New developments described in magnetic susceptibility, portable integrators, recording balances, rnuintenunce OF silver concentration in hypo solutions analytical instrunientaM tion . ofis ourconcerned with improved OST
methotls for increasing the speed, precision: or con\renience of conventional techniques or for rendering them completely automatic. There are dozens of phenomena awaiting further development to make them useful to the analyst. In our opinion, magnetic susceptibility is: one of these. Although the classical Gouy balance is still the most prpcise means for determining magnetic susceptibility, no analyst in his right niind would consider its use in solving routine analytical problems. &4s a conspquencp, this technique is used only for fundamental studies on structure or to supply confirmation for theories of electronic and molecular structure. Some 11 years ago Broersma made a careful st'udy of the induction method and, later: Selwoocl investigated its possibilities. The induction method of determining magnetic susceptibility depends on the change in induction of a coil in a magnetic field which occi!r? when a sample is placed xithin that coil. I n Broersma's apparatus, two mat,ched secondary coils of 1400 turns each, connected in opposition, were located within R single layer solenoid excit,ed by a constant a x . supply. Unavoidable imbalance in the two secondaries was cancelled out, by removing turns from the coil with the higher output and final balance was obt'ained by cnreful positioning of a small iron slug in the vicinity of one coil. By using amplification and a suitable detector any residual unbalance in the secondary output could be measured to within 10-8 volt. Now, after establishing zero output to this precision, a sample can be introduced into one of the secondaries and its change of induction is a directly calculable function of the magnetic susceptibility of the sample. The rectified output can be used to drive a recorder. A recent modification of this technique has been described by Norder [Rev. Sci. Znstr. 31, 8-19 (1960)] for operation at temperatures as high as 500' C. Although the applications were not of an analytical nature, Sortler has used the equipment for measiirement of the paramagnetic susceptibility of metals during exposure to gasas, specifically hydrogen. Inaemuch as t h e
n.ork required remoi-a1 or addition of gas during measurement and at fairly high temperatures, the paper illustratee the great flexibility and speed of the induction method. Most applications of the induction method have dealt with substances of fairly high paramagnetic suscept'ibility. Precise measurements with feebly paramagnetic or diamagnetic substances. would present considerable difficulties The importance and utility of such measurements would seem to justif!. further investigations.
Battery-operated, Field-type Integrator
The continuing effort t o develop new arid better integrators is understandnble. I n many cases a mere recording of dat,a is only part of the problem, because the final information may require an integrated value and to provide this is important because the necessary computation, by conventional means: may involve much more timc and effort than the measurement itself. A sensitive and simple integrator has been described by Frank [ J . Sci. 1 m t r . 37, 276 (1960)] which is battery-opcrated and suitable for field use. The sensing element is a standard millivoltmeter or microammeter, the pointer position of which is sampled at regular intervals by electromechanical means, the required movement of a feeler t ' l m being converted into rotation of a counter. The integrator accepts signals of varying polarity and also indicates the extreme signal level during any chosen period. It, was designed for the integration of signals obtained from netradiometers or solarimet'ers and: according to the author, "without such aid the analysis of the records to give daily radiation amounts is very laborious, particularly SO when broken cloud causes the radiation to fluctuate very midely. Some solutions to this problem are undesirably complex and expensive-eg., the use of amplifiers and integrating motors and integrators attached to recording potentiometers." According to Funk, elect'romagnetic integrators of the Gilbert type did not, prove entirely reliable. I n his instrument the meter pointer is clamped peri-
odically and a feeler moves up to !oca!c the pointer. An aluminum vane, mo\.ing between the poles of a magnet, provides damping of t'he feeler to lessan the impact of the feeler arm. During the return of tmhefeeler, a wheel which has moved with it ie prevented froni returning by a ratchet. Twice per revolution of this wheel, pins actuate :I contact which energizes a register. To accommodate signals of reversed polarity a contact slightly above zero is arranged to close and any subsequent signals below zero will actuat,e a reversing mechanism. Battery drain in the instrument is minimized by opernting most relays and all registers froni capacitative di2charges.
New Recording Balance
Still another form of recording balance has been developed. Van d~ Breggen and Wouterload [ J . Sci. ITistr 37, 297 (1960)] employ an electromiiscnetic system for balance restoration ant1 a novel displacement detector. Thc latter consists of a difierent,ial capacitor-i.e., two capacitor plates with :L movable plate between them. Motion of the central plate toward one of the fixed plates, and therefore away from the other, increases the capacitance 011 one side and decreases it correspontlingly on the other. By energizing thc capacitor with an r.f. oscillat,or through two crystal diodes, a d.c. output signal is obtained, t,he magnitude of which dcpends upon the deflection and the sign of which depends upon the direction. I n this way no separate phase-sensiti7.t rectification of an a s . transducer output signal is required. The oscillator employs a twin-diode pentode to provide a 2-megacycle output of 12 to 15 volts. One of the diode plates permits easy control of the output voltage. A high-gain starvation-type d.c. amplifier supplies sufficient current for the forcerestoring coil, or 5 ma. per gram or 1 ma, per 200 mg., as well as output for a 0 to 10 mv. recording potentiometer. I n studying the dehydration of copper sulfate pentahydrate, the int'ermediate format,ion of trihydrate was established by the authors, confirming the observations of Taylor and Klug and also Schultz and Dekker. The exVOL. 32, NO. 13, DECEMBER 1960
a
107 A
INSTRUMENTATION ixence of this compound is not generally accepted. Silver Concentration in Hypo Baths Maintained Automatically
The c l i m i i ~ ti? intiispensable in alnioit every phase of atomic energy respareh ant1 inore likP1y than not he will
bc an a n d y s t . Once again his arinamentariiini of tecliniyiies has been drawn iipon, i n t'liis case t o niaintain con;tant silvcr concentration in nnclear rniiilsion fixing baths. In such baths if the si1vc.r concentration is too lonthe siirfacc grains in the Pinuision will bc rrchctl :i\v:iy) and it n.ill bc difficult or iiiipoisible t o follow near-minimmn tracks through the toll 10 to 15 micron emiil~ion. 0 1 1 the othcr hand dktortion can rcsiilt from the taming efi'ect of the. uilvcr t l i i o d f a t e complex if the silver conccntration is too high. Satisfactor!- results are ohtaineti if the ailT,cr con c c ~rnt t ion is niaint aine ti in the region of 2 t o 6 granis of sil\-er per liter t Iiroiighoiit fixa t ion. Dahl-,Tensen of Cop~nhagrnhi< tlcvisrti an automatic niraiia of doing this [ J . Sei. I!lsfF. 37, 360 (1960)l. Tlic silver cwiceiitrntiori in tlie hypo bath is meaqrircxl with a silver elcctrodc-calomel c,lccrrotle pair! t,he potential of wliich i, m c i w u r d with :I pH nicter. Dnhl-Jencen lins given :t
1 10 A * ANALYTICAL CHEMISTRY
covenient p.rnpli relating rlectrode potmtial to hryo aiitl d y e r concentri!tioils. He has shonii thnt t h e potcntial i p not affected by thc addition of sodium hi?rilfitc, codiiim sulfite, sodiirin arctare, ;jcctie wid, or gelatin. Changc,; of pH wili li:i~-(> n o effert grwter than *1 my, The output of the pH nic'ter is fed to a liiic-opc~ratetlelectronic relay containing two tIvin triodes. \-olLage tlerived from a Helipot is fed to the second grid of tlie first triotk t o set the control point of t h e relay. The Helipot is calibratetl to indicate the desired electrode potential at whkh ctontrol is to take place. The relay twntacts opcrate a solenoid valve t o atlniit hypo to the bath. Once the initial t1c;ireti charactcrietics are set into t h c system, silver-hypo coiiiplexe,~will iliffuie out of the emulsion ant1 as ?ooii :IF the 110.. tential changes by 5 niv. iron1 the set, point, hypo i d 1 be aclniittetl. The niicroscopic exuniiiiiintion a n d measurement of nuclear e!iiulsions are tedious and, \-er>. frequentl>,, tlie original experiments in wliich tlie eiiiulsionv are exposed a r e exacting and not too easily repeated. This analytical assurance thar an iinportaiit stage of the emulsion processing s1i:tlI he perfect is an important contribution t o the wholc problem.
