Table V. Column Performance Index a s a Function of Sample Injection Volume and k'

I'LCT column Combination coliunn InJ. \Ol, __._____

,'1 Lo.:: '2' hIcd. b' Lov; $' hied. k '

0 . 0 1 1 1 . 2 1 6 . 5 1 3 . 2 1 0 . 8 0 . 0 4 1 0 . 1 1 3 . 6 9 . 2 1 0 . 6 0 . 1 6 . 3 5 . 4 9 . 2 9 . 1 0 . 1 5 . . . . . . 8 . 3 7 . 5

As shown in the last line of Table 111, even an injection volume of 0.15 pl reduces the plate number by less than 50% from the maximum obtained with a 0.01-pl injection. On a PLOT column, such an injection would extrapolate to a 90% reduction in plate number.

Column Per formance Index. The increased sample ca- pacity of the combination over plain PLOT columns can also be expressed in terms of the increased performance index, I , which takes into account separation times as well as plate number and partition factor. Table V shows that for injection volumes up to approximately 0.05 pl, perfor- mance values are slightly lower in the Combination column, but, in contrast to the PLOT column, decrease very little as the injection volume is further increased. This means that

for large injection volumes, the combination columns give better resolution per separation time. This better time-res- olution of the combination columns is illustrated in Figure 2.

CONCLUSION The sample capacity of PLOT columns can be signifi-

cantly increased by using a precolumn of 20- to 30-cm length and 0.061-in. i.d. containing a packing of 3 to 6% liq- uid phase.

LITERATURE CITED

(1) J. G. Nikelly. Anal. Chem., 45, 1264 (1973). (2) J. G. Nikelly, Anal. Chem., 46, 290 (1974). (3) L. Mlkkelsen, F. J. Debbrecht, and A. J . Martin, J. Gas Chromatogr., 4,

(4) A. 9. Christophe, J. Chromatogr. Sci., 8, 614 (1970). (5) Perkin-Elmer Corporation, Norwalk, Conn., Chromatogr. Newslett., 1,

(6) J. G. Nikelly, Anal. Chem., 44, 623 (1972). (7) Max Blumer, Anal. Chem., 45, 980 (1973). (8) J. G. Nikelly, Anal. Chem., 45, 2280 (1973). (9) J. G. Nikelly and Max Blumer, Amer. Lab., 5, 12 (1974).

263 (1966).

27 (1972).

(IO) C. Horvath, "The Practice of Gas Chromatography," L. S. Ettre and A.

RECEIVED for review May 6,1974. Accepted September 15, 1974.

Zlatkis, Ed., Wiley-lnterscience, New York, N.Y., 1967, p 150.

Turbidimetric Determination of Chlorine in Chlorobutyl and Other Chlorine Containing Polymers at Low Levels

J. 2. Falcon, J. L. Love, L. J. Gaeta, and A. G. Altenau The Firestone Tire and Rubber Company, Central Research Laboratories, Akron, Ohio 443 17

The synthesis of organic and inorganic polymers with halogen containing substituents has brought about the need for the determination of these substituents a t low lev- els.

Described here is a relatively quick and simple turbidi- metric method, compared to the long and tedious micro- Carius method, for determining very small amounts of chlorine in elastomeric polymers. The method involves the Schiiniger flask combustion of a fairly large sample with the chloride resulting being absorbed in dilute nitric acid solution. The spectrophotometric absorbance of the silver chloride suspension, after the addition of dilute silver ni- trate, compared to that of various standard chloride solu- tions determines the chlorine in the polymer.

This method is also applicable to phosphazene base poly- mers in that the determination of chloride is feasible even in the presence of phosphorus and fluorine.

The reproducibility of turbidimetric measurements? con- ditions for maximum and constant opalescence, and the ef- fect of diffuse daylight on the opalescence were studied ( I ). The principal source of inaccuracy and irreproducibility in the conventional t.urbidimetric method is the lack of rigid control over the conditions governing the formation of the suspension prior to measurement of absorbance (2 ).

EXPERIMENTAL Apparatus. The conventional 1- and 2-liter Schoniger type oxy-

gen combustion apparatus supplied by A. H. Thomas Co. were used.

The absorbance measurements were obtained using a Beckman Model DB-G grating spectrophotometer with a one-centimeter cell and a tungsten lamp as the energy source. Other absorbance values involving a range of wavelengths were made on a Beckman DK-2 ratio recording instrument.

Reagents. Standard aqueous potassium chloride solution was prepared in demineralized water to contain 1000 ppm chloride. Dilutions were made from this standard stock solution to those containing very low chloride levels.

To form the silver chloride suspensions, 0.01 M aqueous silver nitrate solution was made with demineralized water.

A solution of approximately 0.01M aqueous nitric acid solution which was 0.01M in potassium nitrate was also prepared.

Procedure. The preparation of the calibration curve involved the preparation of six standard chloride solutions to contain 0 to 4 ppm chloride. Acidified 0.01M silver nitrate was added to each so- lution to form a silver chloride suspension. All the solutions were swirled briefly and allowed to stand at least 35 minutes with occa- sional shaking. The absorbance of each of these solutions was then measured a t 420 nm in a 1-centimeter cell. A straight-line curve passing through the origin resulted covering chloride concentra- tions from 0 to 4 ppm. The ideal absorbance was found to be ap- proximately in the range of 0.03 to 0.2 absorbance unit,

It must be emphasized that during the preparation of standards, swirling each solution for a few seconds is sufficient. Prolonged ag- itation tends to agglomerate the silver chloride particles.

In the analysis of a polymeric material for chlorine, any one of three techniques may be applied depending upon the expected chlorine content. If it is suspected to be over I%, the micro-Carius method ( 3 , 4 ) or the conventional Schoniger combustion technique (5, 6 ) should be used; but if the amount of chlorine is expected to be below 1%, then the proposed turbidimetric-spectrophotometric method is recommended.

ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975 171

I ' 5 MIN. 1 0 : I ,

370 380 400 420 440 WAVELENGTH (mu)

Figure 1. Time effect on absorbance of turbidity

Generally, about 100 mg of a weighed sample are combusted in a 2-liter Schoniger flask containing 10 ml of O.OlM nitric acid. After the combustion, the flask is allowed to stand a t least 15 minutes with occasional shaking before 5 ml of 0.01M silver nitrate solu- tion are added.

If the polymeric sample is difficult to combust, the normal plati- num sample basket may be wrapped with 52-mesh platinum gauze to prevent hot and partially burned sample from dripping out of the sample basket. For subsequent combustions of this sample, a smaller sample size and the use of twice the normal volume of ni- tric acid absorbent are recommended.

The measurement of the absorbance of the silver chloride sus- pensions should be done within 35-60 minutes of turbidity devel- opment of both the sample and standard chloride solutions. The absorbances of the standard solutions are measured first, followed by the sample solutions; then the standard solutions are measured again.

The calculation of results is made in two different ways. For samples of low halogen level and if the combustion results in a clear solution, no removal of a solution aliquot is necessary, and the calculation is as follows:

x 100 (1) ppm chloride X 15 sample weight (mg) X 1000 % C l =

The value for ppm chloride is obtained from the calibration curve. If combustion of the sample is such that a clear solution is not

obtained and an organic film results in the combustion flask, an 8- ml aliquot is usually removed and only 4 ml of silver nitrate need be added to the clear aliquot. Here again, the ratio of sample solu- tion aliquot volume to that of silver nitrate is still 2:1, and the cal- culation for chloride is the same as Equation l .

If it becomes necessary to use a different aliquot than indicated above because of a very low chloride level, the calculation for chlo- ride is: % C1 = ppm chloride x

(volume of aliquot + volume of AgNO,) volume of aliquot 1

sample weight (mg) X 1000

I t should also be kept in mind that more than one determination is generally made to determine the optimum aliquot for the halo- gen present.

For samples of higher chloride content, volumes of absorbent can vary from 15 to 50 ml. This volume is designated V. Take a measured aliquot volume designated A . Add an appropriate mea- sured volume of water for dilution if needed which is called W, be- fore the addition of 5 ml of silver nitrate. These volumes are to- taled and designated TV. Then

10 x 100 (2)

(3 ) A + W + 5

A T V = V X

and

(4) v /c c 1 = ppm chloride X T V sample weight (mg) x 10

RESULTS AND DISCUSSION When the absorbance measurements are done within the

time limits mentioned above, the probability of reproduc-

~~

Table I. Chlorine Values us. Combustion Efficiency in Phosphazene Polymers

Completion of combustion, 95

Polymer 100 96 92

1 0.0008 0.0008 0.0008 2 0.0025 0.0024 0.0023 3 0.0040 0.0041 4 0.0083 0.0081 O.OO77 5 0.017 0.015 0.016 6 0.025 0.024 0.023

0.0038

ibility of the measurements is very good. Absorbances of standard solutions prepared on six different occasions within six weeks agreed very well, a t chloride concentra- tions up to 4 ppm.

Turbidity develops rapidly and becomes stable a t the 4- ppm chloride level within about 15 minutes. However, the silver chloride particles tend to agglomerate slowly upon standing for 1 hour or more. This generally leads to a lower absorbance than one would expect upon extrapolation of the absorbance values a t the lower chloride concentrations. Levels greater than 4 ppm should not be used because of agglomeration problems resulting from the unavoidable ag- itation during the transfer of the solutions to the absorb- ance cells. Therefore, a smaller sample size is advisable.

There is a small increase in absorbance when the stan- dard solutions stand for 1 hour from the time of initial preparation. This increase is very small and does not lead to any significant error in the analysis of the sample.

Turbidity development a t the 0.5-ppm level was also studied. I t is incompletely developed within 30 minutes. Stable turbidity is obtained after 1 hour with essentially no change in the absorbance between 1 and 2 hours,

The effect of time on the turbidity a t medium chloride levels (2 ppm) was studied by scanning spectrophotometri- cally from 440 to 370 nm (Figure 1). The absorbance is fair- ly constant within 30-60 minutes when the suspension is not agitated. This change in absoibance with wavelength is thought to be due to greater sensitivity of the radiation at higher frequency, to a greater number of different size sil- ver chloride particles (7-9).

The effect of color of the sample solution is least at 420 nm. Although the solutions resulting from sample combus- tions reported in this paper were not colored, color should always be considered and corrected for, as much as possible when it is present. A slightly yellow solution may result from the combustion of a hydrocarbon polymer vulcan- izate. This effect should be eliminated by the use of a smaller sample size. If this condition persists, the sample should be treated previous to combustion of free hydrocar- bon material which does not combust clearly and complete- ly.

It has been our experience that most fluorinated phos- phazene polymers do not appear to combust completely. But it has been found experimentally that apparent incom- plete combustion does not seem to significantly affect the chloride values obtained (Table I). However, it does cause some organic film to be formed on the inner walls of the combustion flask as well as on the surface of the absorbent. After 15-minute standing with occasional shaking, an ali- quot of the clear solution can be removed. The silver ni- trate is then added to this clear aliquot. If the precipitant were present in the absorbent during the combustion of the sample, low values for chloride could result due to silver chloride particles adhering to the film.

The data presented in Table I1 are indicative of the agreement between calculated and experimentally found

172 ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975

Table 11. Chlorine Levels of Butyl and Chlorobutyl Polymers and Mixtures

C1 concn, 96

Polymer Calcd Found

Butyl 0.000 0.000 C hlorobutyl 1.060 1.050 C1 - BU + BU 0.019 0.021 C1 - BU + BU 0.027 0.031 C1 - BU + BU 0.043 0.037 C1 - BU + RU 0.068 0.065 C1 -Bu + BU 0.170 0.160

values for chlorine. Although potassium chloride was cho- sen as the chloride source for purposes of calibration, or- ganic sources of chlorine were also considered. In order to obtain organic samples with low chlorine levels, chlorobutyl polymer samples were blended with butyl rubber, giving an expected range from approximately 0.02 to 0.2% chlorine. The halogen content of the chlorobutyl polymer was deter- mined by the conventional Schoniger technique. All the chlorine values for those compositions expected to be less than 0.2 YO were obtained by this turbidimetric method.

The phosphazene polymers investigated had a wide range of chlorine content depending upon their origin and subseauent treatment. There are no theoretical chlorine

Table 111. Chlorine Levels of Phosphazene Base Polymers

c1 concn, %

Polymer Calcd Found

A (Fluoro) ... 0.020 B (Fluoro) ... 2.830 C (Fluoro) 0.16 0.130 D (Non fluoro) ... 0.0005 E (Non fluoro) ... 0.0026 G (Non fluoro) ... 0.035 F (Non fluoro) ... 0.012

ratios. The value of 0.13% chlorine was obtained by the tur- bidimetric method.

Polymers D, E, F, and G are non-fluorinated phospha- zenes which combust completely and their halogen con- tents were determined turbidimetrically.

The applicability of this turbidimetric method is illus- trated by the wide range of chlorine values at such low lev- els obtained for these polymers as shown in Tables I1 and 111.

ACKNOWLEDGMENT Tne authors thank The Firestone Tire & Rubber Co. for

the opportunity to publish this work.

LITERATURE CITED values for t,hese polymers. However, Table I11 shows the experimentally obtained values for some fluorinated and non-fluorinated polymers of this base type.

Polymer A is a fluorinated phosphazene base polymer whose halogen Content was determined by analyzing 15-20 different samples using our turbidimetric method with a maximum deviation of about 10%. The micro-Carius method was used to determine the chlorine

(1) A. B. Lamb, P. w. Carleton, and w. B. Meldrum, J, Amer, (-he,,,. sot., (*) R. L. Coleman, w. D. Schults, M. T. Kelley, and J. A. Dean, Anal. Chem.,

(3) F. Pregl and J. Grant, "Quantitative Organic Microanalysis," 4th ed., ~h~

(4) ~ ~ i n " d e : ~ ~ m ~ n ~ , : ' ~ ~ ~ ~ ~ ~ ~ t h O d S of Analysis," 6th ed., D. Van Nostrand,

(5) /bid,, DD 329-331.

42, 251 (1920).

44. 1031 (1972).

Blakiston Company, Philadelphia, Pa., 1946, p 85.

content of Polymer B because of its relatively high halogen level, with a maximum deviation of about 5%. The mixture of these two polymers (designated Polymer c) was pre- pared to arrive at a composition with an intermediate chlo-

(6)

(7) p. V. wells, Chem, Revs,, 4, 331 (1926), (8) E. J. Meehan and G. Chiu, Anal. Chem., 36, 536 (1964). (') F. H. Firsching* Anal. Chem., 32, 1876 (1960).

M. Kolthoff and P. J. Elving, "Treatise on Analytical Chemistry," Part 11, VOl. 14. John Wiley and Sons, New York, N.Y., 1971, 1 pp 13-16,

rine value. The expected value (0.16%) was calculated from the separate analyses of Polymers A and B and their blend

RECEIVED for review December 26, 1973. Accepted August 26, 1974.

Aquametric Determination of Hydroxyl Groups with Trifluoroacetic Acid. Application to Polyesters

Claude A. Lucchesi, Barry Bernstein, and Ronald P. Bangasser

Department of Chemistry, Northwestern University, Evanston, 111. 6020 1

The determination of the hydroxyl content of organic chemical systems is important in many areas of chemistry and industry because it is either a measure of the concen- tration of substances containing hydroxyl groups or be- cause it relates to the performance properties of the chemi- cal system, as in polyesters. Most chemical methods for de- termining hydroxyl groups involve acylation with acid anhydrides or acid chlorides and differ mainly in the type and concentration of catalyst specified. The most widely used acylation reagent is acetic anhydride, but other anhy- drides, such as phthalic and pyromellitic, as well as acid chlorides, such as acetyl and 3,5-dinitrobenzoyl, are also in

use (1-3). These methods all involve differential titrations, i . e . , the difference between the titration of a hydrolyzed re- agent blank and a smaller sample titration is a measure of the hydroxyl content of the sample. Water and easily hy- drolyzed esters may interfere; and when acids or bases are present, an independent titration and correction must be made. In the method reported here, the water formed uia esterification with a large excess of trifluoroacetic acid (TFA) is determined with Karl Fischer reagent (KFR) and is equivalent to the hydroxyl content of the sample:

ROH + CFSCOOH + CF3COOR + HZO

ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975 173