INSTRUMENTATION by Ralph H. Müller
Increasing Simplicity Plus Automatic Recording w i t h N o Loss in Speed or Precision Distinguishes N e w Line of Refractometers T^1 OR MANY YEARS we have been re-*- porting periodically on new developments in refractometry, a task which is becoming more and more difficult in these days. Some of the earlier differential refractometers were very elaborate optical instruments further complicated by servo-systems and as-
Figure 1 .
Digital refractometer (Model 107)
sociated equipment. By achieving high sensitivity combined with small sample holdup they more than justified the complicated instrumentation. What has struck us as particularly significant in the more recent developments is the increasing simplicity of such instruments with no significant loss in speed, precision, or convenience in operation. At the Houston Symposium on Instrumental Methods of Analysis sponsored by the Instrument Society of America, wè were particularly impressed by the recording refractometers made by Waters Associates of 45 Franklin St., Framingham, Mass. They manufacture several refractometers for a variety of applications. Although we have compiled a complete bibliography of all references on recording refractometers and thought we were thoroughly acquainted with the subject, we were not quite prepared to see a high precision differential refractometer
Figure 2. Schematic of digital refractometer (Model 107)
with automatic temperature control all tucked away inside a standard recording potentiometer! This and other instruments were demonstrated and explained to us by L. E. Maley of Waters Associates. Through the courtesy of Maley we describe and illustrate some of these developments. One of the simplest of their offerings is the Model 107 diy: ;al refractometer, a view of which is shown in Figure 1. Figure 2 is a schematic diagram from which it can be seen that a light beam is directed through a slit, mask, and lens to a sample cell. The beam passes through the sample liquid and the reference standard to a mirror mounted behind the sample cell. The mirror reflects the beam back through the cell to a beam splitting mirror. Each half of the beam then falls on a cadmium sulfide photocell. When equal amounts of light fall on each photocell, the null meter reads zero. If the light beams
Figure 3. Schematic of differential laboratory recording refractometer VOL. 3 3 , N O . 8, JULY 1961
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INSTRUMENTATION
Figure 4.
Refractometer optical bench
are not equal, the meter deflects correspondingly to the right or left. After the reference cell has been filled with the a p p r o p r i a t e standard liquid a n d the i n s t r u m e n t set for the desired range, a test sample is introduced into the sample cell with a hypodermie syringe. A n y unbalance between s t a n d a r d and sample, as indicated by the null meter, is corrected by turning the control k n o b on the righthand side of the instrument case until the meter indicates zero. This knob rotates the glass beam deflector and restores the beams to balance. T h e digital readout geared to this balancing system -indicates the difference between sample a n d s t a n d a r d within an accuracy of one out of 1000 units. (As most readers know, a plane parallel glass plate, either v e r y thin, or quite thick, when placed in a light beam will cause a lateral shift in the light beam when it is r o t a t e d a b o u t its axis. This principle is widely used in self-balancing refractometers.) Some of the specifications quoted for this instrument are : Sensitivity ± 0 . 0 3 % of full scale; accuracy ± 0 . 1 % of full scale, cell volume, 0.75 c c , materials in contact with sample—Teflon, Pyrex, stainless steel. Although operable in one minute, the w a r m - u p time for full accuracy is less t h a n 10 minutes and the zero drift is less t h a n 0 . 1 % in 24 hours. T h e cell t e m p e r a t u r e is slightly above ambient and for the rated accuracy no temperature control is necessary. T h e instrum e n t can b e set such t h a t 1000 units on the digital readout is equivalent to a refractive index range as narrow as 0.0005 R . I . or as wide as 0.2 R . I . Fixed factory calibrated range and accessories to obtain any range between 0.0005 and 0.2 R . I . are available. 86 A
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Figure 5.
Laboratory Recording Refractometer I n the W a t e r s Associates new laboratory recording refractometer, the schematic of which is shown in Figure 3, the optical arrangement is practically the same. Here the amplifier drives the recorder motor and hence t h e glass beam deflector in a direction tending t o restore photoelectric balance and simultaneously moves the pen across the chart. Because of the small size of t h e light source and the photocells, it is possible to mount' these three units plus the beam splitter mirror in one small aluminum block 1 inch high, 2 inches wide, a n d 2 inches deep. T h e source and detector block, as well as the sample and reference cell and the zero and null balancing glasses, are firmly attached to a solid aluminum b a r 1 inch thick, 3 inches wide, and 9 inches long. T h e solid aluminum b a r serves as the optical bench and prevents any physical misalignment of the optical system. T h e heat exchanger is a solid aluminum block attached to the b o t t o m of the optical bench. T h e liquid sample passes through this block before it reaches the sample cell. Figure 4 shows the refractometer assembly. A choice of reference cells enables one to govern the range and sensitivity. T h e sensitivity of the refractometer increases with an increase in the angle at the vertex of the reference cell. If the instrument is set u p for a wide range, a narrow angle is used. For narrow ranges and high sensitivity, a 90° angle is used. T h e thickness of the null balancing glass plate also enters into this choice. T h u s for a full scale range of 0.0003 R . I . a 90° cell angle would be chosen and a " t r i m m e r " plate 0.06 inch thick. At the other extreme, for a full scale range of 0.02 R.I., a cell angle of 1.8° would be chosen with a
Schematic of in-line refractometer
plate thickness again of 0.06 inch. F o u r other choices are available for intermediate ranges. Sensitive refractometers can measure refractive index changes as small as 0.000002 and in general, for organic liquids, the a t t a i n m e n t of this precision requires t e m p e r a t u r e control to a b o u t ± 0 . 0 0 2 ° C. Fortunately, in differential refractometers, precise t e m p e r a t u r e control is not so i m p o r t a n t b u t rapid changes between the reference liquid a n d t h e following sample m u s t be avoided. I n this instrument a proportional t e m p e r a t u r e controller is used. T h e incoming sample passes t h r o u g h t h e bott o m heat exchanger. T h e t o p heat exchanger, which contains the measuring cell, is automatically controlled. A thermistor, connected in a W h e a t s t o n e bridge circuit, responds to the prevailing t e m p e r a t u r e . As t h e t e m p e r a t u r e changes, t h e bridge becomes unbalanced and the d.c. o u t p u t of t h e bridge, after amplification, is applied t o the control winding of a saturable reactor. T h e latter supplies more or less heat to a heater cartridge embedded in the heat exchanger. I n this way, energy is s u p plied smoothly and proportionally to the heat exchanger. T h e entire refractometer assembly, including the temperature controller, is easily accommodated within the case of a standard recording potentiometer.
In-Line Refractometer A third i n s t r u m e n t offered by W a t e r s Associates is an "in-line refractometer" which can be used for t h e continuous monitoring of process streams. As shown in Figure 5, this refractometer
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ANALYTICAL CHEMISTRY
uses a different principle—i.e., a meas urement of the critical angle between a prism face and the flowing liquid. I t is suitable for process stream applica tions where the liquid stream may be very dark, extremely viscous, contain a large volume of solid material, or must be held at a high temperature or pres sure in order to prevent an adverse re action such as polymerization, crystalli zation, or freezing. It will be seen from Figure 5 that the optical principle in volved is essentially that of the classical Abbé refractometer. The photometric system is essentially the same as in the other two refractometers. This recording process refractometer has a range of 0 to 0.005 R.I. to 0 to 0.02 R.I. with a sensitivity of 0.25% of full scale and an accuracy of 0.5% of full scale. The zero drift is less than 1% of full scale in 24 hours. Maximum sample temperature is 300° F . and maximum sample pressure is 250 p.s.i.g. The speed of response is 90% in 20 seconds. I t is well known of course t h a t the refractive index is also a function of wave length (dispersion). In the Waters instruments this problem is disposed of very neatly by the use of cadmium sulfide photocells which have a fairly sharp response peak at 6150 Α., so t h a t they are essentially mono chromatic detectors. Applications. Among the hundreds of applications of these refractometers are the measurement and control of brine mixtures, acid-water, alcoholwater, and glycol-water mixtures, sugar concentrates, and aromatic hydrocar bons. The monitoring of the blending of gasoline additives and hydrogénation control of food oils and fats are further uses. Of enormous value is the use of recording refractometers to analyze the effluents of chromatographic columns. At Houston we were shown a special cell for the differential refractometer with a sample volume of only 0.07 cc. The research man might give some thought to the use of these instruments in kinetics studies. The old timers will snort at these developments as just more gadgetry to do the same things we have always been doing, only at higher cost. Maley has voiced our own opinions when he said, " I t is characteristic of modern analysis instrumentation that whereas little improvement can be made in an almost perfect classical measurement, a new approach can open u p vast and unexplored possibilities. The automatic recording of refractive index falls readily into this classification. Refractometry has been one of the last of the classical optical measuring techniques to become recording."
