
1473 ultraviolet light by acids having conjugated unsaturation follows the Lambert-Beer lax a t low concentrations:
D
= log,,
where D = absorbance IO = intensity of radiation transmitted by pure solvent, Z = intensity of radiation transmitted by solution, I = length of solution in emtimeters through which light passes, c = concentration in grams per 1000 ml., and k = extinction coefficient. The concentration of a pure substance follows from Equation 1; the per cent of a component, X, in a mixture with other components from Equation 2
%X =
Kx x 100
K x = measured extinction coefficient of the unknown K = extinction coefficient based on the pure compound. Spectrophotometers in common use-e.g., the Beckman D U instrument-do not give absolute I values. Rather they give "per cent transmittance,,' 100 I/Zo, or absorbance by Equation 1. The extinction coefficients vary with the concentration units employed: In oil v o r k it is common to use grams per 1000 ml. Other workers employ moles per liter, and grams per 100 ml. I t is therefore of utmost importance in spectroscopy either in reporting data, or in evaluating results of other investigators t o have all terms clearly defined. Much confusion has arisen when concentration units are not clearly stated, or when fractional transmittances are not correctly converted to absorbance. For these reasons these equations are expressly stated a t this point. The extinction coefficient of mixtures with several components absorbing at a given TTave length is the sum of the fractional extinctions:
+ K B ( B ) + KC(C') = K i ( A ) + KL(B) + KI-(C)
kmlxture =
KA(A)
(3)
a t another u-ave length k'mixture
TThere ( A ) , ( B ) ,(C)
F. Qualitative Detection of Linolenic and Linoleic Acids by Bromination. This procedure is given by Markley ( 1 7 ) .
IO D - = k X c X I, k = I C X I
(4)
= weight fractions of the components.
For alkali-isomerized linolenic-linoleic-oleic acid mixtures,
K values a t the absorption peaks are the following: Dissolve 2 grams of anhydrous acid in 20 ml. of anhydrous reagent grade et,hyl ether. Cool to 0" C. or lower and add bromine very slowly from a finely drawn pipet with very good stirring to avoid local overheating. When excess bromine is present as shown by a reddish yellow color, let stand 15 minutes a t 0" C. Discharge excess bromine by addition of isopentene. Keep a t ice temperature for several hours t o complete the precipitation of ether insoluble hexabromides. Filter (save filtrate), wash with cold ether, and recrystallize from benzene. Hexabromostearic acid, melt'ing point 185" C., indicates linolenic acid in the original mixture. Material that fails to dissolve in hot benzene may be the octabromide of arachidonic acid, melting point 227-228" C. decomposed. Allow the ether filtrate to evaporate and extract the residue repeatedly with boiling petroleum ether, pentane, or heptane, preferably with a Soxhlet extraction unit. Chill the extract t,o deposit tetrabromides, and recrystallize the solid from petroleum naphtha containing 10% ether. Tetrabromostearic acid, melting point 115' C., indicates linoleic acid in the original mixture. T h e tetrabromide is frequently low melting unless carefully purified. IS it melts 5" to 10" C. low, i t should be mixed with an authentic 115' C. melting sample. If the melting point rises, the chances are high that the bromide originated from linoleic acid. Pure linoleic can be prepared from cotton seed oil or safflower oil (20) and from it a good tetrabromide. If the amount of linolenic acid is negligible as shown by absence of hexabromide (or if ultraviolet measurement following alkali isomerization shows negligible absorption a t 2680 A . ) an improved procedure for linoleic tetrabromide formation is that of White and Brown (97, 98) which increases the yield of tetrabromide and correlates it with the linoleic acid originally present.
G . Determination of Linoleic and Linolenic Acids by Ultraviolet Absorption (Alkali Isomerization). The absorption of
At 2330 A .
Diene from linoleic acid, k = 91.9 Diene from linolenic acid, k = 62.2 Mono-me (oleic acid) absorption is negligible At 2680 A. Triene from linolenic acid, k = 51.1 Diene from linoleic acid and mono-ene from oleic acid absorptions are negligible. [ k values are from Brice @).I Sccordingly, from Equations 3 and 4
% linolenic acid and
% linoleic acid
=
(%) 2680
=
- % linolenic 91.9
X 100
) X 100
X 0.622
(6)
For oleic acid the iodine number of the mixture is required (Procedure D). The iodine consumed by linoleic and linolenic acids deducted from the total iodine is that due to oleic acid:
% oleic acid mixture)
=
- 2.74 ( % linolenic) 89.9
- 1.81 ( % linoleic)
) x 100
Residual saturated acids in mixture = 1 0 0 ~ o(unsaturated acids)
(7)
(8)
If more complex mixtures containing arachidonic acid are encountered, then the procedures of Brice (i?)should be Sollowed.
PROCEDURE. The alkali solution employed is potassium hydroxide (7.5 grams) in redistilled ethylene glycol (100 ml. maintained a t 180" C.). A weighed sample of mixed acids is introduced and allowed to react for 25 minutes. Thereafter dilutions
ANALYTICAL CHEMISTRY
1474 are made with 99% ethyl alcohol prior t o ultraviolet measurem m t s . Details of the procedure are given by Mitchell et d. ( 6 7 ) and should be followed with scrupulous care. Dilutions required for the method are such a s t o give absorbances of 0.2 t o 0.8 (Equation 1) corresponding to transmittance of 65 t o 16%. Thp concentration of acids is then of the order of 0.25 t o 1.5 grams per 1000 ml., depending on the diene content. Set the spectrophotometer t o give readings at 2300 A. and 2680 A. Determine either per cent transmittance 100 X I/Io, or absorbance. B y Equation 1 calculate k 2 3 3 0 and k*e*o. Substitute b values in Equations 5 and 6 and calculate linoleic and linolenic percentages. Determine iodine number on original acid mixture and calculate oleic miti content from Equation 7 . CONCLUSIONS
Thi, proposed system of characterization summarizes anti codifies information on organic acids (Order I ) for easy identification. Four main classes of acids have been established within which the gathering of a minimum of quantitative data leads t o identification of individual members. The “drying oil acids” because of their great practical importance received special treatment. Their identification as single acids or in mixtures generally requires tests for quantitative unsaturation, isomerization, and ultraviolet absorption. The system provides a n improved and simplified process for identifying of unknown compounds which has general application because so many types of compounds are convertible t o acids. ACKNOWLEDGMENT
The authors wish t o thank Richard King for purification and determination of the values for a-licanic acid used in this studv. LITERATURE CITED
(1) .Im. Oil Chemists’ SOC.,“Official and Tentative Methods,”
rev. ed., Methods KA-951 and CD-125. 1952. (2) Brice. B. -4.. Swain, M. L., Herb, S. F., Xichols, P. L.. Jr., and Riemenschneider, R. W., J . Am. Oil Chemists’ Soc., 29, 279 (1952). (3) Dorinson. A , , McCorkle, AI., and Ralston. A. W., J . Am. Chem. Soc., 64, 2739 (1942). (4) Doss, 11. P., “Properties of the Principal Fats, Fatty Oils, Waxes, Fatty Acids and Their Salts,” New York, Texas Co., 1952. (5) Earle, F. R.. and hlilner, R. T.. Oil arid Soup, 17, 106 (1940). (6) Ellis, B. A. Analyst, 61, 812-16 (1936). (7) Huntress, E. H., and hlulliken, S. P., “Identification of Pure Organic Compounds,” p. 91, 179-85, Xew York, John Wiley 8: Sons. 1941.
(8) Jackson, J. E., Paschke, R. F., Tolberg, W., Boyd, H. AT., and Wheeler, D. H., J . Am. Oil Chemists’ Soc., 29, 229 (1952). (9) Kamm, O., “Qualitative Organic Analysis,” 2nd ed., New York, John Wiley & Sons, 1931. (10) Kass, J. P., and Burr, G. O., J . Am. Chem. Soc., 61, 1062 (1939). (11) Kass. J. P., Kichols, J., and Burr, G. O., Ibid., 63, 1060 (1941). (12) Kass. J. P., and Radlove, S. R., Zbid..64, 2253 (1942). (13) Kaufmann, H. P., Baltes, J., and Buter, H., Der., 70, 903 (1937). (14) Kirk, R. E., and Othmer, D. F., “Encyclopedia of Chemical Technology,” Vol. 6, pp. 269-83, Sew York, Interscience Publishers, 1951. (15) Kuhn. R., Ber., 69, 1764 (1936). (16) RIacKay, A. F., and Bader, A. R., J . Org. Chem., 13, 77 (1948). (17) BIarkley, K. S.,“Chemical and Physical Properties of the Fatty Acids,” pp. 605-6, Kew York, lriterscience Publishers, 1947. (18) Matthem, K. L., Brode, W. R., and Brown, J . B., J . Am. Chem. SOC.,63, 1064 (1941). (19) AIattiello, J. J.. “Protective and Decorative Coatings.” Vol. I V , New York, John Wiley & Sons, 1944. (20) LlcCutcheon. J. W., Org. Syntheses, 22, 76 (1942). (21) Nikusch, J. D. v., Angew. Chem., 62, 475-80 (1950). (22) AIikusch, J. D. v., Fette u. Seifen, 54, 751 (1952). (23) hIikusch, J. D. v., J . Am. Oil Chemists’ SOC.,28, 133 (1951). (24) Zbid.,29, 114 (1952). (25) i\Iikusch. J. D. v.. and Frazier. Charles. IND.ENG.CSEM.. ANAL.ED., 13, 782 (1941). (26) Ibzd., 15, 109 (1943). (27) Mitchell, J. H., Kraybill, H. R., and Zscheile, F. P.. Ibid., 15, 1-3 (1943). (28) lIorrel1, R. S.,and Davis, W. R., J . Chem. Soc., 1936, 1481. (29) Kichols, P. L., Herb, 8. F., Riemenschneider, R. W..et al., J . Am. Chem. Soc., 73, 247 (1951). (30) Nunn. L. C., and Smedley-Maclean. I., Biochem. J., 32, 1974 (1938). , (31) O’Connor, R. T., and Heinselman, D. C., J . d m . Oil Chemists’ Soc., 24, 212 (1947). (32) Paschke, R. F., Tolberg, W.,and Wheeler, D. H.. I b i d . , 30,97 (1953). (33) Riemenschneider, R. W., Wheeler, D . H.. and Sando, C. E., J . Bl’ol. Chem., 127, 391 (1938). (34) Shriner. R. L., and Fuson, R. C., “Systematic Identification of Organic Compounds.” 3rd ed., p. 146, New York, John Wiley 8: Sons, 1948. (35) Strain, H. H., J . Am. Chem. Soc., 63, 3448 (1941). (36) Wendland, Ray, J . Chem. Educ., 23, 3 (1946). (37) White, LIary, and Brown, J., ,J. Am. Oil Chemists’ Soc., 26, 385 (1949). (38) Ibid.,29, 292 (1952). ~I
>-
RECEIVED for review March
1 , 1954. Accepted Jnne 21, 1954. Journal Series, General Mills Research Laboratories.
Paper 180
Fire Assay for Palladium 1. G. FRASER and F. E. BEAMISH, University o f Toronto, Toronto, Ontario, Can. This investigation was carried out to determine the distribution and losses of palladium in the various phases of a fire assay. Of five platinum metals, palladium is the least subject to loss in the slag. While the losses are small, they are in practice irrecoverable by reassay. This work forms part of a systematic investigation of the efficiency of collection by lead. It was undertaken because the fire assay is the only method generally applied to ores and concentrates and hitherto no data have been recorded to indicate the accuracy of the method.
F
OR many years the analysis of ores for the platinum metals has been carried out by fire assay procedures. The literature reveals little justification for this method except b y analogy t o the success obtained R-ith gold and silver. I n recent years serious doubts have arisen concerning the collection of rhodium, ruthenium, osmium, and iridium b y lead (1-3, 5 ) . With these
metals losses t o the slag were significant even in the idealized case where no base metals were present. These losses were shown to vary according to the nature of the slag. Base metals when present in the slag caused high iridium losses (30 to 50%). This n a s about ten times the loss of any of the other metals thus far investigated. The literature ( 1 ) reveals one experiment where osmium was fused with a basic flux without collection b y lead. Only of the osmium could be recovered from this slag with three reassays. I n a similar esperiment with iridium ( 3 ) only 75y0of the iridium was recovered. The cupellation of osmium was shown to be useless as nearly complete volatilization of osmium occurred ( 1 ), With ruthenium the loss by volatilization n a s negligible b u t a 10% loss occurred t o the cupel ( 5 ) . It has been shown ( 3 ) that iridium was lost mechanically during the cupellation because of the formation of black scales on the surface of the bead. I n the present paper the authors present data on the collection of palladium by lead from various fluxes, and compare the losses
