INSTRUMENTATION N e w automatic titrator, designed for actuation of a buret, can be a d a p t e d to motor-driven syringes in a u t o m a t i c titrations con I NTEKEST tinues a t a high level a n d a n u m b e r of autotitrators a r e commercially avail able. A new addition to commercial instruments offered for this purpose is announced by Ε . H . Sargent and Co., Chicago, 111., to be in production a p proximately August 15. T h e S a r g e n t - M a l m s t a d t automatic t i t r a t o r is based upon work originally described b y M a l m s t a d t [ANAL. C H E M . ,
26, 1348 (1954)]. I n this instrument t h e valve is actuated b y t h e second d e rivative of t h e voltage-volume curve in a potentiometric titration. Whereas most titrators provide a u t o m a t i c stop ping a t a predetermined voltage, this device operates on a pulse corresponding t o t h e second derivative a n d stops t h e titration automatically. This mode of operation assumes t h a t t h e second de rivative criterion minimizes errors ordi narily associated with small drifts in the electrode potentials. I n principle, therefore, small deviations from t h e true theoretical potentials a r e of no con sequence. T h e circuitry involves a d e termination of t h e potential between the indicator and reference electrode. This potential is amplified and succes sively differentiated twice by resistancecapacitance networks. Signals which are so differentiated electronically are not precisely proportional t o first or second derivatives, b u t introduce no distortion into t h e end-point phenom ena on a time basis a n d therefore cause no delay or anticipation in t h e end point. T h e second derivative voltage is a p plied t o t h e grid of a t h y r a t r o n tube which forms one element of a relaxation oscillator operating a t approximately 10,000 cycles per second. When t h e derivative signal rises above a certain value determined by t h e characteristics of t h e tube a n d its biasing voltage, t h e oscillator begins t o oscillate a t a con s t a n t amplitude a n d continues t o do so until t h e voltage falls to t h e same level a t which point oscillation ceases. A filter circuit now removes t h e half-wave oscillations, producing a square-wave voltage. Subsequent differentiation of this square-wave produces t w o pulses, one positive a n d one negative. T h e ascending portion of t h e second or posi VOLUME
tive pulse corresponds to t h e inflection point of t h e original potential-volume curve. This pulse is applied t o a second t h y r a t r o n t u b e which acts as a switch t o shut off t h e b u r e t valve. T w o filtering circuits which are es sentially t i m e delays a r e included in t h e design of t h e switching t h y r a t r o n stage. One of these circuit delays serves t h e conventional purpose of delaying power application t o t h e t h y r a t r o n until 10 seconds have elapsed. This prevents the initial voltage surge, as t h e elec trodes assume a different potential, from causing a false end point signal. A second timing circuit is also included a t this point to prevent t h e second or switching t h y r a t r o n t u b e from firing unless a certain minimum time elapses between t h e negative a n d positive pulses. These circuit elements permit a certain a m o u n t of discrimination b e tween t h e true ejid point signal a n d u n avoidable "noise." T w o factors are mainly responsible for background noise. One of these is small electrode instability a n d t h e other is imperfect solution mixing. Ob viously, t h e signal t o noise ratio should be as large as possible a n d this is readily achieved i n all except extreme cases. Noise signals which are due t o elec t r o d e potential instability a r e caused chiefly b y electrode surface conditions and m a y b e minimized by proper clean ing a n d care. Noise signals which arise from mixing can be minimized b y efficient stirring, b u t i t will be recog nized t h a t this factor limits t h e maxi m u m r a t e a t which a reagent m a y be added without overrun. I n this instru m e n t these factors are minimized b y t h e o p t i m u m alignment of ring platinum electrode, stirrer shaft, a n d b u r e t t i p . Actual recordings of t h e signal indicate t h a t in this instrument t h e automatic end points are independent of t h e r a t e of titration over a n approximate range of addition from 1 t o 6 ml. per m i n u t e . At faster r a t e s overrun can be detected, b u t this results in an approximate maxi m u m of 0.1 m l . a t r a t e s of 10 t o 12 m l . per m i n u t e . Although this instrument is designed for t h e automatic actuation of a buret, it can be a d a p t e d t o motor-driven syringes with slight wiring changes in
2 7 , N O . 9, S E P T E M B E B
1955
by Ralph H. Muller
the control unit. Current timers for coulometric titrations can also b e accommodated. T h e above description of t h e cir cuitry involved in this a u t o t i t r a t o r might give t h e impression t h a t t h e i n s t r u m e n t is v e r y complicated. A c t u ally, t h e entire control unit is housed in a v e r y small, compact box. M o s t of t h e conventional electrometric schemes have been studied with this equipment a n d actual performance is shown to be noncritical, being simplified b y t h e fact t h a t absolute voltages are not of importance in this scheme of end point selection. I t lias also been applied t o some nonaqueous solutions, principally acid-base. From t h e great interest and activity in t h e field of autotitrators, one can assume t h a t m a n y other developments will be seen in t h e future. This is j u s t another example of a n excellent aid t o the busy analyst, a n d t h e availability of this autotitrator will be welcome news t o analytical chemists. Chromatographic Analysis of Hydrocarbon Mixtures We a r e indebted t o B . W. Bradford, Imperial Chemical Industries, L t d , j Billingham, England, for further infor m a t i o n on t h e "Chromatographic A n a l ysis of Hydrocarbon Mixtures." H e refers us t o a publication with D . H a r v e y a n d D . E . Chalkley in t h e Journal of the Institute of Petroleum [41, N o . 375, 80 (1955)]. This article refers t o gas phase chromatography with techniques similar to those performed b y t h e instruments available in this country—namely, t h e Perkin-EImer Fractometer a n d t h e Burrell KronaoT o g . Extensive details a r e available i n this paper on t h e absorption columns and automatic thermal conductance 47 A
INSTRUMENTATION
instrument abstracts
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Corporation/Pasadena/California
Double Monochromator Provides Low Stray Light, High Resolving Power Spectrophotometer users can m e a s u r e high a b s o r b a n c e values w i t h t h e s a m e speed, a c c u r a c y a n d relia bility as other m e a s u r e m e n t s by using a C a r y Recording S p e c t r o p h o t o m e t e r which incorporates a double mono c h r o m a t o r to virtually eliminate s t r a y light a n d provide u n u s u a l l y h i g h re solving power. W i t h single m o n o c h r o m a t o r in s t r u m e n t s , s t r a y light interferes so g r e a t l y with m e a s u r e m e n t s a t high absorbance values t h a t t h e d a t a ob tained and conclusions d r a w n a r e often meaningless. F r e q u e n t l y , such d a t a a r e reported in t h e l i t e r a t u r e a n d it is impossible t o confirm t h i s d a t a w i t h careful w o r k using a relia ble i n s t r u m e n t . S t r a y light can be corrected for, b u t a g r e a t deal of e x t r a d a t a and te dious calculations a r e required which l a r g e l y n e g a t e s t h e convenience of a recording s p e c t r o p h o t o m e t e r . In t h e C a r y Recording Spectrophotometer, the problem of s t r a y light is handled by prevention r a t h e r t h a n correction. T h e double m o n o c h r o m a t o r reduces
s t r a y light to a negligible a m o u n t over t h e entire r a n g e of t h e i n s t r u ment—less t h a n 1 p p m for t h e m o s t part. H i g h r e s o l v i n g p o w e r in t h e double m o n o c h r o m a t o r r e s u l t s ' n o t only from dispersion which is twice a s g r e a t , b u t a b e r r a t i o n s in one-half of the m o n o c h r o m a t o r m a y be concelled in t h e o t h e r half. Also, by com bining two different t y p e s of dispers ing elements, t h e a d v a n t a g e s of each m a y be combined to give b e t t e r per formance over a wide r a n g e . In t h e C a r y Model 14, for e x a m ple, a prism and a g r a t i n g a r e com bined t o give h i g h resolving power over t h e entire ultraviolet, visible and near infrared regions. Resolving power is about 1A in the visible and ultraviolet and 3A in t h e n e a r infra red—nearly ten t i m e s t h e a c c u r a c y offered by single m o n o c h r o m a t o r in struments. F o r complete design a n d per formance information on C a r y R e cording S p e c t r o p h o t o m e t e r s , w r i t e for bulletin ACS-7.
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The HC1 spectrum, above, is the rotational fine structure, and the pattern of a large peak followed by a smaller one is due to the presence of two chlorine isotopes. These peaks are separated by about 10A (3cm-1) VIBRATING REED AMPLIFIER IMPROVES MASS SPECTROMETER PERFORMANCE
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T h e C a r y V i b r a t i n g Reed A m p l i fier, Model 36, is being used in a n increasing n u m b e r of m a s s s p e c t r o m e t e r installations w h e r e h i g h molec u l a r w e i g h t a n a l y s e s m a k e rapid s c a n n i n g of m a s s n u m b e r s desirable. The Model 36 combines rapid r e sponse with high sensitivity. Response is critically d a m p e d , with a n 0.1 sec. n a t u r a l period (98.6 percent response in 0.1 s e c o n d ) . T h u s a r a n g e of 100 m a s s n u m b e r s can be a c c u r a t e l y scanned in a s little as one m i n u t e . Sensitivity a n d r a n g e a r e such t h a t a s little a s 10- l s a m p e r e s a n d u p to 1 0 - " a m p e r e s can be m e a s u r e d to a Cary instruments SPECTROPHOTOMETERS VIBRATING REED AMPLIFIERS
reproducibility of 0.2 p e r c e n t with o u t c h a n g e of r a n g e . Stability of t h e Model 36 is superior too —zero drift is less t h a n 10- l 5 a m p e r e s . A n o t h e r C a r y i n s t r u m e n t of in t e r e s t to m a s s s p e c t r o m e t e r u s e r s is the Model 30 V i b r a t i n g Reed Elec t r o m e t e r . I t is preferred for m e a s s u r i n g ion c u r r e n t s in w o r k w h e r e e x t r e m e response speed is n o t re quired, such as isotope d e t e r m i n a tions. Sensitivity of t h e Model 30 is 1 0 - " a m p e r e s , and like the 36 it h a s high stability — less t h a n 5 χ 1 0 - " a m p e r e s zero drift. W r i t e for bulletin ACA-7. :
RAMAN SPECTROPHOTOMETER ELECTROMETERS INFRARED ANALYZERS ULTRAVIOLET ANALYZERS
APPLIED PHYSICS CORPORATION / 362 WEST COLORABO STREET / PASADENA 1 / For further information, circle number 48 λ on Readers' Service Cira", sate 51A
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CALIFORNIA
instruments for analyzing the column effluence. Extensive d a t a and record ings are shown for the analysis of hydro carbon mixtures. These d a t a and records leave no doubt about the elegance of this method of analysis. T h e authors point out thatcompared with other methods of frac tionation this technique has t h e merit of indicating continuously the purity of the fractions as they emerge. As the peak shape is very sensitive to overlapping of components and no phenomenon com parable with azeotrope formation in distillation has yet been observed, they also point out t h a t as with all physical methods of analysis, accuracy can al ways be increased by closer calibration with synthetic mixtures, and when this is done the results are certainly as accurate as those obtainable by other methods, such as optical or mass spectrometry. The method of gas phase chromatography has considerable ad vantages in speed and resolving power for components in t h e lower molecular weight range. Considerable attention has been de voted in this article to t h e accurate determination of retention volume and time for various components in com plex mixtures. These values are of great importance in evaluation of t h e results of an automatic recording. Transis tors T h e transistor in its many forms is rapidly taking over t h e functions of conventional electron tubes. One de vice after another is appearing, in which those minute devices provide great simplification and space economy. T h e instruments of t h e analyst will be "transistorized" to an appreciable ex tent in the near future. We have set ourselves the task of catching up with progress in this field, and t h e possibili ties are indeed exciting. Useful and practical details on transistor circuitry are now appearing as these devices have become readily available. M u c h of the previous information was devoted al most exclusively to theoretical aspects of solid-state physics, a situation which was understandable because the complex n a t u r e of the various phenomena involved required understanding rather t h a n immediate application. In addition to amplifying properties, special transistors are sensitive t o light and to nuclear radiations. Operating voltages are low and power consumption is extremely small. The wrist-watch radio receiver, so dear to t h e h e a r t of moppets and older comic-book addicts, is almost realized. I n the transistor version, this audible hazard has been brought down t o pocket size. A tran sistor version of t h e motor car headlight dimmer has been described, even though ANALYTICAL
CHEMISTRY
complete process instrumentation
INSTRUMENTATION this general minimizer of highway accidents has not found widespread use. One gains a faint indication of what may be expected for the future of analytical instruments. Although we may be appalled by the necessity of learning so much new circuitry, the development is very timely. On the demonstrable theory that almost anything can be done electronically, the practical result is often forbidding. The electronics expert is never stumped by a problem, but his answer to one's request may well appear in the form of a 6-foot-high console with several hundred tubes. Blowers, for the ventilation of a multitube chassis, have become a standard component. One can all too easily fall in error with the assumption that ten vacuum tubes are at least ten times as unreliable as one, but the fact remains that ultimate limits do exist with respect to weight, cost, and waste energy (heat) dissipation. That barrier was reached long ago in electronic computers and interest in equivalent low-energy devices is perhaps greater in the computer field than in any other. Solar Battery
F&P Ultramax Separatory Funnel Here is one of t h e most successful developments in laboratory apparatus—the F & P Glass Separatory Funnel. • No lubricants required • Corrosion resistant • No product contamination Funnel sizes: 30 ml to 2 liters
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The solar battery of the silicon type, developed by the Bell Laboratories, has now attained an efficiency of 11%. In keeping with its farsighted policies, the Bell System has already installed experimental frames on poles in Georgia for charging the storage batteries of rural telephone lines. The theoretical yield for a perfect photocell is roughly 1 kw. per square meter in bright sunlight; the actual yields, at present, are of the order of 100 watts for this area. Costs are still high and until these can be reduced, large scale power applications will not be feasible. The National Fabricated Products, Inc., Chicago, has been licensed by Western Electric to manufacture these cells. The advent of these devices (small cells) is of immediate interest to the research iuau because photoelectric techniques have unlimited applications in the laboratory. The advances in solid state physics, both theoretical and practical, have been so numerous that it is almost certain that simple devices will eventually supplant our present complex and cumbersome electronic equipment. The vacuum phototube with associated amplifiers, or the photomultiplier, which furnishes its own amplification by secondary emission, both represent elegant, long tune developments. It is becoming apparent that their days are numbered. The transistor, thermistor, phototransistor, and the newer silicon cells are eminently practical examples of what may be expected in the near future. ANALYTICAL
CHEMISTRY
