titrated in the presence of copper by adding thiourea to complex the latter. Iodide serves as a n almost specific masking agent for mercury(I1). Copper can be titrated a t p H 3.0 to 3.5 nithout interference from aluminum if fluoride is added. This titration fails a t higher p H values. Data for these and other titrations using masking agents are preiented in Table I'. DISCUSSION
In order to obtain rapid equilibrium near the end point in titrations where some copper must be present, the copper should be added near the end point. The titration of thorium is a n exception; here the equilibrium is rapid even when
the copper is added a t the start of the titration. I n titrations where a masking agent such as citrate is present, it is advisable to delay adding the naphthylazoxine indicator until a few tenths of a milliliter before the expected end point. K h e n determining metals that form weak azoxine complexes (rare earths, for example), zinc can be added instead of copper in order t o obtain a sharp end point. Zinc gives better results than copper for titrations to be carried out in the presence of calcium, but the addition of copper affords a greater toleration for chelating agents. LITERATURE CITED
(1) Flaschka, H., Abdine, H., ;IIilzrochim Acta 1956, 770.
(2) Fritz, J. S., Ford, J. J., A s . 4 ~ . CHEW 2 5 , 1640 (1953).
(3) Fritz, J. S., Lane, W. J., Unpublished
work. (4) Irving, H., Rossetti, H., Acta Chem. Scand. 10, 7 2 (1956). ( 5 ) Kinnunen, J., Merikanto, B , Chemist Analysl 44, 50 (1955). (6) Malat, M.,Suk, V., Jenickova, A., Chem. Listy 48, 663 (1954). ( 7 ) Wanninen, E., Ringbom, A. R., A w l . Chim. Acta 12, 308 (1955).
RECEIVEDfor review October 1, 1956. Accepted January 19, 1956. Division of Analytical Chemistry, 131st Meeting, ACS, Miami, Fla., April 1957. Contribution No. 504. Work performed in Ames Laboratory of U. S. Atomic Energy Commission.
Determination of Monomeric Epsilon-Caprolactam and Moisture in 6 Nylon HENRY H. SCHENKER, CLYDE C. CASTO', and PAUL W. MULLEN Textile Fibers Departmenf, E. 1. du font de Nemours & Co.,Inc., Wilmington, Del.
b A rapid procedure for the simultaneous determination of 6-caprolactam monomer and moisture in 6 nylon flake and yarn is described. They are separated from the sample by vacuum extraction at 200" C., the monomer and part of the moisture being condensed in a water-ice cold trap. The monomer is determined by refractive index in an aqueous dilution of the condensate, while moisture is obtained by difference. The procedure requires less than 3 hours as compared to the 24-hour water-extraction procedure for the water-exdetermination of total tractable material and, when used in conjunction with the water-extraction procedure, it furnishes an estimate of the dimer content plus any low molecular weight, water-soluble, linear molecules present in the polymer.
I
s THL polymerization of +caprolactam (hereafter referred to as monomer) to 6 nylon (a polycaproamide produced by polynir~ization of e-caprolactam), it is important to be able to determine the amount of unpolymerized lactam
'Present atldreks, Research Department, American Potash & Chemical Corp., Henderson, S e v .
remaining in the polymer, Hanford and Joyce (1) employ a 30-minute vacuum extraction a t 180' C. to determine monomer in the polymer. The monomer is condensed on a dry iceacetone cold finger, dissolved in acetone, recovered by evaporation of the acetone, and finally dried and weighed. Van Velden and coworkers (3) have determined, separately, monomer, ring compounds, and short linear molecules in 6 nylon. Hot methanolic extraction up to 36 hours is followed by evaporation and careful drying. Monomer is determined by sublimation and xeighing. Thf ring compounds and short linear molecules left after sublimation of the monomer are separated by a strongly basic ion Pxchange resin. The amount of n-ater-soluble material in 6 nylon can be detcrmined by Soxhlet evtriaction for 16 hours in the case of "*-inch flake or 8 hours in the case of yarn. This procedure is based on the loss in weight of the sample and involves careful drying of Soxhlet thimbles and of the extracted sample. An independent sample moisture analysis is required to allon- results to be expressed on a dry sample basis. These steps are subject to considerable error and require about 24 hours. Furthermore, an estimate of monomer plus dimer (dimer, as used in this paper, is a mixture of cyclic dimer, trimer, etc.) and mter-soluble
linear molecules is obtained, whereas results for monomer alone are often desirable. This paper presents a vacuum extraction technique which is rapid, accurate, and specific for monomer in the presence of 6 nylon, dimer, and moisture. The extraction is carried out at 200" C., and the monomer is condensed in a cold trap, dissolved in water, and determined by refractive index measurement. Since the total sample Tveight loss represents monomer plus moisture, the latter is obtained by difference. APPARATUS
The glassware is constructed of I'yrev chemical glass No. 7740 and is illustrated in a schematic assembly in Figure 1. The sample tube is 180 mm. long overall and 25 nini. in outside diameter with a 3 5 / 2 5 socket joint. The glass bridge is constructed of tubing 20 mm. in outside diameter n ith 35/20 and 28,' 15 ball joints. The trap is 32 mm. in outside diameter and 255 mm. long over-all; outside diameters for the central tube, the side arm, and the nipple are 20, 11, and 15 mm., respectively, and the socket joints are 28/15 and 18/9. The bottom end of the central tube of the cold trap is displaced toward one wall to allow easier entry of lvater into the tube when dissolving the monomer and when shaking and mixing the solution. The KO. 28 and S o . 35 VOL. 29, NO. 5 , MAY 1957
825
pinch clamps (Catalog No. 3241, Arthur H. Thomas Co., Philadelphia, Pa.) have all possible excess metal removed to allow good contact between the glass joints and the special 115-volt, 180watt, Glas-Col mantles (Glas-Ccl Apparatus Go., Iuc., Terre Haute, Ind.). These mantles contain iron-constantan thermocouples to indicate the temperature of the glass bridges. The following numbers refer to the main heating unit (Figure 2). An aluminum block, 10 X 28 X 12 inches, 6, is insulated with 2 inches of Johns-Manville marine sheathing (Johns-Manville Co., Philadelphia, Pa.), and contains 16 holes, 1 inch in diameter and 5-% inches in depth. The block is heated by ten 800-watt Chromalox rods (Edwin L. Wiegand Go., Pittsburgh, Pa.) which are grouped hy pain into three base-heat and two controlheat circuits, 7; & Tagliabue Celectray indicating potentiometer controller, 8 (C. J. Tagliabue Corp., Newark, N. J.), connected to an iron-constantan thermocouple, which is placed in the middle of the block, controls the temperature of the block a t ZOOo =t5' C. On eich of the long sides of the block are mounted an 8-position vacuum manifold, 3, and a n electrical manifold containing eight 115-volt receptacles and eizht iron-constantan thermocouplelead reveptavlcs, 4. The receptncles and vacuum prAious .ire aligned niih the holes in the Idock. The 116w l r r( crpt*cles are divided into groups of four, each group being connected in parallel to one 115-volt, 7.5-ampere variable transformer, 10 (Catalog No. 9708-G, Arthur H. Thomas Co., Philadelphia, Pa.). The thermocouple receptacles are connected through a 16 position switching arrangement to a pyrometer, 2, made especially for heab ing mantles (Catalog No. 11-472-60, Fisher Scienti6c Co., Pittsburgh, Pa.). Sixteen l6ounce Dewar flasks serve as the cold trap containers, 5. Two Cenco Megavac pumps, 9 (Catalog No. 92015, Central Scientific Co., Chicago, Ill.), connected in parallel, maintain vacuum in the manifold, and the actual pressure in the manifold and the glass equipment ir determined by a 240-mm. Bennert closed-tube mercury manom-
Table I. Refractive Indices of Aqueous Monomeric r-Caprolactam Solutions (25.0' C.) no Gramsl5.00 MI. S o h
TO POWERSTAT-
H E A T E D BRIDGE
# 3 5 PINCH C L A M P
lf28
PINCH C L A M P T O VACUUM
INSULATION SAMPLE TUBE
EVACUATED TUBE
Figure 1.
Vacuum extraction apparatus assembly
Figure 2.
Extraction equipment
eter (Catalog No. 6365, Arthur H. Thomas Co., Philadelphia, Pa.) graduated to 0.1 mm., 1. Refractive index readings are made on a Bausch & Lomb Abbe-type m fractometer (Bausch & Lomb Optical Co., Rochester, N. Y . ) . The prism temperature is controlled a t 25.0' 0.1" C. by a water bath (Precision Seienti6c Co., Chicago, Ill.) containing a centrifugal pump and a cooling jacket through which flows 68' F. water; the bath temperature is controlled by a Magna-Set thermoregulator (Precision Thermometer & Instrument Co., Phila-
*
826
ANALYTICAL CHEMISTRY
HEATING M A N T L E
delphia, Pa.). The refr&mullmut;b II calibrated a t the lower end of the scale with atmosphere-equilibrated distilled water and a t the upper end with a standardglass. SAMPLING
All flake samples must be ground to pass an '/,-inch screen and all yarn samples must be free of finish oils which are volatile under the conditions of the analysis. A sample size of 10 to 15 grams is used for all flakes and yarns containing less than 5% monomer,
i'
p z
0
0
200oc.
240.C.
I I
I
I
2 3 4 HEATING TIME ( H O U R S )
Figure 3.
Vacuum extraction of
I
I
5
6
6 nylon flake
Ten-gram sample of '/B-inch flake, 5 mm. Hg pressure, 225' C. bridge temp.
-I
1.2v)
g 2
1.1-
1.0-
-a 0.90
p"
0.8-
W
0.7V
0.6-
t
-
YARN B
a
stopper the tube with the proper rubber stopper. Place the tube in a desiccator to cool. Examine the bridge carefully. The presence of condensed monomer in the bridge invalidates the analysis. Remove the top plug from the cold trap and add about 1 ml. of warm, distilled water with a medicine dropper to wet the monomer. Let the tube stand about 10 minutes and wash the remaining monomer into the calibrated nipple with distilled Kater. When all of the monomer is dissolved, add additional water until the volume of the solution in the nipple is exactly 5.00 ml. Replace the top plug, shake the trap to mix the solution thoroughly, remove the side plug, and pour the solution into a small beaker. Take three independent refractive index readings a t 25.0' f 0.1' C., approaching the end point from each side, and determine the average. I n a similar manner, determine the average refractive index of the distilled water; if this differs from 1.3323, correct all sample refractive indices to the 1.3323 basis. Convert the average sample refractive index to grams of monomer using a calibration plot or table relating grams of monomer per 5.00 ml. solution to refractive index. When the stoppered sample tube is cool, remove it from the desiccator and reweigh.
Calculations.
a
I
0
Figure 4.
I
0.5
I
I
I
I
I
I
2.0 1.0 1.5 HEATING TIME ( H O U R S )
I
I
2.5
I
3.0
Vacuum extraction of 6 nylon yarn
Ten-gram sample, 5 mm. Hg pressure, 200' C. sample temp., 225' C. bridge temp.
LThile 5-gram samples are employed for materials containing more than 5% monomer. PROCEDURE
Prepare a calibration curve or table relating grams of monomer per 5.00 ml. aqueous solution to refractive index. Typical data taken from such a linear plot are shown in Table I. Weigh a n empty, rubber-stoppered, dry sample tube on an analytical balance, place the proper amount of sample in the tube, restopper, and reweigh. Place a dry, cold trap in the wet-ice bath and connect the trap to the proper vacuum manifold position by the 18/9 joint, using a No. 18 pinch clamp. Connect a bridge, contained in the special Glas-Col mantle, to the sample tube using a No. 35 pinch clamp and place the tube in the proper hole in the block, swinging the 28/15 ball over the corresponding cold trap socket; quickly secure the bridge to the trap with a No. 28 pinch clamp. Fold the mantle flaps snugly around the joints and clamps.
Slowly open the vacuum connection on the manifold and allow the pressure to descend to less than 20 mm. of mercury. Connect the 115-volt and thermocouple plugs to the proper receptacles and turn the proper variable transformer to give a mantle temperature of 225" zk 15" c. Allow flake samples to heat for 2 hours, and yarn samples for 1 hour, a t a system pressure of less than 20 mm. of mercury, a bridge temperature of 225" += 15" C., and a block temperature of 200" =I= 5" C. Higher pressures indicate air entering the system, causing incomplete removal of monomer. Low bridge temperatures may cause condensation of monomer in the bridge. High bridge temperatures may cause polymerization of the monomer. At the end of the extraction period, close the vacuum valve, turn off the variable transformer, and disconnect the cold trap from the vacuum system. Disconnect the bridge from the cold trap and stopper both cold trap openings with plugs made from the appropriate ball joints. Disconnect the bridge from the sample tube and quickly
yo moisture
=
(A
- C - D )X
yomonomer (dry basis) D X 100 A
-B
100
A - B =
100
100
- Yo moisture
where A = weight of stoppered test tube plus sample, grams B = weight of stoppered test tube, grams C = weight of stoppered test tube plus dry extracted sample, grams D = weight of monomer extracted, grams RESULTS AND DISCUSSION
The relationship between refractive index (sodium D light) a t 25" C. and grams of monomer per 5 ml. of aqueous solution is linear. Typical data taken from such a plot are shown in Table I. Assuming that an Abbe refractometer reading is accurate and precise to fO.0001 unit, these data indicate that the amount of monomer in 5 nil. of aqueous solution can be determined to k0.003 gram or to *0.030/, absolute for a 10-gram polymer sample. A study was made of the effects of variations in flake particle size, sample temperature, bridge temperature, and system pressure upon the vacuum extraction of monomer and moisture from 6 nylon polymer. Experimental equipment was generally similar to that described under Apparatus, but differed VOL. 29, NO. 5 , MAY 1957
a
827
from it in these specific points: The connecting bridges and ball and socket joints were wrapped with flexible heating tape (prior to the design and fabrication of the special Glas-Col mantles); iron-constantan thermocouples connected to a potentiometer nere used to measure the temperature of the bridges; and a nianostat was included in the vacuum system to maintain copstant pressure throughout the extraction. Effect of Flake Particle Size. For a given set of experimental conditions, the extraction rate was inversely proportional t o the size of the flake particles. A nominal flake size of 1 8-inch was adopted as standard because of the possibility of smaller particles being sucked from the test tube into the bridge and cold trap when vacuum was applied to the system. Effect of Sample Tube Temperature. The sample temperature had to be chosen so t h a t all monomer present in t h e sample could be removed in a reasonable length of time with no appreciable addition or loss of monomer due to sample degradation or polymerization. A sample temperature of 200" C. was adopted because earlier, independent polymerization studies had indicated that the rates of monomer and polymer formation a t this temperature M ere negligible All the monomer could be extracted from *-inch flake in less than 2 hours (Figure 3) and from yarn in less than 1 hour (Figure 4); a t 200" C. the sample does not become molten, eliminating the safety hazard of the shattering of the glass sample tube as the sample cools to room temperature Effect of Bridge Temperature. The bridge temperature must be niaintained a t a level high enough t o pievent monomer condensation on the bridge walls and low enough to prevent polymerization. A series of vacuum evtractions was performed on several portions of one polymer sample and on each successive run the temperature of the bridge n as raised until consistent results n ere ohtained. Evperiniental conditions nere: 10-gram samples of s-inch flake. sample tube temperature of 200" C., system pressure of 5 mni. of mercury, and 2 hours' heating time. Table I1 illustrates that bridge temperatures in the range 210" to 225" C. are adequate and that the sample may be presumed to contain about 8.89% extractable monomer. Effect of System Pressure. Because variations in pressure may occur during the vacuum extraction process, it was necessary to determine the highest permissible system pressure consistent with rapid and quantitatire removal of the monomer from the sample. Ten-gram samples of '/g-inch flahe and sample and bridge temperatures of 200" and 225" C. respectively. nere used. hIonomer present was 8 89%. The 828
ANALYTICAL CHEMISTRY
z
s
I
t0
2t-
I
I
I
I
I
I
I
I
I
I 1 , f
II-
E
10-
5
98-
2 s* m
7-
-E
6-
fn
3
5-
-
4-
&0
3-
K
----- THEORETICAL EXPERIMENTAL -
I
$
0
I
I
I
I
I
I
I
I
I
I
I
I 2 3 4 5 6 7 8 9 1 0 1 1 1 2 I 3 1 4
7. WATER EXTRACTABLES (DRY BASIS)
Figure 5. Comparison of vacuum- and water-extraction methods
data in Table I11 indicate that, at 1 and 7 mm. of mercury pressure, complete extractions were obtained after 1.5 hours. while a t 20-mm. mercury pressure about 2 hours were required. Consequently, a 2-hour extraction period at a system pressure of 20 mm. of mercury, or less, was adopted for flake polymer samples. Pressures greater than 20 mm. were not investigated, since a t these pressures, the flow of
Table
II. Vacuum Extraction of 6 Nylon
Bridq Temp., C.
Monomer Recovered, 7;
132 168 210 225
8.50 8.70 8.89 8.89
Table 111.
Vacuum Extraction of Nylon Polymer
Pressure, LIm. Hg
-1
1 hr. 8 46
i
20
8 65
Table IV.
Heating Time 1.5 hr. 2 hr. 8 90 8.87 9.11 8 59
8.83
8.98
A. A series of four 6 nylon flake samples was analyzed by the vacuumextraction technique. The vacuumextracted residues mere subjected to extraction by water. The mater extract n-as analyzed for the amount of monomer solids present by evaporating the water, and heating a t 100" C. under full vacuum, a procedure lvhich removes any water or monomer present. The melting point of the residue was about 280' C. indicating that the residue was largely dimer ( 2 ) . The results, summarized in Table IV, showed that essentially all of the monomer \vas extracted by vacuum estraction.
Difference between Vacuum- and Water-Extraction Methods
Sample S o . 16-7E 16-8E 16-9E 16-10E
6
monomer vapor through the bridge to the cold trap is too slow. Comparison of Water- and VacuumExtraction Methods. Samples mere analyzed by water-extraction and vacuum-extraction methods. The data in Figure 5 clearly indicate that the vacuum-extraction method gives lower results than the water-extraction method. It was postulated that water extracts monomer, dimer, and Ivatersoluble linear molecules from 6 nylon polymer, xhile the vacuum-extraction method removes only the monomer. To determine the validity of the foregoing postulate, the following experiments were performed:
Original Vacuum Extraction, 2.40 4.15 15.67 11.90
yo Water Extractables in Vacuum-Extracted Residue Sample weight loss (dimer Recovered from monomer, if water extract present ) (dimer only)
+
5
1.74 2.02 2.21 1.59
1 79 1 84 2.15 1.42
B. Further proof that dimer is not extracted by the vacuum-extraction method was obtained by placing a known amount of purified dimer in a sample tube and vacuum-extracting it for 2 hours at 200' C. and a t 1 mm. of mercury pressure. This procedure did not remove any detectable amount of dimer from the sample tube, as determined by weight loss of the sample tube, and no condensed material was found in the cold trap. Because vacuum extraction will not extract dimer from polymer samples, n-hereas water extraction does, the difference in results obtained by the
two procedures on a given sample indicates the amount of dimer, plus any mater-soluble, linear molecules present in the sample.
and 0.4iyc moisture was found with standard deviations of 0.03 and 0.06Ycj respectively. LITERATURE CITED
PRECISION
A statistical study was carried out on 16 analyses on each of two flakes of different monomer and moisture contents. I n the higher range, a n average of i.99% monomer and 1.70% moisture was obtained with standard deviations of 0.26 and 0.25%, respectively. For a polymer of lorn monomer and moisture content, an average of 0.53% monomer
(1) Hanford,
W. E., Joyce, R . RI., J . Polymer Sci.3, 167 119.18). (2) Hymans, P. H., Rer. trav chini. 72, /98-812 (1953).
(3) Velden, P. F. van, Want, G. 11. van der. Heikens. D.. KI&sink. Ch. .I.. Hermans, P. H., Staverman, A . J.; Ibid., 74, 1376-94 (1955).
RECEIVED for revie% September 4, 1956. Accepted January 8, 1957.
Applications of Anion Exchange Resins to Determination of Boron JOHN D. WOLSZON' and JOHN
R.
HAYES
The Pennsylvania State University, Universify Park, Pa. WILLIAM H. HILL Graduate School o f Public Health, University of Pittsburgh, Pittsburgh, Pa.
,The behavior of boric acid and of several known interferences has been investigated using both weak and strong base anion exchange resins. A simple, rapid chromatographic technique employs chloride ion as an elution indicator, applicable to the concentration and separation of boron from dilute solutions of low total salt content. Boron is separated from most of the known titrimetric interferences by a mixed bed ion exchange column. The procedure is rapid, requires no special treatment of the sample prior to the exchange process, and compares favorably with existing techniques. However, it cannot be recommended for separation of boron prior to colorimetric determination.
S
rapid and accurate procedures are available for the deterniination of boron. but there is no single mcthod of suitable accuracy which can 11c applicd to a variety of sample types without the prior wparation of boron from its determinative interferences. Scyarativc techniques which have been employed include ion exchange, previpitation, extraction, distillation, paper chromatography, and electrolytic proEVERAL
Present address, Marshall College, Huntington, W. Va.
cedures. Of these, only the distillatioii of boric acid as the methyl ester and the ion exchange procedures have shown general applicability. However, the methylborate distillation technique is time-consuming and tedious, and gives slightly low boron recovery even when multiple distillations are carried out. The utilization of ion exchange resins for the separation of boron has been confined, with one exception (8),to the separation of boron from cationic interference by retaining the cations on a strong acid cation exchange resin while the boric acid and other anions pass into the effluent. Such procedures are not applicable to samples for titrimetric determination which contain reasonable concentrations of anionic interferences whether these are of the buffering or complexing nature. $1~0, cations forming anionic complexes might not be removed by the cation exchange resin. A strong acid efluent is obtained 1%hich, on neutralization, produces a high ionic strength medium and thereby decreases the accuracy of a titrimetric process involving the use of color indicators. STRONG BASE RESINS
I o n Exchange Chromatography. Two general methods use strong base resins t o separate boron from
its inteifeiences. The first of these renioyes all anions fiom t h e sample and quantitatively elutes them in ordei of their increasing affinity foi the iesin. T h e second uses t h e resin in a n anionic form such t h a t the boric acid from the sample solution is not held by the resin, n hik the, interferences are retained. Schutz ( 8 ) reports the use of strong base anion exchange r e m s for the quantitative separation of boron. He uses the resin Anibcrlite IRA400 in the formate form to separate boric and phosphoric acids in the determination of boron in fertilizers. The phosphoric acid is retained by the resin while the boric acid is mashed into the effluent by distilled water. Samples are of lo^ total salt content and relatively large resin bed capacity. Positive errors of 0.1 to 0.4 nig. of boron are obtained on 1-gram samples containing 25 to 40 mg. of boron. Prior to the above puhlication, similar separations had been attempted in this laboratory using the chloride and the nitrate forms of the same resin. The results obtained are accurate to about 1 0 . 2 nig. of boron, but serious contamination of the boron-containing effluent is noted. Less accurate results were obtained as the total foreign ion concentration increased. Preliminary studies of the behavior of boric acid and the common anionic VOL. 2 9 , NO. 5 , M A Y 1957
829
