INSTRUMENTATION by Ralph H. Müller
Figure 1. The pH meter is used for electrochemical analyses at Frick Chemical Laboratory, Princeton University
M a n y Novel Uses of pH Meter Developed novel uses for the Leeds & Several Northrup standard Model 7664 p H meter have resulted from work conducted a t the Frick Laboratory, Princeton University, by Furman, Bricker, and Adams. These results, compiled by the Leeds & Northrup staff, presumably could be duplicated by other comparable p H meters of precision, reproducibility, and high input impedance. Potentiometric End Points with Controlled Current Input. The p H meter is used for measuring end points in titration cells in which a small but essentially constant current is passed between a pair of electrodes. A constant current of about 2 μ&. is supplied by a simple auxiliary circuit, compris ing a battery and a series resistor of high value connected across the elec trodes. The pH meter is connected across the electrodes and indicates the voltage changes which arise as the titration proceeds. The character of the observed volt age changes at the end point depends upon whether both electrodes, or only one, function as a detector. Choice of the electrode type determines its function. Because current is passing, the detector is in a polarized condi tion. Such a condition markedly in fluences the electrode behavior and accounts for the useful properties of the system. Two Indicating Electrodes. If two platinum electrodes are used in the solution, changes in potential occur at
both. The p H meter shows the net change of potential of the two elec trodes. If the reactions are reversible with respect to both electrodes, the pH meter shows a sharp voltage peak at the end point. Such a response re sembles the first derivative curve of an ordinary potentiometric titration. Hence, this method has been termed a derivative polarographic end point. If one of the electrode systems is irreversible, a sharp peak is not ob tained. The response is an abrupt step change a t the end point to a higher or lower voltage level. This type of measurement and sev eral applications have been described by Reilley, Cooke, and Furman
[ANAL. CHEM. 26, 1673 (1954)] could be used for this purpose. This end-point method is similar to the common dead-stop method which also uses two platinum electrodes. In the dead-stop method, however, a constant voltage of low value (0.01 to 0.1 volt) is applied to the electrodes, and the resulting current is deter mined. I n the present method the converse is true. pH Measurement with a Constant Current Electrode. Bricker is using the Model 7664 meter to measure the pH of a solution by means of a con-
[ANAL. CHEM. 23, 1223 (1951)]. Ap
plication of this method to the titra tion of glucose with ferricyanide has been discussed by Adams, Reilley, and Furman [ANAL. CHEM. 24, 1200
(1952)]. The instrument is recommended for making end point measurements in a Karl Fischer titration which requires a polarizing current, and uses two platinum detecting electrodes. T h e fact that no salt bridge is required is an advantage. Indicator electrodes can, therefore, operate without mainte nance problems in hot solutions. Another advantage is the applicabil ity of the method to automatic batch titrations where the large voltage pulse at the end point can be utilized to terminate a titration. A circuit such as that described by Carson
Figure 2 . Automatic potentiometric polarography equipment includes L&N model 7 6 6 4 indicator (on stool) VOL. 2 9 , N O . 9 , SEPTEMBER 1957 ·
7 7A
INSTRUMENTATION
Ilpjz •Hi
instrument abstracts
ΓΙ!
Applied Physics
Corporation/Pasadena/California
At E a s t m a n K o d a k
Gary Model 14 Spectrophotometer measures absorbance to 7.1 without correction for stray light.
Run A represents the normal check of a Fabry-Perot interference niter similar to those used in densitometry of color films where the passband and shape of the niter curve is important up to densities of 7.0. This particular filter curve showed a nonsymmetrical peak at 639 mu. For a closer examination of this peak a second expanded curve of this wavelength region, Run B, was made in super-position on the first curve. In the Laboratories of the Eastman Kodak Company interest in absorbance values over seven —less than . 0 0 0 0 1 % transmission—is more than idle curiosity. Recently, Kodak physicists, using one of their Cary Model 14 Spectrophotometers, were pleased to find they could measure densities to 7.1 with out correction for stray light. In contrast, there are numerous instances where m o n t h s of hard work were wasted because unsuspected stray light of single rnonochromator instruments caused large e r r o r s s o m e t i m e s e v e n b e l o w 1.0 a b sorbance. Double monochromators cost m o r e to design and build. But they provide advan tages that can be had in n o other way. Besides low stray light, the double rnono chromator adds the dispersion of its sep arate sections and is a r r a n g e d t o cancel severe optical aberrations, giving increased resolution. In the Model 14, a silica prism and a 1 5 , 0 0 0 l i n e diffraction g r a t i n g add t h e higher ultraviolet dispersion and the low stray light of the prism to the excellent visible and infrared dispersion of the grat ing. Each complements the other to produce e x c e p t i o n a l p e r f o r m a n c e from I 8 6 0 to 26,500 Angstroms. A l l Cary i n s t r u m e n t s are t r u l y direct reading. Freedom from stray light provides one of the most dramatic examples of what this can m e a n to a user, but there are others. S o m e of t h e u n u s u a l f e a t u r e s of C a r y Recording Spectrophotometers which con tribute to accuracy by helping avoid correc tions are listed at right.
COUNTER-DIAL WAVELENGTH SCALES-Easily
read, no interpolation, corrections negli gible for most work. MULTIPOT-CORRECTED 1 0 0 % LINES-A11 Cary
S p e c t r o p h o t o m e t e r s h a v e M u l t i p o t s for c o m p e n s a t i n g s a m p l e a n d reference cell differences, and for compensating m i r r o r unbalances which inevitably occur in time. SPECIAL RECORDER FUNCTIONS AVAILABLE -
Log absorbance recording for qualitative — quantitative analyses. Kubelka—Munk func tion recording for paper and textile dye work. Expanded absorbance or transmission scales for weak a b s o r b e r s or d i f f e r e n t i a l photometry. ACCURATE PHOTOMETERS-Slide wires accu rate to within limits of recorder readability. P h o t o m e t e r accuracy r i g i d l y tested w i t h standard filters. H i g h power p e n motor and low friction p e n c a r r i a g e . C o n s t a n t con trolled pen d a m p i n g and fast response over entire absorbance range. HIGH ACCURACY ELECTRICAL ZEROING-Spur-
ious electrical pickup reduced by careful design and testing. PHOTOMETRIC PRECISION TO STATISTICAL
LIMITS—Statistically efficient p h o t o m e t e r systems, r e a c h i n g t h e t h e o r e t i c a l l i m i t s achievable with the best m o d e r n multiplier phototubes and semiconductor photocells. C o m p l e t e specifications are a v a i l a b l e o n all Cary R e c o r d i n g S p e c t r o p h o t o m e t e r s . Write to Applied Physics Corp., 362 West Colorado Street, Pasadena, California, for Bulletin AC-29.
For further information, circle number 78 A on Readers' Service Card, page 99 A 78 A
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ANALYTICAL CHEMISTRY
s t a n t c u r r e n t electrode (see Figure 1). A m a n u s c r i p t describing t h i s work h a s been s u b m i t t e d for p u b l i c a t i o n . H e is able to m a k e a reliable p H m e a s u r e m e n t in t h e r a n g e of 5 t o 14 p H b y using a p l a t i n u m wire c o a t e d with m e r c u r y as t h e d e t e c t o r electrode. T h i s electrode is polarized anodically by a c o n s t a n t c u r r e n t of t h e order of 1 μ&. A calomel reference electrode completes t h e cell. Bricker uses a low-voltage b a t t e r y a n d a series re sistor of large v a l u e t o s u p p l y t h e constant current. The p H meter measures t h e voltage across t h e elec trodes. , Bricker explains t h e p H response of t h i s electrode b y p o s t u l a t i n g t h e anodic conversion of m e r c u r y to mer cury oxide. T h e p H of t h e solution controls f o r m a t i o n of t h e m e r c u r y oxide a n d v o l t a g e of t h e m e r c u r y elec t r o d e . T h i s electrode covers a p H r a n g e where performance of t h e glass electrode is inferior. Current Scanning Polarography. T h e m e t e r is being used t o m a k e m e a s u r e m e n t s b y a t e c h n i q u e de veloped a t P r i n c e t o n called p o t e n t i ometric p o l a r o g r a p h y with controlled c u r r e n t scanning, or m o r e simply, cur r e n t scanning p o l a r o g r a p h y . This t y p e of m e a s u r e m e n t is r e l a t e d t o o r d i n a r y p o l a r o g r a p h y such as car ried o u t w i t h t h e Leeds & N o r t h r u p E l e c t r o c h e m o g r a p h , which imposes a linearly changing v o l t a g e on a d r o p p i n g - m e r c u r y electrode a n d records the r e s u l t a n t c u r r e n t . T h e c u r r e n t level is t h e i n d e p e n d e n t \'ariable, a n d the r e s u l t a n t v o l t a g e is m e a s u r e d . C u r r e n t scanning p o l a r o g r a p h y can be carried o u t with a solid electrode or with t h e d r o p p i n g m e r c u r y elec t r o d e . T h e original description b y A d a m s , Reilley, a n d F u r m a n e m p h a sized its applicability t o m e a s u r e m e n t s with solid electrodes [ A N A L . C H E M . 25, 1160 (1953)]. T h i s m e t h o d
gives slightly b e t t e r results t h a n does conventional p o l a r o g r a p h y , a n d it provides a m e a n s for e v a l u a t i n g t h e performance of a n electrode in a coulometric g e n e r a t i o n process. T h e electrode sj^stem in c u r r e n t scanning p o l a r o g r a p h y usually c o m prises t w o solid electrodes, which carry t h e scanning c u r r e n t , a n d a reference electrode, which p e r m i t s a n e x a m i n a t i o n of t h e p o t e n t i a l of one of the solid electrodes. T w o p l a t i n u m wire electrodes in a stirred solution are c o m m o n l y used for t h e solid elec trodes. H o w e v e r , o t h e r solid elec t r o d e c o m b i n a t i o n s can be used. T h e scanning c u r r e n t c a n be s u p plied b y a n y circuit which provides a djus ta bility a n d control of t h e cur-
INSTRUMENTATION
RESEARCH AND PILOT PLANT EQUIPMENT
for DRYING and CALCINING
Gas Fired Continuous Rotary Dryer
The gas fired continuous rotary dryer—pictured above — is used for drying ores, chemical salts, sand, clays, fertilizers and various other materials that can be brought into direct contact with the products of combustion. Inlet gas temperatures to 900° F.; velocities up to 600 R.P.M. using propane or other bottled gas. Approximately 2 4 " wide; 5'6" long; 2 ' 9 " high. For either purchase or rental.
Steam Heated Continuous Rotary Dryer Gas Fired Rotary Continuous Combination Calciner and Cooler Electrically Heated Rotary Continuous Combination Calciner and Cooler Direct Fired Rotary Kiln Steam Jacketed Batch Dryer (with Rotary Sweep)
Parker and Adams [ANAL. CHEM.
Get full details. Write for Bulletin No. 117 today!
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ANALYTICAL
rent value, up to a maximum current of the order of 100 Ma. In the work cited above, the scanning current was obtained from a constant voltage source of high value. Regulation and measurement of the current were ob tained by using known resistors in series with the voltage source. A more recent development uses a vacuum tube circuit to supply the scanning current and a microammeter to measure the current. The pH meter is connected to the electrode under examination and to the reference half cell. Change in voltage of the selected electrode is indicated continuously by the p H meter. Measurements are made man ually by adjusting the scanning cur rent and recording the corresponding voltage and current values. A series of such points at successively increas ing currents is obtained and plotted. The resulting curve can be interpreted to give considerable analytical in formation.
CHEMISTRY
WARE!
28, 828 (1956)] point out certain ad vantages of the current scanning method in the analysis of phenylenediamines. Automatic Potentiometric Polarography. Adams has developed auto matic equipment to carry out current scanning polarography. His equip ment uses the Model 7664 p H meter with a Speedomax recorder to record the current voltage curves (see Figure 2). The scanning current is controlled by a motor-driven potentiometer which regulates the output current through an impedance-converting unit em ploying a transistor. The resulting current output shows good linearity with time. He has made such meas urements with the dropping mercury electrode. Chronopotentiometry. In chronopotentiometry a constant current is applied to a pair of electrodes in a cell. One electrode is selected for examination and may be a pool of mercury or a platinum plate. After the circuit is closed, the potential of this electrode increases with time. Change in voltage is observed with the pH meter. The meter is connected to the electrode in question and to a third electrode, a reference half cell in the solution. The plot of changing electrode volt age with time has a characteristic sigmoid form when a substance which can undergo a reaction at the electrode is present in the solution. The time that is required for the voltage value to pass through the limits defined by the substance is termed the transition time. The square root of the transi-
tion time is proportional to the con centration of the substance being measured. Hence, the technique can be used to measure concentration of a substance in solution. Delahay and Mamantov [Anal. Chem. 27, 478 (1955)] presented the theoretical principles on chronopotentiometry. An experimental evalu ation of the method has been given by Reilley, Everett, and Johns [Anal. Chem. 27, 483 (1955)]. The latter workers used a Model 7664 p H meter and an electronic recorder to furnish a record of voltage time changes. Transition time can be measured with the p H meter alone. An observer with a stopwatch times the interval taken for the reading to pass between two predetermined values, and point out that the p H meter might be modi fied to start and stop a timer auto matically at the beginning and the end of the transition time interval. In either instance, concentration of the substance being measured can be calculated from the elapsed time read ing. Electrolytic Resistance Measure ments. The p H meter can be used to make ordinary electrolytic conduc tance titrations and measurements of specific resistance or conductance. A cell with two pairs of electrodes is used. A direct current of constant magnitude is passed between the pair of secondary electrodes. The p H meter is connected across the primary pair of electrodes and measures the RI drop. Resistance of the solution is obtained using Ohm's law from the voltage drop as measured on the pH meter. Taylor and Furman [ANAL. CHEM.
24, 1931 (1952)] have described the use of a Leeds & Northrup p H meter for this purpose. They describe a simple conductance cell. Tungsten metal is a preferred material for the primary electrodes because it acts somewhat as a nonpolarized electrode. They titrated a number of common substances and obtained satisfactory results. These workers cite certain advan tages over the usual alternating cur rent procedure. The resistance meas urements are made more conveniently, because the p H meter is a direct read ing instrument. Moreover, the method is useful when a p H meter is on hand and the specialized alternat ing current conductiometric equip ment is not available. They found that the electrolysis error caused by the flow of direct current was neglible. Accuracy and precision com pared favorably with usual conduc tance measurements.
" W h y is flow control so important in Gas C h r o m a t o g r a p h y ? " Because part-per-million analysis of gases and liquids boiling up to 350° C depends upon a precise, reproducible rate of flow through the chromatographic column. Inadequate regu lation, or pressure surges caused by sample introduction, may cause drift or misleading peaks on chromatograms. The Beckman GC-2 Gas Chromatograph, with its exclusive dual-capil lary orifice system and its precision pressure regulator, controls the flow of carrier gas to 0.5 cc/min. Surges caused by sample introduction are isolated in the sample stream and never reach the reference stream or detector cell. This dual system also makes it possible to operate the column under vacuum—an exclusive feature that can double the sensi tivity of the instrument. The precise flow control of the Beckman GC-2 results in resolution and reproducibility unparalleled in any other chromatographic instrument. Reproducing flow conditions is simply a matter of reproducing regulator settings. The GC-2 can handle gases and liquids with boiling points up to 350° C. In most cases, it can pinpoint and measure sample constitu ents present in only one part per million. " W H A T IF MOST OF M Y WORK IS WITH LOW BOILERS?"
In this case you may find the low-cost Beckman GC-1 ideal for your needs. Designed especially for analysis of fixed gases and liquids boiling up to 80° C, this instrument provides a degree of precision and reproducibility exceeding most other chroma tographic instruments—yet sells for about half the price of the GC-2. If you would like more information on gas chromatography—or the Beckman GC-1 or GC-2 -write for Data File L-36-15.
Beckman®
Φ
Beckman GC-2
Scientific Instruments
Division
2500 Fuller ton Road, FuUerton, California a division of Beckman Instruments, Inc. For further information, circle number SI A on Readers' Service Card, page 99 A V O L . 2 9 , N O . 9 , SEPTEMBER
1957
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81
A
