'VOL. 13 (r955) ANALYTICA CHIMICA ACTA

A HIGH-FREQUENCY TITRATION APPARATUS

by

J . C. CLAYTON, J . F. IIAZEL, NV MI . ~Ic\ .11313 AND C I. SCIJ\ABLE

Jeparlueent of Chersslry, Univcrsily of 1'cuusvivanZa, Philadelphia, 1'a (U .S A )

INTItODUC'rION

Extensive use of high-frequency titrimeters has been restricted by the high cost,

complex circuitry and difficulty of construction and modification of present instru-

ments. To overcome these obstacles an easily-constructed inexpensive 1114h-frequency

titration apparatus was proposed by 11ALL° .

An instrument similar to that of I[ALI, was built but after using it for a number of

preliminary titrations, several limitations became evident . The most serious of these

was the fact that when no series capacitor was used with the titration cell, oscillation

-*%-as often completely clamped out by the conductance of the solution under test . This

possibility was mentioned by HALL . When the switch selecting the series capacitor was

set so that the oscillator could be made to fall into oscillation throughout the titration,

the instrument was relatively insensitive . Moreover, if any of the series capacitors were

used, each of the two leads to the titration cell was sensitive to capacitance changes

between it and the ground (chassis), making effective shielding of the cable from the

instrument to the titration cell impossible .

A final disadvantage was the fact that the method employed by I-LAr.r. to

determine

changes in effective capacity, described in detail by IJL`Imui, requires that the capa-

citance of the variable condenser be decreased slowly until the circuit falls into oscilla-

tion, causing a rise in negative grid bias, indicated by a closing of the electron-ray tube.

At this point, turning of the variable capacitor knob must be stopped .' If the capacitor

has been turned past the point where oscillation begins, it is not possible to turn back

(increase the capacity) to that point to get the reading, since the circuit falls intooscillation nearer the resonant frequency than it falls out of oscillation .

In order to overcome these limitations, the present apparatus was designed and

constructed from readily available parts . A number of features of HALL'S apparatus

were incorporated in the circuit of this instrument . This titrimeter, used frequently in

our laboratory for over two years, has been found to be satisfactory and reliable .

Recently HALL el al.8, published a modification of his original apparatus employing

separate tubes for oscillator and oscillation indicator .

References p . 493

4s7

488 J . C . CLAYTON, J . F. HAZEL, W. 11 . MCNABB, G . L. SCHNABLE

VOL. 13 (1955)

APPARATUS

The instrument has been designed to respond to either effective capacitance or effective resistancechanges in the titration cell circuit . The schematic diagram of the apparatus is given in Fig . z anda photograph of the complete titration assembly is shown in Fig . 2 . The circuit employs a type GVfi(metal) tube as a crystal oscillator . The titration cell is coupled to the tank circuit of the oscillatorby means of a coaxial cable and radio-frequency connectors . A type GE5 electron-ray tube servesto indicate the grid bias of the oscillator tube . A type GX5GT/G tube is used as a full-wave rectifierto supply the I3 vriltage for the tubes . A switch is provided which allows the use of either this Bsupply or an external supply,

001fly" Jdr110V

60 cy A C

F63vc.t,Xfib

7~:66000v ct

x 1meg

6x5 GOO

*'of,L-,u~t

L

oh

C1=100mmfd Z~.orC2=400 mmfd

- ~,-M X1500L 1

~,, 47= 40 turns M20

johnsFenameled wireon 11440 formfor 2000 KC.

x xcrystalexternal ground

1BOv voltmeterD.C

F.g i . S •.•hcm ttic diagram of the apparatus .

Fig . 2 . Complete titration apparatus .

Jacks are provided for the measurement of the grid bias voltage with an external voltmeter .The pasition of variable capacitor C1 , a zoo mmfd semi-circular variable capacitor, is indicated ona 4-inch dial with vernier . In a titration any change in the effective capacitance in the titrationcell circuit is compensated f nr by an equal and opposite change in the capacity of C 1. C2 , a 400 mmfdRe/erences p . 493

lion

22. 9,biasbattery

'VOL. 13 (1955)

A HIGH-FREQUENCY TITRATION APPARATUS 489

variable capacitor in parallel with C 1, allows a titration to be started at a convenient setting of C iwith various types of titration cells . Each division of the dial of C1. readable to tenths by meansof the vernier, corresponds to about i mmfd .

Changes in the effective conductance in the titration cell circuit are measured as changes in thegrid bias of the oscillator tube as indicated by the electron-ray tube and measured by a sensitiveexternal voltmeter. Ri allows the magnitude of the grid bias measured by the external voltmeterto be diminished by a constant amount of from o to 22 volts . This allows the use of a lower rangeon the voltmeter for titrations where bias does not change much . This feature was used by HALL.

The apparatus has sockets for crystals having 4- and I- inch pin spacing wired in parallel so thateither type of crystal may be used . Standard ii--inch 4-prong plug-in coils are used . Thus the fre-quency of the oscillator can be changed simply by plugging in the appropriate coil and crystal .

It was found that a warm-up period of several hours before use minimizes drift due to thermalvariations in the apparatus . A timer was set to turn on the instrument in advance so that it was readyfor operation when needed .

Titration cells

A cell adapter consisting of variable capacitors and ngle-pole double-throw switchesin a metal cabinet permitted various types of cells to be conveniently used with thistitrimeter2. For titrations in the dilute concentration ranges cells similar to that des-cribed by HALL AND GIBSON were used 7. This cell employs two metal bands, one abovethe other, around a glass tube (Fig . 2) .The titration cell circuit of this type of cell isprimarily affected by changes in the conductance of the solution contained in it . Contin-uous magnetic or air stirring can be used .

For measurements in the high concentration regions the cells were of narrow glasstubing. Metal bands again served as external electrodes. The sample solution wascontained in a beaker in which the glass tube was immersed . After each increment oftitrating reagent was added to the sample, the solution was thoroughly stirred, aportion was drawn tip into the glass tubing and its effective conductance or capacitancemeasured by the high-frequency method. The liquid was again returned to the bulksolution and the process repeated .

e

e

Since the conductivity of aqueous solutions is dependent on the temperature andincreases rapidly with rise in temperature, some type of temperature regulation wasdesirable. Moreover, in any conductance-loss type of instrument, the changes in ampli-tude of the oscillator, which are plotted versus concentration, are the result of changesin energy dissipated as heat in the resistive solution in the titration cell . "This effectresults in a rise in the temperature of the solution in the titration cell and was studied byFORNMAN AND CInSP 5 .

With the larger diameter test-tube titration cells satisfactory temperature regulationwas obtained by making the lower electrode a thin copper jacket or simply wrappingcopper tubing around one of the metal electrodes . This electrode was grounded andconstant-temperature water was circulated through it (Fig. 3) . For the narrow tubingtitration cells the beaker was surrounded by a glass jacket through which constant-temperature water was pumped . Since the active electrode was removed from thebulk of the solution and the measurements were taken rapidly, this arrangement wassatisfactory .Re/erences P. 493

490 J . C . CLAYTON, J . F . HA%EL, W. M. AICNABB, G . L. SCHNABLI

r

V

S

..

Fig 3 . Thcrnn')statecl cell for highly conductive solutions .

s rlleasurcnrcnls o/ change in a//eclivc ca fiacily

Effective capacity changes Can be measured by the method used by I3l.XDER,Fiscmr.lz and l-L\1.L' 4 0° . A suggested alternate procedure involves adjusting Cl toreproduce a certain grid bias voltage, indicated by the external voltmeter . In thismethod an arbitrary value on the voltmeter scale, usually about one-fourth of maximumgrid bias, is selected as a reference point . This is shown as P in Fig . 4. Decreasing thecapacity of variable capacitor C l causes the6V6 tube to fall into oscillation . The capa-citor is then adjusted until the voltmeterpleasuring grid bias reads this arbitrary value .The Capacitor setting corresponding to thisarbitrary grid bias value can be approachedfrom either side and is very sharp since aslight change in capacitance causes a largechange in grid bias voltage in this region .

aThis method is faster and less tedious thanthe method used by BENDER, TISCHER and

PHALL, where the variable capacitor is slowly

zrotated until the circuit falls into oscillation,indicated by the sudden closing of the elec-tron-ray tube. After adjustment of thevariable capacitor until the arbitrarily chosengrid bias value is obtained, the dial reading

Max. Mon.

is taken . These readings are made after each

Tuning capacitance

addition of titrating agent .

Fig. q . Tuning curve of a crystal oscillator.References p . 493

VOL. 13 (2955)

VOL . 13 (11955)

A HIGH-FREQUENCY TITRATION APPARATUS

Measurements o/ change in c//ective conductance

Changes in the effective conductance in the titration cell circuit are indicated bychanges in the grid bias of the oscillator tube as indicated roughly by the electron-raytube and measured by the external voltmeter. After each addition of titrating reagent,C1 is adjusted to maximum grid bias voltage (resonance) and voltmeter readings aretaken. Where changes in effective conductance are small, Ri can be adjusted so thatthe external voltmeter reads zero or some small value . A lower meter range can thenbe used, resulting in greater sensitivity . For extreme sensitivity it is possible to use upit meter, for example a Beckman model H pit meter, as a vacttum tube voltmeter .This has a 4O0 mV scale .

But in general it is not necess,uy to use a vacuum tube voltmeter as the externalmeter. A very high input impedance is not needed, since in effect the meter is in parallelwith the 47,ocw ohm grid bias resistor in the circuit . Good results have been obtainedusing standard volt-ohm-milliamnieters for external meters . Simpson model 26o andTriplett model (130 meters have been used successfully . Both have a D .C . sensitivityof 20,ooo Olin-is ,'volt . Thus, on a 5o-volt range these meters have a resistance Of T,000,000ohms, which in parallel with the 47,oc() ohnt grid bias resistor has negligible effect onthe operating characteristics of the oscillator . Furthermore, vacuum tube voltmeters,especially line operated meters, show some drift, and this can be avoided by the useof sensitive conventional voltmeters .

Response curves

The difference between measuring changes in effective capacitance and conductanceis easily seen by comparing the response curves of both methods . The response curveis a plot of the property measured by the high-frequency titrimeter versus the nor-

10IIIII1

10-'

10 - '

1

10

10'

40'

104Specific conductance (mhos)

492

Fig . 5 . Effect of electrode separation distance on high-frequency response curves. Nine-mm flintglass tubing. 2 .3-cm Aluminium foil electrodes .

A. 30-cm Electrode separation distance .B. 4G-cm Electrode separation distance.C. 6t-cm Electrode separation distance .

References p . 493

492 J . C . CLAYTON, J . F. HAZEL, W. Al . MCNABB, G . L. SCHNABLE

28 mality or electrolytic conduc-tance of the sample solution .

24

Measurements of changes ind effective capacity in the ti-_02

tration cell circuit, whenplotted against concentration

0 16

or specific conductance, displayan S-shaped response curve ;

1

measurements of effective con-ductance changes yield a

e

U-shaped curve. Only in thevicinity of the inflection point

44

-3

-2

-1

0

of the S-shaped curve or inLog KC/ normality

the regions on either side of

Fig. G. Effect of cell diameteron high-frequency response curves

the minimum intheU-shapedA 4o-mm Glass tubing .

curve is the instrument re-13 . r3-mm Glass tubing .

sponse sensitive to concen-tration changes.

Since the useful concentration regions of the U-shaped curves are wider than thoseof the S-shaped ones, this limitation is less severe for measurements of effective changes .Moreover, it has been founds that by progressively varying the electrode separationdistance, for a given cell tube and sample solution, (Fig. 5) or varying the diameter ofthe glass vessel, for a constant electrode length and electrode separation distance(Fig. 6) the useful concentration regions of the response curves can be shifted over awide range .

Titration curves

Fig. 7 illustrates the type of titrationcurves obtained with this high-frequencytitration apparatus . Curve A representsthe titration of o .2N sodium hydroxidewith iN hydrochloric acid ; curves C andD show the titration of o.iN nitricacid with o .25N sodium metasilicate .In these acid-base titrations the plottedend-points agreed quite closely withtheory. Curve B illustrates a precipi-tation reaction, the titration of o .iNsilver nitrate with o.rN potassiumchloride .

Superficially these high-frequencycurves resemble conductometric titra-tion plots, but there are apparentdifferences . In the conductometricmethod the portion of the curve nearthe end-point is usually rounded dueReferences y. 493

VOL . 13 (1955)

Fig . 7. Typical titration curves .

VOL. 13 (1955)

A HIGH-FREQUENCY TITRATION APPARATUS

to solubility, hydrolysis or dissociation of the reaction product . The significant pointsare those removed from the end-point where these phenomena are suppressed . Inthese high-frequency titration curves measurements removed from the equivalencepoint can be neglected . It is in the region near the equivalence point that the plots arelinear. These points, therefore, determine the end-point .

S U AVIARY

A high-frequency titrimeter which responds to either effective capacitance or to effective conduc-tance changes in the titration cell circuit is described . The instrument is simple, inexpensive antireliable. An easy, yet accurate, method of measuring effective capacitance changes is used . Typicaltitration curves are given . Some novel methods of temperature control are presented including theuse of the electrodes themselves as a means of regulating the temperature of the solution .

RESUME

Un titrimGtre a haute fri;quence, rLpondant aux variations, soit de capacitance effective . snitde conductance effective Tars d'une titration, est dtcrit . L'appareil est simple, pen co£tteur et stttr.UJne mtthode, facile et cependant precise, permet de mesurer les variations de capacitance effective.Des courbes de titrations typiques stint donnCes . Quelques mCthodes nouvelles de controle de tent-

t.sont pr6sentGes, comprenant l'ernploi des electrodes elles-nit•m es pour r 'gler Ia temp'rat tirede la solution .

Z US AM1IENFASSUNG

Es wird I-iochfrcqucnz-Tttrinteter, welches Variationen, set es der effektiven lCapazitanz,see es der effektiven 1{onduktanz, wahrend ciner Titration entspricht, beschriebcn . Der Apparatist einfach, billig and sicher. Es ist cite einfache and dennoch pr .irtse Methode, um die Variationender effektiven and gebrauchten Kapazitanc zu rnessen. Einige neuc Methoden ftir die Temperatur-kontrolle werden vorgeschlagen, welche (lie Verwenclung tier Elektroden selbst, um die Temperaturder Iosung zu regulieren, vorsehen .

REFERENCES

P. BENDER, J. Cheer . Tdirc., 23 (1940) 179-J . C . CI.AYTox, J . F. HAZEL, W. M . AMCNAUtt AND G. L. SCIINAULE, Cheenist Analyst, 43 (1954) 54 .J . C. CLAYTON, J . F. HAZEL, W. 1l . MCNABIS AND G L . SCIINAULE, paper presented before theInstrumentation Symposium of the Electronics Divisien at the 105th Meeting of the Electro-chemical Society, Chicago, May, 1954 .It. I3. Fiscunit, Anal. Client , t9 (19 .1i) 835 .J. FORMAN AND D. J . CRisr, Trans. Faraday Snc ., 42A (1946) 186-J . L. HALL, Anal . Chem.. 24 (1952) 12 .{4 .J. L. HALL AND J . A. GittsoN, JIB, Anal. CGein ., 23 (1(j51) grll6 .J . L. HALL, J . A . GIIisoN JR . . H. 0 . PHILLIPS AND F . I:. CRITCi1PIELD, J. Chem . Edttc., 31 ( 1 954) 54.

12a

sa67a

493

Received March 7th, 1955