photometer was used without an auxiliary polarizer. PRECISION
High precision in measuring absorbance is usually not necessary. I n fact, slight variations in film thickness often make high precision impossible. The measurement of dichroic ratio is especially sensitive to thickness variations, whereas sample rotation during
emic F. E. CRITCHFIELD and
measurement of the parallel-perpendicular ratio tends to minimize their effect. Two significant figures in the P.P.R.I. and dichroic ratio are sufficient for most work.
LITERATURE CITED
w.,
MCOCk, , T. c., T ~ Faraday SOC.41, 317 (1945). (2) Hemberg, G., “Molecular Spectra and Molecular Structure,” Vol. TI, p. 414, Van Nostrand, Princeton, N. J., (1) B
~ c. ~
1945.
(3) Krimm, S., ACKNOWLEDGMENT
The author thanks W. E. Davis for his derivation of the equations for the theoretical value of the P.P.R. index.
(1954).
J. Chem. Phys.
22, 567
for review 10, 1961. Accepted September 25, 1961. Division of halytical Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961. &X2EIVED
nalysis of P D. P. JOHNSON
Development Department, Technical Center, Union Carbide Chemicals Co., Soufh Charleston, W. Va. Application of chemical methods to alyses ob polymers is made difficult b y their limited solubility and chemical resistance. However, chemicat methods often can be used for determining trace concentrations OS impurities such as metals, monomer ratios of copolymers, trace concentrations of polymers, and end groups in polymers. The method selected for the preparation of the sample usually governs the success of the analysis for metals in polymers. Procedures used include dry and wet aohing and solution techniques. Chemical methods can be used to determine the monomer ratio of copolymers, if solubility difficulties can be overcome and the functionality being determined is not chemically resistant to the reagent. The copolymer of ethylene and ethyl acrylate can b e analyzed by a chemical method. The recent problem of determining trace concentrations of polymers is motivated by the increased use of polymers for packaging foods. The determination of low concentrations of polyethylene in liquid fats is one example of a method of this type. The sxcessful application of chemical methods to the determination of end groups in polymers is usually dependent upon the molecular weight of the polymer, the sensitivity of the method and the reactivity of the end group. End groups that have been determined include hydroxyl and epoxy. HE application of chemical methods to analytical problems associated with polymers is difficult, principally because of low solubility and chemical resistance. To analyze a polymer by chemical methods, the component or functionality being determined must be brought into intimate contact with the reagent. In contrast to more simple molecules, this is usually difficult.
4
o
ANALYTICAL CHEMISTRY
Depending upon the analysis being performed, contact with the reagent is brought about by ashing or extraction of the polymer or via solution techniques. Even if the polymer can be dissolved in a suitable solvent, it may have no functionality of sufficient reactivity to allow chemical analysis. I n spite of these difficulties, chemical methods can be used for the solution of the following types of problems: determination of trace concentrations of metals; determination of the monomer ratio of copolymers; determination of trace concentrations of polymers; and determination of the end groups of polymers. DETERMINATION OF TRACE METALS IN POLYMERS
Trace (low p.p.m.) Concentrations of metals in polymers may be detrimental to the stability of the polymer, affect its physical properties, or prohibit its use in food packaging applications. For any or all of these reasons, the determination of trace metals in polymers is a common problem. The success of any method for determining metals in polymers is usually governed by the procedure selected for the preparation of the sample prior to the actual determination, Methods commonly used include dry and wet ashing, extraction, and solution techniques. Extraction of metals is usually not satisfactory because of the inability of the extractant to penetrate the polymer completely. Solution techniques are valuable when they can be applied, because of their simplicity. However, satisfactory solvents cannot always be found and, even if they can be, interferences may obviate the technique. In addition, the metal may be in a form that is insoluble in the solvent for the polymer.
Because of the limitations of the above techniques, wet- or dry-ashing procedures are generalIy used. Each of these techniques has its advocates and, therefore, considerable controversy has existed about their relative merits. Recently, a n excellent comparison of the procedures was reported, in which the various sources of errors were ascertained by radioactive tracers (8). A summary of this study is presented in Table I. Wet-ashing techniques can be used for all of the elements studied. Only with mercury is recovery Ion-, the loss being due to volatilization. Although losses are negligible in wet-ashing techniques, the procedures cannot always be used because of trace metal contaminants in the large quantities o f reagents used for the oxidation, For this reason, dry-ashing procedures are advocated by many workers. Besides being lengthy, the major objections to dry ashing are losses due to the volatility of certain metals and to fusion of the metal with materials in the crucibles. Each of theqe losses can be inhibited b y the use of sulfuric acid as an ashing acid; however, as shown in Table I, serious losses can occur even under these conditions. Mercury and selenium are particularly bad in this respect and cannot be determined readily. Losses due to volatility can also occur with other elements, particularly lead, zinc, and iron, in the presence of high concentrations of chloride ions or organically bound chlorine as in poly(viny1 chloride) polymers. Even when sulfuric acid is present, care must be taken t o prevent this loss by maintaining the temperature of the oxidation below 500” C. I n summary, wet-ashing techniques are relatively free from losses, but large amounts of metals in the reagents may produce excessively high blanks. Low
~
~
~
blanks are obtained by dry-ashing tcchniques, but losses of volatile metals can be serious. Therefore, neither of the techniques is a panacea, and the optimum technique must be selected to solve the particular problem a t hand. DETERMINATION OF MONOMER RATIO OF CO POLYMCRS
The ratio of monomers present in copolymers has a distinct bearing upon the properties of these polymers. Often careful control of this factor is essential t o the production of a satisfactory copolymer for a particular end we. Frequently chemical methods can be used for determining this ratio; however, the application of such procedures is not always simple. Eiemental analyses frequently can be applied with a minimum of difficulty, because these procedures usually involve destruction of the polymer and solubility problems are not encountered. Examples of such methods include: the analysis of vinyl rhloride copolymers via Parr bomb chloride determinations and the determination of the monomer ratio of acrylonitrile copolymers by a Xj eldahl nitrogen determination. When attempts are made to apply chemical methods other than elemental analyses to problems of this type, solubility difficulties are frequently overwhelming. Work in our laboratories on the analysis of copolymers of ethylene and ethyl acrylate illustrate horn difficult the solubility problems can be (3). Union Carbide recently introduced the ethylene-ethyl acrylate copolymers commercially. The physical characteristics of these polymers are roughly similar to polyethylene, because of the lorn concrntration of ethyl acrylate present in the polymer. During the development of the materials, R saponification method was required to determine how much ethyl acrylate was being introduced into the polymer structure. The qoiubility characteristics of the ethylene-thy1 acrylate copolymerq are similar to those of polyethylene itself. Therefore, suitable solvents are restricted to hydrocarbons, particularly the aromatic hydrocarbons. Of course, sodium or potassium hydroxide required for saponification is immiscible with such solvents. Attempts to saponify the ester functions in a two-phase system consisting of potassium hydroxide in triethylene glycol and polymer in a hydrocarbon were unsuccessful because of inadequate contact. Attempts to saponify the ester directly in refluxing MOH in triethylenp glycol were also unsuccessful, for the same reason. These difficulties indicated that complete solution of both the polymer and the base would be required.
After considerable investigation, mostly by trial and error, the following solvent system for the KOH and polymer was successfully used: W~ter Triethylene glycol 1-Hexanol Xylene
Vol. % 2 13 30 55
Using this solvent system, potassiuni hydroxide is sufficiently soluble so that a 0.5N solution can be prepared. At refluxing temperature the copolymer is soluble and quantitative saponification can be effected in one hour. Some reproducibility data obtained by the method are listed in the following table: Combined Ethyl Acrylate, Wt. % in Copolymer
Sample
15.66 10.20 2.97
1 2 3
15.53 10,50 2.90
This saponification technique was later used to calibrate a simpler infrared method. This is a good esample of how chemical techniques can be used to calibrate physical methods which are usually simpler to apply but often difficult to calibrate. DETERMINATION OF TRACE CONCENTRATIONS OF POLYMERS
The determination of trace concentrations of polymers is motivated by the increased use of polymers in contact with food. Frequently it is necessary t o determine how much polymer gets into food. I n many cases the direct determination of the polymer is practically impossible, and migration studies in simulated food solvents are carried out, so that predictions can be made as to the level that mould mist in food. In conjunction with the use of polyethylene as a food packaging material we were concerned with the determination of the amount of polyethylene that would migrate from film into vegetable oil.
Table I.
Element Antimony Arsenic Cadmium Cobalt Copper Chromium
This problem was solved by a turbidimetric method (4). The oil extract of the film is treated with a mixture of hexane, ethanol, and 2-propanol, which causes a portion of the dissolved polyethylene to precipitate. The turbidity of the sclution is then measured and referred to a calibration curve to dctermine the concentration of polyrthylene dissolved in the oil. The calibration curve is preparrd by extracting the film with n-hexane a t various temperatures. Portions of these extracts are evaporated to determine the concentration of dissolved polyethylene. Another portion is added to yegetable oil and precipitated with the alcohol mixture, and the turbidity is determined. Since only the higher molecular weight species of the dissolved polyethylene are precipitated by the alcohol, the amount of nonprecipitating polymer must be the same in the standards and the samples. This factor is controlled by using the same filmsolvent ratio in the standards and the sample and by preparing the standards a t various temperatures instead of using aliquots of a hexane solution obtained a t a single temperature. If the latter approach is used, the amount of nonprecipitating polymer varies with the size aliquot used. The method was used succesefully to determine the migration of polyethylene into vegetable oil, and from this, some idea could be obtained of the extant of the migration of the polymer into fatty foods. Although the determination of trace concentrations of polymers is relatively neiT, increased emphasis will be placed upon such determinations, as the use of polymers for food packaging increases. DETERMINATION OF END GROUPS IN POLYMERS
Frequently a need arises to determine the end groups in polymers, to determine the molecular weight, or to determine the nature of these end
Comparison of Ashing Techniques for Heavy Metals
Ashing Technique Dry 94% recovery with H2SOp OK with H2S04 recovery with H2SQaa
Wet OK OK OK
OK OK OK with H2S01 OK OK OK Iron OK with HBOab OK Lead OK with H2SOlb OK, coprecipitation can occur 95% recovery with HNO~-HISOP Mercury No gooda Selenium No gooda OK with HNOs-H~SO~-HC104 OK with H&Q, OK Silver Zinc OW OK a Losses via volatilization. b Losses can occur with large concentrations of chloride, although HaSol inhibits losses.
VOL. 33, NO. 13, DECEMBER 1961
0
1835
group@. Chemical methods are usually limited to polymers of low molecular
weight determinations of LO-gram samples of oxyethylene compounds is shown in the following table:
ly is the conceptration of the
sing the sample size to overcome problem. The determination of
Approx. No. Av. Mol. Wt. 2 , 000
8,000 13,000
17,000
30,000
HO-[CBaCHzOCH&-] "OK
Net Titer, Ml, 20 6.0 3.1 2.4 1.3
e,
7%
0.3 1.1 2 2
2.5
3 1
reagent. Therefore, the reaction proceeded rapidly and no difficulty \vas experienced in determining the unreacted hydrochloric acid. A technique such as this could conceivably be applied to other problems dealing with insoluble polymers; however, sample size would no doubt be limited because of the high viscosity of the swollen polymer-solvent mixture. I n 8ome cmes the methods cannot be
These data show that as the molecular weight increases the
since this causes so For practical purposes this technique is of little value above a molecular
reactivity of the functional group in the polymer.. LITERATURE C l E D
tion of approximately 10 ml, is desirable
net titration as low
A studv of the nrecisi
prevent excessive errors. One unique solution to solubility problems was recently reported for determining unreacted epoxy groups sins (1). These resins were e m the NCl-diox cleave the oxirane nely divided polymer swelling occurred and intimate contact was obtained between the sample and the END
(1) Dannenburg, H., Harp, W. R., Jr., ANAL.CHEM.28,236 (1956). (2) Gorsuch, T. T., Analyst 135,84(1959). (3) Johnson, D. P., unpublished data,
Union Carbide Chemicals C Charleston, W. Va. ( 4 ) Johnson, D. P., Critchfield, F. E., J.Agr. Food Chew., in press.
RECEIVEDfor review July 10, 1961. Accepted September 13, 1981. Division of Analytical Chemistry, 139th Meeting, ACS, 5t. Louis, Mo., March 1961.
OF SYMPOSIUM
ity of Condense
b Condensed 14-labeled palmitic a down b y the bangmuir-B on gold surfaces which had previously been treated electrolytically in on oxidizing or reducing environment. The stabilities of the resulting monolayers were determined b y measuring double layer capacities as a function of time and by radiochem terminations. The most stab1 A disruption of the surface, either by forming or removing a monolayer of gold oxide caused the adsorbed layer to become less stable.
connection with a study of the anodic oxidation of gold surfaces (7),i t was desired to compare the stabilities of condensed monolayers of palmitic acid on oxidized and reduced gold N
1836
0
ANALYTICAL CHEMISTRY
this purpose, a slightly uir-Blodgett (& 10) electrode through a solution-air interface at which a condensed monolayer of carbon-14-labeled palmitic acid was present. By suitable application of a controlled potential to the gold electrode prior to its withdrawal, its surface could conbe placed in a reduced or o electrode with rbed lmitic acid could then be r either oxidizing or reducns, and the double layer observed as a func. While the double layer e regarded as a n abthe amount of palsurface, i t is a conndestructive method of following surface changes as a function of conclusion of a n experiunt of adsorbed palmitic
acid could be determined by dissolving it from the surface and using scintillation counting to determine carbon-14. EXPERIMENTAL
Prior Electrolyte Treatment of Surrevious studies on the electrolytic oxidat old in aqueous hloric acid (6, as kno potential of 1.55 volts (us. h electrode) in 0.01M HCIOl or 1.65 volts in 1M HC104, a sur responding approximat ld(II1) oxide could be put face by anodic treatment. A t higher potentials, a dee oxidation to form a massive 1 Au208 occurs. The
1
Present address, The Dow Chemical
Co., Midland, Mioh.
