Chemical Instrumentation 5. Z. LEWIN, N e w York University, Washington Square, New York 3, N. Y.
principles, characteristics, and limitatiuns of chemical instrumataCion will be surveyed. We define chemical instrumentation as comprising dl those devices that have found important uses i n chemical laboratories, ranging from balances and burets to servomechanisms and spectrometers. Our emphasis will be on commercially available equipment, and we shall attempt to provide summaries of the de& features of the current offerings of manufacturers. Approximate prices &ill be quoted in order to indicate the order of magnitude of cost of these vorim frafures. The sppciul uduoaloges a n d ' m di.~adrunlaqtsinltrnnl ~ I I these dcnirms will be crificaNu diseuss~dto ormide /he rendrr nith t l . ~!&d of knowledge he should have i n order intelligently to choose and use his equipment.
Balances
feature
the knifc cdgos wear more rapidly than they would in nn equivalent variable-load two-pan bslance. For this reason, many ~ingle-pan, constant-load balances are equipped uith sapphire bearing surfaces and/or knife edges in place of the agate used in two-pan conventional balances.
I n this, and the subsequent articles i n this series, the basic
1.
.-a
(Continued)
Single Pan, Constant-Load Balances Recent years have seen a steady growth in the popularity of the singlepan, constant-load anelyticd balance, mainly because good design has made it possible for this type of instrument to provide great speed in weighing without too great a sacrifice of economy and accuracy. These devices are based upon the lever principle but have a constant weight permanently zffixed to one end of the beam in place of one of the pzns. This eliminates the need for a knife edge a t that end, and hence only two of the three conventional knife edges are required-one a t the fulcrum, and the second tut the other (sample) end of the besm. This is shown sohemittically in Figure 7. With no sample on the pen, the balance is zeroed by means of weights suspended from the working arm of the lever such that the torque they produce equals that of the fixed mess. For convenience, and to keep the total mass acting down on the oentral knife edge small, the lever employed is generally not of the equal-arm type; instead, the fixed mass is usudly a t s. greater distance from the fulcrum (and, hence, smaller) than are the balancing weights. When a sample is placed on the pan, the torque on the working arm of the lever is now tw great, and the beam deflects. For exact rehalancing, a totsl mass must be removed from the sample side of the lever equal to the mass of the sample (see Fig. 8). In actual practice, only enough weight is removed t o bring the deflection of the beam within the renge of an optical measuring system, and the difference between this position and the original sero is directly read in mass units. I n order for this system to provide the desired speed and convenience, it must be damped to eliminate oscillations of the
Figure 8. Weighing b y Substitution. A. No Specimen on Pan; All Balance Weighk Are Down. B. Specimen on P m ; An Equal 8.1once Weight I. Lifted.
8 Figwe 7. Fundamental Distinction Between Twopan and Single-pon Designs. A. Two Pons; Three Knife Edger. B. One Pon; Two Knife Edge*.
beam. However, since the total mass acting downward is always approximately constant, the proper location of the damper t o give critical damping does not need to be varied t o match the weight of the sample. Another advantage of this substitution weighing method is the faet that the sensitivity of the balance is constant for all sample weights, since the total load on the heam always remains substantially eonstant Furthermore, since all manipulations are performed at the same end of the beam, there can be no errors arising from inequalities in the lever arms of the beam. On the other hand, the substitution method ha8 certain inherent disadvantages. The beam is always under full load, even when no sample is on the Dan, and hence
Sapphire is much harder than agate; i t is also much mole expensive. The unsymmetrical design of the constant-load beam is utilized to reduce the totsl weight on the beam, and thereby minimize the rate of u-ear of the edges and bearings. By also using long knife edges, the (constant) load per millimeter of knife edge in these balanccs can actually be made smaller than the average losdlmm of the classical t n - p a n balance. The Christian Beckcr Model NA-1 uses a unique system to protect and preserve the knife edges and boarings. This instrument cantsins two beams, mounted one above the other. Weighing down to the one-gram level is performed on the "rough" besm; u-hen the unknown has been balanced to within one gram, a control is used to raise the rough heam, while lowering the "fine" beam. The final weighing down to the 0.1 mg level is made on this fine beam. 4 further disadvantage of the single-pan design is the faet that a counterpoise cannot be used, so that weighing errors due to adsorbed moisture on large esposed surfaces cannot he eliminat,ed, and (Continued on page A68)
Volume 36, Number 2, February
7959
/
A67
Chemical instrumentation ~-
~
air btmyanc,y must L ~ P cor.orrected lor by calculation. Taring is distinct from comlterpaising, in that a. tan: is intended merely to compensate for the wtiyhl of a part of the sample or its container, whereas a counterpoise duplicates its uolume and stvface area. Taring can be readily aceomplkhed in the aingle-pan devires. Most models permit it retieulc 01. ruled acalr to be displaced l,p tlre operator for this purpose. Aft,er a weighing bottle, auteh glass, rte., ha8 Imrn placed on the pan, tlre position of t,hr reticule is a 6 justed relative to the zero of the optical scale to whatever extent is neuemsry to twing the reticule zero hack to ooineidence with the optical scale zero ( e e Fig. 9), thus s,ht,raeting out the effect of the oontniner from the weighing that is sohsequently to lw carried out. The sample added to the container is then weighed by the substitution method ail alrrsdy dexrilwd. Some single-pan bd~slsnepshave provision for adding a limited number of additional weigl~teto the fixed-weight side oi the heam for taring porposea. REFERENCE SCALE
Figwe 9. Principle of the Optical System of a Projection Balance.
I n most of the corl.ently available singlepall analytical balances, the weights on t,he sample side of the lever consist of a set of riders, metal rings, or rods that are controllud by an exten~sllymounted control with % digital indicator of t,he total ~ p i g h t , When the beam is grossly o~&ofbalwae, the direction of imbalance is visibly evident by the tilt of the beam, the movement of the optical projection wale, or may be signaled sotomatically 11y indicator panel lights or au equivalent device mounted on the case. Since the balance is critically damped, i t is a very rapid and convenient process to dial out the approximate weight of the sample. Mounted a t one end of the h e a n is a retioule containing a scale graduated in mass units, as illustrated in Figure 9. A lump and optical system facues this scale onto a ground ghss screen that can be convcniently viewed by the operator. A refemnce scale in independently focused on this screen, and a prism may be adjusted to set the retioule scale to read zero with respect to this reference scale when no sample is on the pan, or the reference scale itself may be moved, aa i n illustrated in the figure. With the sample on the psn, the reticule scde will be displaced from the reference zero hy an amount t,lmt r e p w
A68
/
Journal o f Chemical Education
sents the unhalitncc remaining xltor dislinp out t,hc major part of t.he sample weight. I n bhis a a y , a mmplete weighing t,o six significant figures ran he cornplcted in 30 ~ocondsor lorn. The single-pan balances are no more complicated than t,he classical two-pan instruments wibh rerpect to fondam~ntal principles, hut thr optienl and mechxnicsl construct,ion t ~ ~ n d tos he very much more complex, and this accounts (or the rather large increase in price of t.hcsa devices ovrr the elnssicnl in~t,rnment,s. I t has heon maintained that thc greater number of weighings that can he medc per working day with t,ho single-pan balnne~s can more than offset tho highel. cost. At least one nnivrrsitv rurrcntly lues some of these hslaneea in t.he undwgradxate quantibxtivn analysis coursrs to replace 2-3 time8 thcir n~tmherof two-pm balancefi. Table 2 summariscs the rlenign characteristics of tho currently most widely usod commercinl hnlancc.~of t,ht singlepan, c ~ n s t a n t ~ l o at,ypo d with sensitivities suitable for macro-quantit,ative work.
Recording Balonces The moat rrrent innovations in instl.nmentation for sn2~1yhicalweighing have been the introduction of commercial halanees that record wights automatieslly and continuously, or that ubiliae electronic teehniqws of measur~ment. The desirability of an nut,amated approach to precise weighing for many IPSPXI.C~ and control purposrs has been recognized for R fairly long time, and there havc been many attempts t,o develop such hshnees. Unt,il recently, the only rornmereial recording balance was that produced by the SoeiBt6 A.D.A.M.E.1,. in Prance, according to the design of P. Chnvenard (US. dist,rihutor, it. Y. Fernnr co., Maldon 48, Mass.; Price $2835). In this instromont, s. mirror mounted on the balance beam reflects a light ray ont,o a. drum of photagraphic paper. A s the hxlanco beam dcflrcts, due to aright rhnngra, the light
Figure 10. Bdance.
Design
of
the Chevenard Thermo-
ray traces n. record of t,hrse deflections on t,he paper (Fig. 10). The capacity of the helanee is JO grams, and the sample to be studied may have m y weight between 0.1 and L O g. The starting position of the light spot on the photographic paper is adjusted by appropriate countorweights placed on the hnlancc. Weight changer; are proportional to distance on the paper, I mg corresponding bo a dir;placement of 1 mm. The sensit,ivity of d ~ t r e t i a n of weight changes is of tho order of 0.2 bo several milligmns, depending upon ;tmb h n t condition^. The Chevcnard hlll~rlne (f.'onlinud on p o g ~,470) Volume 36, Number 2, February 1959
/
A69
Chemical Instrumentation is specifically designed for the recording of weight changes occurring during the heating or cooling of a specimen, and has a regulated cleetric oven with a bifilsr winding mounted over the sample in such a way its to minimize thermal and mechanical disturbances of the balance. This instrument has been extensively used since its snnouncement in 1944, and has served to establish the new technique of thermogravimetry as an important and powcrful tool in physical and analytical chemistry. I t s cheracteristies are treated in detail in the book of C. Duval (see Bibliography). However, this balance has a m ~ m k . rof disadvantages, not tho least of which is the inconvenience associated with the use of a light beam and photographic accessories as the sensing system for small displacements.
Secondary Signal displacement of armature to L
left
+
right
Figure 1 1 . Schematic Diagram of o Differential Transformer, and the Variolion In h e Output Signol From Its Secondory Winding a. a Fvncti0n of h e Position of the Arrnohlre.
An accessory has recently been announced that permits conversion of a. Chevenard balance from photographic to pen-recording (price of Chevenard balance modified for pen recording: $4130). The mirror is replaced by a. pair of eketrical contacts. Deflection of the beam causes the contack to make or break. When the oont~ctora t the end o i the balance beam touches its mate, a servomotor drives the latter upw.ard until the contact is hraken, whereupon the motor direction is auto ma tie all^ reversed and the moving member ia driven downward until it again makes contact. Thus, the moving contact constantly hunts about the position of the beam contact; the motion of the moving contact is mechsnieally linked to the motion of a pen on s paper chart. The pon therefore tracks the movement of the end of the halance beam. The Feruer Company also markets an accessory, called the "Microverter" (pries:
A70
/
Journal o f Chemicol Education
$426) consisting of a differential transformer, the core of which is to he mounted in place of the mirror. The movement of this core relative to a fixed winding generates an electrical signal that can be used to drive a pen recorder. The principle of the differential translormer is illustrated in Figure 11. This device consists of a primary winding, through which s n ac signal is passed, and a split secondary winding, the two parts of which me connected in electrical opposition. When the soft iron core is s p m e trically placed between the two secondary coils, the induced voltages in these exactly cancel each other, and the output is zero. S s the iron core is displaced from tho halanoed condition, the output increases, a t first linearly, but with increasing deviation from linearity for large displacements. An all-electronic approach to a. recording balance has recently been placed on the market by Ainsworth (Model BR-AU; price $3690). In this instrument, s. center-tapped induction coil serves as the nnhdmee detector. This coil consi.it8 of two windings mounted one above the other, and a metal slug (armature) that is suspended d o n g the common axis of these windings, placed symmetrically with respect to both. This is illustrated schematically in Figure 12. The two coils are connected in a bridge circuit, so that if the armature haa a precisely symmetrical magnetic (due to mutual and self-induction) effect on both, the hridge iia balanced when the other two arms have equal resistances. When the armature, which hangs freely from one side of the balance beam, moves so that i t is in closer coupling with one m.nding than the other, the bridge becomes unhnlaneed, and if the movement is not too large, the bridge unbalance rises linearly with the distance from the null position.
U Figure 12. Diogrammotic Representation of the Ainsxorth Recording Bolonce.
In the Ainsworth recording balance, one arm of the bridge circuit contains the slide wire resistor o i a continuous belanee recording potentiometer. The servomechanism of the potentiometer sutamittieally and continuously sdjusts the position of the slide wire contactor to reduce the unhalanee signal t o zero. This oontaetor is mechanically linked t o a. pen which traces a record of its movements on a strip chart. When the beam deflection exceeds 0.02 inch (corresponding t o 102 mg) the slidewire contsctor reaches a position where it operates a relay that causes a 100-mg weight t o drop onto the beam, and thus restores the heam and induction coil armature to the null position, ready for the next 100 mg of weight increase. If the weight decreases, these processes operate in reverse to remove weights from (Cmtinued a page A72)
Volume 36, Number 2, Februory 1959
/
A71
Chemical instrumentation the pan. Thus, the instrument operates on the null-hslance principle down to the 0.1 g level, and on the direct deflection principle for the Isst 100.0 mg of the totill weight The capacity of the balance is 200 g; electrical weight loading occurs a t 0.1 g intervals up to a maximum of 4 g. The sensitivity is 0.1 mg. The American Instrument Co., Silver Spring, Maryland, has also come forward with n recording balance, but this instrument differs markedly from those a.hihieh have ulrpady heen described. I t is called the "Thermo-Grav" (price: about $LZ,000), and is designed specifically for the recording of thermogranmetric cmves up to 1000°C of samples maintained either in a vacuum or a controlled atmosphere. The temperature of the sample is sensed by a themlocouple, the electrieitl signal of which drives a pen on a chart recorder along the X-axia; the weight of the s a m p l ~ia sensed electrically by means of a null balanciug bridge circuit, the error aignal of which drives the recorder pen along the Y-axia (see below). Thus, the reeorder plots a, graph of the sample weight aa a function of temperature. [Since the recorder pen simoltaneously responds to independent signals in two of the Cartesian coordinate dirwtions, it is called an "XY-recorder!'I Alternatively, the sample may be milintwined a t a fixed tempe~ature,in which e w e the recorder can be arranged to plot the s;mple weight ns o. Rmctioa of time.
$
SPRING
Figure 13. Diagrammatic Representation of the Aminco ThermoGrov.
In the "Thermo-Grav," the lever principle has been abandoned in favor of the extension spring. Hanging from the spring are a soft iron armature, a support for weights, and a sample crucible, as illustrated in Figure 13. Electrical controls permit the furnace to be heated linearly s t rates from 3'C per min to 18°C per mi". Two furnaces are provided to permit consecutive runs to be made without having to wait for the furnace that has just been used to cool down to room temperature between runs. The eleetrauie detection of the weight
A72
/
Journal of Chemical Education
both as a function of time. Units me beam release and srrestment, governed by of tho specimen ia h a s d upon the elecavailable with capacities of 16 to 50 g, a periodic timer. This device protects trical null halance principle. The two and maximum furnace temperatures rangthe knives and repositions the heam hefore halves of the secondary winding are coning from lO0ODC (e.g., Model TR-01, drift or vibration affects the balance nected in series aiding (not series opposi0.1 mg sonsitivity, 10 mg deflection range, accuracy. The helanee capacity is 120 g, tion, as in the differential t,rmsformnr +0.1 g electric weight loading, price and the mrtnimum rate of weight change doscrihed). The voltage a t the $4075) to 1450°C. that can he followed is 400 mglmin. midpoint of this econdssy winding, which Still mother type of electrical system is Model It-1 has 1 mg sensitivity and lists depends upon the position of the iron core involved in the (non-recording) Cahn xt S3425; Model R-01 has 0.1 mg sen& hanging from the spring, is compared Eleett~ohalnnce(Model 1400, price $695). tivity and is quoted a t t3fi50. electrically with a voltage tltpped off by a Here, the null h a l ~ n c e is determined eontactor on a patent,iometer slidewire, These recording Idances are also availvisually, hy bringing a pointer to a fixed and n servomnehmism moves the dideable in the form of thermo-recording units, reference point on a scale, hut the torque wire contactor until R null in this "error with n furniloe mounted ahove the beam, t,o accomplish this is applied hy mrans of signal" is achieved. The po~itionof the and a two-pen recorder to provide a simulpotentiometer contaet,or is thus a linear (Continued on page A741 taneous record of w i g h t and temperature, function of the oore po~ition,and hence of the sample weight. The linearity of this TO CHASSIS type of circuitry tends to hc hetter than that of the differentid transformer. The "Thermo-Grav" has a crtpaeity of 10 grams, and will drtect weight changes from 0 to 200 mg with sn accuracy of about 2 mg. A recording hxlanco hased on the lever principle, hut employing a non-transformer (i.e., non-magnetic) hype of pick-up for detecting tho hesm deflection is ~ v ~ i l able from Stant,on Instruments, Ltd. N ( U S . distributors: Burrell Corp., 2223 Fifth Ave., Pittsburgh, 1's.). This balance employs a st,andard Stanton analytr ical balance (Model AD-2 or AD-41, to the beam of which is fixed one plate of M iNDiRECTLY a n electronic condenser, as shown scheGROUND VIA S matically in Figure 14. This plate of the D BALANCE CAS capaoitor is fastened to the end of the heam and moves up or do&S the beam deflects. The other (follower) plate of the Figure 14. Diogrommotic Representation of the Stonlon Recording Bolonce, Bared on a capacitor is located immediately ahove Variable Cop.cit0r Transducer. the heam, and is pivoted in line with t,he plane of tho hslxnee center knife. This plstc i~ BWVO-driven,and follows ?very minute movement of the beam to maintain a t all timan u. constant capacity gap. This capacity gap, which is of the order of magnitude of 4 micromicrofarads, form8 one arm of a sensitive, compensated capacity bridge circuit, the error signal of which drives the ~ervomechanism. Ko attempt is made to measure capacitances directly or to eslihrate the capacitance in terms of ssmple weight, for such calilrrations arc prone to instability and drift. Thore is no direct grounding connection, or m y other form of mechanical contact with t,he balance heam. Poise and sensitivity are adjustable in the normal way, and the full original accuracy of t,he hdance is preserved. I n the Stanton recording balances, the recording pon is directly linked to the upper followor plate, and advantage is taken of its servo drive to operate electric weight, loading a t the end of each full beam mowment. When the dcfleetion hecomes great enough to exceed the full ohert range, a relay is tripped, and a weight (0.1 g in Model R-1; 0.01 g in R-01) i~ automatically added or removed to bring the beam hack to its starting position. The chart range is 0-100 mg on Model R-1 and 0-10 mg on R-01; when the deflection exoeeds this range, s. switch is tripped, and the pen is moved back to the zero line again. Thus, the pen may travel across the chart paper ten times in either direction heforc ~xhausting the electric weight loading. The balance is of t,he three-knife-edxe typt., with air damping, and automatic: motoriard control of the
6
Volume 36, Number 2, February 1959
/
A73
Chemical Instrumentation a direct current flowing through a coil that ie part of the sample supporting mechanism. The electric current takes the plscc of conventional weights and riders, and can be edibrsted in mass units by means of standard weights. This instrument is bafied upon the common D'Arsonval meter oonstmction; the sample essentially deflects the meter painter, and a suitable electric current through the coil, which is suspended in the field of a permanent magnet, is utilieed to counterbalance this force and bring the pointer hack to zero. This is illustrated in Figure 15. The advantage of this elcetromagnetic approzch is its convenience, and the fact that it affers freedom from environmental effects which produce the slow drifts of rest point and sensitivity observed with mechanical balances. The instrument is r s p ~ d ,and is readily adaptable to remote operation. It han a maximum capacity of 175 mg, end the range of sample weights is sdlustahle from 0-1 to 0-100 mg by oontrolling the coil volt, age. The maximum sensitivity is 1 microgram.
Figure 15. Diograrnrnatic Representation of the Cohn Electrobdmce.
Sartorius offers an electromagnetic balance based on a similar electrical principle t o that just descrihed (Model Electrons I, price $1935). This device has s quartz beam, provision far taring, and a stabilized amplifier for precision eontrol of the balancing current. Tho capacity is 1 gram, with a maximum sensitivity of 1 microgram per sesle division. The electrical torque is achieved by the use of alternating currents, which permits greater sensitivity and better control than the de method. The Fisher Scientific Company now offers sn accessory (price, $625) that may be attached to an wdinary analytical balance to permit i t to be used in conjunction with 8. potentiometer chart recorder ns im automatic recording halanee. The accessory consists of a differential transformer and amplifier for sensing deflections of the halance beam, and a n electromagnet assembly for applying farce to the henm to hring it hack to its equilihrium position. The full scale range is oontinuously variahle from 10 mg to 5 g, and the linearity is hetter than 1% of full scale if there are no nearby magnetic substsnccs or fields (e.g., iron salts).
(Conhued on page A78)
A74
/
Journal o f Chemical Education
Chemical Instrumentation Sartorius also offers a reoording scces sory designed specifically for the Sartorius Selecta, hut usable \\ith other balances as d l . I t consists of a low voltage light source, a. collimator system, two rightangle prisms, a vane t h a t is t o be mounted over one of the beam stirrups, and a photccell. A stabilized amplifier provides a n output t h a t is proportional to the intensity of the light striking the photocell. This, in turn, is s. function of the position of the van? with respect t o the light source, and hence of the angle of deflection of the balance heam. Thus, movements of the beam are translated into electrical s i p & that e m he frd directly t o a n indicating mrter or s recording potentiometer. .4n "nutomntie," electronic, or recording balance cannot he m y more accurate
introduced by the circuit components. The justification for cluttering up a. rrnsitive, prpeise weighing device like an nnalytiertl balance with complicated electrical circuitry and recording mechanisms can only he the special capabilities inherent in t h i ~type of automation. These are, in descending order of relative importance, (a) increased ratc of obtaining data, ( b ) adaptability t o remote or programmed operation, and ( c ) convenience to the operator. These capabilities, parliculndy the first two, are 80 important, in s nnmher of experimental situations (such as, for example, thermogravimct,ry, corrosion studies, admrption measurem e n t ~ ,radiochemistry) as t o justify the effort that he8 gone into the development of ~ l t t o m a t e dhalancen. Alt,hough ;a number of different instrumental approaches t o automated weighing have heen deserihed above, the possi-
Table 2.
Bibliography BIETRY,L., "Why Substitution WeighChimie, 11, No. 4, !12-!)6. ing?" [Available from Mettler I n s t m m ~ n t Corp., Box 242, Hiphtstorvn, S o w ~ e r s e yI . I~ASHOF, T . W., A N D MACIIRDY,12. R , "Testine a Quick-Weiehine Balance." Anal. CThem., 2 6 , 707-712 (1'J5 ALIPERIS, I., "Performance of s PenRecording Chevenard Thermol,~lmm~," Anal. Chem., 29, 48-54 (19571.
Design Characteristics of Single-pan, Consfont-load Analytical Balances
Make & Model
Capacity
American Bnl. Co. AinsnorthRight-A-Weigh Christian Bcrlcer N.4-l Fislrw "Gt.am-:~tir"" Mcttler H-5 hleatler B-5 Met,tle~. B-5 C1000 .\likromn AW-lob
200 g 200 200 200 160 200 1OO0 200
Xikrowa AW-50" Sauter Monopan a
bilities have by no means been exhausted. For a n example of recent work with the variable inductance transducer, see Cochran, C. N., Rev. Sn'. Instr., 29, 1135 (1958). A recent application of a photoconductive detection element for direct measurement of the deflection of a halxnre beam i~ described by Cannon, P., ibid., 29, 1115 (1958). A relatively simple recording thermobalance based on n torsion wire and phototube-controlled scrvo~neehanism is described by WendIsndt, W. W., Anal. Chem. 30, 56 (1958). Nllmerou~other references to nutomstod hxlnnees e m he found in the recent. litet.aturc. Two makes of single pan balances tlrat m a p e d inclusion i n Table I of port I a m the Stanton 12cleeleas-04latie s w i m (distrib71101: B w i e l l C o ~ p . ,Piltsbwgh, Pn.) and the Afondial belanee (distrihrdor: Luz Scientific I n s t n ~ m a tCo., N . Y . ! .
100 200
Knife edges
I!ireet read~ n range g
Price
100 mg I 00 1000 I15 1200 115 115 1010
$805 8!l5 1185 8!15 (is0 8!15 1450 8!15
Sapphire Snpphire Agst? S8~pphil.e Sapphire Sapphire Sapphire Stainlcssfiterl (Sapphire he:~ling~i
102 110
Sapphire
!]!I5 800
Made by Mett,ler for distrihutinn h.v Fisher Srinllifir Co. Standard Scientific Supply Corp., New Y o l k
W.S. distrihutov:
The present nrticle concludes the ncrvey qf mmmcrn'ally available laborabv, anal-
?,tical balances tho1 was initiated i n the J o n u a q issue. Nezt month's disrzmion will be the first of two articles denling with temperature-eontrollad baths. The March installn~enttreats the principles of temperaturc control, and of the design of bath components. The 4 p d avticle covers the characteristics of comniareiall~, available thermostat baths, thernmegtrlators, relaus, heaters and stirwxs.
A76
/
Journal of Chemical Education
,
w i n g Devices; pH Plctcrs.
"
Septembe~ and Orloher-
I n p i r i e s and comments ronrrrniny chemical instntmentatia are irwitrd from readers. If of s u f i c i a t yenerd interest. such inquiries or comments will b~ yioen space in these eolumns.
