This behavior is borne out by the comparison of data presented in Table 11. Tests with powders having extensive internal pores resulted in specific surface values lower than indicated by gas adsorption.

NOMENCLATURE

A = Cross-sectional area of bed, cmz L = Length of bed, cm M = Molecular weight of gas, gram/mole P = Gas pressure upstream of valve, mm Hg Q = Gas flow rate, mole/cm2 second q = Gas flow rate, cm3/second

R = Gas constant, 8.31 X IO7 dyne cm/mole OK S, = Specific surface area of powder, m*/gram se = Specific surface area of powder, cm2/gram T = Absolute temperature, OK

t = Temperature, "C AP = Pressure loss across powder bed, dyne/cm2 Ap = Pressure loss across powder bed, mm Hg E = Bed porosity [ = (apparent volumeabsolute vol-

p

ume)/apparent volume] = Absolute powder density, gram/cm3

RECEIVED for review November 7, 1966. 1967.

Accepted March 14,

Polarographic Determination of Aminocaproic Acid in Nylon Polymer

Thomas A. Robinson' Fibers Division, Allied Chemical' Corp., Petersburg, Va.

DURING THE POLMERIZATION Of E-CaprOlaCtam to nylon-6, c-aminocaproic acid is formed by the hydrolysis of the amide group of the monomer and remains in the final polymerization product in low concentrations. In a study of the kinetics of the polymerization of caprolactam by Hermans, Heikens, and Van Velden, the concentration of aminocaproic acid in equilibrium polymer ranged from 0.0008 to 0.005 millimole per gram of polymer (1). This low concentration presents a problem in the quantitative determination of aminocaproic acid in nylon.

In their study, Hermans and coworkers used paper chro- matography to analyze for the aminocaproic acid. The experimental polymers were extracted with methyl alcohol t o remove the amino acid, along with caprolactam and oli- gomeric material. The extract solutions were concentrated and spotted on paper. .By use of the proper eluant, the ami- nocaproic acid was separated from the other components. After using ninhydrin to develop the spots, these were visually evaluated by several observers against known aminocaproic acid standards (2). In our evaluation of Hermans' method, it was found that the spot size and color intensity varied too much for it to be useful. Attempts were made t o correlate spot area to concentration, but again reproducibility was lacking.

Turian, Tyurin, and Zhantalai have reported a polaro- graphic procedure for the analysis of caprolactam (3). The monomer was first hydrolyzed in 1 N sulfuric acid and con- verted to aminocaproic acid. After neutralization and the addition of sufficient base to make the solution 0.2N in sodium hydroxide, the aminocaproic acid was reacted with a large

1 Present address, Agricultural Division, Mobil Chemical Co., Richmond, Va.

(1) P. H. Hermans, D Heikens, and P. F. Van Velden, J . Polymer Sci., 30, 81-104 (1958).

(2) Ibid., 16, 451-7 (1955). (3) Ya. I. Turian, Yu. M. Tyurin, and B. P. Zhantalai, Z/I. Analit.

Khim., 16, No. 3, 352-8 (1961).

excess o formaldehyde to form quantitatively the Schiff base as shown below:

NHz(CH2)COO- + CHzO + CH,N(CHz),COO- + H,O (Schiff base)

Although it is possible to carry out the polarographic reduc- tion of the Schiff base in a n alkaline medium, the authors reported that the limiting current is increased 5-8 times when the solution is buffered to a pH of 6-8 or the solution is acidic, pH 1-2. It was also recommended that the formal- dehyde concentration not exceed 4-5 % in the final solution, as contaminants might interfere. Turian and coworkers indicated that the aminocaproic acid could be determined in the range of 0.038-35.0 millimoles per liter. Therefore, this procedure had very good possibilities for the analysis of aminocaproic acid in nylon polymer.

EXPERIMENTAL Apparatus. In this study, a k e d & Northrup Electro-

Chemograph was used. The electrode system was a dropping mercury electrode us. a saturated calomel electrode (Sargent No. S-29405). The instrumental settings and variables were: current span, 1-2 Fa; damping, 3 (maximum); DME drop time, 4-5 seconds; voltage span, -0.70 to -1.40 volts; nitrogen purge, 20 minutes before each sample and 30-45 seconds between each scan; and number of scans, 3-4 per sample.

Reagents. Reagent grade chemicals were used whenever possible. The aminocaproic acid was recrystallized twice from water. The caprolactam was commercial grade.

Sample Preparation. Twenty grams of nylon polymer were ground t o 20 mesh and water was extracted by boiling reflux for 12 hours. The hot solution was filtered and concentrated by evaporation t o approximately 5 ml. Some oligomeric material precipitated and was removed by filtration. The filtrate was placed in a 50-ml volumetric flask and to it were added 5.0 ml of formaldehyde, 15.0 ml of citric acid-sodium hydrogen phosphate buffer, p H 8.0, and 2.0 ml of 2N po- tassium chloride. The sample solution was diluted to volume with distilled water. Standard solutions were pre- pared in a similar manner, using aliquots of a known amino- caproic acid solution.

836 e ANALYTICAL CWEMISTR7

e z Y I

I

U a

- 1 i.

0 . 2 MICROAMPERE - 8.0

I

- 0 . 6 - 0 1 - 0 8 -0.9 -1.0 -1.1 -1.2 -1.3 -1 .1

V O L I S

Figure 1. Effect of pH on reduction of Schiff base

RESULTS AND DISCUSSION

Effect of pH. Atterripts t o analyze 0.1N sulfuric acid solu- tions of the Schiff base, as recommended by Turian and co- workers, were not successful. I t was observed that the re- duction wave shifts t o a more positive potential with increasing acidity. The literature reports a half-wave potential of - 1.30 volts for the Schiff base in 0.1N sodium hydroxide. When the solution of the Sch ff base was buffered t o p H 8, it was observed that the half-wave potential shifted t o - 1.06 volts. In a pH 2.2 solution, the Schiff base is reduced a t -0.77 volt. The limiting current also decreased with increasing acidity, dropping about 48 between p H 8.0 and pH 2.2 (Figure 1). Analysis of solutions which were 0.1N in sulfuric acid did not reveal a reduction wave for the Schiff base. I t was also found that it was not necessarq to make the solutions 0.2N in sodium hydroxide for the SchiiT base to form and aqueous extract solutions were used directly, Formaldehyde concentrations up to 10% were used without any interference. An increase in limiting current of the Schiff base was observed each time the same aliquot of so1,ition was polarographically scanned. This continual increase , n limiting current has been attributed t o a n increase in reduction products in the immediate area of the electrode. Purging the solution between each scan with nitrogen gas gave more reproducible limiting currents.

Standard solutions of aminocaproic acid were prepared and the Schiff base formed. A plot of the log amino acid con- centration cs. the log diffusion current, id, gave a straight line relationship in the range of 0.05-0.25 millimoles per liter. However, the polarographic scans of the sample solutions revealed a second reduci.ion wave a t -0.80 volt and the wave for the Schiff base strongly depressed and shifted to -1.19 volts (Figure 2) . Because the sample solutions contained caprolactam and low molecular weight oligomers, additional tests were made to evaluate the effect of these materials. I t was found that both caprolactam and oligomers depressed the wave for the Schiff base and some solutions containing oligomeric material also exhibited a second wave a t -0.80 volt. Attempts were rrsade to compensate for the wave de- pression by the preparzition of standard aminocaproic acid solutions which contained both caprolactam and oligomers. This procedure, however, gave poorly defined reduction waves, particularly for the lower concentrations of aminocaproic acid and in general was not satisfactory. It was noted that a large excess of caprolactam depressed the wave observed a t -0.80 volt. This allowed the easier measurement of the limiting current for the Schiff base, although there was some distortion of the wave form.

- t

0.1 MICROAMPERE

1. Wl lHOUI ADDED C A ? I O L A C I A M

2 0 5 s C A l R O l A C I A Y ADDED

I I I I I I I I 1 -0.7 -0.8 -0.9 -1.0 -1.1 -1.2 -1.3 -1.4 -1.5

VOLTS

Figure 2. Reduction of Schiff base in polymer extract solutions

Table I. Analysis of Known Aminocaproic Acid Solutions by Standard Addition Technique

mm/liter mm/liter Av . Av. dev. Taken, Found,

0.50 0.50 0.49 0.55 0.51 0.02 0.52 0.48

0.20 0.23 0.26 0.22 0.22 0.02 0.19 0.18

Table 11. Aminocaproic Acid Content of Production Grade Nylon Polymer

Millimoles/litera Millimoles/gram 0.44 0.0011 0.72 0.0018 0.52 0.0013

Av. 0.56 Av. 0.0014 0.28 0.0007 0.20 0.0005

Av. 0.24 Av. 0.0006 0.60 0.0015 0.56 0.0014 0.44 0.0011

Av. 0.52 Av. 0.0013 a Concentration in sample solution.

Standard Addition Method. Because the addition of caprolactam and oligomers to standard aminocaproic acid solutions introduced too many variables to allow for mean- ingful results, a standard addition technique was adopted. I n this procedure, a polarographic scan was made on 4.0 ml of the solution containing the Schiff base and unknown amounts of caprolactam and oligomers. Additional capro- lactam (0.5 gram) was added t o depress the wave at -0.80 volt, when present. To the test solution was then added 0.100 ml of a 40.0 millimole per liter standard aminocaproic acid solution, which reacted with the large excess of formalde- hyde to form additional Schiff base and the scan was repeated.

VOL. 39, NO. 7, JUNE 1967 e 837

An ave:age of inree scans was used :a determine ihe diffusion current before and after the aadirion of the standard soiution. In this manner, the number of variables was reduced and the aminocaproic acid content of the extract solutions was determined by the following equation:

C,hrr c, = ~

H(V + G) - hV where C, = concentration of standard solution

V = volume of sample solution c = volume of standard solution added h = diffusion current before addition of standard

H = diffusion current after addition of standard

The results of the analysis of known aminocaproic acid solutions may be seen in Table I.

solution

solution

Samples of production grade nylon polymer were analyze< in the manner described above. As may be seen in Table I I , the aminocaproic acid content was found to be in the iower end of the concentration range reported by Hermans and coworkers. This is a reasonable figure as production grade polymer is leached to remove most of the water soluble material, while those polymers examined by Hermans and coworkers were not washed and would contain a higher percentage of the amino acid. There was observed a sample to sample variation in the analysis of the production grade polymer. This is probably due to slight variations in extrac- tion during the sample preparation.

RECEIVED for review January 6, 1967. Accepted March 13, 1967. Division of Analytical Chemistry, 152nd Meeting, ACS, New York, N. Y., September 1966.

separation of Petroleum Hydrocarbons by Selective Adsorption with Sephadex LH-20

Beveridge J. Mair, Philip T. R. Hwang, and Raffaele G . Ruberto Petroleum Research Laborator), Carnegie Institute of Technology, Pittsburgh, Pa.

SILICA GEL and alumina appear to be the favorite adsorbents for the fractionation of petroleum hydrocarbons. The separation of paraffins and cycloparaffins from aromatics can be effected quantitatively by adsorption on silica gel and a nearly complete separation of aromatics into mononuclear, dinuclear, trinuclear, and higher nuclear aromatics is possible with alumina. However, the separation of the sub types such as the paraffins from the monocycloparaffins or the alkylbenzenes from the monocyclanobenzenes is much more difficult and for this purpose neither silica gel nor alumina is effective. For many binary mixtures of a paraffin with a cycloparaffin, with silica gel or alumina as adsorbents, one of the components is preferentially adsorbed in a part of the concentration range, there is one concentration a t which neither component is preferentially adsorbed, and in the remainder of the range the second component is preferentially adsorbed ( I , 2). Thus a complete separation of paraffins from cycloparaffins is not possible irrespective of the number of stages in the fractionating column.

This report describes the separation of paraffins from cyclo- paraffins and alkylbenzenes from cyclanobenzenes by selective adsorption on Sephadex LH-20. Heretofore, Sephadex LH-20 has been used primarily with the technique known as gel filtration chromatography for the separation of molecules according to their size. However, it has been recognized that Sephadex LH-20 possesses selective adsorptive properties. For example, Wilk et ul. (3) in experiments with Sepliiiclcx LH-20 expanded with 2-propanol have hewn th;it thc rctcri- tion volumes increase with inc.rc;ising i'iiig coiitcnt for the

.

(I) 13, J . blziir, A . I . C ~ i i l ~ ~ r i ~ i u l i , : i r i c I I . I ) . l<cl\\iiii. /ti,/. / / i .c .

(2) A . I . . t iirshlcr. " I lic C'l ici i i i \~i > 0 1 I'c~rolcuiii 1 i ) d i t w i r b t v i \ . "

( ? i ki, MiI1. 1 . Koclliii; arid t i . HtwLk.. .i. ~~/ i r~vmi ;o ,~ . , 24. 4 l J

C/wni.. 39, 1072 ( l947j .

Kciiiliciltl. N ~ N Yorh. 1054. ('li:iii. S . 17. I(i5

i ! 906

Table I. Separation of Binary Mixtures Components Formula Separation

C16H31 Fair n-Hexadecane n-Decylcyclohexane Ci6Haz n-Undecane CiiHir 1 -Methyl-cis-4-isopropyl-

cyclohexane CaHzo Good

Excellent n-Undecane CiiHza cis-Decah ydronaphthalene CioHia n-Pentylbenzene 4,7-Dimethylindan 7-Phenyltridecane Dodecahydrochrysene Excellent

following aromatic hydrocarbons: benzene, naphthalene, anthracene, fluoranthene, chrysene, 3,4-benzpl rene. and 1,2- benzpyrene. This is exactly opposite to the order expected if the adsorbent was functioning in accordance H i t h the theory of the gel filtration technique. .41so. the unique ability of Sephadex LH-20 to e x t u c t pol! nucle,ir aronxitic hydrocarbons from their aqueous suspensions h,is been noted by Wilk and Rochlitr (4 and e\pl;iincil in terms of strong 11)-

drophohic inter.1 L L '1 1011s.

83% ANA'YT'CA: CHEklsTRY