would have t o be included in the subsequent quantitative determination.
Chemical Laboratory, is acknowledged and appreciated.
ACKNOWLEDGMENT
LITERATURE CITED
The advisory assistance of C. F. Pickett, Director of the Coating and
( 1 ) Shreve, 0. D., in “Organic Analysis
A . Weissberger, eds., Interscience, Sew York, 1956. (2) Swann, RI. H., ANAL. C H m . 29, 1352 (1957). (3) Swann, M. H., Adams, If. L., Weil, D. J., Ibid., 27, 1604 (1955). (4) Ibid., 28, 7 2 (1956).
”
Vol. 111, p. 485, John Mitchell, Jr., I. bI. Kolthoff, E. S. Proskauer, and
R E C E I ~ Efor D review Sovember 8. 1957. Accepted March 6, 1958.
Performance of a Wide-Range High-Frequency Titration Apparatus ARTHUR H. JOHNSON’ and ANDREW TIMNICK Kedzie Chemical Laboratory, Michigan State University, East lansing, Mich.
b In a high-frequency titration apparatus, when the usual multiturn coil, into which the titration vessel is introduced, is replaced b y a singleturn loop, the distributed capacity associated with the inductance is reduced to a minimum and the instrument appears to respond only to changes in the solution conductivity. Practical instrument response is observed with aqueous solutions in the titration cell varying in concentration from 0.003 to 5.OM in sodium chloride. Direct titration of 3N hydrochloric acid solution with 4N sodium hydroxide was accomplished.
R
ELATIVELY FEW high-frequency
instruments have been constructed in which the solution was placed in a vessel in the field of the coil of a parallel resonant circuit. The few that have been developed have been of two types: those in which the parallel resonant circuit was a part of the circuit of a radio-frequency oscillator (5, 8-1 1 , I S ) , and those of the wavemeter type-i.e., parallel resonant circuit excited by radio-frequency energy from an external oscillator (3, 4,l a ) . All the instruments used coils of the multiturn type, which have associated with them relatively high values of distributed capacity. The distributed capacity shunts the inductance of the coil, making the coil equivalent to a parallel resonant circuit having a natural resonant frequency. The natural resonant frequency of the coil sets an upper limit to the frequency a t which the coil is useful in providing an inductive reactance. Inserting a solution of high dielectric constant within the coil markedly affects the natural resonant frequency of the coil. Thus, the response of a n instrument of this 1 Present address, Bauer and Black, Chicago, 111.
1324
ANALYTICAL CHEMISTRY
type is a function of at least tn-o variables, the dielectric constant of the solution and the high-frequency conductivity of the solution, as the inductance of the coil is not a constant and the conductivity of the solution varies with the addition of reagent. The instrument described (6) is of the loaded coil type, in which part of the tank circuit is a rigid single-turn loop constructed of 3/16-inch copper tubing. The diameter of the loop is substantially larger than the diameter of the polyethylene titration vessel that is inserted into the loop along its axis. The tank circuit is a partially distributed and partly lumped constant system, and has some of the characteristics of a quarter wave length line shorted at one end. The vessel and its contents are inserted near the closed end of the line (the loop) where the effect of changes in dielectric constant-that is, changes in lumped tuning capacity-have a minimum effect on the resonant frequency of the line. At the closed end of the line, changes in solution conductivity-that is, changes in load-have the maximum effect on the Q of the resonant circuit. Thus, a practical separation of two variables, which contributed to highfrequency titration instrument response, has been attained. The instrument responds only to changes in the solution conductivity for all practical purposes. Loading of the oscillator by the solution is controlled in three ways: by withdrawing the titration vessel vertically along the axis of the loop-Le., controlling the volume of the solution in the field of the loop; by increasing the diameter of the loop-Le., reducing the coupling between the solution and the oscillator; and by varying the feedback to the grid of the oscillator tube by changing the ratio of the capacities of the feedback capacitors, C1 and C?.
INSTRUMENT CONSTRUCTION
The instrument is a modified version of the 120-mc. instrument using a capacitative-type cell ( 7 ) . The loop and its shield have been substituted in the position previously occupied by the capacitative cell. The loop leads are terminated in banana plugs, which in turn are plugged into insulated banana jacks, in the side of the chassis, that serve as terminals to the grid and plates of the 955 tube. The feedback and tuning capacitors, C1 and Cz, are short lengths of Amphenol RG 8/U coaxial cable with one end terminated in Amphenol-type PL-259 male coaxial cable connectors. A range of capacity of C1 and Cz is obtained by simply varying the length of the coaxial cable attached to the male connectors. The male connectors plug into the female coaxial cable connectors mounted on the top of the chassis used to receive the ends of the 120-mc. half-wave line when the capacitative cell is employed. The interchangeable parts (feedback capacitors and half-wave line, loop shield assembly, and capacitative cell) make rapid conversion from one type of instrument to another a simple process. KO changes in the oscillator wiring are required-it., no soldered connections need to be broken.
d schematic diagram of the circuit of the instrument is shown in Figure 1. Figure 2 is a photograph of the instrument with the cover plate removed from the loop shield assembly to show the position of the polyethylene titration vessel relative to the loop. The loop shown is 7.5 cm. in diameter. and the titration vessel, 6 cm. in diameter. The diameter of the shield assembly is 17 cm., and the spacing between the cover plate and the bottom plate of the shield assembly is 7 cm. Concentrically secured around the aperture in the cover plate is a ring 2 cm. wide, having a n inside diameter of 6 cm. One and one-half cm. of the ring protrudes below the plane of the cover
Figure 2.
Figure 1. Oscillator circuit of wide-range titration apparatus Cl,C1. RG 8/U coaxial coble of varying lengths Cn, CI,Cs. CI. Ci. 100 wf., mica RI. RA. 15.000 ohms. 1 WOW RL '1000ohms, 1 wktl Ra. 100 ohmi, 2 wan$ L2. 10 turns No. 22 enameled m p p e r wire wound oround Rs
Wide-range titration apparatus
VI. 9 5 5 t v b e 1%. Loop of 3/m-iwh silver ploted copper tubing Idinensions In Table I1
plate into the interior of the shield assembly. The polyethylene titration vessel slides snugly into the ring and is held firmly in one position during a titration, For some titrations, the bottom of the vessel rests on the bottom plate of the shield assembly; in others, the vessel is withdrawn upward along the axis of the loop and is supported in the raised position on a spacer ring inserted between the protruding lip of the vessel and the ring fastenpd in the aperture of the cover plate. The two positions of the vessel described above appear in Figure 3. I n the raised position (insert), loading of the oscillator by the solution is reduced and the range of the instrument is extended to regions of higher conductivity. For expediency, grid current changes were measured with the Sargent Model XXI polarograph (7). Components such as used and described by Anderson et al. ( I ) could also have been used. Trials have been made in which a zero suppressor type circuit has been incorporated to follow grid current changes. R, is replaced by a precision variable resistor t o regulate sensitivity. The potential drop across Rt is then compensated by voltage from a 1.5volt dry cell, regulated by another properly selected precision potentiometer. Compensation is indicated by
a microammeter which serves as a null indicator. Thus a very inexpensive self-contained titration apparatus can be constructed. RESPONSE CURVES
Sensitivity curves for the instrument were obtained with aqueous sodium chloride solutions (Figure 4). The corresponding instrument adjustments appear in Table I. I n obtaining the curves, t a o loops of different diameters were employed. Various amounts of feedback were used by varying the ratios of the capacities of C, and (7%. For some curves, the titration vessel was in the position shown in the lower portion of Figure 3, and for others, it was in the position shown in the insert. A study of the curves and Tahle I reveals that the instrument may be adjusted to have a maximum sensitivity in any concentration range desired. h i s i n g the vessel 2 cm. shifts the range of maximum sensitivity from that shown by curve 1 t o the range shown by curve 2. Curves 1 and 8 show the effect of in-
creasing the diameter of the loop. Curves 1and 4 show the effect of varying the feedback in the oscillator circuit through change8 in the capacity ratio of C, and C2. Obviously, the combinations of loop diameter, vessel position, and feedback capacity used in obtaining the sensitivity curves by no means exhaust the possibilities. Intermediate curves can be obtained with intermediate values of cell parameters. The voltage impressed across the 1000-ohm resistor in series with the grid-leak resistor by the polarograph places a small amount of fixed bias on the oscillator. The effect of varying amounts of fixed bias from 0.2 to 0.4 volt on the sensitivity curves is shown in Figure 5. An increased bias reduces the slope of the sensitivity curve slightly. Ordinarily, variations in fixed bias are much less than the extremes shown. Therefore, it may be assumed that adjustments of the bridge control on the polarograph have a negligible effect on sensitivity curves. A point of interest in connection with the curves is that in order to reach the more concentrated ranges, the loop diameter wa8 increesed, resulting in a lowered operating frequency. I n instruments previously reported (S), t o extend their range of sensitivity to regions of higher conductivity, the operating frequency of the oscillator had to be increased. Thus, it appears that operating frequency is of little significance in the present instrument,
28
i24 0
0
.
216Y
0
Y
0
e 0 0
-
4-
Y
e 0001
Figure 3.
Titration vessel
nseert show vessel in raised position
001 01 MOLARITY, NACL
Figure 4.
I IO
Sensitivity curves VOL. 30, NO. 8, AUGUST 19'58
1325
INSTRUMENT ADJUSTMENTS
Table I.
Curve
Wide-Range High-Frequency Titration Apparatus Adjustments Pertaining to Sensitivity Curves
C1a
1 2 3 4
2
5 6
5
2 2 3
5 5 2
7
8 9 10 110
2 2 2
Capacitor
e
Vessel Sensitivity,c Position Fa./Mm. 1 0.15 2d 0.15 0.06 1
&
7
-7
2 2
1
-1
->
1 1
1 1 1
1 1
1
83
90 94 94 94
0.06 0.06
1 2d
0.06
1
82
1
2d 2d
82 82 82
3
19
8.8
7.7 14.0 12.9 10.4
0.06 0.06 0.40
1
2
Bridge Settigg,c % of 1-V. Span 8.5 8.5 6.5 7.0 7.2
0.06 0.15
?d
Length of Coaxial Cable, Cm. 10 11
30.6
Diameter, Cm.
bLoop
1
10
7.5
2
Male connector only
Polarograph adjustments for grid current measurement. Titration vessel is raised and supported 2 cm. above normal position L? and RSshort circuited by connecting cathode directly to ground. Table II.
0
1
3 5 2
1
5
c
Cp 1
Frequency, Loopb Mc. 7 88 * 7 88
Titrations of Hydrochloric Acid with Sodium Hydroxide
(Acid aliquot diluted to 150 ml. before titration) End Point, Nl. HighIIC1, M1. HC1, S XaOH, N Theoretical frequency 33 30 0 4772 33 22 0 7949 19 94 33 28 0 4772 33 22 0 7949 19 94 41 65 0 4772 41 52 24 96 0 7949 75 9Ob 75 80 3 4 100 03" Direct titration, no dilution. Determined by comparison of solutions using methyl red indicator
1L 15
1003
ML
3N
HCL
,"i 40.14
a-
wl- ll
ti I 10
I
HL. N A O H
Figure 6. High-frequency titration of 3N hydrochloric acid with 4N sodium hydroxide (titration details in Table II) Figure 5. Effect of varying bias on respase of instrument
and any region of conductivity may be reached by varying loop diameter, vessel position, or feedback. The operating frequency of the oscillator is the frequency assumed by the oscillator with the particular dimension loop and tuning capacity selected in order to work in a specific range of conductivity. As is true with other instruments, keeping the level of the solution above 1326
ANALYTICAL CHEMISTRY
a grounded band in the cell assembly eliminates instrument response to changes in liquid level. The minimum volume of solution required t o eliminate response due to changes in liquid level in the present instrument varies with the position of the titration vessel. When the bottom of the titration vessel is resting on the bottom plate of the shield assembly as shown in lower Figure 3, a 150-ml. sample is required. Smaller samples may be used in all other vessel positions.
The 7.5-em. loop and the 10-em. lengths of C1 and Cz are plugged into their respective positions in the instrument. A vacuum tube voltmeter is connected to read the negative grid bias developed by the oscillator. A ground lead is connected from the chassis of the instrument to the same ground connection used for the polarograph external ground, placing the chassis of both instruments at the same potential. The vessel containing the sample to be titrated is slowly lowered into the field of the loop and the grid-bias changes on a vacuum tube voltmeter are observed. As the vessel is being lowered, the grid bias voltage decreases gradually and will be at a minimum when the vessel is a t its lowest point. If, in the course of lowering the vessel into the field of the loop, a sudden decrease in grid bias is observed and the voltmeter indicates a steady value upon further lowering of the vessel, the solution is too concentrated and is overloading the oscillator. If the vessel is within inch of its lowest position when the oscillator goes out of oscillation, longer lengths of C1 and CL)are substituted in place of the 10-em. lengths used up to this point. If the oscillator will not remain in oscillation with the vessel in the lowest position when the lengths of CI and Cz have been extended t o 25 em., the 10-cm. loop should be employed and the above procedure repeated. The instrument has its greatest sensitivity when the vessel is lowered to its maximum, and therefore, the initial adjustments of the instrument should be made t o permit titration of the sample with the vessel in its loncst position. However, satisfactory results may be obtained Kith the vessel in other positions. To select an intermediate position, the vessel is lowered slowly into the field of the loop until the oscillator goes out of oscillation. Then the vessel is raised very s l o ~ d yuntil a sudden increase in grid bias indicates the oscillator has resumed oscillation. The proper width spacing band is selected and slipped into position on the vessel to maintain the vessel in the position attained in the above adjustment. With extreme concentrations, such as 3M hydrochloric acid, the 10-em. loop and intermediate lengths of C1 and Cz are used. I n addition, a 100ppf. mica condenser is installed betiveen the cathode of the 955 tube and ground. The condenser is shown in the schematic diagram of Figure 1 as C,. I n the titration of a strong acid, adjustments are made so that the oscillator is just in oscillation a t the beginning of a titration t o obtain a complete titration curve. I n the titration of a weak acid, the adjustments are made so that a t the completion of the titration
the oscillator is a t the point of going out of oscillation. The polarograph is adjusted so that a constant voltage of the same polarity as the bias voltage developed by the oscillator is impressed across the 1000ohm resistor, Rz, in series with the gridleak resistor. Any voltage from zero up to the span voltage setting of the polarograph may be selected by adjustment of the polarograph bridge control. Better control of the voltage is attained with lower values of span voltage. Thus, if a voltage of 0.1 volt is to be applied to the resistor, Rz, and the span voltage is 1 volt, the bridge is set to 10% of the span. If a 0.4-volt span is used, the bridge setting is 40y0. It is easier to adjust the bridge to the proper value a t the lower span voltage because the increment per dial division is considerably less, and the backlash and overtravel in the adjustment of the bridge control produce correspondingly smaller variations in applied voltage. The polarograph bridge control setting depends upon the type of system being titrated. For a strong acid, the per cent of span voltage is so selected that a t the beginning of a titration the recorder pointer is near the 280-mni.
limit. If a weak acid is titrated, the applied voltage is such that the pointer is near the zero limit. A current measuring sensitivity of 0.06 pa. per mm. is satisfactory for most titrations. The range used in this study was 0.06 to 1.5 pa. per mm. TITRATIONS
To check the performance of the wide-range high-frequency titration apparatus, various acidimetric titrations were carried out. The results for the titration of aliquots of standardized hydrochloric acid diluted to 150 ml. and titrated with standardized sodium hydroxide are shown in Table 11. The most noteworthy result listed is the one for the direct titration of 100.3 ml. of 3N hydrochloric acid solution with 4:V sodium hydroxide. For this titration, the titration vessel was raised 2 cm. above the normal position. The normal oscillator circuit was altered by installing a 100-ppf. capacitor between the cathode of the 955 tube and ground (shown as C7 in the circuit diagram). The titration curve is shown in Figure 6. The greatest concentration previously titrated directly mas 1.15N hydrochloric acid (9).
LITERATURE CITED
(1) Anderson, K., Bettis, E. S., Revinson, D., - 4 ~ 4CHEM. ~ . 22,743-6 (1950). (2) Blaedel, W. J., Malmstadt, H. V., [bid., 22, 734-42 (1950). (3) Fujiwara, S., Hayashi, S., Ibid., 26, 239-41 (1954). (4) Ishidate, M., Masui, M., J . Pharm. SOC.Japan 73, 867 (1953). (5) Jensen, F. W., Parrack, A. L., I N D . ENG.CHEM., A N A L . ED. 18, 595-9 (1946). (6) Johnson, A. H., U.S. Patent 2,703,863 (March 1955). (7) Johnson, A. H., Timnick, -4., NIL. CHEM.28, 889 (1956). (8) Leifer, A., Grove, E. L., Kassner, J. L., Division of Analytical
Chemistry, 127th Meeting, ACS, Cincinnati, Ohio, March 1955. (9) Masui, M., J . Pharm. SOC.Japan
74, 530 (1954). (10) illilner, 0. I., ANAL. CHEM. 24, 1247-9 (1952). (11) Xakano, K., Hara, R., Yashiro, K., Zbid., 26, 636 (1954). (12) Sargent, E. H., and Co., “Scientific
-4pparatus and Methods,” Summer, 1949. (13) West, P. W.,Burkhalter, T. S., Broussard. L..h A L . CHEM.22. 469-71 (195oj
RECEIVEDfor review April 9, 1956. Accepted March 28, 1958. Division of Snalytical Chemistry, 129th Meeting, 9CS, Dallas, April 1956: Abstracted in part from thesis submitted by A. H. Johnson for the M.S. degree in chemistry, Michigan State University.
Colorimetric Determination of Carboxylic Acid Derivatives as Hydroxamic Acids VIVIAN GOLDENBERG Department of Biochemistry, Hillside Hospital, Glen Oaks, N. Y PAUL E. SPOERRI Department o f Chemistry, Polytechnic Institute of Brooklyn, Brooklyn, ,Aqueous alkaline hydroxylamine is widely used to form hydroxarnic acids which may b e estimated by their color reaction with ferric ion, in the analytical determination of carboxylic acid derivatives. This study determined the influence of substituents on the ease of hydroxamic acid formation b y monocarboxylic acid derivatives. The esters tested required 2 to 120 minutes for maximum hydroxamic acid formation. The d a t a show a correlation beween structure and rate of hydroxamic acid formation. Bulky and electron-donating groups inhibit hydroxamic acid formation by substituted benzoic acid esters. Replacement of the hydrogen atoms of formamide by a methyl or phenyl group results in decreased rate of hydroxamic acid formation. In setting up the conditions for the determination
N. Y .
of a-hydroxy, a-amino, and a-chloro aliphatic esters as hydroxamic acids, the hypochromic effect of these substituents must b e considered.
C
acid derivatives react with hydroxylamine to form hydroxamic acids which may be estimated by their color reaction with ferric iron. This reaction is used extensively in analytical chemistry. I n 1934, Feigl and coworkers (4, 5 ) introduced the use of the ferric-hydroxamic acid reaction as a spot test for carboxylic esters. Lipmann and Tuttle (14) based a specific quantitative method for acyl phosphates on their reaction with hydroxylamine in water a t p H 6. Hill ( 2 1 , 22) applied the ferric-hydroxamate reaction t o the quantitative deARBOXYLIC
termination of long-chain fatty acids and esters. I n 1949, Hestrin (20) developed a rapid colorimetric micromethod for the quantitative estimation of short-chain carboxylic acid esters, lactones, and anhydrides based on their ready conversion to hydroxamic acids when added t o alkaline hydroxylamine in aqueous medium. He concluded that the hydroxylamine reaction of soluble carboxylic acid esters was completed within 1 minute and predicated his quantitative test on this short reaction time. The hydroxylamine reaction has also been used for the determination of carboxylic acid amides (3, 6 ) . Bergmann (2) employed essentially Hestrin’s procedure (10) with longer intervals of reaction a t various temperatures. He studied the comparative VOL. 30, NO. 8, AUGUST 1958
1327
