Stable High Frequency Titration Apparatus in the 100-Mc. Frequency Range ARTHUR
H. JOHNSON‘
and
ANDREW TIMNICK
Kedzie Chemical Laboratory, M i c h i g a n State University, East Lansing, M i c h .
mounted on a brass plate 3/s-inch thick. The coaxial receptacles were so spaced that, t h e spacing of t’he banana plugs soldered t o t,he center conductors of the female receptacles was the same as the spacing of the terminal plugs of the coils previously used. Figure 3 s h o w the half-wave line mounted in position. During operation the line was clamped t,o a vertical rod fastened t o t h e Jrork bench. The 955 tube, V l , and its associated components were mounted on a small subchassis of 3/16-in~haluminum plat,e attached t o t h e main chassis of the instrument by means of suitable brass standoffs. Access t o the tube mas provided t’hrough a hole in the top of the main chassis around which a suitable shield assembly was secured. The titration vessel, 6 cm. in diameter and 8.5 em. high, was constructed from a 250-ml. polyethylene bottle. -4collar located about 2 cm. from the top of the vessel regulated t’hevolume of the vessel u-hich could be lowered int.0 the bands. Two copper bands, 1 em. high and G cm. in inside diameter, were mounted 2 em. apart by polystyrene retainers; the retainers were rigidly mounted inside a metal box which served as a shield for the bands and the titration vessel. Leads from the two bands were soldered to banana plugs insulated from the box. A third plug connected to the box grounded the box to the chassis of the oscilla-
Replacement of the multiturn coil bj a coaxial half-wal e line in the oscillator circuit of a high frequency titration apparatus raises the operating frequencj to 130 mc. R? following the oscillator tube grid current change, using a 3lodel S X I Sargent Polarograph, practical instrunient response is obserbed with aqueous solutions in the titration cell varjinp in concentration from 0.001- to 1.V in sodium chloride. With perchloric acid in glacial acetic acid solutions, response is observed for concentrations up to 0.041.1in perchloric acid.
D
U R I S G consideration of the construction of a high frequency titration apparatus for a systematic study of pos-
sible applications of high frequency titrations, the instrument described by Anderson, Bettis, and Revinson ( 1 ) was selected 011 the basis of its stability, sensitivity, simplicity of construction, mid flexibility for possible modifications. I t s performance as a “caapacitative” type instrument operating in the frequencjrange from 12 to 130 nic. is described. Oscillator frequency change is the response measured. Response curves for an instrument operating at 130 me., obt,ained by measuring the oscillator tube grid current change, are also included. Ext.ension of the operating frequency t o the 130-me. range was accomplished by replacing the conventional multiturn coil in the oscillator circuit d t . h a coaxial half-wave line. The t.erm “capacitative” is used to denote the type of cell emploj-ed and not the response measured. The term i? suggested t o differentiate between the type of cell in 1Thic.h the cell bands or plates are in contact with thp iralls of the titixtion vessel, const,ituting capacitor construction (the type used in this study), and the type in which the titration vessel is placed in the coil of the oscillator circuit, a “coil loading“ type instrument originally designed by Jensen and Parrack (j),
Figure 1.
FR EQU ETCY M E A SL‘HING I %‘STRUM ENT
C,.
Schematic diagram of 120-mc. titration apparatus
Cell assembly
CI,C j , Ca, C1. 100 micromicrofarads. mica
Instrument and Cell. A capacitative-type instrument similar t o the prototype mentioned above ( 1 ) was constructed with some changes in mechanical arrangement and cell design. Essentially the instrument circuit was that shown in Figure 1, without R2 and C,. For operating frequencies up to GO me., L1 was a multiturn coil, and in the 120-nic. region L I s-as a half-wave line constructed of RG S/U coaxial cahle. Three individual coils were wound so that the instrunient operated at 12.2, 38.8, or 60 mc. (45 turns of ?;o. 30, 15 turns of S o . 21, 8 turns of S o . 24 insulated copper wire, respectively 1. Each coil was wound on a 1/2-inch coil form mounted on a suhstantial insulating base. T w o banana plugs were attarlied to this base, and the ends of the coil were terminated in the plugs. The coil shields were made of heavy gage copper and n-ere firmly att,ached to t,he bases of the coil forms. T h e roil and shield) its well as band-type cell const’ruction, are shown in Figure 2. K h e n plugged into position, a spring collar attached to t,he (ahassisgripped the Ion-er edge of the shield firmly, rompleting the shielding to the chassis, as 71-ell as preventing any ph>.siral movement of the coil assemhly during a titration. The half-wave line vas a 90-em. length of RG 8/U flexible coaxial cable, the ends of which were terminated in Amphenol type PL-259 male coaxial cable connectors. T h e cable was bent hac~liupon itself t o bring t,he two cable connectors side by side, and then t,he parallel sect,ions of cable were t’aped together. hdded ripidit>-\vas given to the line by taping it to a short length of 1 :?-inch wooden don-ling. T h e male coaxial cahle connectors a t the ends of thp line plugged into female coaxial receptacles
20 microfarads, 450 volts L I . R G 8/U coaxial half-wave line, approximately 90-cm. total lenath (see text) L1. 10 turns No. 22 wire wound around Ra. La. Filter choke, Gracoil 200925 Rt. Rd. 15.000-ohm. 1-watt C:, Ca.
v2.
5Y3
Vz. T’R 150/30
tor. T h e cell assembly mas designed so that the polyethylene vessel could be removed by simply sliding it vertically out of the bands and the remainder of the cell assembly. The fit was snug enough to prevent any movement of the vessel relative to its surroundings during a titration. For operation a t 120 me., the bands around the titration vessel provided more capacitance than could be tolerated. They were replaced by two plates 3 cm. high, 2.5 em. long, and 2.5 em. apart. The plates were mounted in the same plane and curved t o fit the contours of the polyethylene vessel. T h e polystyrene spacers that had previously supported the bands were left in position t o center the vessel and t o prevent its physical displacement during a titration. Details of component arrangements are shown in Figure 1. With an empty titration vessel in the cell assembly and the
___1 Present address, Bauer and Black, Chicago, Ill.
889
ANALYTICAL CHEMISTRY
890 cell assemblv nlueeed into the oscillator, the omratinp frecmencv of the titrafidn ai:aratus was measured with.& U.S.-ArGy BC1255-5 heterodyne frequency monitor. T h e frequency was found to be 120 me., a good cheek on the frequency expected on the basis of the leneth of the half-wave line used. The f r e q u e n 6 change of the oscillator resulting from solution composition change in the titration vessel was followed. A U. S. Army BC-221-D heterodyne. frequency meter was employed. Bemuse the highest fundamental frequency of the calibrated oscillator in the frequency meter was 20 mc., higher frequencies necessitated that higher harmonies of the frequency meter oscillator be used to obtain an audible heat note. A short antenna
solutions, as should be attained according t o Blaedel and Malmstadt (8;4). Frequency drift of the frequency meter oscillator and the titration apparatus oscillator combination was followed far a 30minute period. An increase in frequenoy was noted during the first and last &minute intervals. The frequency appeared to remain constant during the time interval from 9 to 22 minutes. Slight scattering of experimental points in the titration curves indicates desired stability of the instrument. Frequency drift was' observed when the filled titration vessel was inserted into position in the cell assembly. It was assumed that temperature change was the cause. The effect was detected by following the temperature change of a dilute sodium chloride solution in the vessel inserted in the cell assembly and the corresmnding frequency change of the oscillator of the titrat,ion apparatus. After the first 15 minutes, the temperature rise and the frequency increase had passed ihrough a maximum and then leveled off. The maximum i m a reached ttt 5 minutes, during which the temperature rise ,\-as 0.47' C. and the meter reading increase was approximately 1 division. F a r t h e n e s t 15 minutes, the solution tempera ture remained constant a t 0.11" C. above the initial temperature, and the frequeney meter reading remained a t about 0.3 division above the initial reading. The results of this experiment are presented in Figure 6 . On subsequent titrations the vessel containing the solution RBS introduced into the cell asfiemhly about 10 minutes prior to reagent additions. GRID CURRENT MEASURING INSTRUMENT
Figure 4. Cell construetion details for operation at 120 m c .
Figure 3. Half-wave line and conneotion to titration apparatus
t
o.N.CI
The frequency measuring instrument was modified 80 that grid current ohanges could be measured. The parallel comhination of R2 and Ca was placed in
I N H.0
* H C l O e I N G U C I A L HOAD
t
connected to the frequency meter provided sufficient signal for measuring purposes a t the three lower frequencies listed. For frequency change measurements a t 120 me., a loose coupling between the plate of the oscillator tube and the frequency meter mixer was made through a small capacitor. Connection v a s made by means of coaxial cable from the frequency meter to the titration apparatus a t the female coaxial receptacle located on the side of the chassis shown in Figure 3. No pulling of the oscillators was detected. The output of the frequency meter was amplified in the audio channel of an R.C.A. Rider Channelyst. Sufficient volume was provided in the earphones after amplification. A vimal indication of eero beat conditions could be obtained on the appropriate magic eye in the Channelyst if desired. Sensitivity and Stability. A sensitivity curve for the 120-mr. titration apparatus was obtained for sodium chloride in water and ia shown as curve A in Figure 5. Practical sensitivity is attained for a specific conductance range corresponding to that of sodium chloride concentrations from 0.005 to 0.08M in aqueous
C O N C E N T R A T I O N , M O L E S PER L I T E R
Figure 5.
Response curves
Frequenoy change responae at 120 ma. and grid current change at 130 rnc.
89 1
V O L U M E 28, N O . 5, M A Y 1 9 5 6 4270
05
4269
04
4260
03
6 4267
02
4
A
Table I.
--
Titration of Phosphoric Acid" with Sodium Hydroxide T y p e of Titration H F , 137 mc. HF, 137 mc. PH
Series b A
X a O H , 111.
I 4266
i z t h y l orange indicator H F , 137 mc.
01
B 4265
PH
00
0
5
10
15 20 25 TIME (MINUTES1
30
35
40
Figure 6. Frequency change associated with temperature change of solution in titration cell assembly
2 75 2 75 2 72 2
z.2
2 ,a
2.66 2 65
+
a 100 ml. of phosphoric acid stock solution 35 ml. of water titrated with 0.1958.V sodium hydroxide. b A and B, different solutions of pliosi~lioricacid.
2750 2740
2730
2720 0
P
9 rn
2710 2700
0
I
2690
y.
2680 2670 2660 2650
0
5
10 HL
Figure 7.
HAW
High frequencj titration of phosphoric acid with sodium hqdroxide
series with the grid leak resistor, I?, (Figure 1). -4 part of the grid bias developed by the oscillator appears at the terminal of R, and is normally lrss than 0.2 volt. The wall-tj-pe galvanometer and its associated circuitry as used by Anderson and others ( 1 ) for measuring grid current changes was replared 11y the Sargent 1Iodel X S I Polarograph. Leads from the polarograph normally connected to the polarographic elect'rode assembly Tvere connected to be terminals of RP. K i t h a span voltage of 1 volt on the polarograph, any voltage of the same polarity a8 the bias voltage b e h e e n 0 and 1 volt could be placed across R2 through t'he leads. At the beginning of a titration, the applied voltage and current, measuring sensitivity were selected so that the recorder indicat,or could he moved to such a position that the response encountered in the titration covered the 280-mm. range of recorder scale without any readjust,ments. By preliminary titrations or esperience the proper initial adjustments w r e made. Sensitivit,y curves for operation of the instruSensitivity. ment a t 130 mc. (half-x-ave line approsimat,ely 85 cm.) n-ere ohtained with aqueous sodium chloride and with perchloric acid in glacial acet,ic acid. The results are shoim as curves B and C in Figure 5 .
0
2
I
HL
NAOH
Figure 8. High frequency titration of acetic acid with sodium hydroxide
0
5 ML
10 NACL
Figure 9. High frequency titration of silver nitrate with sodium chloride
TITRATIONS
Solutions prepared and standardized by common methods were used for various types of titrations with the frequency measuring instrument operating at, 12.2, 38.8, or 60 mc. The appearance of t,he titration curves was similar to those reported by Anderson and others ( 1 ) . Figure 7 compares the titration of phosphoric acid with sodium hydroxide at, 12.2 mc. t o a t,itrat,ion at, 120 mc. The discrepancy betn-een the t,heoretical and the experimental end points is due to the fact that phosphoric acid
x a s standardized to the phenolphthalein end point. Subsequent check titrations show much better agreement of the high frequency end point with other titration methods. Results are listed in Table I. The titration curves show that straighter lines are obtained a t 120 me., facilitating the location of the end point. Figure 8 is the reproduction of a curve obtained for the titration of acetic acid with sodium hydroxide a t 120 mc. Perfectll- straight lines
892
A N A L Y T I C A L CHEMISTRY
were obtained. Figure 9 shows titration curves obtained for titrations of silver nitrate x-ith sodium chloride.
comes apparent t,hat b ~ .synchronizing the movement of the plunger of an automatic buret, n.ith the chart drive of the recorder, a recording automatic tit,ration apparatus could result.
DISCUSSION
Blaedel and llalmstadt ( 2 , 3 ) had shown that, in order to estcJnd the useful range of i'i.equency measuring instruments to a more practical region of conductivities, frequencies of 100 mc. and above were necessary. Th? highest frequencj- obtainal)le using conventional multiturn coils in the instrument reported here is approximatel!. 80 mc. Substitution of a half-n-ave coasial line for the conventional coil resulted in an operating frequency of 130 m r . Highel, i'requenries could probably be attained with shorter lengths of coaxial cahle and other slight modifications. This will be investigated. There is n o rndiral departure here from the ordinary simple oscillator constructional practices. .411 electronic components are readilJ- available and any mechanical fabrication is easily ~)erforrned. Construction of a duplicate oscillator in xliirh some changes in laJ-out were made required only 6 houis. The adaptability of a re~ordingpolaro$raph to grid current rhange measurements reduces construction time. Also, it tie-
AChNOWLEDG\ZE\T
The authors wish to thank R. E Hooser and D. -1.Costniizo for carrying out the check phosphoric acid titrations. LITERATURE CITED
.Inderson, K., Bettis. E. S., Revirison, D., A h - a ~(.'HEM. . 22, 743-6 (1950).
Blaedel, W.J..Alalinstadt, H. I,.,Ibid.. 22. 734-42 (1950). Ibid., pp. 1413-17. Blaedel, W. J., llalmstadt, H. V., Petitjean. D. L.. dndersoii. R. K., Ibid., 24, 1240-4 (1952) Psrrack. .I.I... ISD. E x . CHEM.,A h - k ~ . ED. . 18, Jerisen, F. W.. 595-9 (1946).
RECEIVED for review Septeiiihei 8.
19.33, -4ccepted December 27, 19.55. Division of Analytical Chemistry;, 128th Meeting, .
