October, 1946
ANALYTICAL EDITION
neutralization reactions will proceed with greatest ease in the presence of water. iZcctone i q wed, because it is a solvent for both liquefied hydrocarbons and water, apparently has no effect upon the standard acid, and lowers the freezing point of the solution to below -60" C. The titer of 0.02 N sulfuric acid In acetone dropped less than 1% in 144 hours. Other solvents, ketones and alcohols, were not so satisfactory for one or more reasons involving high freezing point, poor miscibility, or instability of the titer of the standard acid prepared with them. I t was found t h a t samples cannot be kept or weighed in iron containers, though free of brass valves and fittings, since reaction or adsorption occurs, reducing the base content of the hydrocarbon. Solutions of ammonia in n-butane, kept in iron cylinders, changed from an initially determined value of 0.00081% t o loner and lower values until after 16 hours' storage only 0.0001 R remained. Solutions of methylamine in n-butane, similarly stored, changed from 0.00149 to 0.00103% in 16 hours. Similir iolutions
59s
prepared with these bases, as well as with bases up to and including isopropylamine, but in pressure glass tubing, retained their initial concentrations after storage, demonstrating that loss of base was not due to reaction between it and the hydrocarbon solvent. The Florence flask, equipped with condensing coil, eliminates the need for metal sample containers. ACKNOWLEDGMENT
The authors express their sincere appreciation to S.11.Garrison for her valuable assistance in the preparation of the manuscript. LITERATURE CITED
(1) Levin, Uhrig, and Roberts, IND. EXG.CHEM.,ANAL.ED.,17, 212 (1945). (2) Uhrig, Roberts, and Levin, Ibid., 17, 31 (1945). PREBENTED before the Division of Petroleum Chemistry at the 108th MeetN. Y .
ing of the AMERICAN CHEMICAL SOCIETY, New York,
Use of High-Frequency Oscillators in Titrations and Analyses FRED W. JENSEN
AND
A. L. PARRACK
Agricultural and Mechanical College of Texas, College Station, Texas Energy absorption from high-frequency fields forms the basis for a new method of conductometric analyses. Distinct advantages are that this method eliminates the use of electrodes, and that. values may be read directly on a meter. It is very flexible and highly sensitive. A n y changes in ionic or dipole content in ionized or un-ionized solutions may be followed. The electrical circuit i s described and a variety of types of titrations are presented.
/
E
LECTItICAL methods for titrations and analyses are generally limited in their scope and application by the necessity of using electrodes of various types. Galvanic methods usually are restricted to ionized solutions and ordinarily are not usable in organic liquids, particularly if the organic liquid is a very poor ionizing medium. Conductometric methods have a wide field of application but frequent' readjustments are usually required. By using the field of a high-frequency oscillator it is possible to produce ionic or dipole motion without the introduction of electrodes. The energy required to produce this motion causes a change in the oscillator current which is easily measured. Thus when an oscillator is loaded by the introduction of a liquid or a solution into its tank circuit, its characteristics are altered. Chiefly three factors govern the magnitude of the loading: the type and circuit constants of the oscillator, the volume of the solution and its location viithin the tank circuit, and the conductance of the solution due to its ionic or dipole concentration. The changes in conductance during a reaction cause variations in loading, whereby the course of the reaction can be followed. Oscillators differ widely in the manner in which they load. I n Figure 1are shown types of loading curves which can be obtained from various oscillators or even from the same oscillator by changing its frequency or its circuit constants. Conditions which produce linear curves of steep slope are desirabie, since oscillators producing such curves show high and uniform sensitivity throughout their loading range. However, oscillators with such high sensitivity also load out of oscillation readily. Therefore provision for restoring oscillation is highly essential. A study of the characteristics of various types of oscillators ( 5 ) indicated that the tuned-plate tuned-grid oscillator was the most adaptable to analytical work, since it could be made highly sen-
/
LOA31 PJ G Figure 1.
Typical Loading Curves
sitive and yet maintain reasonable stability, and could be readily brought back into oscillation when necessary. ELECTRICAL CIRCUIT
I n Figure 2 is shown the electrical circuit of a tuned-plate tuned-grid oscillator with a sensitive metering system in the positive power-supply lead. A regulated power source furnishes high voltage. Oscillations occur when the two tank circuits, LICI and L z C ~are , resonant a t approximately the same frequency. The condenser, C1,is the primary factor in determining the frequency of oscillation, while condenser C ) controls the excitation to the grid of tube A . Any triode vacuum tube having good oscjllating characteristics may be used. Current flowing through resistance R1 produces the grid-bias voltage. The trimmer condenser, Cs,by-passes high-frequency energy around RI and is adjusted to give maximum sensitivity t o loading in the plate tank circuit. Introduction of the glass tube, T,containing the solution into coil L1 accomplishes the loading. Condensers C, to Cs, inclusive, and choke coil La reduce stray high-frequency
.
596
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 18, No. 10
currents and eliminate body-capacity effects, thus stabilizing the circuit. The oscillator current is read on milliammeter M s , which also assists in determining the manner in which the circuit goes in or out of oscillation. By adjustment of resistance R4 in the series Ra-Rd-Rs, the voltage across resistance R? can be balanced by an equal and opposite voltage across R3 and a poition of X,.
i
I
I I I
1 1
%PAC,
1
T
I
Figure 3.
I
I I
Variation of Oscillator Tuning of C!
Current with
I I I
I 1 I
I I
I I I I I
I I I I I
I
I
I I
I I
I
I
I
I
I I I
I I
I I I
I I
I
I
I
I
I I
I
I
I
---I
LI
Figure 2.
and extends \vel1 b c l o ~t'hc liqrriil I length of this shield ia dctrxminetl justed so that changes in the liquid v have no effect upon the loading. The choice of frcquency i.5 not critical, except that an owillator of a given design may havo inaxiinum sensitivity a t a certain frequency. Frequencies in the range of 15 to 20 megacycal(:s give satisfactory results. -It Fiighcr frequencies the control of stray currents is more difficultj while coils and condensers tend t'o become excessively large at iowcr frcqiiencies. Figure 3 illustrates the manner in which a t uned-plat c t rined-grid oscillator responds to tuiiing by C1. The circuit is in oscillation only when thc two tank circuits are resonant a t approximately the same frequcncy. Increase in loading during titrat,ion causes a change in tho impedance and in the rworiant frcquciicy of the plate tank circuit. Hon-ever, its original resonant, I'requimcy can be restored by changing the capacitance in tho plat? tank circuit. Thus, with CI set to give good loading charactcri.stics as a t A , a sufficient
Electrical Circuit
K i t h the protective shunt, Rs, in the circuit and K i t h switch Si closed, the microammeter, X I , shows the approximate state of balance. Closer adjustment of R4 then permits removal of Re from the circuit. Reversing switches SI and S p serves to i n crease the range of microammeters MI and d 1 g without reducing sensitivity. Thus, if MI and Mz have ranges of 0 t o 200 and 0 t o 50 microamperes, i t is possible t o read over a range of 400 microamperes without resetting, and still maintain a n accuracy in reading of 0.2 microampere. The values of the various circuit const,ants are not highly critical. The following will give good results.
R1. Resistance, 40,000 ohms R2. Resistance, 1,000 ohms R3, RE.Resistance, 10,000 ohms Ra. Wire-wound potentiometer, 10,000 ohms CI, C2. Variable condenser, 100 micromicrofarads Ca. Adjustable mica condenser, 100 micromicrofarads
I603
Co 12007
W
5 2
2 2
a o a : 0
-
2
0031
z
800 woo2 0
z
f
C4-Co. Mica condenser, 0.01 microfarad
SI, 8 2 . Double-pole double-throw anticapacity switch $11. Microammeter, 0 t o 200 microamperes
Ji2. Microammeter, 0 t o 50 microamperes l i s . Milliammeter, 0 t o 150 milliamperes
i:
L1, Lz. Coil, 4 turns of No. 6 copper wire, spaced 1.5 in(:hei: coil diameter, 2.25 inches La. Radio frequency choke, 2.5 millihenries A . Vacuum tube, type 801 High sensitivity is obtainable by increasing the size ?E Rp up to certain limits, or by adjusting Cpand C3 t o such values that tuning with C1 causes the circuit t o go in or out of oscillalion n-itli extreme rapidity. As sensitivity is increased a slow drift in the meter reading frequently becomes apparent. Good shielding and grounding are essential. The shield around T fit- snugly
0
coo
I
0
20
10
'/OLUME Figure 4.
IN
3:
CG.
Titration of Hydrochloric A c i d with Sodium Hydroxide A.
High-frequency method.
B.
Conductance method
ANALYTICAL EDITION
October, 1946
5, -K
aiice, since the solution contains a relatively high concentration of sulfuric acid. Thus as seen from curve A on Figure 8, the end point appears but no reversal takes place. If, however, the conductance of the potassium permanganate solution is increased liy the addition of inert potassium sulfate, the typical reversal occuri a t the end point as shoivn by curve B. Jander and his eo-workers (1, 5, 4 ) , in their work on conductometric titrations in solutions of high foreign salt content, add the concentrated titrating agent from a specially constructed microburet in order to avoid large volume changes. However, the sensitivity of the high-frequency method is sufficiently great, so that adding relatirely large amounts of inert salt with the titrating agent does not destroy its accuracy. Titrations in organic solvents or mixtures of organic solvents frequently present interesting problems. Since the degree of ionization is usually low, loading due t o rotation of dipoles may constitute an appreciable part of the total effect. Furthermore, i f the titration involves more than one solvent, changes in the
I n
I.&. 0
z d
2300C
a ..'a 3
5
5 r
3
597
200c
In
$
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'2
5
z
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160
+-z
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ul
r
5 L.
0
0
5
15
20
IF
25
2
120
X
VOLUtVE
figure 5.
I'J
LC
Titration of o-Phthalic A c i d in Acetone with Sodium Methylate in Methanol A . High-hoquency method.
B . Conductance method
w CL I
a 0 X
0 H
B(
incrcaxc i n loading (liii,ing t i t i x t ~ ( J I I riiay ('au,w t lie circuit to go uut of oscillation as at t3. Tlicti tiy ,-imply i~c~turiing with C1, t h e c'irwit can readily k)~, brouglit havk int11 owiilation. Since sIio\vii 011 I:igure 3 are noilrly parallel u p 10 i h t t point vihere the virc figirist o go out of oscillation, the LlOpCY of the titration c u r l e not materially changed by reiuriiiig with C1 in thin i n i , pttrticularly i f the retuning is don(: h f o r e the circuit goes out [ i f oxillation. I n actual practice C', ( : i ~ i ibe very cnn~cnicntlyused as a mcans of resetting or decreases beyond the range of the when t!ie current incr rriicroammeters. Minor adju~tnientscan be made with B4. UTlth iliis tcchnique no observable cli:tngc:s in t.he .lopes of the titration r'iirvcs are produced by rcstit tiny.
5 + z
'd 3:
\*
n
3
0 4'
\/
0
0
10
VOLUME TYPICAL A N A L Y S E S
I n carrying out a titration tlic solution in t.ubc T is suspended within coil L, and the motor stirrer is started. B y means of Ci, and also (73 if necessary, the cit,cuit is adjusted so that it go and out of oscillation v-ith the desired sensitivity as ob on M I . Balance on meters 1111 and JI, is obtained by atljustment of 124. If the temper:iture of the solution differs from that of the interior of the system. :i slow drift of the meter needle is apparent until thermal eyuilihrium is rpached. Ordinnrily this effect can be neglected in titr:ition. In the various types of titrations, illustrawl in Figures 4 to 9, mere diluted with 100 cc. of water and were titratcd with rjolutions also in the order of 0.1 N . End points appear as changes in the slopcs of the curves. If the disappearing ion. has a higher conductance than the ion replacing it, the excess of the titrating agent beyond the end point, produces a reversal i n the curve. Comparison is made between (his method and the usual caonductance method on Figures 4 antl 5 . The scale for the niicroatnpc:re ordinate is greatly rcducetl lor comparison purposes. In cases such as the titration of air:idificttl ferrous ammoniiirii wlfato solution n-ith potassium permanganate solution, addir ion of the titrating agent beyond .wliiiiori antl lon.crs its concluctthe cnd point actually dilutw
2 j cc. of solution i n the ordcr of 0.1
t h c b
I
1
20
IN
30
GG.
Figure 6. Titration of Sodium Carbonate with Hydrochloric A c i d (A) and of Potassium Chloride with Silver Nitrate ( B )
degree of ionization and in the dipole content during titration materially affect the slope of the curve. The end point on curve B , Figure 9, is completely masked, owing t o the increase in the number of methanol molecules Tvhich are more polar than the acetone molecules, and the increase in the solubility and the degree of ionization of the sodium benzoate as methanol progressively enters the solution. When the sodium methylate is made up i n benzene with only sufficient. methanol t o maintain solubility, a sharp end point is obtained as seen by curve C. Initially, conductance increases until the limit of solubility of the sodium benzoate is reached, and thereafter decreases because of the removal of benzoic acid as insoluble sodium benzoat,e. This suggests a possible method for the determination of solubilities. Results of these titrations are listed in Table I. The known end point values are those obtained by standard volumetric methods or calculated from the known strengths of the solutions. T h e observed end point values are obtained by iise of t h r oscillator.
INDUSTRIAL AND ENGINEERING CHEMISTRY
598
Vol. 18, No. 10
+
Table
I. Comparison of Known and Observed Values
Substance Titrated HaPOi
Titrating Agent NaOH
Known End Point, co.
Observed End Point, cc.
25.25
12.66 25.20 37.87 12.18 24.40 24.98 25.03 25.10 24.90
NagCOs
HC1
12.20
FeSOa(NH4)d301 FeSOi(NHi)zSOc NanCaOi KC1 CsHiCOOH in acetone
KMnOa KMnOa-K&04 K>fnOa-K&01 AgNOs CHsONa in benzene ?\'a&Os CHsOPia in MeOH
25 IO0 25,04 25.13 24.92
HCI
CsHi(C0OH)n in acetone
23.98
trating agent a t theequivalence point. KisusedasK.(V, V*)a where V1 is the volume of the solution before titration. The term (z K / z ) represents the amount of chloride ion for which no silver ion has been added. A close approximation of the condu'ctance of the solut,ion a t any time prior to the equivalence point is shown by Equation 1.
-
Conductance =
24.00 25.50 12.00 24.00
25,50 24.00
If the derivative is taken and equated to zero, the quadratic Equation 2 results. 1600
m W
I n the solution of this equation coefficient b is very small in comparison to and therefore a t minimum conductance:
120c
[L
W
a
z a 0
x =
[L
d--c/a
(3)
0
=
or
so1
z +
(4)
z W
[L
LL I)
40( 2000
0
0
10
20
VOLUME
IN
30
40
CC.
Figure 7. Titration on Phosphoric A c i d with Sodium Hydroxide ( A ) and of Hydrochloric A c i d with Sodium Carbonate ( B )
1500
a w
a H
a
0 E
'
0
I IOOC
z DILUTE S O L U T I O N S
I-
Adjustment of CZand Ca and tuning with C1can cause the circuit to go in or out of oscillation with such extreme rapidity that even slight changes in conductance produce relatively large changes in the oscillator current. Curve A , Figure 10, shows the titration of 28.00 cc. of 0.01000 'Y potassium chloride solution, diluted to 300 cc., with 0.01000 A- silver nitrate solution. The minimum conductance point a t 23.90 cc. does not coincide with the true equivalence point of 25.00 cc. for several reasons. The silver chloride becomes more soluble as the equivalence point is approached and dilution of ions takes place as titration proceeds. However, a simple approximate correction can be made in the following manner:
W
If z is the number of gram-ions of chloride ion present a t any (x-K/x) are the amounts of time, then K/x, N P , , and NVI silver, potassium, and nitrate ions, respectively, where N is the normality of the titrating agent, and V Iis the volume of the&
-
z (L (L
3
501
0
0
Figure 8.
10
20
30
VOLUME IN GG. Effect of High Ion Concentration
A . Titration of 0.1 N lenous ammonium sulfate with 0.1 N potassium permanganate B . Titration of 0.1 N ferrous ammonium sulfate with 0.1 N potassium permanganate and 0.4 N pobsrium sulfate C. Tibation of 0.1 N sodium oxalate with 0.1 N p o t a r dum permannanatr'and 0.5 N potassium sulfate
October, 1946
ANALYTICAL EDITION
VOLUME 0
10
20
30
VOLUME IN CC.
Figure 9.
Titration of Benzoic A c i d Acetone
in
IN
599
GC.
Figure 10. Titrations in Dilute Solutions A . 0.01 N potassium chloride with silver nilrate E . 0.01 N potassium chloride with silver acetate C. 0.001 N hydrochloric acid with sodium hydroxide
Figure 11 illustrates another type of analysis which may be performed readily by this method. The curve shows the variation in oscillator current as the concentration of hydrogen chloride in dry benzene is changed. Since a change in hydrogen chloride concentration from 0 to 0.275%by weight produces a change of 100 microamperes, i t is possible t o read a change in hydrogen chloride content in the order of 0.0006% by weight. To some e x t e n t t e m p e r a t u r e changes affect the oscillator current. With a n increase in temperature the ionic conductances become greater, causing a n increase in the current. The temperature effect on the dielectric constant causes a change in the opposite direction. I n addition t o titrations, this simplified conductance method can be applied for following reaction rates, control work, etc. Wherever changes in ionic or dipole conductances occur, they may be readily observed by this means.
A , E . W i t h sodium methylate in methanol C. With sodium methylate in benzene
~
MacInnes, Shedlovsky, and Longsworth (6) give the ionic conductances a t 25” C.! of potassium, chloride, silver, and nitrate ions at 73.50, 76.32, 61.90, and 71.42 reciprocal ohms, respectively. The value of K , for silver chloride is calculated as 1.7 X 1O-lo from its solubility ( 8 ) a t 25’ C. Using these values in Equation 4 and replacing VI by its approximate equivalent of 0.0239 liter (the volume a t minimum conductance), the amount of chloride ion a t minimum conductance is 1.24 X gram-ion. K/zis0.15 X 10V5gram-ion and (z - K / x ) is 1.09 X 10-5 gram-ion. This is equivalent to 1.09 cc. of 0.01000 N silver nitrate solution. Addition of this correction t o the 23.90 cc. gives an end point of 24.99 cc. In Equation 4 the term (A?]- - Xxo3-) appearing in the denominator represents the difference in the ionic conductances between the disappearing ion and the ion replacing it. If instead of the nitrate ion, the acetate ion, which has a conductance 40.87 reciprocal ohms (6), is used, this difference becomes greater, thereby decreasing the value of z. On curve B , Figure 10, the minimum conductance point is much sharper and appears a t 24.68 cc. when silver acetate is used. Calculating the correction from Equation 4 but replacine, the ionic conductance of the nitrate ion with that of the acetate ion, the en6 point is found as 25.0.5 cc. It can be seen in Equation 4 that small values of K,, as well as large differences in conductance between the disappearing ion and the ion replacing it, \ d l produce small corrections. I n acid-base titrations the hydrogen ion with a conductance of 349.72 reciprocal ohms (6)disappears and is replaced by a positive ion of much lower conduct,ance. I l s o , since thc ion prodthe corrections will become very uct constant is 1.0 X small or negligible. As shown on curve C, Figure 10, the titmtion of 25.00 cc. of 0.000998 1V hydrochloric acid solution in 300 cc. of redistilled water with 0.001003 N sodium hydroxide solution produces a minimum conductance point a t 24.80 cc. The known end point is 24.83 cc. The correction of 0.003 cc. is negligible. The change in the slope of the rurve at 32 cc. is due t o titration of carbon dioxide which has dissolved during titration. I t s positiop varies with the time used for titration. Cases may occur in which the disappearing ion has a lower conductance than the replacing ion. Then, since the curve has a positive slope throughout,, no minimum point is produced and Equation 4 does not apply. However, the excess of titrating agent entering the solution as the end point is passed causes a break in the curve and no correction is necessary.
WT. Yo OF Figure 11.
HCI
Determination of Hydrogen Chloride in Benzene ACKNOWLEDGMENT
The authors are indebted to the Engineering Experiment Station of the college for valuable assistance and t o M.A. Mosesman, W. K. Anderson, S. TI-. Levine, J. H. Griffin, V. L. Turner, J. TT. Johnson, and R. 11.Hill for aid in experimental work. LITERATURE CITED
(1) Immig, H., and Jander, G., Z. Elektrochem., 43, 207-11 (1937). (2) “International Critical Tables”, Vol. VI, p. 256, New York, McGraw-Hill Book Co., 1929. (3) Jander, G., and Ebert, ii., Z. Elektrochem., 41, 790-94 (1935). (4) Jander, G., and Harms, J., Angew. Chem., 48, 267-71 (1935). (5) Jensen, F. W., and Parrack, A . L., Texas A. and M. College, Eng. Expt. Sta., Bull. 92 (1946). (6) MacInnes, D. .4.+ Shedlovsky, T., and Longsworth, L. G., Chem. Rev., 13, 29-46 (1933). PREBEXTED before the regional meeting of the in Austin, Texas, December 7, 1916.
~ M E R I C A XCHEXICAL SOCIETY
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