V O L U M E 25, NO. 10, O C T O B E R 1 9 5 3 (6) Davies, C. W., Biochem. J . , 45,38 (1949). ( 7 ) Frohman, C. E., Orten, J. AI., and Smith, A. H., J . Biol. Chem., 193,277 (1951). (8) Goodban. A. E.. and Stark. J. B.. unoublished method. i9) Goodban, A . E:, Stark, J. B., and bwens, H. S.,J . A g r . Food Chem., 1, 261 (1953). ( I O ) Hartley, G. S., and Roe, J. W.,T r a n s . Faraday SOC.,36, 101 (1940).
Hulme, A. C., and Swain, T., S a t u r e , 168,254 (1951). Klose, A. A., Stalk, J. B., Purvis, G. G., Peat, J., and Fevold, H. L., I n d . Eng. Chem., 42, 387 (1950). (13) Kunin, Robeit. and lIcGarvey, F. X . , Ibid., 41, 1265
(11) (12)
1511 (18)
Lugg, J. W. H., and Overell, B. T., Australian J . Sci. Research,
(19)
Marshall, L. hI., Donaldson, K. O., and Friedberg, F., ANAL.
(20)
llarvel, C. S., and Rands, R. D., Jr., J. Bm. Chem. SOC.,72,
1, 98 (1948).
CHEX,24, 773 (1952). 2642 (1950).
Xachod, F. C., ”Ion Exchange, Theory and .4pplication,” New York, Academic Press, 1949. (22) Porter, W.L., B N ~ L CHEM., . 23, 412 (1951). (23) Pucher, G. W.,Sherman, C . C., and Vickery, H. B., J . Bid. (21)
Chem.. 113. 235 11936). (24)
Stark, J. B., Goodban, A. E., and Owens, H. S., A s ~ L .CHEM., 23, 413 (1951).
(1949 j.
Kunin, Robert, and McGarvey, F. X., Ibad., 45, 83 (1953). Kunin, Robert, and Myers, R. J., “Ion Exchange Resins,” Xew York, John Wiley and Sons, 1950. (16) Kunin, Robert, and Myers, R. J., J . Am. Chein. SOC.,69,2874 (14) (15)
(1947).
(17) Legault, R. R., Nimmo, C. C., Hendel, C . E., and Notter, G. K.. I n d . Eng. Chem , 41, 466 (1949).
125) Stalk, J. B., Goodban, A. E., and Owens, H. C’hem., 43, 603 (1951).
S.,Iizd. Eng.
(26)
Stark, J. B., Goodban, d. E., and Owens, H. S.,J . Agr. Food.
(27)
Willard, H. H., and Young, P., J . Am. Chem. Sac., 52, 132
Chem.. 1. 564 (1953). (1930).
RECEIVED for review .4pril 18. 1953.
Accepted July 28, 1953.
Chemical Analysis of GR-S by Complete Solution Procedures Lniformity of quality in the manufacture of GR-S synthetic rubber must be maintained to meet t h e consumer specifications set u p for t h e material. The tests currently used by t h e plant laboratories and by t h e consumer to determine organic acid and soap i n GR-S involve a n extraction with ethanoltoluene azeotrope. The work described was undertaken to develop accurate and precise methods for t h e determination of theseconstituents, which would not be limited by incomplete extraction inherent in the present procedures. The organic acid and soap are determined by titrating aliquots of a tolueneethanol solution (,? to 1)of a single weighed sample.
Tests for stabilizer and bound styrene made on t h e same solution are incorporated into a continuous scheme of analysis. I n addition, with the aid of corrections for ash and for moisture and other votatile components, a complete determination of t h e gross polymer and nonpolymer constituents of GR-S niay be made. When GR-S containing small quantities of mineral acid is tested, both t h e solvent and the method of preparing t h e solution are modified. The titrations for soap and for mineral and organic acids are made with t h e same indicator, rn-cresol purple, and serve as examples of titrations in organic solvents.
Gross Constituents in GR-S Containing Soap FKEDERIC J. I,INNIG, JEAN &I. PETERSOR-, DEMBY JI. EDWARDS, A X D FALTER L. ACHERMAN Yational Bureau of Standards, Washington, D. C.
I
N these complete-solution procedures for the analysis of organic
acid, soap. stabilizer, and bound styrene in GR-S syrithetic rubber, organic acid and soap are determined by the titration of aliquots of a solution of the original sample. The hound styrene is determined by measuring the refractive index of the polymer purified by precipitation from the titrated soap aliquot. T h r stabilizer content is determined spectrophotometrically on a third aliquot. The tests are applicable only to uncompounded GR-S coagulated with salt and acid and without excess mineral acid. They are inadequak for GR-S coagulated with alum, and for GR-S containing oil or gel. The determination of organic acid, soap, and stabilizer in a solution of the m-hole sample eliminates errors caused by incomplete extraction inherent in all extraction procedures. It is t,hus unnecessary t o obtain a thinly sheeted sample and to maintain the solvent a t the proper temperature for maximum extraction. The use of aliquots of the same sample eliminates t,he necessitj- of wrighing separate samples for each test. Purification of the sample used for measuring refractive index by precipitation from the titrated soap aliquot 3 1 9 0 saves time.
With corrections for ash and for moisture and other volatile components (if present), a complete analysis of the gross polymer and nonpolymer constituents of GR-S may be made. Minor chemical constituents, such as mater-soluhle ash, different types of soap, bound modifier, short stop, and trace elements are not considei,ed. Maron, Ulevitch, and Elder (9) have descrihrd conductoniet,ric procedures for the determination of organic acid, soap, and alkali in GR-S latices. For solid GR-S, the Specifications for Government Synthetic Rubbers (11) prescribe an extraction procedure employing ethanol-toluene azeotrope in the analysis of some of the constituents considered here. The method is discussed critically by Kolthoff, C a m , and C‘arr ( 4 ) . However, a procedure in which constituents are det,ermined on a solution of the sample has not been found in the published literature. RUBBER SAMPLES
Four samples previously used as reference lots in the cont,rol testing of GR-S production mere used t o obtain data. These samples are described in Table I. The two latices polymerized at 50” C. (122” F.) using fatty-acid soap as the emulsifier pro-
ANALYTICAL CHEMISTRY
1512 duced fatty acid on coagulation with salt and sulfuric acid and left some residual fatty-acid soap. The fatty acid and soap were determined in the solid polymer used here &s stearic acid and sodium stearate, respectively. The two latices polymerized a t 5' C. (41' F.) using rosin-acid soap as the emulsifier produced rosin acid on coagulation with salt and acid and left some residual sodium rosinate for analysis.
Table I.
Description of GR-S Samples
Sample
X-603
Sample X-418BL
Control Test Sample4
Sample X-558
Temperature of polymerization,
c.
50 50 5 5 T y p e of 80ap F a t t y acid F a t t y acid Rosin acid Rosin acid T y p e of stabilizer PBNAa BLEb PBSAa BLEb Phenvl 8-naphthylamine (phenyl 2-naphthylamine), Stabilizer of indefinite composition resulting from reaction of acetone a n d diphenylamine.
PROCEDURES
Solution of the Sample. Approximately 5 grams of material sheeted to less than 0.75 mm. on a laboratory rubber mill are cut into strips or small pieces to facilitate solution. The specimen is accurately weighed and added piece by iece to a 400-ml. rubber
extraction flask containing 200 ml. of sogent prepared by mixing 5 parts by volume of toluene and 1 part of absolute ethanol. Solution is obtained by refluxing in a rubber extraction apparatus. Swirling of the flask from time to time prevents the rubber from sticking to the glass. When the specimen is completely dissolved, the solution is transferred t o a 250-ml. volumetric flask, a cylindrical funnel being used to aid the transfer. About 15 ml. of solvent are added to the extraction flask, and refluxing is continued for an additional 3 to 5 minutes with frequent swirling of the flask. Two additional rinsings are made with 15-ml. portions of solvent, refluxing if necessary to remove any rubber adhering to the flask. All rinsings are added to the solution in the volumetric flask. Undissolved rubber in the bottom of the flask may be detected by the presence of bubbles which persistently adhere to the surface of the glass after removal from the hot plate. It has been found that a piece of filter paper 9 cm. in diameter placed in the bottom of the flask helps to prevent the sample from sticking to the glass surface. The volumetric flask and its contents are allowed to cool to room temperature, and the volume is adjusted by adding solvent. Aliguots from this solution are used for the analysis of stabilizer, organic acid, and soap. Immediate transfer of these aliquots is recommended in order to obtain representative portions of the finely divided suspension which later settles. This immediate transfer is also desirable because of the unusually large thermal expansivity of these nonaqueous solutions. The volume of the solution delivered by the pipets should be determined experimentally by weight calibration because rubber solutions such as these do not exhibit the same flow properties as aqueous sdutions. A blank is prepared by refluxing 200 ml. of the solvent and treating it in the same manner as the rubber solution. Stabilizer. A 3-ml. aliquot is removed from the rubber solution and diluted t o exactly 100 ml. with methylcyclohexane having a transmittance greater than 90% of that of water at the wave length a t which the stabilizer is to be measured. The appropriate wave lengths are specified in Table 11. The 3-ml. aliquot of the blank is treated in a similar manner.
Table 11. Polymer Absorption Correction Polvmer-Absorntion Coriection per 'Gram
Stabilizer
of Sample in.250 MI. of solution" 0.116
Wave Length, m p PBNA 309 =k 1 BLE 288 =!= 1 0.225 Stalite 288 f 1 0.225 a These values a r e taken from t h e specification ( 1 1 )
The absorbance of the diluted solution is compared with that of the diluted blank by means of an ultraviolet spectrophotometer equipped with 1-cm. matched quartz cells. Two measurements are made on each diluted solution. In the rare case in which the absorbance does not fall between 0.4 and 0.8, either a larger aliquot
should be used or the solution should be diluted to bring the measurement within this range; the calculations should be adjusted accordingly. 2500 ( A - P ) Stabilizer, $7 O - V X W X E A = measured absorbance P = correction for absorption of the polymer, calculated from Table I1 V = volume in milliters of the aliquot diluted to 100 ml. W = weight of the original dry sample E' = specific extinction coefficient of the stabilizer The specific extinction coefficient is preferably determined on a lot of the stabilizer used in the preparation of the rubber to be tested. If the particular lot of stabilizer is not available, it is helpful to make the determination on a mixture of two or more representative samples of the type of stabilizer used. In the case of BLE (reaction product of acetone and diphenylamine of indefinite composition) and Stalite ( a heptylated diphenylamine of indefinite composition), the samples should be thoroughly blended by agitation because of the inhomogeneity of these materials. TheFe materials should not be blended by heating since heating causes a change in composition. An accuratelv weighed specimen [approximately 0.12 gram of PBNll (phenyf 2-naphthylamine) or BLE, or 0 16 grsm.of Stalite] is transferred quantitatively to a 500-ml. volumetric flask. The specimen is dissolved, and the solution made up to volume with the same batch of 5 to 1 toluene-ethanol solvent used to prepare the original rubber solution. Khen the solution has been throughly mixed, 3-ml. aliquots are pipetted into each of three clean 100-ml. volumetric flasks. The pipet used should be calibrated for the stabilizer solution in the same manner as the pipets used for the rubber solution. After the solutions have been made to volume and miaed thoroughly, the absorbances are determined a t the appropriate wave lengths indicated in Table 11, using for comparison the solvent blank prepared in the manner previously described. Two measurements are made on each of the three diluted aliquots, and the specific extinction coefficient is calculated from the mean of these values as follows: 50A Specific extinction coefficient = -
sxv
A = measured absorbance S = weight of stabilizer V = volume in milliliters of the aliquot diluted to 100 ml. The specific extinction coefficient should be determined a t frequent intervals to avoid the effects of variations in the stabilizer, solvent characteristics, or behavior of the spectrophotometer. Generally, one set of determinations each day is satisfactory. Organic Acid. A 100-ml. ali uot is transferred from the rubber solution to an Erlenmeyer i a s k and diluted with 70 ml. of fresh solvent. Seven drops of m-cresol purple indicator (0.3% solution in 95% ethanol, each 0.1 gram of indicator neutralized with 2.62 ml. of 0.1 N sodium hydroxide) are added and the solution is titrated with approximately 0.1 N alcoholic sodium hydroxide (prepared by dissolving 5.2 ml. of an aqueous 50% sodium hydroxide solution in l liter of absolute ethanol) to the first change in color. A 100-ml. aliquot of the blank is diluted and titrated in the same manner. l ! f b x Nb x K X 2.5 Organic acid, % =
W
116 = milliliters of standard Bodium hydroxide solution used for the titration after correction for the blank N b
K W
= normality of the sodium hydroxide solution =
a factor; 28.4 when the organic acid is determined as stearic acid and 34.6 (determined empirically) when as rosin acid (11)
= weight of the original dry sample
Soap. The 147 ml. of rubber solution remaining in the volumetric flask are transferred to a 400-ml. Erlenmeyer flask. The volumetric flask is then rinsed with two 50-ml. portions of fresh solvent, and the rinsings are added to the solution. Ten drops of the m-cresol purple indicator solution are added to the rubber solution, and the titration is made with approximately 0.05 N alcoholic hydrochloric acid (prepared by adding 4.2 ml. of concentrated hydrochloric acid to one liter of absolute ethanol) to the first change in color. This solution is to be saved for the determination of bound styrene. The remainder of the blank is treated in the same manner and titrated.
M, X N o X K X 1.70 Soap, % =
W
V O L U M E 25, NO. 10, O C T O B E R 1 9 5 3
M, =
milliliters of standard hydrochloric acid solution used for the titration after correction for the blank N , = normality of the hydrochloric acid solution K = a factor; 30.6 when the soap is determined as sodium stearate and 36.8 (determined empirically) as rosinacid soap (11) W = weight of the original dry sample Bound Styrene. Twenty-five milliliters of the aliquot previously titrated for soap are slowly poured into a flask containing 100 ml. of an 85 to 15 2-propanol-water mixture. The flask and its contents should be agitated constantly during the entire period of addition of the rubber solution. About one half of the precipitated polymer is placed bet-iveen pieces of aluminum foil measuring approximately 5 by 10 cm and the pieces of foil containing the polymer are placed between the cold platens of a Carver press. The platens are then brought together for a few minutes under a pressure of 35 to 70 kg. per square em. (500 to 1000 pounds per square inch). After the pressure has been released, the aluminum sheets are separated, and the thinly pressed polymer, which a t this time should be less than 0.75 mm. in thickness, is placed in a 25O-ql. extraction flask containing 25 ml. of anhydrous ethanol-toluene azeotrope (prepared by mixing 30 volumes of anhydrous toluene with 70 volumes of absolute ethanol) The azeotrope containing the pressed specimen is refluxed for 15 minutes. The extracted specimen is then dried in a vacuum oven for 1 hour a t 100” C. The index of refraction of the dry specimen is measured in the manner described by Arnold, Madorsky, and Wood ( I ) . The relationship for converting the index of refraction to bound styrene content is given in tabular formin the Specification. for Government Synthetic Rubbers (11). DISCUSSION OF PROCEDURES
Solution of the Sample. A 5 t o 1mixture of toluene or benzene with ethanol has been found to be the most satisfactory solvent for this purpose. The ethanol facilitates solution of the soap and the salt of the indicator used in the titrations. The ethane] also prevents cloudiness which would otherwise result during the titration of organic acid with alcoholic sodium hydroxide, since the latter contains small amounts of water introduced during its preparation from aqueous 50% sodium hydroxide. The ethanol used to prepare the rubber solvent must not contain more than 1% water, or the 3-ml. aliquots employed in the determination of stabilizer will become cloudy on dilution with methylcyclohexane. The use of methylcyclohexane as a solvent in place of toluene or benzene in the preparation of the original solution causes cloudinesb, which makes detection of the end points in the titrations of organic acid and soap considerably more difficult. Chlorinated hydrocarbons such as chloroform and ethylene dichloride, while excellent solvents for rubber, are not satisfactory in these tests because they form mineral acid and colored reaction products in the presence of stabilizers iuch as phenyl 2-naphthylamine. These effects would, of course, give erroneous results in the determination of organic acid, soap, and stabilizer. Although both toluene and benzene behave equally well as solvents, the former is preferred because it is cheaper and less toxic. The use of a mixture of benzene and ethanol as a solvent in the analysis of fatty acid and soap in GR-S was &st suggested by Frieden ( 3 ) . Stabilizer. Highly purified niethylcyclohexane was chosen as the diluent for the stabilizer aliquots because in the pure state it has a very high transmittance over the range of wave lengths used. When optical grade solvent is not available, the technical material may be purified by first distilling it through a short column and then passing it through a column of activated silica gel about 3 feet long. These solutions have been found to follow Beer’s law of optical transmittance quite closely over the range of concentration encountered in this procedure. Thus it should be satisfactory to determine the absorbance of three aliquots of the solution of stabilizer using the same volume of aliquot in each case. This volume should, of course, be one which meets the optical requirements indicated in the procedure.
1513 The 3-ml. volume recommended in the procedure has been found satisfactory for both the rubber and the stabilizer solutions The use of three 3-ml. aliquots should yield greater precision than the use of single 2-,3-, and 4-ml. aliquots as prescribed by the specification (11), provided of course that the solutions follow Beer’s law, m they do in this case. The use of the 2-, 3- and 4-mI. aliquots would be necessary only for determining adherence to Bedr’s law. A self-zeroing pipet devised by Tryon (14)is extremely useful in pipetting the small aliquots used for the determination of stabilizer because it includes a flushing feature which eliminates drainage errors that become quite appreciable in this work. The values for polymer absorption given in Table I1 for the present calculations may not be used for concentrations greater than 3.0 grams per liter because of deviations from Beer’s law (13). In this work, the concentration of polymer in the diluted aliquot actually measured in the spectrophotometer is of the order of 0.6 gram per liter, a value well within this limit. The correction for this quantity of polymer is small relative to the absorbance of the diluted 3-mI aliquot (0.4 to 0.8); for a 5.0-gram sample it is 0.017 a t 309 mp and 0.034 a t 288 mp. These corrections were determined for methylcyclohexane alone. However, indications are that the small quantities of toluene and ethanol present in the diluted aliquot do not greatly affect these small corrections (13). Organic Acid and Soap. rn-Cresol purple was found to give the most readily discernible end points for these titrations; using a statistical method ( 7 ) , this indicator w m found to be accurate as well. The salt of the indicator was chosen because it is more soluble in 95% ethanol than the acid form. Dilution of the 2% rubber solution with solvent prior to making the titration reduces the masking effect of the dissolved rubber on the color changes; the rather high concentration of indicator also allows for a more distinct color change a t the end point. The titrations may be performed conveniently using a 5-ml. buret graduated in 0.01 ml. When using an untitrated sample for comparison, an experienced operator can readily detect the color change using increments of less than 0.02 ml.; with increments of 0.02 ml. the color change becomes apparent even to an inexperienced operator in all but a few cases. If extremely dark rubber solutions are to be titrated, it may be necessary to determine the end point with the aid of some type of photometer using a filter that absorbs in the region of 530 mp (10) When such an instrument is employed, the solution should be agitated in such a fashion as to prevent absorption of carbon dioxide in the open vessel ordinarily used in these extended titrations. This may be accomplished by covering the solution and stirring it with either a stream of inert gas or a well submerged stirrer that does not beat air into the solution. Bound Styrene. Experiments performed in this laboratory by Tryon (IS)have shown that fractional precipitation of GR-S will yield a precipitate having a higher refractive index and therefore a higher bound styrene content than the total polymer. These findings are supported by the work of Yanko (15) who showed that the bound styrene content of GR-S fractions increased with increase in the number average molecular weight of the fraction. Thus, in developing a precipitation method for the purification of GR-S it was necessary to employ a precipitant which would assure a relatively high recovery of polymer. It was also found that a short extraction of the precipitated polymer with ethanol-toluene azeotrope would aid in the complete removal of occluded solvent and other impurities. The major portion of these occluded impurities can be r e moved by pressing the precipitated polymer into a thin sheet prior to extraction, and the use of this thinly sheeted material facilitates the extraction of the residual impurities. In order t o use these purification techniques it was necessary to obtain a p r e cipitated polymer that was not too sticky to handle. To allow for the complete use of the original solution for the de-
ANALYTICAL CHEMISTRY
1514 terniination of organic acid, soap, and stabilizer, it seemed desirable to precipitate the polymer from a portion of the titrated solution rather than from the original solution. The soap aliquot was chosen rather than the aliquot titrated for organic acid because it was believed that the precipitation technique would remove organic acid more readily than soap. Since the original aliquot is diluted prior to the soap titration, it became necessary to incorporate water in the precipitating medium in order to ensure a satisfactory recovery of polymer. Thus, mixtures containing 15% water with the three alcohols, methanol, ethanol, and 2-propanol, were studied as precipitants. Preliminary experiments showed that the method employing the 2-propanol-water mixture as a precipitant was the most satisfactory from the standpoint of general ease of manipulation. This precipitant recovers about 90% of the polymer portion of the sample. This purification procedure affords a saving of time over that required by the extraction procedure of Arnold, RIadorsky, and Wood ( 1 ) . e-pecially when all the constituents are to be determined. RESULTS
Precision. T o determine the precision of these methods duplicate determinations were made daily for a number of different days on each of the four rubber samples described in Table I. Mean values obtained for these tests are given in Table I11 along with the number of determinations made for each series. The standard deviation of the mean value, where given, is designated as sm, and the standard deviation of a single random determinationestimated fronian analysis of variance of each series is designated as sI. A less technical estimation of the precision of the method may be obtained from the spread of the values indicated by the difference between the high and low values. Accuracy. A careful statistical study (7) has shown that the accuracy of the organic acid and soap titrations is better than 1% relative, or about 0.03 to 0.05 and 0.005 to 0.01% absolute. respectively. when the acid radical is aliphatic and its equivalent weight is known. Usually there is an additional bias introduced by the difference between the actual equivalent weight of the
Table 111. Precision of Test 3Iethods in Complete Solution Procedure for Analysis of GR-S % Constituent Organic acid Mean Sma 87
High
Low
Number of determinations Soap Mean Srn S7
High Low Number of determinations Stabilizer Mean Sm
High
Low
S u m b e r of determinations Bound styrene Mean Sm SI
High Low S u m b e r of determinations
Control test sample KO.4
X-603
X-418BL
4.80 0.01 0.04 4.86 4.74
5.12 0.03 0.06 5.18 5.02
5.66
5
3
0.221 0.01 0.02 0.253 0.189
0.005
5
3
1,105 0.01 0.02 1.139 1.081
1.285
10 0.098 0,007 0.02 0.120 0.081 10 1.314 0.004 0.010 1.329 1.300 10
23.99 0.03 0 08 24.13 23.90
5
24 01 0 08 0 2 24.24 23 i s
... ...
5.71 5.62
X-558
5.85 0.02 0.05 5.91 5.77 10
...
0:oos 0.003
0 048 0,002 0.008 0.063 0.040 10
...
1:is1 1.276
3
1.183 0,005 0.01 1.197 1.161 10
20.22
...
:
20 26 20.14
23.45 0 02 0.07 23,55 23.31
5 3 10 10 Standard deviation of the mean. b Standard deviation of a single random determination including both within-day and day-to-day variability. a
fatty acid in the rubber and the equivalent weight of stearic acid used in the calculation. Preliminary potentiometric studies showed that the titration of rosin acid using this indicator yields results that may be low by about 0.05% absolute. K Ostudy has been made of the titration of rosin-acid soap. It is believed that the accuracy of these tests is quite satisfactory for control of production and for purchase specifications. For comparison, values for the stabilizer content of these polymers were determined by the spectrophotometric procedure dercribed in the Government Specifications for Synthetic Rubbers (11). This procedure is a modification of one (2, 6) for which Laitinen, Nelson, Jennings, and Parks (6) observed good agreement, within experimental error, between quantities of stabilizer added and found. The values obtained by the specification procedure are in good agreement with those given in Table 111.
Table IV.
'Analysis for Stabilizer and Bound Styrene by t h e Specification Methods - Stabilizer, % Bound Styrene, ?&
Sample Designation X-603 X-418BL Control test sample KO,4 X-558
Mean value 1.309 1.111 1.286 1.161
Standard deviation 0.004 0.004 0:OOl
Mean value 23.91 23.99 19.89 23 54
Standard deviation 0.03 0.04
...
0.02
The relationship between the bound styrene content and t,he refractive index of GR-S polymerized at 50" C. (122" F.) is hased on unpublished work done at the rational Bureau of Standards that involved refractive index measurements and elemental analysis of purified GR-S polymers for the det,ermination of the styrene content. This relationship is believed to be accurate to within 0.1%. Though the relationship for GR-S polymerized a t 5' C. (41" F.) has not yet been established, indications are that with the quantities of styrene currently present in most GR-S (20 to 25%), the relationship used here provides a good iipproximation within a few tent.& of 1%. For comparison, values were obtained for the bound styrene content of these samples using the extraction procedure shown by Arnold, Madorsky, and Wood (1) to be satisfactory for the purification of this type of GR-S. These data (Table I\-) are in satisfactory agreement with those in Table 111. Analysis of Gross Chemical Constituents. T o illustrate the method of arriving a t a total analysis of GR-S, the values given in Table I11 for organic acid, soap, and stabilizer are repeat,ed in Table V under the columns labeled solution. \-dues for the moisture and ash contents of each sample are also given. Rubber usually contains a small amount of moisture, but in this case the samples contained an inappreciable quantity because of previous milling to obtain samples of satisfactorj- homogeneity for use as standard lots. For samples cont'aining appreciable quantities of moisture this constituent may be determined by the azeotropic distillation procedure described by Tryon (12) and later adopted b y the specification (11). The hot-mill method ~vouldgive values which include other volatile components such as monomer stprene! which could be determined by difference. The ash contents of the samples were determined according to the method of Linnig, Milliken, and Cohen (8). To determine the total nonpolymer portion of the samples, a correction was made for the ash as sodium carbonate resulting from the soap in the rubber. This corrected value probably r e p resenk the actual inorganic salt cont,ent (plus silica) of the original sample modified by slight variations in composition which may occur during the ashing process. The correction is made as follows: Inorganic ash % = ash content - 0.1730 X soap content 0.1730 =
NatCOa, grams 2Na02C(CH?),&H,. grams
V O L U M E 2 5 , NO. 10, O C T O B E R 1 9 5 3
1515
spectively, using the indicators employed by Kolthoff, Carr, and Carr The polymer content was determined by Tyeighing the extracted specimen Sample Sample Sample Control Test after i t had been dried to constant weight in a X-418BL, Sample A o . 4, X-558, X-603, % 70 7c _ _ _ _%_ vacuum a t 50" C. The time required for drying S o h - Extrac- Solu- ExtracSolu- ExtracSolu- Extracwas about 23 hours. The nonpolymer content Constitrtrnt tion tion tion tion tion tion tion tion was determined by difference with corrections Organic acid 4 80 1.72 5.12 5.12 5 66 .j 59 5.85 .5 80 Soap 0 098 0 . 1 4 6 0 . 2 2 1 0.254 0 00.5 0 038 0.048 0 086 applied for the ash content of the extracted niateetabilizcr 1.314 1 . 1 4 0 1.105 0 . 9 4 3 1 283 1.104 1.183 1 094 Moisture or volatile rial using the values for inorganic saks givrn matter 0 00 , , . 0.00 ... 000 ... 0.00 ... in Table V. Ash (total) 0 98 ... 0.42 . .. 0 78 ... 0.85 ... Ash (froin soap As a check on the completeness of extraction, the 0 017 _. 0.038 . . . 0.009 .. 0 0083 . . . calcd.) refractive index of the dried specimen was measAsh (inorganic salts by difference) 0 96 0.38 ... 0 78 ... 0 84 . ured and the corresponding bound styrene content Totalnonpolyiiier 7.17 7'26 6.83 6 83 7 73 7.93 7 92 8. . 13 Total polynier 02.83 .,. 93.17 ... 92.27 ... 92.08 .. determined. The mean values obtained for these Bortnd atyrene (in polyitier) 23.99 23 90 24 01 23 94 20 22 1 9 . 9 5 2 3 . 4 5 23 87 tests are given in Table V under the columns Bound styrule (in labeled extraction. Consistent,lyhigher values were rubber) 22 27 ., 22 37 ... 18 66 ... 21.59 .., Bound bittadiene obtained for soap, probably because a different (in rubtier)" 70.56 .. , 70 80 .. . 73.61 , .. 70.49 ... indicator x-as used in the extraction procedure. a This fignre inclrtdes boitnd modifier which usually aiiiounts t o about 0.67,. The consistently low values obtained for hoth _ _ _ _ _ ~ _ ~ _ _ _ _ ~ _ ~ _ . ~ stabilizers were probably due to incomplete extraction. Consistently higher values were 011Addition of the value for inorganic ash to the values for nioistairied for total nonpolymer by the extraction procedure. These ture, organic acid, soap, and stabilizer gives a value for the total probably were due to the small quantity of low polymer that the authors of the procedure indicate is removed by the extrarnonpolymer portion of the sample. The polymer portion is detion solvent. S o consistent differences existed for organic acid termined by subtracting this value from 100. The bound strrene or bound styrene content. in the polymer, as determined by experiment, is then reduced to Further attempts to check the values for the total polynier conthe bound styrene in the whole rubber. tent of t,hesesamples were made using a modification of the method For comparison these samples were analyzed using the extracof Kolthoff, Carr, and Carr (4)for determining the low-polytion procedure suggested by Kolthoff, Carr, and Carr (4) for the mer content of ethanol-toluene-r~-aterextracts by precipitating determination of the polymer and nonpolymer portions of GR-S. from a chloroform solution with iodine chloride and subsequeiitly T h e extraction solvent was prepared by adding 10 parts by volume weighing the dried precipitate. In this modification, t,he whole of water to 100 parts of ethanol-toluene azeotrope, composed of 70 sample was dissolved in chloroform and treated with iodine chloparts of ethanol and 30 of toluene. The thinly sheeted rubber ride. These values were grnerally ahout 3Oj, lower than those sample was cut into small strips and duplicate tests were made b>calculated by difference or ohtained experimentally liy the estracextracting 6-gram portions for 2 hours with 100 ml. of this solvent. tion procedure. T h e extract was poured into a 250-ml. volumetric flask, 100 ml. of This discrepancy is undouhtedly an effect that depends on the fresh solvent were added to the extraction flask, and the extracquantity of material analyzed hecause the test has been used very tion continued for an additional 2 hours. The second 100-ml. successfully for estimating small quantities of low polymer. The portion was added to the volumetric flask and the extract made discrepancy is obviously far greater than could reasonably be esup to volume with fresh solvent. pected. Accordingly, the iodine chloride precipitation method The stabilizer content of the samples was determined spectre is not suitah]? for the determination of the total polymer portion photometrically on a 3-ml. aliquot of the extract. Organic acid and soap were determined on 100- and 147-mI. aliquots, reof GR-S. Table V. Comparison of Analyses of Gross Chemical Constituents of GR-S by the Complete-Solution Procedure and Extraction Method (4)
(e).
,
(Chemical Analysis of GR-S by Complete Solution Procedures)
Titration of Mineral and Organic Acids in Toluene-Ethanol Solution FREDERIC J. LINNIG AND ALICE SCHNEIDER, .Vutional Bureau of Standards, Wushington, D. C .
I
N T H I S method for the determination of mineral acid and or-
ganic acid, which are sometimes found together in GR-S synthetic rubber, both types of acid are determined in a solution of the sample by making a single titration to two different end points with the same indicator. The presence of mineral acid, in addition to the organic acid always present in GR-S, is caused by the use of a greater quantity of sulfuric acid during the coagulation than is required for reaction Kith the soap in the latex. This excess mineral acid is known to retard the rate of cure of the polymer and thus to change the processing characteristics of the material. PROCEDURE
A 2-gram specimen of sheeted GR-S rubber is dissolved in 140 ml. of toluene in the manner described above. When the rubber is completely dissolved, the solution is allowed to cool to room temperature, and 30 ml. of 95% ethanol are added with constant swirling of the flask during the addition to prevent large quanti-
ties of polymer from precipitating. The flask should be swirled until all flocks of precipitated polymer have completed redissolved without heating. About 7 drops of the m-cresol purple indicator solution are added, and the solution titrated with approximately 0.1 X alcoholic sodium hydroxide. A pink color indicates the presence of mineral acid, and the titration for this acid proceeds until the color definitely reaches the yellow range. From this end point, the titration for organic acid continues to the first change toward purple that appears as a darkening of the solution which persists after swirling the flask. The titrations are conveniently performed with a 5-ml. buret graduated in 0.01 ml. For the determination of the blank corrections, 140 ml. of solvent are refluxed and treated in the same manner. The first blank titration is made by adding standard 0.05 Y alcoholic hydrochloric acid until the solution changes from the yellow color, normally obtained with the indicator and solvent, to a pale salmon. The second blank is determined by back-titrating the same portion of solvent with 0.1 6 alcoholic sodium hydroxide to the first change to purple which does not disappear on swirling the flask.
