INDUSTRIAL AND ENGINEERING CHEMISTRY
454
as creosote and anthracene oil are unsuitable for use as Diesel fuels b u t when blended in approximately equal amounts with the synthetic Diesel oil a high-grade fuel is produced. Part of the Fischer product can be utilized for the production of synthetic lubricating oil. There are two mays in which this can be done. Suitable paraffin hydrocarbons can be chlorinated and these compounds coupled with aromatic hydrocarbons. The other method involves the polymerization of heavy olefins by the use of aluminum chloride. The lubricating oils by polymerization are of high quality and are being produced commercially in Germany. When these oils are subjected to a n oxidation test, they show a smaller formation of carbon but a larger increase in viscosity than lubricating oils from natural sources. The resistance t o oxygen can be improved by mild hydrogenation. The most interesting aspect of this process in the United
VOL. 32, NO. 4
States is its application to synthetic liquids produced from methane. This may be an important factor for the conservation of natural resources, since there is a large amount of natural gas available a t present.
Acknowledgment Acknowledgment is made to Anglo-Transvaal Consolidated Investment Co., Ltd., for permission to publish some of the information in this paper.
Literature Cited (1) Egloff, Nelson, and Morrell, IND.ENO.CHEM.,29, 555 (1937). (2) Fischer. J . Inst. Fuel, 10, 10 (1936); Wilke, Chem. Fabrik, 11, 563 (1938). (3) Fischer and Tropsch, German Patent 484,337 (1925). (4) Snodgrass and Perrin, J . Inst. Petroleum Tech., 24, 289 (1938).
Development of Rancidity in Stoddard Dry Cleaning Solvent ADRIAN C. SMITH, CHARLES S. LOWE,
AND GEORGE P. FULTON
National Association of Dyers and Cleaners, Silver Spring, Md.
The accumulation in Stoddard dry cleaning solvent of substances associated with rancidity was studied by chemical and physical methods. The increase in fatty acid content was found to be due largely to free fatty acids present in soap additions, rather than to fatty acids present in the soil from the garments, and could not be used as a criterion for rancidity without qualifications. The Kreis test, in a modified form, has been found useful in detecting incipient rancidity. Peroxides formed during the cleaning operations decomposed in the drying cabinet and gave rise to aldehydes and low-molecular-weight acids. Potentiometric titration curves on residues from used dry cleaning solvent indicate the buffering effect of these decomposition products.
N
UMEROUS studies have been carried out to determine
the nature, causes, methods of detection, and control of rancid odor developed in edible fats, soaps, textile oils, and similar products. The application of the results of such studies to the odor problem of the dry cleaner, mho must remove cheaply and efficiently soil which may contain any or all of these substances from all types of textile fabrios without permitting any odor to remain on the garments, has not
been undertaken. The necessity for such an investigation has materially increased within the past few years as a result of the marked tendency toward clarification of solvent by pressure filtration alone, without alkali treatment or vacuum distillation. Elimination or less frequent use of the latter methods of purifying the solvent permits a more rapid accumulation of products associated with rancidity, with subsequent odor trouble. The purpose of this paper is to describe a series of experimental cleaning operations carried out under controlled conditions and to correlate chemical and physical tests made on the solvent after each load with detection of objectionable odors on the garments.
The Dry Cleaning Process Owing t o lack of information in the scientific literature regarding the technique of dry cleaning, a brief resume of average cleaning practice with factors pertinent to this investigation is given here. CLEANING OPERATIONS.Mechanical action in a horizontal cylinder-type washer with reversing drive is employed to bring about intimate contact with the dry cleaning solvent and produce force necessary to dislodge loose soil. -4charge of 2 pounds of filter powder for each 100 gallons of solvent is added directly on the garments to increase the efficiency of filtration. This initial run with filter powder requires about 10 minutes and is known as the break. The solvent is circulated through the filter screens, previously coated with filter powder, from the start of the break run, in order to carry away loose soil as quickly as possible and thus minimize any tendency toward reabsorption of soil by the garments. The filter circulation is then shut off and soap is added, usually in the ratio of 1 pound of soap to 25 gallons of solvent. The amount of soap added varies somewhat with the type of
APRIL, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
FIGURE 1. PILOT-PLANT MODELWASHER soap used and the nature of garments to be cleaned. The soap run requires about 30 minutes, varying slightly with the type of work being cleaned. The soap run is followed by the important rinse run, which should be made with completely clarified solvent for the best results. However, the practice of rinsing while simultaneously filtering out the insoluble portion of the soap is growing in popularity, since this operation costs less and does not require so great a volume of solvent. Rinsing with clarified solvent requires about 20 minutes. After extraction of most of the solvent by centrifuging, the garments may be dried in a tumbler a t 160" F. with injection of live steam for the first 5 to 10 minutes and near the end of the run to dissipate static charges that may attract lint, or they may be hung in a drying cabinet maintained a t 120160" F. A rapid circulation of air is provided in both instances to hasten drying. The temperature employed varies with the nature of the garments; 140" F. is usually used on dresses and light-weight garments which will dry more rapidly. MATERIALS.The majority of dry cleaners today use a petroleum distillate fraction known as Stoddard solvent for the cleaning medium. Specifications (8) require a 50 per cent distillation yield a t 350' F. and a maximum end point of 410' F. The flash point must not be lower than 100' F. to minimize fire hazard. Tests for acidity and sulfur compounds should be negative, and absorption of unsaturates by sulfuric acid (93-94 per cent pure) should not exceed 5 per cent by volume. Chlorinated solvents such as carbon tetrachloride, trichloroethylene, and perchloroethylene, generally classed as synthetic solvents, were introduced into the trade about 1930. Since the cost of such solvents is roughly five to ten times that of petroleum solvents, a closed-type unit which provides for condensation and reclamation of fumes preventing excessive evaporation and spread of toxic vapors is often used. I n such units, washing, extraction, and drying are carried out in the same cylinder. I n addition to giving good cleaning action, dry cleaning soaps must rinse easily from the garments and be capable of being completely removed from the solvent with the type of clarification available. The ordinary type of paste soap contains a relatively high percentage of alkali soap, together with a certain amount of free fatty acids which aid in dispersing the soap in the solvent. Dry cleaning solvent is also frequently incorporated in the soap itself. Paste soaps show a
455
greater tendency to retard flow in the filter chamber than so-called soluble or filter soaps. Soluble soaps are somewhat varied in nature. They may contain cosolvents, such as butyl Cellosolveor isopropyl alcohol, as well as amine soaps. Since the soluble soaps do not normally raise the filter pressure, they are referred to as filter soaps. If an attempt is made to remove such soaps by alkali clarification, emulsification of the solvent with the alkali solution may take place. Cosolvents having a boiling range below the end point of the solvent cannot be removed by vacuum distillation. Filter screens are precoated with diatomaceous earth before dirty solvent enters the filter. This charge amounts to from 5 to 7 pounds of filter powder for each 100 square feet of filtering area. Sweetening powders or decolorizing agents are sometimes employed. They may be finely ground clays or activated carbon which function by adsorption to remove fatty acids and colored substances. CLARIFICATION OF SOLVENT.Alkali treatment, vacuum distillation, adsorption, pressure filtration, or combinations of these operations are used in purifying the solvent. Alkali treatment is employed in many plants today and has advantages in low initial, operating, and maintenance costs. Animal and vegetable fatty acids are saponified to water-soluble soaps which settle to the bottom of the tank. Activated carbon is always used with this treatment to remove colorand odor-forming compounds. After leaving the alkali tank, the solvent is returned through a cotton bag or gravity screen filter to the washer. Sodium hydroxide (IO" to 15" Be.) is widely used, although ortho-, meta-, and sesquisilicates are employed in some instances. This method of clarification does not remove unsaponifiable oils. It also permits an accumulation of cosolvents from filter soaps, which may cause emulsions and subsequent high solvent losses. Vacuum distillation is carried out at a pressure of 3 to 4 inches of mercury and 30 t o 40 pounds steam pressure. Moisture and entrained particles are removed by passing the distilled solvent through damp cotton rags. Impurities having a higher boiling point than 410" F., such as mineral oils, greases, and waxes, and high-boiling fats and oils from soaps, collect in the still. This method of clarification always yields a clear, colorless solvent. The combination of alkali treatment and vacuum distillation is sometimes used and generally yields a solvent quite free from impurities. Pressure filtration with metal-screen or clot h-bag flters offers the most economical and rapid removal of suspended matter but will not remove soil from solution. Some further treatment, such as distillation, alkali treatment, or adsorption, must be used in conjunction with this method in order to remove soluble fats, oils, and greases.
Analytical Methods of Detecting Incipient Rancidity
It is not a simple matter to determine the presence of rancidity in its earliest stages of development. It may be caused by the action of enzymes or microorganisms, by atmospheric oxidation, or by a combination of these effects. It is associated with the production of numerous chemical products, which may interact with one another and which will vary with the nature of the fat or protein, and with the type of reaction and environment. Greater progress has been made with regard to detecting incipient rancidity induced by atmospheric oxygen than with that brought about by enzymes or bacteria.
456
INDUSTRIAL AND ENGIKEERING CHEMISTRY
For the purpose of this investigation it is assumed that enzymic or bacterial effects are minimized, inasmuch as dry cleaning solvent is considered to be a practically sterile medium. It is generally held by most investigators in this field that the first step in the development of rancidity is the addition of oxygen to a n ethylenic linkage with peroxide formation, followed by scission of the chain, to give lower molecular weight aldehyde derivatives bearing the characteristic rancid odor.
74 I
-FLOW METE
!
OF SOLVENT SYSTEM FIGURE2. FLOWDIAGRAM
Tests which have been applied in detecting products associated with rancidity include: determination of acid number (mg. of potassium hydroxide per gram of sample); the Kreis test (3),shown by Powick (10) to be specific for epihydrinaldehyde; the Schiff test for aldehydes adapted to quantitative measurement by Schibsted (11); the determination of aldehydes by reaction with sodium bisulfite according to the technique developed by Lea ( 5 ) ; iodine number; Issoglio's method of determining oxidizable steam-volatile substances as modified by Kerr (2); determination of peroxide oxygen by methods developed by Lea ( 7 ) , Wheeler (16), and Young, Vogt, and Nieuwland (16). Stout and Tillman (12) found the acid number of the nonvolatile residue from dry cleaning solvent to be of value in determining the usability of the solvent for further cleaning. An azeotropic mixture of carbon tetrachloride and ethylene dichloride was employed as the dry cleaning solvent in this instance. No soap was used in their cleaning cycles. Lea (6) found little change in free acidity of oils during lightaccelerated oxidation a t room temperature, with the result t h a t a fat or oil may have a low acidity, yet exhibit extremely rancid characteristics. In general, a measure of free acids present represents potential rancidity which can, under the proper conditions, be converted through peroxide formation t o aldehydes largely responsible for rancid odors. Of the tests mentioned, probably that proposed by Kreis (3) which depends on the development of a red color produced b y the condensation of phloroglucinol with epihydrinaldehyde i n the presence of hydrochloric acid, has been most widely used for the .detection of incipient rancidity. In order to avoid interference by condensation with other aldehydes yielding weakly colored products, Neu (9) employed a technique wherein only the rancid vapors come in contact with the phloroglucinol. This was accomplished by thoroughly mixing the sample with powdered pumice and glass wool, inserting it in a calcium chloride U-tube, and passing carbon dioxide gas bearing hydrogen chloride vapor over the mixture into a solution of phloroglucinol in ether. By comparing the
\-OL. 32. NO. 4
intensity of the color produced with a standard in the ZeissPulfrich photometer, the test can be made quantitative (4). This test in a slightly modified form has been found useful in testing for rancidity in dry cleaning solvents. The Schiff test for aldehydes was found to give positive indications of rancidity in several dry cleaning soaps before being used in cleaning operations. The test is very sensitive to impurities in the reagents and not adapted to quantitative measurement unless the sulfur dioxide content and ratio of reagent to sample are carefully controlled (11). Attempts to determine aldehydes quantitatively in Stoddard dry cleaning solvent by the reaction with sodium bisulfite (6) have been unsuccessful. Changes in iodine value with relation to odor and flavor have been shown to be insignificant. Nonrancid fats and oils show a small oxidizability value (Issoglio test) which increases only slowly as rancid odors develop. Peroxides have been determined from the amount of iodine liberated from potassium iodide resulting from their decomposition in hot glacial acetic acid (7). Peroxides in oils have been reported as being stable up to 170" C. (13) although most of those generally encountered are quantitatively decomposed under these conditions. This method is open to criticism in that as the iodine is liberated, it may be absorbed by unsaturated linkages in fatty acids which have not reached the peroxide stage. Hamilton and Olcott (1) have shown that in the presence of glacial acetic acid and in an inert atmosphere, the iodine numbers before and after destruction of peroxides by potassium iodide were identical. Since the iodine number is a measure of unsaturated linkages, there was no addition of iodine to these linkages during the peroxide reaction with potassium iodide. These data mrere obtained for
CONC. H C l PUMICE STONE GLASS WOOL
LJu
TRAP CONC. HCL
u
ALC. PHLOROGLUCINOL
FIGURE3. KREISTESTAPPARATUS rancid samples of oleic acid and are reasonably assumed to apply to other unsaturated fatty acids under similar conditions. Oxidation of ferrous to ferric iron in acid solution has been employed in the estimation of peroxides in hydrocarbon oils (16). Since this reaction is not so sensitive as that with potassium iodide, only the more reactive peroxides are detected in this way. Since most peroxides decompose readily, the peroxide content as measured represents the amount of excess peroxides formed over that which has decomposed. I n general, good correlation has been found betn-een the k e e p ing qualities of fats and oils and their peroxide content, but the relation to incipient rancidity is not so well defined.
Experimental Details Cleaning was carried out in a pilot-plant model washer (Figure l ) , 24 inches in diameter and 6 inches in width, with a maximum capacity of 7 pounds of clothes. Clarification wa6 accomplished by pressure filtration through Monel metal screen5 ( A , Figure 2) representing approximately 9 square feet of filtering area. The filter chamber (Figure 1) was equipped with sight glass and pres-
APRIL, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
451
sure gage. The direction of flow through the system is indicated in Figure 2 . Twenty gallons of solvent were required for normal operating conditions. A flowmeter (Figure 1) permitted determination of change in rate of flow with increase in pressure in the filter chamber. The normal rate of flow before noting any pressure increase was 157 gallons per hour. A petroleum solvent meeting Stoddard specifications (8) was used as a cleaning medium. Two commercial soaps were used during the investigation, the approximate compositions of which are given in Table I. The soaps selected were of the filterable type and representative of the dry cleaning soaps on the market today. The filter powder was a grade of diatomaceous earth used in the industry. A clay-type sweetener was employed during two of the series of runs.
TABLEI. APPROXIMATEPERCENTAGE COMPOSITIOK OF SOAPS USED Nonvolatile matter Free fatty acid (as oleio acid) Type of fat or oil TYDe .. of B o w Water (by xylene method) Stoddard solvent Isoprupyl alcohol Isopropyl acetate Isomers of amyl group Carbonate
Soap 1 18.7 8.1 Mutton tallow Ammonium and potassium 15.0 66.7
....
Presedt'
Soap 2 16.2 2.8 Peanut oil Sodium
20.2 Present 20-30 20-30 Present
..
5
I
i
10
IS
20
i
1
25
30
1 --I,,
35
40
45
j
t-
10.41 FIG.5
I
19 0
0.3
$0.3 m
Ly
4
z
0.2 3 ',
'3) 0.2
z
Ly
00.1
0.1
2
0.2
s
c)
The garments cleaned were of all types, and material including wool, cotton, silk, and rayons. Not over 7 pounds of work were cleaned on any one load, the average being 3 to 4 pounds. The cleaning operation was carried out as follows: The clothes were given a 10-minute break run with 0.5 pound of filter powder, solvent being circulated through the precoated filter screens. Soap sufficientto form a 0.5-1 per cent solution was dissolved in a portion of the solvent and added to the clothes. During the soap run the solvent was circulated through the by-pass (Figure 2), and no flow through the washer took place. After 25 minutes 0.5 pound additional filter powder was added. After 5 minutes of additional agitation, the solvent was again allowed to circulate through the washer and filter. This flow of solvent served to rinse the soap from the garment's. Approximately 40 minutes were required for the solvent to clear as viewed in the sight glass. Solvent was removed from the garments in a centrifugal extractor, and drying was carried out in a drying cabinet a t 140" F. Sample cloths, conditioned to remove sizing and other impurities, were included in each load, extracted, and dried along with the garments for test purposes. During the course of the first three series of loads, fresh solvent was added at irregular intervals t o compensate for that remaining in the garments, that utilized for sampling, or that lost by evaporation; this is the practice followed bv the average dry cleaner. During the fourth series of loads this variable was held constant by adding 700 ml. of fresh solvent after each load for this purpose. Tests carried out on the solvent were made immediately after each load was completed in order to eliminate accelerated oxidation attributed to standing in light or possibility of hydrocarbon gum formation with the accompanying characteristic odor. The acid number was determined after each load by titration of a 50-ml. portion of the solvent with alcoholic potassium hydroxide. Residues were determined by evaporation on a steam bath to constant weight. During the course of the investigation imppovements were made in the technique of carrying out the Kreis test, and the iodometric determination of peroxides was adapted to the estimation of peroxides in dry cleaning solvent. Attempts were made to apply the Kreis test as a test tube reaction to samples of the used solvent. This was done by shaking 5 ml. of solvent with 2 ml. of concentrated hydrochloric acid and adding 2 ml. of 0.1 per cent solution of phloroglucinol in ether. Although some positive tests were obtained, the results did not correlate well with organoleptic tests. Where the solvent was off-color, this interfered considerably with the test. This test was also made on the ether extract of cotton cloths which had been cleaned and dried along with the garments. Although this method proved more satisfactorythan the test on the solvent itself, colored substances in the solvent which were deposited on the cloths again interfered. Satisfactory results could not be obtained by following Neu's technique of mixing the residue with pumice and glass wool. By making use of a slight modification of this method, very consistent results were obtained. Instead of mixing the sample with pumice stone and glass wool, cotton
I
1
I
ASACID NUMBER 1 Ba RES1 DUE G. PER 100 ML.1 0.
6
9
12
15
18
21
24
27
o,l
E
30
__-___ NUMBER OF LOADS CLBANED-
FIGURE 4. ACIDAND RESIDUEBUILD-UP, NOSOAP OR SWEETENER FIRSTSERIES; FIGURE 5. ACIDAND RESIDUEBUILD-UP,SOAP No. 1 AND SWEETENER,SECOND SERIES; FIGURE 6. ACID AND RESIDUEBUILD-UP, SOAPNo. 2 A N D SWEETENER, THIRD SERIES; FIGURE 7. ACIDAND PEROXIDE BUILD-UP, SOAPNo. 1 AND No SWEETENER, FOURTH SERIES
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
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VOL. 32, NO. 4
acid were shaken with 10 ml. of solvent sample. A positive test was indicated by the appearance of a red color on addition of 1 ml. of ammonium thiocyanate solution. I t was found that the discolorations in the used solvent samples made quantitative determinations of peroxides by this method difficult.
11 10
Discussion of Results 9
1:
I
Q
6
57'
i'
I
I
I
I
'
I
FIG. 8
CURVES OF RESIDUES FROM VARIFIGURE 8. PH TITRATION OUS LOADS FIGURE 9. CHANGEOF APH/V NEARTHE NEUTRAL POINT IN THE TITRATION OF RESIDUES FROM VARIOUS LOADS cloths (18 X 24 inches) were employed t o absorb the solvent sample and were hung dripping wet in the drying cabinet maintained a t 140' F. By eliminating the extraction process, which removes approximately 85 per cent of the solvent, a greater concentration of solvent residues was obtained on the cloths. Cotton was used since it has been the experience of cleaners that rancid odors apparently cling to cotton better or are noticed more quickly than on any other fiber. These cloths were then inserted in a 3-em. diameter tube through which carbon dioxide gas carrying the acid vapors was passed (Figure 3). The carbon dioxide was supplied from a tank and was permitted to flow a t a moderate rate. After passing through concentrated hydrochloric acid, the gas stream was led over a mixture of concentrated hydrochloric acid, pumice stone, and glass wool to aid in further saturation of the gas with the acid vapors After contact with the cloth, the stream bearing the substance active in the condensation was passed into 5 ml. of 0.1 per cent alcoholic phloroglucinol solution. I t was found that better results iyere obtained if moisture was removed from the stream by passage over calcium chloride just before reaching the sample. This method of applying the Kreis test has been used in testing over four hundred solvent samples from all sections of the country, including both petroleum solvents and chlorinated hydroearbon solvents, and has been found more satisfactory in differentiating rancid and nonrancid solvent than organoleptic tests. The method of detecting peroxides was to add 50 ml. of glacial acetic acid and 10 ml. of 20 per cent potassium iodide solution t o 50 ml. of solvent sample, shake 5 minutes, add 100 ml. of distilled water, and titrate with 0.05 N thiosulfate, using starch indicator. The qualitative test for peroxides based on the oxidation of ferrous to ferric iron was studied during one series of runs. Ten milliliters of a dilute ferrous sulfate solution acidified with acetic
Figure 4 shows the build-up of free acid content and oil residues in dry cleaning solvent during a series of runs in I\ hich no soap or sweetener was used. After forty-three runs the acid nuniber had reached a value of 0.22. The same acid number was attained in later series of runs during which soap was added after only seven, four, and four loads, respectively, had been cleaned. It is evident that a large part of the acid build-up comes from the free fatty acid present in soap additions. The sharp increase in the amount of residue from load 22 to 23 was due to the cleaning of a pair of garage overalls which were fairly saturated with oil and grease. Further data on this series of runs is given in Table 11. No odor was detected on any garments cleaned during this series. Figures 5 and 6 give similar data for series of loads in which soaps and a sweetener were used. The sharp jumps in both residue and acid number curves were due to irregularity in addition of fresh solvent. The filter was cleaned in one series after load 15, in the other after 16. Addition of solvent a t these points to compensate for that removed along TTith the muck caused a sharp drop in acid number and residue content. Odor was detected on the garments cleaned at load 20 of the second series (Figure 5 ) and load 23 of the third series (Figure 6) by the writers and was confirmed by several experienced cleaners. I n both instances odor developed shortly after the filter was cleaned out; this indicated the possibility that the thick filtering layer built up from constant additions of filter powder and soap may have been adsorbing odorforming substances.
TABLE 11. WEIGHTAND TYPEOF GARMENT CLEANED DURING FIRST SERIES O F LOADS,WITH h'0 SOAP OR SWEETENER Load NO.
1 2 3 4 5 6
7 8 9
10
11
12 13 14 15 16 17 18 19 20 21 22
Weight of Garments Cleaned
Lb.
02.
5 7 5 5 3 5 4 3 4 4 3 3 3 4 4 2 3 3 3 4 4 3
3 0 8 14 6 6
10 5 5 14 14 12 0 S 0 0 5
3 7 5 3 9
Type of Material
Load No.
Wools Silks Wools Wools Silks Wools W7001s Wools Wools Wools Wools Wools Wools Wools
23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43
Wools Wools \Vools Wools Wools Overalls W'ools rnools
Weight of Garments Cleaned Lb. Or. 3 9 3 6 3 5 3 13 3 3 12 3 12 3 4 12 3 0 3 2 2 0 2 S 5
3 3 3 3 3 3 4 5
0 S 9 2 8
13 0 0 0
Type of Material Overalla
wools Wools T3'00lS
Wools Wools tools Wools Wools WOO18
Silks
Wools Wools Wools Wools Cotton rnools Wools Wools Wools Wools
The acid number cannot be used as a criterion of rancidity without reservations, inasmuch as the acid number for the runs on which odor was first detected was lower than sereral values obtained before the filter was cleaned. A positive Kreis test was obtained on the solvent by the test tube method, beginning a t load 19 of the second series, one load before odor was detected on the garments. Similar tests on samples from the third series were negative. Positive tests were obtained beginning a t load 19 (Figure 6) by making the test on the ether extract of cotton cloths which had been cleaned with the garments. A satisfactory method of de-
ISDUSTRIAL AND ENGINEERING CHEMISTRY
APRIL, 1940
termining peroxides in dry cleaning solvent had not been found a t the time these runs were made. Figure 7 shows the increase in peroxide and acid content during a series of runs with soap X o . 1 and no sweetener, and with elimination of irregularity in addition of fresh solvent. d distinctly rancid odor was noted on the garments cleaned during the run 36; those from the runs 32 to 36 mere slightly rancid. A marked increase in peroxide value is noted almost immediately after filter is cleaned, coinciding with the first detection of odor. This sharp rise was probably associated with the catalytic effect produced by a certain concentration of peroxides in accelerating further oxidation. The peroxide oxygen was calculated on the assumption that one oxygen in the molecule was titratable. At the point a t which odor was definitely detected on the garments, the solvent had a peroxide content of 14.4 mg. peroxide oxygen for a 50-ml. sample. Additional data on this series of runs are given in Table 111. TABLE111. DATAON Load N0.G
1
2
3 4 5 6 7 8
9 10 ~. 11 12 13 14 15 16
17
18
19 20
Weight of Garments Cleaned Lb. 02. 4 3 3 3 3
0 0
2 3 3 3 2 2 2 2 2 2 3 2 2 3 2 3
8 7 0
10 9 7 0 0
Type of Material
Temp. Solvent F.
Pressure in Filter Chamber Lb./sq. in. 0 0 0
0 0
Rayons Silks, acetate rayon Silks. acetate rayon Wools Silks, rayons w-ools Wools wools Wools Wonk
82 84 84
80 81
84 86 89
86 89
0 0 0
1
1 0
0 0 0 0.25 0 0
0 0
0 0 13 2 12 22 2.5 9 23 2 3 24 11 1.5 25 0 15 26 10 0 27 Cotton, wools 82 10 7.5 28 U~ools 84 3.75 9 29 Wools 80 4 2.5 30 8 Cotton, wools 85 3.75 31 7.5 0 Cotton, wools 87 32 Silks 86 1 10 10 33 2 Wools 14 88 1.25 34 W0"lr. 87 10 2.5 35 7 2.5 CottGn, wools 88 36 Cotton, wools 85 10 7.5 37 8 Wools a5 13 38 0 Wools 87 1 39 this series 0 0.5 per cent b y volume of soap No. 1 added to each load: was completed during the period May 23 to June 13, 1939. 21
Figure 8 shows titration curves obtained on different samples from the fourth series of loads by means of a glass electrode potentiometric method. Fifty-milliliter samples were evaporated on the steam bath, the residue was dissolved in 50 ml. of methyl alcohol and titrated with 0.05 N alcoholic potassium hydroxide, and pH values were determined after each addition of 1 ml. The pH values were only relative since the dissociation effect of the alcohol is not the same as that of a pure water solution. Since the shapes of the titration curves for residues containing only a small amount of acid are similar in all respects t o that of oleic acid, we may assume that the acid has a dissociation constant approximating that of oleic acid.
FOUXTH SERIESOF LOADS
0 15 14 6 9 11 9 11 10 12
4 3
THE
459
Positive Kreis tests, made by the modification of Seu's method described above and having the characteristic absorption band of the condensation product, were obtained on the sample cloths cleaned, starting with run 32. The treatment which these cloths underwent (drying without extraction a t 140" F. in a current of air) would provide favorable environment for the decomposition of peroxides to aldehydes, on which the Kreis test depends. This would also suggest t h a t failure to obtain a positive Kreis test on solvent samples as a test tube reaction was due to the fact that the peroxides had not decomposed. The possibility of a relatively high but unoxidized fatty acid content remaining in the garments with solvents of high acid number, leading to subsequent odor trouble on wearing the garments, should also be considered. No definite data to this effect are available a t present. The ferrous iron test for peroxides began to show a positive reaction a t load 31, agreeing well with the first detection of odor and with the results from the Kreis tests.
F L O W IN GAL.PER HR.-+
FIGURE 10. FLOWus. PRESSURE I n addition to showing the build-up of total acid content in the residues by the horizontal displacement of the curves, the shape of the curves changes as the acid content increases, and the values of the tangents decrease between pH 7.5 and 10. This behavior is shown more clearly (Figure 9) by plotting the ratio of the rate of change of pH to the volume of potash against the volume of potash added. The curves thus obtained for the earlier runs give sharp inflection points similar to oleic acid, broadening out as the acid content increases. This broadening effect indicates a gradual change or buffering effect brought about by an increase in thc number of weak acids present, presumably formed from high-molecular-weight acids through intermediate peroxide formation and decomposition. Figure 10 shows that a linear relation was obtained between pressure produced in the filter chamber and rate of flow during the fourth series of runs. This relation was unaffected by reducing the filtering area by half. I t is evident that, operating under a pressure of 13 to 14 pounds, with this system, the rate of flow is cut by 50 per cent. For higher rates of flow a greater pressure would be required to produce the same effect. Many dry cleaners have used the sulfuric acid absorption test to check the condition of their solvent with respect to rancidity. This test was designed to give an indication of unsaturates in new solvent. Where fatty acids and other impurities are present, the results are likely to be misleading. Unless temperature, rate of settling, and concentration of the acid employed are controlled, in many instances results cannot be duplicated within 5 to 10 per cent. Figure 11 shows the sulfuric acid absorption of Stoddard solvent to which varying percentages of U. S. P. oleic acid were added. The pure solvent showed an absorption of 2.5 per cent. Addition of 0.5 per cent oleic acid increased the absorption by approximately 8 per cent. When fatty acids are not present, the results of this test cannot be duplicated unless the acid used
INDUSTRIAL AND ENGINEERING CHEMISTRY
460
has been carefully standardized. Thomas, Bloch, and Hoekstra (14) showed that the amount of absorption in gasoline fractions is dependent on acid concentration. Table IV gives the results of the sulfuric acid absorption test on samples of new Stoddard solvent from several refineries using various
PERCENT HpSO+ ABSORPTION-
FIGERE 11. SORPTION
ON
SULFURICACID ABSOLVENTC ONTAIKIKG OLEICACID
concentrations of the acid. The tests were carried out according to the method used in testing dry cleaning solvent for Stoddard specifications ( 8 ) . TABLE IV. VARIATIONOF ABSORPTION OF KEWSOLVENT NITH CHANGE I N CONCEXTRATION O F SULFURIC ACID Solvent
No. 1 2 3 4
63.3% acid 1 0 0 0
Absorption of New Solvent 7 94.6%' 93.8% 94.3% acid acid acid 1 2 2 0 0 0.75 0 0 0 0
0
0
96.8% acid 8 4 3 2
VOL. 32, NO. 4
acids present in the soil from the garmepts. Rancid odors on the garments were detected on runs made shortly after the filter was cleaned; this indicated that relatively thick layers of filter powder and soap adsorb part of the odor-forming substances. The acid number cannot be accepted as a criterion of incipient rancidity without reservation. The Kreis test and peroxide determinations in the modified forms described can be applied successfully to dry cleaning solvents and are useful in detecting the earlier stages of rancidity. The Kreis test, as carried out by a modification of Neu's method, has been applied in analysis of over four hundred samples of used dry cleaning solvent and found to give satisfactory results in detecting incipient rancidity. An exact correlation between the first appearance of a positive Kreis test or the peroxide content of the solvent with odor trouble has not been made because of the variables involved in organoleptic estimations. Three series of test runs were made with soap, and in each case a positive Kreis test was obtained before odor was detected on the garments. Application of electrometric titration technique to alcoholic solutions of residues obtained from evaporation of used solvents indicates the buffering effect of low-molecular-weight acids formed as rancidity develops. The sulfuric acid absorption test for unsaturated substances, on which many dry cleaners have relied for checking the condition of their solvent with respect to rancidity, has been shown to be unsuitable for this purpose.
Acknowledgment The authors wish to thank the United States Hoffman Machinery Corporation for supplying the model washer and filter system used in this investigation.
Literature Cited Hamilton, L. A , , and Olcott, H. S., ISD EXG.CHEM.,29, 217 (1 937).
Ke'&,R: H., Ibid., 10,471 (1918). Kreis, H., Chem.-Ztg., 26, 897 (1902). Lampitt, L. H., and Sylvester, N. D., Biochem. J . , 30, 2237 (1936).
It is evident that a variation in concentration of the sulfuric acid used, such as is found in c. P. acid obtained from reputable chemical manufacturers, will lead to inconsistent results even on new solvent, and when applied to used solvent the test does not give a n accurate measure of fatty acids present which may cause rancidity. Where the variables mentioned were not controlled, results obtained ranged from 10.5 to 20 per cent absorption.
Lea, C. H., IND. ESG. CHEM., Anal. Ed.. 6,241 (1834). Lea, C. H., Proc. Roy. SOC.(London), 108B, 175 (1931). Lea, C. H., "Rancidity in Edible Fats", p. 107, New York, Chem. Publishing Co., 1939. Natl. Bur. Standards, Stoddard Solvent-Commercial Standard '33-38, Feb. 10, 1938. Neu, R., Chem.-Ztg. 61, 733 (1937). Poaick, W. C., J . Agr. Research, 26, 323 (1923). Schibsted, H., IND. ENG.CHExr., Anal Ed., 4, 209 (1932). Stout, L. E., and Tillman, A. B., IND. ENQ. CHEM.,28, 22
Summary
Taffel, A . , and Revis, C., J . SOC.Chem. Ind., 50, 87T (1931). Thomas, C. L., Bloch, H. S., and Hoekstra, J., IXD. E m . CHEM.,Anal. Ed., 10. 153 (1938). Wheeler, D. H., Oil & S o a p , 9, 89 (1932). Young, C. A., Vogt, R. R., and Nieuwland, J. A , , IKD.ENQ. CHEM.,Anal. Ed., 8, 198 (1936).
The increase in fatty acid content of dry cleaning solvent during a series of cleaning operations is due largely to free fatty acids present in soap additions, rather than to fatty
(1936).
