Topics in..
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Chemical lnstrumerrtation feature
Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
These articles, most of which are invited contributions by guest by calling attention authors, are intended to serve the readers of THIS JOURNAL to new developments i n Me theory, design, cv availability of chemical laboratory instrumentation, or by presenting useful insights and explanations of topics that are o j practical importance lo those who use, or teach the use of, modern instrumentation and instrumental techniques.
XXXVIII. Refractometers 1. E. MALEY, Anacon, Inc., Ashlond. Mass. 0172 1 Refraetometry is a well-known but little understood technique of analytical measurement. A determination of the refractive index of a. liquid is often made as a check on its composition if i t is a solution, or its purity if i t is a single compound. Likewise, the continuous measurement of rs liquid flowing through a tube whether i t be a process stream or the eluate from a, chromatographic column can provide a. direct indication of a change in composition or quality. Relatively few of the refractometers being used today in the laboratory differ significantly from the original designs of Ahbe (1874) and of Pulfrich (1887). I n fact, these names have almost become generic terms to describe certain types of refractometers.
from one medium into another of dif-
second medium is optically denser than the first, the ray will become more nearly perpendicular to the dividing surface. The angle between the ray in the first medium and the perpendicular to the dividing surface is' called the angle of incidence, i, while the corresponding angle in the second medium is called the angle of refraction (see Fig. 1). The refractive index, n, expresses the ratio of the velocity of light in the two mediums which form the boundary through which the light is passing. Snell's law expresses n as the ratio of the sine of the angle of incidence to the sine of the angle of refraction:
Theory A ray of light which passes obliquely
MEDIUM
Figure 1.
Bmsis principle of refrodive index.
Refractive index = n =
sin i sm r
L. E. Maley is a gradunte of Caluregie Instilute of Technology with o. B.S. in Electrical Enginowing and of Doquesne University with s. Doctorate of Laws. I n 1948, Mr. Maley joined Minneapolis-Honeywell. Ine. as a. prodrmtion and application engineer. He left, Minneapolis-Honeywell in 1950 to join the Instrument Division of Mine Safety Appliances Co. From 1950 until June, 1960, Mr. Meley was in charge of the sales and application engineering of analytical instruments for Mine Safety Appliances Co. Mr. Maley left Mine Safety Appliances Co. in 1960 to join Water Associates, Inc. Waters Associates was a. new company embarking in the field of continuous refraelometers. Mr. Maley was Vice President, Secretary and x Director of Waters Associates, Inc. until 1967 when he left Waters Associates to form Anacon, Inc. Mr. Maley has been actively engaged in the design and application of analytical instruments and sampling systems for over 16 years. He ha? authored numerous recognized technical articles wch as: "Analysis of Fractionation of Polymers," Jaornal of Polymer Science, No. 8, pp. 253-268; "Liquid Chromatography Monitoring with a New Refractometer," Pittsburgh Conference on Analytical Chemistry, March, 1961; "Design of Multiple Stream Sampling Systems," Control Engineering, November, 1961.
(Continued n page A468)
C Circle No. 152 on Readers' Service Card
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Chemical lnstrumentafion I t is normal practice to refer the index of refraction to that of a vacnum which is arbitrarily defined as having a refractive index of unit,y; reference to air introduces less than 3 parts in 10' error. The refractive index n, therefore, is a dimensionless const,ant whose value for light of s given wave length is determined by the character and state of the liquid or solid medium and of the reference medium, air. If the refractive index of various liquids or liquid solutions is to he compared with each other, then i t is necessary to specify the state of the reference medium m well as to control other variables which affect the velocity of light in the sample to he measured. The symbol nhD for example, means the refractive index for the D lines of sodium measured a t 20°C. Most refractive index values given in the handbooks have been measured under these conditions and are so referenced.
LENS LIGHT
Figure 2.
Critical ongle refroctometry.
Figure 3.
Differential refractometer.
SOURCE
Types of Refractometers Two types of refrectometers are commereially avaihhlo, the di&renlial and I n the differential the critical-an.& refractometer, a light beam is transmitted through a partitioned cell which refracts the beam a t an angle which depends on the difference in refractive index between the sample liquid in one part and a standard liquid in the other. Differential refraetometers can be accurate to 1 X 1 0 ' units of refractive index difference between the two liquids. In the critical-angle refractometer the light incident on the surface of the solntion changes sharply from reflected lo t,rsnsmitted light a t a critical angle. Several different versions of the differential and critical-angle refractometers are commercially available far the laboratory and for continuous process monitoring.
A.
Critical-Angle Refractometers
Critical-anglo refractometers measure the refractive index of liqoids a t an interface with air or, more commonly, with a glass prism. As shown in Fig. 2, a light, beam is directed a t the interface a t various angles in the vicinity of the critical angle. The critical angle is the angle from the perpendicular a t which the beam changes from light transmit,ted into the liquid to light totally reflected a t the liquid surface. At angles smnller than the critical angle the light is transmitted into the liquid. The critical angle depends not only on the solution composition hut also on the prism material. The refractive index can he calculated from: i, = arc sin (n,/nJ where
critical angle in radians within the elass n, =index of refraction of the glass nr = index of refraction of the liquid
i,
=
The significant fesbure of a crilical angle refractometer is that, i t measures the refract.ive index s t the swface of a sohtion. Since surface reflection requires no penetration of the light beam into the
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solution, this instrument may he used for highly opaque samples and various murky solutions and suspensions, as well as transparent samples.
8.
Differential Refractometers
Differential refractometers are intended primarily for the analysis of liquid mixtnres. They are applicable to any mixture whose refractive index is a single-valued function of the composition; this includes nearly all simple binary systems. In refractive index meltsotwnents.
recognized and precautions are taken lo minimize the temperatwe effect. Each of the instmments employs n. sample cell through which :h light beam is transmitted. The beam is refracted t,hrmgh an angle whose size depends on t,he refractive index difference between the sample and x standard which constitutes a part of the cell. The angle of refraction is measured with a photoelectric pickup. The aoolieabilit~vof a refractometer to
strument, and n calibration curve of composition versus refractometer reading is plotted. Various rangas and nominal values of refractive index can be accommodated by the various instruments, and some of them will provide any range down to An = 10" a t any nominal value. The minimum detectable index difference for sustained periods of operation is '/la of the range or lo-' units of refractive index for the best of the new instruments. I n the most precise instruments sensitivity is limited t o this value mainly by mechanical deformation of the instrument caused by changes in ambient and sample temperature. The principle of the differential refractometer is very simple. First consider thepnsgsge of a beam of light through a simple right-angled prism of refraotive index n, and s. second prism of idenlical composition in close proximity to the first but with t,he refracting mrfaces reversed (Fig. 3a); the beam will p a s s through the combination without net refraction. If, however, the second prism is made of material having a very slightly different refractive index from the first., dn, then the beam will be deflected n through t,he angle B (Fig. 3h). I f dn is
+
(Conlintwd on page A470)
Chemical Instrumentation very small, B will also be very small and the distance z, measured at some arbitrary point along the beam, will be proportional to dn. Now replace the solid prisms with hollow ones filled with liquids having dn and we refractive indices n and n have a simple differential rrefrrtctometer. The principle of the differential refractometer is not new and such instruments have been in use for plant control for a number of years, but the literature shows thrtt there is a steadily increasing awarness of the potentialities of the technique, both for laboratory and plant use. Operation of B typicsl differential refractometer may be better understood by referring to the schematic diagram (Fig. 4). A light beam generated by an incandescent lamp passes through the slit and mask that confine the beam to the region of the cell. The lens renders the light rays from the slit parallel. This parellel beam passes through the cell to the mirror. The mirror reflects the beam back through the sample cell to the lens which focuses it upon the edge of the besmsplitter mirror, MS. At this point, the location of the light besm depends upon the relative refractive indices of the sample and the standard. As the index of refraction oi the sample changes with respect to the standardliquid, the beam emanating from the lens shifts laterally with respect to the beam splitter. When this occurs, the amount of light reaching one photocell changes with respect to the other photocell and an electrical signal is generated in proportion to this difference. The bean, deflector driven by the servo motor and its controlling amplifier-photocell circuits keeps the light beam falling equally on the two photocells. This occurs in the following manner. When light falls equally on the two photocells, there is no signal input to the amplifier. A shift in the beam, due to refractive index change of the sample, puts more light on one photocell, impressing a. signal on the amplifier. The amplified signal drives the motor and t,he beam deflector in a. direction that equalizes the light falling on the two photocells. The more the refractive index of the sample deviates from the reference, the greater the deviation of the light beam and the more the deflector must turn to balance the split light beams. The position of the deflector, therefore, is a measure of the refractive index differential.
+
.
. Figure 4. Differential rofractometer, schomotis. Light fmm lamp bulb I ) parser through the slit and musk assembly S, is collimated b y lens L, then twice traverses the sample and reference prismatic The exiting beam is focused b y the rome lens, and is displaced cells, being reflected b y mirror M laterally as necessary b y the glass blocks Z, the zero adjustment, and D, the balancing deflector. Part of the light is reflected b y the oblique mirror MI onto cadmium rulflde photocell G,while port parses the edge of Ms to impinge on cell CI. The signoh from G and G control a renomotor SM, via the differential amplifier. The motor acts to turn the deflector D ro os to maintain equal illumination on CLand G,and a t the rome time operates a stripchart recorder.
.
1.35 O
4
8
1
2
1
6
M
2
4
SALT IN VAlER,% Figure
I Salt in water.
Process Refradometers
Since the early dsys of World War 11, recording refractometers have been used to monitor thepurity of butadieneandstyrene streams. In addition, process refractometers have been used to control the blending of these two feed components in the manufacture of GR-S rubber. The refractive indices of styrene and hutadiene are approximately 1.5434 and 1.4120, respectively. A change of 0.1'% is easily detectable by continuous refractometry. Applications in the food industry involve materials xnch as soya bean and cottonseed oils having refractive indices
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Figure 6.
Sulfuric acid, hydmchioris acid.
of approximately 1.47. The finished products are shortenings and margarine with indices of approximately 1.43. Close control of the end point is required to obtain a product of the desired quality. A process refractometer makes this control possible without the delays required in making spot checks with laboratory instruments. Many applications involve following the concentration of solutions. I n the concentration of sugar solutions, salt brines and acids, in fact, almost any chemical solutions, the degree of concentration can be easily followed by the change in refractive index (Fig. 5). For example, the refractive index of s. sucrose solution increases by about 0.0002 units per 0.1% increase of sugar concentration. I t is a simple matter to detect such changes by process refractometry. It is also possible to detect changes as little as 0.0270 in aqueous solutions of nitric acid. An even (Continued on page A4721
Chemical lnstrumentcrtion higher ultimate sensitivity is possible with sulfuric acid solutions (Fig. 6 ) . The separation of aromatics from mturates is an outstanding example of the possibility of using refractive index to follow a process, since the change in refractive index between aromatics and saturates is quite marked. The aromatics have a refractive index of about 1.50, and the saturates ahout 1.40. Since these measurements can be made to one in the fourth place, a precision of about 0.1% is easily obtained. Alcohol-water analyses are also carriedout by this method. Water has a refractive index of 1.33299, while methanol is 1.32920, ethanol 1.36176, and isopropanol 1.37757 (Fig. 7). I t is possible to detect changes of s few tenths to a few hundredths of 1% in the concentrations of these various alcohols in water. Many applications of continuous refractometry are carried out in fractionating tower control. One example of snch an application is the separation of cyclohexane and n-hexane. The respective refractive indices of these components are 1.42623 and 1.37486. This difference is sufficient to give measurement,^ of composition changes of less than O.lYo Another typical fractionating column application is the determination of normal butane in isohntane. These two components in the liquid phsse have a refractive index difference of approximately 0.0100. I t is possible to detect changes of the order of 0.1% in this case.
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Figure
7. Alcohols
and acetone.
While refractometry has been applied to many process streams there was a limitation in the scope of these applications until t,he development of the in-line cn'tical-angle refractometers. Many processes are amenable to monitoring and control by refractive index, hut these streams may often he opaque or contain a large concentration of solid materials snch as catalyst fines, pipe scale, undissolved solid product, polymers, etc. For example, in the manufacturing of high boiling petroleum products a relation hetween refractive index and quality exists.
These streams are too dark to he analyaed by t,he differential refractometer hut they can he easily handled with the eriticalangle reflectance type refractometer. The Anscon in-line refractometer (schematic diagram, Fig. 8) has the prism mounted in the pipeline itself. The prism is in intimate contact with the process flnid because its sorfaee is flnsh with the wall of the pipeline. The entire optical system other than the prism is mounted in a n explosion proof housing, clamped to (Continued on page A474)
Chemical Instrumentation
explosion proof housing t o the prism, is in with the sample. Since the prism is mounted in stainless
INLINE REFRACTOMETER BLOCK DIAGRAM Figure 8.
In-line refroctometer.
t h e pipe. I n operation, a light heam from a n incandescent light hnlh is directed through a lens and out the back of t,he
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steel tubing or pipe, the process fluid does not have t o be removed from the pipeline d.. transmitted t o some remote instru-
(Continued on page A476)
Chemical Instrumentation ment. Product is measured m d e r actual process condilions, i.e., a t the same temperature and pressnre conditions prevailing in t h e process. The short section of pipe in which the prism is mounted is provided by the msmlfnctorer s s a part of the instrument, and installation is made simply by flanging the section of pipe into the process line. The light. benm directed from the instnlment housing is refracted a t the interface between the prism and the process flnid and directed back throogh a lens and restorer glass on t o s detector assembly consisting of two cadmium mlfide photocells. One photocell is located in the permanently illominated section, and the other is mounted a t the critical angle point., where the beam changes from light t o dark. As the refraet,ive index of the process Rnid changes, the critical angle changes and causes more or less light to fall on the sample photocell. The other cell, of course, always remains in the f d intenvit,~portion of the beam.
Sample Conditions: Viscosity, Coating Action Since a refractometer operating on the critical angle principle measores the index only a t the interface, care must he exercised to make the material a t the interfrtce representative of the bulk solntion. Agitation or turbulence in the process stream, of course, accomplishen
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the reqnired cleansing of the interface qitite easily, but when agitation is small or when the solution is highly visco~ts, a film can build u p on the pl.ism surface and produce a n erroneous reading. To eliminate t,his possibility, a n sotomatic jet washing system is provided in t,he pipeline head.
Temperature Compensation Another important factor that must be taken into consideration in critical angle measurements is the effecL of s. temperatore change in the process stream. The ~.ef~.active index of fluids, and of the prism itsclf, are sensitive to temperature changes. I n a well-agitated line, window temperat ~ m quickly comes to equilibrium with solution temperature, and measurements can be very accurate and reliible. If the temperatlrre is changing, however, nome compensation must be made. A thermistor probe inserted in the pipe can measure the temperature of the process line and compensate far any temperature changes over a t.eosonnble lange. It is not possible to compensate over a range a s largo ns 100°F (50°C), but it is possible to eompensate for temperstnre changes of If the npproximat,ely 20°F (10'C). process stream is varying radically in temperature, then the head must he provided with thermostatic eontml.
Laboratory Refractometers There are three common t.ypes of labor$tory refractometers-the AbbP, the immersion.or dipping, and the Pulfrich instruments. The Pulfrich uses mono-
Chemical Instrumentation chromatic light, requires more of the sample than the Abbi: type, and is therefore less commonly employed. These instruments have been used extensively in the laboratory for many years. Accurate measurements can be made over a fairly wide range of refractive indices. Same typical accuracies and ranges are shown in Table I. Toble 1. Mox m m A c c ~ r a c yond Range of Cr'tco -Angle Refroctometer%
..
Maximum aCCllrRCV
Ilefractometer
n~
Pulfrich".k.'
+ I X lo-'
AbbO"-J
flX10-'
Precision Abbhd.J
1 2 X 10F
Dipping (immersi~n)~.d
&4 X
Typical ranees of"n 1.33-1.61 1.47-1.74 164-1.86 1.3&1.70 1.45-1.84 1B3-1.64 1.36-1.50 1.40-1.70 13-1.54 (6
prisms)d
Hilger, Veiss, Rellingham & Stanley, Bmlsch and Lomb ' A 0 Insb. Co., f Valentine.
The A b b i Refroctometer The range of the Abbi: is normally 1.3000 to 1.7000, the maximum precision attainable being 0.0001. I t reads the refractive index directly, and requires only a drop of sample. White light is used, and to prevent R colored, indistinct boundary between the light and dark fields two direct-vision prisms, called Amici prisms, are placed one above the other between the objective and eyepiece of the telescope. These are constructed of different varieties of glass and are so designed as not la deviate a ray of light of the wavelength of the sodiwn D line. Rays of other wavelengths are, however, deviated, and by rotaiing the Amici prisms it is possible to comteraet the
Figure 9
Abbe refroctometer.
(Conlinaed o n page A478) Volume 45, Number 6, June 1968
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Chemical Instrumentation dispersion of light a t the liqrlid iuterfcxe. A typical Abbe refl.actometer is shown in Figwe 9.
momted below the eyepiece inside the tube. The lower swfare of the prism is immersed in n small beaker containing the sample, with a mirror below to reflect light up through the l i q ~ ~ i d .
The Immersion Refroctometer This refractometer is thc simplest to me, but reqnires 10-15 ml of sample. A single prism is mounted rigidly in the telescope containing the eampensntor and eyepiece a s shown (Fig. 10). The sealo is
1
4
'1
: '
Figure 10.
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Pulfrich Refractometer The Pdfrich refmetometer is psrticularly nseful for precise measmemerits with solotiom or liquids that are volat,ile, reactive, or hygroscopic, with solid plates, for differential studies, and for dispersion measutwnents. Absolule values obtained with t,he Pulfrich refractorneter are osually not reliable in t,he fifth decimal place, but a comparison at two wavelengt,hs or of two substances can be made to +1 X Solids and liqnids of index between 1.33 and 1.86 can be investigated over a wide temperature range. The refractive index n cannot be rend direclly, but must be determined from a set of observations. Once the apparatus has been set up, a value of n can be obtained in about 5 mi". The critical boond:wy is pnrt,icularly sharp in this type of reflactomeber even with liquid samples, ninee i t is free fmm shielding of grazing rays and the full area of the refracting prism may be illuminated.
Direct Reading Critical-Angle Refractometer A new direct reading laboratory criticalsugle refwctometer was recently intro-
duced by Anacon. This instnlment makes use of the same basic principle, but the change in the light beam is measured by a photosensitive detector and the refractive index value indicated directly on a meter. The Anaeon direct, reading ref~.actometer is fihown in Figure 11. The sample liquid is placed in a cavity where a synthetic diamond prism is mounted. The cavity is covered with a spring loaded, Teflon covered, cap which ensnres that the liqnid wets the prism surface. When the surface is wetted, the light-tadark ratio of the critical angle image ehangm, and the photosensitive detector on which tho image is focused changes itr resistance. This change in resistance is electronically measured :uld a signs1 directed to t,he indicating meter.
Figure
11.
Labomtory
direct
reading
fractometer'
( C o n l i n z d on paye A482)
re-
Chemical Instrumentation Refractometers in Liquid Chromatography The chromatography of rolorless liquid mmoounds has lone been bmdened with
colorimetry, and refraetometry have been adapted in a n effot.t to obtain continuous flowing chromatographs which wodd greatly simplify fraction collection and analysis. An instroment which would
continr~allymonitor ihe refractive index of the coh~mnefluent, has always appeared to be the most desirable for this applicai tion (Fig. 12). Various instrliments along these lines have been described in the literstr~re since Tiselius, Claesson and their collaborators first described t,he use of an optical system registering changer in refractive index based on the "Schlierenu method. Refractometry is appealing as a, continuous monitor for liquid ehl.omatography, and one can say that refractive index is to liquid chromatography as thermal enndoctivity is to gas chromatography. Refractameters will respond to any change in the liquid efluent and therefore can be med as a universal detector. The other analytical techniques mentioned above are specific rather than universal and consequently are limited to particular classes of material. With such instrw
or what reagent must he used to cause a color reaction. If the material does not absorb radiation at the frequency for which the detector is set the peak will be missed entirely or if the reagent is wrong a color change will not occur and again a peak will be missed. A refmetometer responds to any change in the liquid efluent and is therefore a tlniversal, highly sensitive detector.
When to Use a Refractometer Whenever the sample to be analyzed is s simple binary mixture, such as alcohol in water, the first choice analyzer is a re-
fraetameter. Density-measuring analysecs are applicable when the range of compositions is broad. Btlt when t,he range is narrow, down to about 0.4Y0, and certainly whcn a n snalysis of the liquid phase of a snspcnsion or slurry is required, a refractometer again is first choice.
Batch Analysis: Ketchup, Jams, Jellies There are also a variety of applications for batch analysis wherein very viscous food prodncts are being cooked to a desired soluble solids content. A typical example is continuous measurement of soluble solids in ketchup (Flg. 13). Since
Figure 13.
Soluble solids in ketchup.
this is a hatch type operation, a sample is pumped from the bottom of the kettle through the in-line refractometer and hack
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Chemical Instrumentation -
BRlX MEASUREMENT AND CONTROL FOR APPLESAUCE PROCESSING
-
into the finishing kettle. When a product reaches the desired concentration the kettle can be drained and refilled with another batch. With this type of installamonitoring is made of tion a, contin~~ous the product s s it is being prepared.
RECORDING-CONTROLLER
Carbonated Beverages Figwe 14 ilhtstrates a typical installation of an in-line refractorneter a t a carbonated beverage plant. Here the unit measures the amount of sugar in the beverage after it has been completely carbonated and just prior to bot,tling. Some other anolications of interest are measurement of glycerine-water mixtures, formaldehyde-methanol mixtues, syn~
~
..
SWL"
Figure 15.
Figure 14.
Carbonated beveroge plant
Applerouse control system.
bhetic rubber processes, and a variet,y of instant food products, inclnding coffee and tea. An extremely important applicetion is in the hydrogenation of fats and oils. The analysis of these materials as
well as standard lubricating oils in refineries has been done for many years with laboratory refractornetem. It was felt that it would be impossible to analyze (Continued on page A484)
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Chemical instrumentation these pt.oducls eontin~~ously, beca~rse of large qnmltities of entrained metal catalysts, such as nickel, p l a t i n ~ m ,etc. T h e maintenance problems involved in filtering and cleansing these sample stresms for measurement by a differential refractometor or other analytical instr~lments presented such severe problems that analysis cordd seldom be made on a continuoos basis. Tho in-line refrsctometer has been spplicd to these hydrogemlion problems ns well as to measnrement of waxes in oils, mensnrement of catalytic cracking-unit feed stock, mcaswement of octane numbers of gasoline, and detection of interfaces in pipelines. I t provides all immcdiate, simple analysis witho~lta very elaborate, highly complicated, sampling system. In :L nnmber of eases the elimirmtian of s sampling system in itself mare than justifies installation of the refractometer. The field of in-line measurements is becoming more and mare import,ant with the advent of completely controlled aotomatic processes. Figwe 15 shows an automatic control system for applesauce processing and Figure 16 a. monitor for a paper mill recovery bailer. To obtain true automatic control in many eases, analysis must be extremely fast and extremely reliable. The more important factors in analysis instrumentation are reliability and simplicity. I n order to
A484 / Journal o f Chemical Education
,
SPRAY RING
RECOVERY
' L
I
FROM EVAPORATOR
J
BLACK
)%(
BOILER
PEFRACTOMETER
ha
I ASH GREEN LlOVOR
Figure 16.
Paper mill recovery boilers.
simplify the complicated analysis measurements it is virtually essential in many rases that the unit become a n in-line measuring device. By measuring within t.he actnal process, many of the problems of analysis are eliminated nnd the unreliability of complicaled sampling systems is removed. The in-line measurement pmvides x very high degree of reliability and s very high speed of response. I t is anticipated ihat in the next few years we will see more and more analyt,icel devices ap-
plied to direct in-line measurement,. In summary, refractometers have been used fur almost 100 years for the measurement of liquid solutions. Contin~lalimprovements are being made, particularly in the continuous process type refractometers. No significant changes have been made in laboratory refractometers in recent years but the advent of solid state circuitry probably will ennse some major changes in these instrnments in the very near futnre. llefractometers are reliable,
