Table V. Column Performance Index a s a Function of Sample Injection Volume and k' InJ.
I'LCT c o l u m n
\Ol,
,'1
0.01 0.04 0.1 0.15
Combination coliunn
__._____
Lo.::
'2'
hIcd. b'
11.2 10.1 6.3
16.5 13.6
...
...
5.4
Lov;
$'
13.2 9.2 9.2 8.3
for large injection volumes, the combination columns give better resolution per separation time. This better time-resolution of the combination columns is illustrated in Figure 2.
hied. k '
CONCLUSION
10.8 10.6
The sample capacity of PLOT columns can be significantly increased by using a precolumn of 20- to 30-cm length and 0.061-in. i.d. containing a packing of 3 to 6% liquid phase.
9.1 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 P e r f o r m a n c e Index. The increased sample capacity 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, performance 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
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, 263 (1966). (4) A. 9. Christophe, J. Chromatogr. Sci., 8, 614 (1970). (5) Perkin-Elmer Corporation, Norwalk, Conn., Chromatogr. Newslett., 1, 27 (1972). (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). (IO) C. Horvath, "The Practice of Gas Chromatography," L. S. Ettre and A. Zlatkis, Ed., Wiley-lnterscience, New York, N.Y., 1967, p 150.
RECEIVEDfor review May 6,1974. Accepted September 15, 1974.
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 levels. Described here is a relatively quick and simple turbidimetric method, compared to the long and tedious microCarius 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 nitrate, compared to that of various standard chloride solutions determines the chlorine in the polymer. This method is also applicable to phosphazene base polymers in that the determination of chloride is feasible even in the presence of phosphorus and fluorine. The reproducibility of turbidimetric measurements? conditions for maximum and constant opalescence, and the effect 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 oxygen 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 solution to form a silver chloride suspension. All the solutions were swirled briefly and allowed to stand at least 35 minutes with occasional 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 concentrations from 0 to 4 ppm. The ideal absorbance was found to be approximately 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 agitation 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.
A N A L Y T I C A L CHEMISTRY, VOL. 47, N O . 1, J A N U A R Y 1975
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~~
Table I. Chlorine Values us. Combustion Efficiency in Phosphazene Polymers Completion of combustion, 95
I
0 :
I
' 1 5 MIN.
,
370 380
420
400
Polymer
100
96
92
1 2 3 4
0.0008 0.0025 0.0040 0.0083 0.017 0.025
0.0008 0.0024 0.0041 0.0081 0.015 0.024
0.0008 0.0023 0.0038 O.OO77 0.016 0.023
5
440
6
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 solution are added. If the polymeric sample is difficult to combust, the normal platinum 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 nitric acid absorbent are recommended. The measurement of the absorbance of the silver chloride suspensions should be done within 35-60 minutes of turbidity development 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:
%Cl =
ppm chloride X 15 sample weight (mg)
X
1000
x 100
(1)
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 8ml aliquot is usually removed and only 4 ml of silver nitrate need be added to the clear aliquot. Here again, the ratio of sample solution aliquot volume to that of silver nitrate is still 2:1, and the calculation 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 chloride is:
% C1 = ppm chloride x
1
(volume of aliquot + volume of AgNO,) volume of aliquot sample weight (mg) X 1000 10 x 100 (2) I t should also be kept in mind that more than one determination is generally made to determine the optimum aliquot for the halogen 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 measured volume of water for dilution if needed which is called W , before the addition of 5 ml of silver nitrate. These volumes are totaled and designated TV. Then
T V = V X
A + W + 5
A
(3 )
and
v/c c 1 = ppm chloride X T V sample weight (mg) x 10
(4)
RESULTS AND DISCUSSION When the absorbance measurements are done within the time limits mentioned above, the probability of reproduc172
ibility of the measurements is very good. Absorbances of standard solutions prepared on six different occasions within six weeks agreed very well, at chloride concentrations up to 4 ppm. Turbidity develops rapidly and becomes stable at the 4ppm 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 at the lower chloride concentrations. Levels greater than 4 ppm should not be used because of agglomeration problems resulting from the unavoidable agitation during the transfer of the solutions to the absorbance cells. Therefore, a smaller sample size is advisable. There is a small increase in absorbance when the standard 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. It is incompletely developed within 30 minutes. Stable turbidity is obtained after 1hour with essentially no change in the absorbance between 1 and 2 hours, The effect of time on the turbidity at medium chloride levels (2 ppm) was studied by scanning spectrophotometrically from 440 to 370 nm (Figure 1).The absorbance is fairly 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 silver chloride particles (7-9). The effect of color of the sample solution is least at 420 nm. Although the solutions resulting from sample combustions 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 vulcanizate. 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 hydrocarbon material which does not combust clearly and completely. It has been our experience that most fluorinated phosphazene polymers do not appear to combust completely. But it has been found experimentally that apparent incomplete 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 aliquot of the clear solution can be removed. The silver nitrate 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
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
Table 111. Chlorine Levels of Phosphazene Base Polymers
Table 11. Chlorine Levels of Butyl a n d Chlorobutyl Polymers a n d Mixtures
c1 concn, %
C 1 concn, 96
Polymer
Butyl C hlorobutyl C1 - BU + BU C1 - BU + BU C1 - BU + BU C1 - BU + RU C1 -Bu + BU
Calcd
0.000 1.060 0.019 0.027 0.043 0.068 0.170
Found
0.000 1.050 0.021 0.031 0.037 0.065 0.160
values for chlorine. Although potassium chloride was chosen as the chloride source for purposes of calibration, organic 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 determined by the conventional Schoniger technique. All the chlorine values for those compositions expected to be less than 0.2 YOwere 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 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 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 prepared to arrive at a composition with an intermediate chlorine value. The expected value (0.16%)was calculated from the separate analyses of Polymers A and B and their blend
Polymer
Calcd
A (Fluoro) B (Fluoro) C (Fluoro) D (Non fluoro) E (Non fluoro) F (Non fluoro) G (Non fluoro)
... ... 0.16 ... ... ... ...
Found
0.020 2.830 0.130 0.0005
0.0026 0.012 0.035
ratios. The value of 0.13% chlorine was obtained by the turbidimetric method. Polymers D, E, F, and G are non-fluorinated phosphazenes which combust completely and their halogen contents were determined turbidimetrically. The applicability of this turbidimetric method is illustrated by the wide range of chlorine values at such low levels 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. (1) A. B. Lamb, P.
LITERATURE C I T E D w. Carleton, and w. B. Meldrum, J, Amer, (-he,,.
sot.,
42,251 (1920).
(*) R . L. Coleman, w.
D.Schults, M. T. Kelley, and J. A. Dean, Anal. Chem.,
44. 1031 (1972). (3)F. Pregl and J. Grant, "Quantitative Organic Microanalysis," 4th ed., ~h~ Blakiston Company, Philadelphia, Pa., 1946,p 85. (4)~ ~ i n " d e : ~ ~ m ~ n ~ , :of' Analysis," ~ ~ ~ ~ ~6th ~ ed., ~ t hD.OVan d SNostrand, (5)/bid,, DD 329-331. (6) 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, (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).
RECEIVEDfor 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 concentration of substances containing hydroxyl groups or because it relates to the performance properties of the chemical system, as in polyesters. Most chemical methods for determining 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 anhydrides, 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 reagent blank and a smaller sample titration is a measure of the hydroxyl content of the sample. Water and easily hydrolyzed 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
A N A L Y T I C A L CHEMISTRY, VOL. 47, NO. 1, J A N U A R Y 1975
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