Table 11. Precision Data
Sample no.
Solvents
i
2-Meth ylpentane
3-Methylpentane n-Hexane Methylcyclopentane Cyclohexane
1
2
XI
x2
11 40 23 47 42
12 41 26 49 47
CY and Ao, even if the experimental conditions such as heating temperature, time, and injection time are altered. Evaluation of the Method. To evaluate the precision of this periodic introduction method, duplicate measurements were carried out with two kinds of adhesive tape prepared from solutions of hexane isomers and cyclohexane, respectively. The measurements were performed at 70 "C of vaporizing temperature. The data presented in Table I1 indicate that the residual solvents ranging in content from 10 to 50 ppm can be determined within 8.3 % of relative standard deviation. On considering the difficulties of preparing adhesive tape with constant level of the residual solvent, the relatively large deviations are rather satisfactory for the determination in this field. Since no alternative absolute method to estimate the con-
Content found, ppm xa
x4
Av(n)
13 44
12 42
30 46 43
42 42
12 42 27 46 44
30
Re1 std dev, % 7.5 4.7 8.3
6.0 5.2
tent of the occluded solvents in polymers is available, accuracy of this method cannot be discussed. However, the obtained quantitative data can reasonably be considered to be close to the true value, judging from the principle implicit in this method. Though this study was carried out using the solvents with relatively low polarity, further work on the other polar solvents system is currently in progress. This periodic introduction method is also applicable to the investigation of the residual solvent in plastics and organic coatings by modification of the sample holder. RECENED for review June 24, 1970. Accepted August 24, 1970.
Automatic Data Recording of Unsaturation uantitative yd rogenation J. J. Szakasits Houston Research Laborafory, Shell Oil Company, P. 0. Box 100, Deer Park, Texas 77536 The measurement of titrant uptake during hydrogenation of olefinic materials using the Brown catalytic hydrogenation technique has been automated. A highly sensitive pressure transducer with linear output (0-10 volts) in the range of interest is coupled to the buret of the analytical hydrogenator. Hydrostatic pressure in the buret i s converted to a corresponding electrical signal through the pressure transducer, and displayed on a chart recorder. Usefulness of this apparatus i s eminent when olefinic materials possessing differing hydrogenation rates are analyzed routinely. Other advantages include the capability of detecting small amounts of easy-to-hydrogenate olefinic components, a reduction in operator time, and the attainment of a permanent record. The apparatus is sturdy and easily constructed.
A QUANTITATIVE HYDROGENATION METHOD has been successfully adapted ( I ) to determine the unsaturation in gas oils and gasolines with relatively high sulfur content (up to 4 wt This technique is based on the method developed by Brown et al. (Z), and involves the generation of a highly active platinum catalyst on activated carbon in a specially designed hydrogenation apparatus. The Brown analytical hydrogenator is automatic in the sense that the generation of hydrogen takes place at a rate regulated by a mercury valve
z).
(1) J. L. S. Curtis and M. 0.Baker, ANAL.CHEM., 42,278 (1970). (2) C. A. Brown, S. C . Sethi, and H. D. Brown, ibid., 39,823 (1967). 1708
*
which controls the flow of sodium borohydride (NaBH4) to the hydrogenator flask as a function of the internal pressure of the system. The hydrogenator flask pressure is limited by a mercury bubbler permitting the titration to proceed at near atmospheric pressure. The quantity (milliliters) of titrant of known molarity consumed during hydrogenation is read at predetermined time intervals during hydrogenation, and a curve based on these point readings is plotted. An instrument has been developed which automatically records on a chart recorder and simultaneously displays on a digital panel meter the milliliters of titrant consumed during hydrogenation. The hydrogenation process of materials is unchanged from the techniques described in References I and 2. The value of this apparatus is most apparent, when materials possessing differing hydrogenation rates are titrated routinely (10-20 minutes titration). Also, with this apparatus, small amounts of easy-to-hydrogenate components in a sample can be measured. This method requires less operator time and gives a uniform and permanent record. EXPERTMENTAL
In order to be able to measure and record automatically the milliliters of titrant consumed by the analytical hydroanalyzer upon the injection of a quantity of sample, the liquid pressure in the buret is converted to a corresponding electrical signal by means of a highly accurate pressure transducer with linear output in the range of interest. The pressure ( P )
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
REFERENCE REFILL TRANSDUCER
I TEMP BATH A N D STIRRER
fl HYDROANALYZER
Figure 3. Functional block diagram
Figure 1. Automatic hydroanalyzer apparatus produced in the buret at the pressure transducer and the liquid height (h) are related as shown by the following expression, P = hgp, where g is the gravitational constant and p is the density of the liquid in question. It is evident from this relationship that the pressure and height are directly proportional, and the pressure directly relates to volume since g and p are constants in the expression and the buret has a constant cross-sectional area. Apparatus. The apparatus as depicted in Figure 1 consists of an analytical hydrogenator mounted on the side of a cabinet which contains the pressure transducer, its electronics, a digital voltmeter (DVM), and the chart recorder. The analytical hydrogenator is made up of a buret with a refill line, an automatic mercury valve, flask, and mercury bubbler. The buret was modified by the addition of a pressure transducer line connection. Also, the automatic mercury valve was altered to fit the mounting bracket by remov-
ing the existing bubbler line and installing a new line as shown in Figure 2. Both the pressure transducer and the bubbler are connected to the buret by ball joint connections. An easily removable bracket is used to hold the hydrogenator which ensures accurate positioning and makes possible the quick removal of the hydroanalyzer from the cabinet for cleaning. It is important that the buret be returned to its initial position to retain the calibration settings. The pressure transducer (Validyne Model D15) and its electronics, the digital voltmeter, and the chart recorder are connected as shown in the functional diagram, Figure 3. The pressure transducer is constructed of stainless steel with a flat diaphragm of magnetic stainless (range 0-0.5 psi) clamped between two case halves, each of which contains a sensing coil covered with a nonmagnetic stainless layer. The pressure cavity, therefore, is completely stainless steel. A small vent hole with screw cap facilitates the complete filling of the pressure cavity if desired. The pressure transducer is installed in such a manner that the buret pressure acts on one side, while a reference pressure is applied on the opposite side of the membrane in order to obtain a zero reading with a completely filled buret. Silicone oil (the type used in fore pumps) is used in the reference pressure tube. The electronics of the transducer (Validyne Model CD15) were not altered in circuitry; however, they were removed from the case and modified to mount on the panel. The necessary controls for power, zero, span, and auxiliary output for checking the pressure transducer during runs with an auxiliary recorder also were added, as shown in Figure 1.
I RE
n
E
---------3 ern
Figure 2. Hydrogenation valve and buret modifications PRESSURE TRANSDUCER L I N E ( 1 2 / 2 B A L L JOINT N E W BUBBLER LINE ( 1 2 / 2 BALL J O I N T )
Hg V A L V E
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
BURET
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2
4 6 T I M E (MINUTES)
Figure 4. Hydrogenation curves for (A) 1 ml of 1 M 1-hexene in diglyme and ( B ) 1 ml of 1.001M 1,3-pentadiene in diglyme titrant 0.102MNaBH4
I
,
2
4
I
I
6
8
,
'
1
10
12
8
1
8
14
TIME (MINUTES]
A Weston 0-1.99 volt single-range digital panel meter is used rather than a moving pointer type for quick and easy reading of the milliliters of titrant consumed, which is displayed in 0.1-milliliter increments. However, the final result is obtained from the chart recorder, where a continuous line is produced which faithfully represents the NaBH4 uptake during hydrogenation. Calibration. The accuracy of determinations will depend largely on the precision of the calibration of the apparatus. Each recording device, DVM and chart recorder, has to be adjusted for zero position and span in such a manner that the output will be a major divisionimilliliter of titrant. This is desirable but not mandatory, as any other reproducible increment can be read, or one could use a plain chart paper and measure with a ruler. The calibration of the instrument is carried out in the following manner: Power is turned on to allow the components to warm up for a period of approximately 30 minutes to ensure proper operation of the DVM. Next, the DVM and the chart recorder have to be zero adjusted with no input. The digital panel meter zero adjustment is located behind the front bezel. The adjustment is made with shorted inputs, by turning the potentiometer located in the far left corner of the meter. The recorder zero is adjusted by turn-
Figure 6. Hydrogenation curves for an olefinic pyrolysis gasoline ( A ) and partially hydrogenated portion of the same gasoline ( B ) ,obtained manually Sample size 0.5 ml, titrant 0.102M NaBH4
ing the input voltage selector switch to the off position and setting the zero adjust. For normal operation the input selector switch should be at the five-volt position (which gives a 0-10 cc range). Upon completing the outlined steps, the 10-ml buret should be filled wth the titrant. The proper liquid level is adjusted with the valve located behind the bubbler, by setting the meniscus to the buret's zero mark. With a full buret, the DVM and chart recorder should indicate zero. If another value is read, the transducer electronics zero on the front panel has to be adjusted to obtain the zero reading. By slowly emptying the buret through the drain valve at the rear (Figure 3), the DVM and chart recorder should simultaneously indicate the volume of titrant drained from the buret, Any reading other than the ten major divisions on the chart when the 10 milliliters of titrant are drained has to be corrected by the span adjust on the front panel located adjacent to the zero adjust.
b -
5 -
5
Figure 5. Hydrogenation curve for 1 ml of 1.0047M indene 5 in diglyme sample E Size 1.0 ml, titrant 0.102M NaBH4
VI
z-
A . 4
i-
4 -
3
2 SAMPLE SIZE 1.0ml T I T R A N T 0.102M NaBH,
1
I 2
4
6
8
10
12
1lME lMlNUTESl
1710
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
14
16
18
20
It was noted during testing and in use that a complete calibration of the apparatus as outlined here is needed only when the instrument is assembled or major repairs or alterations are made. In routine use, only the zero and span adjusts have to be manipulated in order to keep the instrument in proper calibration. The preparation of the catalyst, titrant, constant temperature bath, and other factors closely affecting the performance of the analytical hydrogenator is carried out in the same manner as previously reported (1, 2). Operational parameters have not been altered in any form, nor has the manner of data interpretation-only its speed and accuracy.
9
8
7
6
5
RESULTS AND DISCUSSION
Synthetic blends were made up to test accuracy and the general behavior of the apparatus. Some typical results are shown in Figures 4 and 5. In Figure 4, the hydrogenation of 1 M I-hexene and 1.001M 1,3-pentadiene in diglyme are shown. The sample size charged was 1 rnl for each of the test blends. These samples titrate readily, as indicated by the sharp rising curves which reach a relatively flat plateau in -15 sec for 1-hexene and -30 sec for 1,3-pentadiene, indicating complete hydrogenation of the sample. In contrast to these simply titratable samples are the types such as indene, shown in Figure 5 . The results obtained with samples having titration characteristics shown in Figures 5 , 6, and 7 demonstrate the usefullness of recording the titrant uptake. Indene, like hexene and pentadiene, has a relatively sharp rise but, after a point of inflection, the curve continues to rise steadily. The sloping portion of the curve is attributed to the slow hydrogenation of the aromatic ring. A comparison of manually and automatically obtained data curves is shown in Figures 6 and 7. The samples are an olefinic pyrolysis gasoline ( A ) and a partially hydrogenated portion of the same gasoline (B). Figure 6 shows the curves plotted from data points taken initially at one-minute intervals, starting from injection of the sample and continued at five-minute intervals after five minutes. The sample size was 0.5 ml. The calculated hydrogen values in mmoles/ml were found to be 4.6 for feed and 0.3 for the product. Figure 7 shows the chart trace obtained with the automatic hydroanalyzer (for the same sample as in Figure 6) from which hydrogen values were calculated, 5.3 and 0.30 mmoles/ml for feed and product, respectively. The sample charged was
4
3
2
SAMPLE S I Z E 0.5ml T I T R A N T 0.102 M NaBH,
1
0
1 2
,
1 4
,
1
~
1
6 8 TIME IMlNUTES''
1
IO
1
, 12
1 14
Figure 7. Hydrogenation curves for an olefinic pyrolysis gasoline ( A ) and partially hydrogenated portion of the same gasoline (B), obtained with the automatic data recording apparatus Sample size 0.5 ml titrant 0.102144NaBHl
0.5 ml. The difference in olefinic content of curves A and B is well indicated in both sets of data. However, the continuous recording shows a small sharp rise (in curve B) and a well-defined point of inflection which is lost in a manual interpretation. The capability of this method of determining small amounts of easy-to-hydrogenate portions in a sample is most significant in certain industrial applications as it was in the examples shown in Figures 6 and 7. The automatic data recording apparatus allows the analyst to select different amplification levels to magnify a particular portion or the whole titration curve for easier data interpretation.
RECEIVED for review June 15. Accepted August 3, 1970.
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