Coulometric titrations using highfrequency determination of the end point

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Authors Fordemwalt, James Newton, 1932-

Publisher The University of Arizona.

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COULOMETRIC TITRATIONS

USING HIGH FREQUENCY DETERMINATION

OF OHE END POINT

ByJames N. Forderawalt

A Thesis

submitted to the faculty of the

Department of Chemistry

in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

In the Graduate College, University of Arizona

VThis thesis has been submitted in partial fulfillment of require­ments for an advanced degree at the University of Arizona and is deposited in the Library to be made available to borrowers under

rules of the Library. Brief quotations from this thesis are

allowable without special permission, provided that accurate

acknowledgment of source is made. Requests for permission for

extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major depart­

ment or the dean of the Graduate College when in their judgment the proposed use of the material is in the interests of scholar­

ship. In all other instances, however, permission must be obtained

from the author.

SIGNED:

ACKNOWLEDGEMENT The writer wishes to express his sincere

appreciation to Dr. Edward N. Wise for his advice,

assistance and encouragement through the course of

this work.

TABLE OF CONSENTS

Chapter PageI. INTRODUCTION 1II. DISCUSSION OF THE PROBLEM 8III. EXPERIMENTAL 10

A. Design and Construction of theConstant Current Power Supply 10

B. Experiments Done Using the HighFrequency Instrument l61. Oxidation of Hydroquinone by

Ceric Ion in Anhydrous Acetic Acid 102. Acid-base in Anhydrous Acetic Acid 193. Reduction of Ceric Ion by Chromic

Ion in Water 194. Acid-base in Water 22

IV. CONCLUSIONS 24V. SUGGESTIONS FOR FUTURE WORK 25

BIBLIOGRAPHY 27

CHAPTER I

INTRODUCTION

Coulometric analysis is a quantitative analytical method in

which an electric current passing through the unknown solution

is used as the titrant. Faraday's law states that 9^,500 coulombs,

when consumed in an electrode reaction, produces one equivalent

of chemical change. This is used to calculate the quantity of

material determined from the quantity of electricity necessary

to bring about the end point. The current flowing in the solution

usually does not perform the actual analytical reaction, or

deposit out a metal, as in the classical forms of electrolysis,

but rather is used to generate, in solution, a reagent which per­

forms the actual titration. It is necessary in the generation

of this reagent that the current efficiency be 100 $. That is,

that there be no side reactions, heating effects or similar losses,

and that the reagent be quantitatively generated.

Oxidation-reduction, or acid-base titrations can be performed

with the proper choice of electrolyte. For a redox reaction, an

electrolyte, such as cerous ion can be chosen which will be oxidized

at the anode .An example would be cerous ion oxidized to eerie ion.

The eerie ion can then oxidize the unknown, such as ferrous iron

or other oxidizable material, and return to the cerous state to be

oxidized at the anode again. In an acid titration, the H+ ions are

discharged at the cathode and free Eg is liberated, leaving OH ions in solution. In either case, it is essential that the electrodes be isolated from each other either by means of a salt bridge or by trapping the non-active electrode in a sintered glass cup, so that the reagent generated at one electrode is not swept past the other electrode and the reverse reaction take place before the electrolytically generated reagent can react with the unknown.

Coulometric analysis has the advantages of being particularly suited for micro and semi-micro work, as well as of eliminating the necessity of preparing and storing standard solutions. It is also quite rapid under ordinary circumstances, and under optimum conditions is as accurate as, or even more accurate than, the classical methods. This is possible since both time and current, the two measured variables in a coulometric titration, can be accurately measured to as many as five significant figures.

Coulometric titrations were first attempted by Szebelledy and 15

Somogyi in 193&, but aroused little attention, as the system they used offered no special advantages and was rather inconvenient to use.

In 19^5 Lingane performed a coulometric analysis byelectrodepositing all of a sample metal from its solution onto a

11mercury cathode using carefully controlled cathode potential. In this method, the current falls asymptotically to zero as the end point is approached. The titration was run until it was felt that an insignificant amount of the metal was left in solution. The quantity of electricity which had passed through the solution was

3determined by means of a Eg and 02 gas coulometer.

In 1947, Swift and associates titrated micrograms quantities14of thiodiglycol with electrolytically generated Br^. The

important feature of this titration, was the introduction of the

use of constant current, and the measurement of time to determinethe number of coulombs consumed. Dry cells were used as the source

of electricity, and the current was controlled manually with a

rheostat. The number of coulombs was calculated as the product of

the constant current times the time for the titration.DeFord and his associates designed an apparatus whereby they

could electrolytically generate their reagent externally. They

developed an inverted "Y" shaped apparatus formed from capillary

tubing. In each leg of the "Y” was an electrode, and as the liquid

flowed from a reservoir into the base of the "Y", the ions generated

at each of the electrodes were swept down the respective tube.

The desired ions were passed into the solution to be titrated. The

ions generated at the other electrode were dumped into a sink and

discarded. Using this system DeFord determined several acids and

bases as well as arsenic with quite reasonable accuracy.

In 1952, Cooke, Reilly and Furman, at Princeton, titrated

manganese as permanganate in solutions of concentrations in the

parts per trillon range, using coulometically generated ferrous 4ion. Solutions of 0.001 to 0.0005 microgram of manganese per

milliliter were titrated with an accuracy of about With more

concentrated solutions the error was considerably less.

A more recent development in coulometric analysis has been the

use of dual intermediates, in which an electrolyte containing both cupric and bromide ions was used to furnish either reducing cuprous ion or oxidizing Br̂ ,, according to whether the active generating electrode was made the cathode or anode, respectively. Using this electrolyte, a slight excess of bromine can be generated and

maintained in a solution of a compound which brominates slowly,

and then the excess Brg can be destroyed by reversing the electrical

connections to the cell and generating cuprous ions.

In 1953> E. N. Wise, at the University of Kansas, developed a completely automatic coulometric titrator, in which photometric

17detection of the end point was employed to terminate the titration. 1

As well as the elements mentioned above. Cl, Br, I, and

mercaptans have been coulometrically titrated on a micro scale.Various systems of end point detection have been used with

coulometric analysis, such as the usual color indicators, ph meters,

potentiometric, and amperometric methods of end point detection.

All of these depend on contacting the solution in some way, and

have met with varying degrees of success in coulometric analysis.

In most coulometric analyses, the exactness with which the end point

could be detected has been the limiting factor in establishing

the accuracy of the titration.

High frequency electrical oscillations have been used by

chemists for almost kO years in the determination of dielectric

constants, which in turn has served to indicate the course of reactions,

to follow the progress of fractional distillations, and to determine

the moisture content of solids. However, it has been only recently

that it has been adapted to determining the end point of titrations.

The high frequency method of determining the end point is in some ways quite similar to the conductometric method of end point

detection. The high frequency system can be considered as being a conductometric system operating at a radio frequency and employing a conductometric tube fitted with external electrodes, in place of

a cell with internal electrodes in contact with the solution. In

this way many of the difficulties which result from the electrodes

contacting the solution are eliminated.

In the early high frequency titrations, the cell was placed in

the tank coil of a high frequency oscillator and then either the

grid voltage or the plate current was measured by some convenient

method. The end point of a titration was seen as a break in a curve of one of these plotted against milliliters of titrant, which

resulted from a change, presumably, in the dielectric constant

of the unknown at the end point. However, this astern was generally un­

satisfactory, as it suffered from instability and lack of

sensitivity.

Investigation has shown that the changes in the solution are

usually due to changes in dielectric constant. Since the capacitance

of a system is directly related to the dielectric constant, the

cell was modified in such a way that full advantage could be taken

of the capacitive effect.Several cell designs were employed, all of which were essentially

the same, and consisted of two plates of a condenser around

the titration vessel. These were connected in the plate tank circuit

of the oscillator. The operation was essentially the same as the

older method, but greater sensitivity and stability were obtained.

Hall and Gibson in 1951 ran a series of titrations at different7concentrations and frequencies. They found that the shape of the

titration curve and the nature of the break at the end point changed

with concentration and with frequency.

Blaedel and Malmstadt, in 1952 developed an instrument in3which they incorporated a differential frequency meter. With

this they measured the change in frequency, rather than the actual

frequency of the oscillator. This was combined with a constant

flow buret, which provided a smooth flow of titrant into the solution.

The end point was observed when a change in the rate of change of

frequency was noted as the titrant flowed into the solution at a

constant rate.

Beryllium has been titrated volumetrically using a high fre­

quency end point by Anderson and Revinson, by dissolving the beryllium

in hydrochloric acid and then titrating with sodium hydroxide. 1

Bein, in 195^ used high frequency for determining the chloride

and sulfate concentration of water entrapped in the shallow marine

sediments of the Gulf of Mexico. 2

Nankano, Hara, and Yashiro, also in 195^, reported very satis­

factory results for acid-base type titrations, as well as for the

determination of dimethylglyoxime and metal chelates, such as12chelates of cobalt, nickel, manganese and lead.

Wagner and Kaufmann in 1953> titrated a number of organic bases

in glacial acetic acid with perchloric acid, using high frequency

methods to determine the end points. The results they obtained were found to be in good agreement with those found by other methods of end point determination. They felt that high frequency methods will probably have wide application in the field of non-aqueous

titrimetry since it overcomes the limitations of lack of suitable

visual indicators and adequate electrode systems, which have thus far hindered non-aqueous titrimetry.

CHAPTER II

DISCUSSION OF THE PROBLEM

In a coulometric titration the only quantity which needs to bemeasured with great accuracy for the determination is the number ofcoulombs which have been passed through the solution. The classicalmethod of measuring this has been by the use of a silver coulometeror a Eg & Og gas coulometer, in which the weight of silver platedout on an electrode, or the volume of & 0^ liberated are measured.These methods, however, are not satisfactory for the very smallamounts of current used in many coulometric analyses, due to thedifficulty encountered in measuring a very small weight of silveror small volumes of & 0 g •

As a result of these limitations, the use of constant currenthas been introduced, in which the current is maintained constantthroughout a titration, and the number of coulombs is taken as theproduct of the current times the time for the titration.

In the early constant current coulometric titrations, storagebatteries were used as a current source, and the current wasmaintained constant manually by means of a rheostat and a galvanometer.However, this is both tedious and uncertain.

Several designs for all-electronic constant current regulatedpower supplies have appeared in the literature. However, most ofthese have not had the range and versatility which have been feltto be desirable for a coulometric titrator. Several features which were felt to be desirable are; 1. close regulation for load

resistances varying from nearly a dead short to several thousand

ohms, 2. easily changed ranges of current, without requiring a

great deal of setting up and adjustment when changing ranges, and

3. current ranges which would correspond to an intergral number of micro-equivalents per second, so the number of micro-equivalents

in the solution could he calculated simply from the time for the

titration.GerameU in 1955, began work on coulometric titrations in non-

aqueous solvents, using primarilarly anhydrous acetic acid. ̂After

solving the difficulties of finding electrolytes and reagents for

the anhydrous acetic acid he was unable to find a suitable system

of end point detection. Redox indicators which he used, either changed color at too low an emf when used for the cerous-eerie system, or were bleached and lost their color during the titration,

so that no change could be observed when the end point was reached.

Potentiometric systems did not give a consistent change in

the emf near the end point during the coulometric titration of

hydroquinone or metol.

CHAPTER III

EXPERIMENTALDesign and Construction of a Constant Current Power SupplySeveral constant current power supplies have appeared in the

literature.̂ ^ however, it was felt that none of these met the requirements of stability, wide range, and versatility desirable for coulometric titrations using constant current.

Features which were felt desirable were: 1. stability, 2. insensitivity to line voltage variations, 3* close regulation over several ranges of current, 4. a method of switching from one current range to another without involving a great deal of resetting and adjustment, and 5* current ranges which could be set so an integral number of microequivalents could be titrated per second.

The stability and insensitivity to line voltage fluctuation was achieved by using separate power supplies for the high current and the control voltage sections of the supply. In addition, for the voltage reference section, a Sola type CVE 7170 constant voltage transformer was used, along with two 0D3 tubes which provided further voltage regulations.

Close current regulation over a wide range of load resistance was accomplished by using a high gain DC amplifier consisting of a 6SL7 driving two 6l6*s in parallel which acts as the current regulating valve. The 6SL7 amplifier is of conventional design.

consisting of the two halves of the 6SL7 direct coupled. The comparsion voltage was obtained from a 5651 voltage reference tube, which is rated as a particularly stable type. The use of two 6l6's in parallel however, is a departure from the conventional design for circuits of this type. It has been the practice to use a 6AS7 with its two triode sections connected in parallel as the current control valve, due to the high current carrying capacity of the 6AS7* However, the 6AS7> in a circuit of this type has a gain of 2, whereas two 616*5 in parallel, using triode connection, have a gain of 8, and are capable of carrying sufficient current for the purpose for which this supply is designed.

Four current ranges were obtained by providing four sets ofsocontrol regulations resistors, with a four-position sw±tchAthat

any range could be selected by turning the switch to the proper position. Minor adjustment of one resistor is usually required when changing ranges.

In addition to the above, automatic start and stop for the timing clock with the initiation and termination of current was provided by incorporating a triple pole double throw relay in the cell and clock-motor circuit. When the clock is turned off, the output of the supply is shunted through a dummy load resistance.When the clock motor is turned on the output of the supply is conn­ected to the titration cell.

The complete power supply was built up on a lif by 19 inch aluminum chassis, and rack mounted, along with the range switching and dummy load resistances,on a 25 inch high rack and panel.

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16EXPERIMENTS USING THE HIGH FREQUENCY INSTRUMENTS

Gemmell, in 1955* "began work on using coulometric titrations in

non-aqueous media. ° He was unable to get satisfactory results in

most cases due to the lack of a suitable system of end point

indication. High frequency indication of the end point has proven

a satisfactory end point indicator for volumetric titration in

non-aqueous media and was proposed as an answer to the problem.One of the systems used by Gemmell was the titration of

hydroquinone with coulometrically generated eerie ion. Cerous

perchlorate was used as an electrolyte. A saturated solution of

cerous perchlorate in the anhydrous acetic acid solvent was required

to carry the current which was desired for the titration.

Complicating the problem was the fact that the available cerous

perchlorate had absorbed so much water that it appeared almost

as a syrup. It was necessary, therefore, in order to maintain an

anhydrous system, to add a considerable amount of acetic anhydride

to the saturated solution of cerous perchlorate in acetic acid to

react with the absorbed water.

The hydroquine was made up by dissolving 9*5505 g- of

hydroquinone in anhydrous acetic acid and making up to 100 ml.

This solution should be exactly 0.100 N. The anhydrous acetic acid

was made by adding a small excess of acetic anhydride to glacial

acetic acid. One to five ml. of the hydroquinone solution could be

then withdrawn by means of a pipette for the titrations.

No available eerie salt was found which was soluble in

anhydrous acetic acid, so no standard eerie solution was prepared.

However, in order that a preliminary check on the effectiveness of the high frequency end point detection system could be made, a

0.100 N solution of hydroquinone and 0.100 N solution of eerie sulfate were prepared in water. It was necessary to add a few

drops of H2S04 to the eerie sulfate to prevent hydrolysis.A series of titrations were run using the aqueous solutions.

The meter reading on the high frequency instrument changed as the

titration progressed, and gave an indication of an end point.

However, this end point indication was found to be independent of

the amount of hydroquinone present, and indeed, was nearly the same

when the titration was run on a blank without any hydroquinone.

It was then concluded that an aqueous system of this type did not

produce satisfactory results. The apparent end point indicated by

the high frequency apparatus had no relationship to the true

equivalence point.

The coulometric apparatus was set up for use with the high

frequency apparatus. The singly trapped cathode and the anode

were introduced. In this first coulometric titration, the amount

of cerous perchlorate necessary for the titration was not known.

Therefore, cerous perchlorate was added until a sufficiently low

resistance of the solution was obtained for the titration. An

attempt to balance the high frequency instrument was then made. It

was found that a balance could not be made, even at full scale on

the balancing dial.

Upon initiation of the generating current, the apparatus ceased

oscillation, or appeared to do so, as the meter dropped to zero and

18would not move from that position for any setting of the condenser balancing dial or of the meter adjustment. The maximum setting of the meter adjust knob gave a zero reading. The electronic indicating eye had opened completely, indicating either a lack of oscillation, or a complete loading of the external circuit.

It was concluded that the cell capacitance was too great when the coulometric electrodes were introduced and the current was initiated. Modification of the cell for handling solutions of higher capacitance was deemed necessary.

The modification of the cell which was found to be most satisfactory is shown in Figure 5. It was found, with the coulometric electrodes in, and the current on, the instrument would balance at approximately the midpoint of the dial.

A series of titrations of hydroquinone with coulometrically generated eerie ion was then performed. End points were found, but were neither sharp nor reproducible. One titration would give a sharp end point, and another, run under as nearly indent!caJ. conditions as possible would give almost no indication of the end point, or just a very slight indication. The balance of the dial for maximum reading was found to be quite critical. If the dial was set on the low side of balance point, the titration curve merely changed slope in the region of the end point. If the dial was set on the high side of balance, the curve was similar, though not as sharp. The maximum change obtained over a single titration was only six divisions on a scale of fifty divisions. See Figure 6.

19ACID-BASE TITRATIONS IN ANHYDROUS ACETIC ACID

For acid "base titrations in anhydrous acetic acid, the apparatus

used was the same as for oxidation-reduction titrations. The

generating electrodes used were corrugated platinum, with the anode

singly trapped in a sintered glass bottomed tube.Perchloric acid was the acid used, and lithum perchlorate

was the electrolyte. Crystal violet was introduced as an alternate

indicator. The high frequency instrument was balanced for maximum

reading on the meter in case.Several titrations were run. The high frequency instrument gave

no indication of an end point. The crystal violet changed color near

the calculated equivalence point, and was assumed to give a fairly

accurate indication of the equivalance point. The titrations were

run well into the basic region, then the generating electrodes

were reversed and back titrations were run to observe the changes on

the high frequency instrument. In each case no significant

indication of the end point was indicated by the high frequency

instrument.

REDUCTION OF CERIC ION BY CHROMIC ION IN WATER

The oxidation-reduction system using coulometrically generated

chromic ion to reduce the eerie ion was investigated. Potassium

dichrornate was used as the electrolyte. An aqueous standard

solution of 0.100 N cerous sulfate was prepared. The generating electrodes were corrugated platinum, with the anode singly trapped.As an alternate method of end point detection, a Beckmann H 2

Univ. of Arizona Library

FIGURE 5

MODIFICATION OF THE HIGH FREQUENCY END POINT DETECTION CELL

n

ACTIVE

ELECTRODE

i

C E L L

B E F O R E

M O D I F I C A T I O N

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M O D I F I C A T I O N

4ACTI VE

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FIGURE 6

GRAPHS OF COULOMB TRIG TITRATIONS USING HIGH FREQUENCY

DETECTION OF THE END POINTS

DI AL SET F OR

M A X I M U M S E N S I T I V I T Y *

CAL CUL AT ED

END POINT *

♦ D I A L SET LOW

M E T E R

R E A D I N G

DIAL SET HIGH

T I M E *

22

potentimometer was used, using standard calomel and platinum electrodes.

A series of titrations of varying amounts of the eerie ionwere run, using the coulometrically generated chromic ion. The

£potentiometric end point was quite sharp and pronouned when the theoretical equivalence point was reached. The high frequency apparatus, however, gave no definite indication of an end point.

ACID-BASE TITRATIONS H WATER Since the hydrogen ion is the most mobile of all ions, if

any ion is to give an indication with the high frequency, the hydrogen ion should. The coulometric and high frequency instruments were set up as before, using sulfuric acid as the acid, and an aqueous solution of sodium sulfate as the electrolyte. The generating electrodes were corrugated platinum, with the anode doubly trapped in sintered glass bottomed tubes. A Beckman model H 2 pH meter with glass indicating and calomel reference electrodes and phenol- phthalein were both used as alternate end point indicators. Before the coulometric titration of the acid was undertaken, two volumetric titrations using standard 0.10N NaOH was performed, in the same vessel, and with the same equipment present as for the coulometric titration. The first, however, was run without the sodium sulfate electrolyte. It was found that the end point was observed by all three methods, the pH meter, the phenolphthalein and the high frequency meter, to be quite sharp, and all came simultaneously.

On the second titration, the sodium sulfate was added. The

end point was still observed to be quite sharp and accurate using

the pH meter and the phenolphthalein, hut no indication of the end

point was given by the high frequency meter.Several coulometric titrations of the same type were then

performed. The results were much the same as with the volumetric

titrations. It was observed that the high frequency gave a curve

in each titration, but which had no significant breaks near the

end point. This was investigated further, using different settings

on the condenser dial of the high frequency instrument and it was

concluded that minor flucutations in high frequency meter reading

were a result of changes in the electrolyte other than of the

concentration of hydrogen ion.

CHAPTER IV

CONCLUSIONS

It was riot found possible to use the high frequency method of end point detection with a coulometric titration, in either aqueous or non-aqueous media, with acid-base or with redox systems. The reason for this is believed to be a masking of the end point detected by the high frequency instrument by the electrolyte necessary for the coulometric titration. The concentration of the electrolyte is relatively constant throughout the titration, and near the end point it becomes many times the concentration of the ion being determined. The high frequency instrument, which operates principally by detecting changes in the dielectric constant of the unknown responds primarilarly to the electrolyte, which is not undergoing change near the end point. As a result, no effect is observed on the high frequency instrument.

It appears that coulometric analysis is incompatible with the high frequency method of end point detection because the electrolyte necessary for coulometric analysis interferes with the high frequency detection of the end point.

CHAPTER VSUGGESTIONS FOR FUTURE WORK

There appears to be little further work to be done using thehigh frequency method of determination of the end point withcoulometric analysis. Perhaps the design of a high frequency

instrument which detects small changes in the presence of

interfering ions, or the choice of coulometric electrolytes which

would not interfere with the high frequency will make possible

the use of this system.Coulometric titrations, as a whole, in the past have been

performed using constant current as a method for obtaining the

number of coulombs passed through the solution. In some titrations,

carefully controlled cathodes potential offers some very important11advantages. A few titrations have been made by this method, but

only comparatively crude mechanical systems have been employed for

the control of the cathode potential. An all electronic controlled

cathode potential coulometric titrator, with an accurate electronic

current integrator would be an answer to the problems which have

arisen in earlier controlled cathode potential coulometric titrations.

Coulometric titrations in non-aqueous solutions also are open

to further investigation. More work is needed on the fundamentals

of conduction of electrolytes in non-aqueous solvents, and better

electrolytes need to be found for coulometric titrations. The

problem of a satisfactory end point detection method in non-aqueous

coulometric titrimetry is still unsolved. The relatively high

resistance of most non-aqueous solutions makes difficult the use of

most common end point detection methods, such as potenti©metric

or amperometric detection, due to the induction currents resulting

from the generating current. Perhaps with more suitable electro­

lytes, this problem may be less serious, and possibly may be solved by use of one of the conventional end point determination methods.

BIBLIOGRAPHY

1.2.3-b.

5-6.

7-8.

9.

10.11.

12. 13-14.

15.16.17.

Anderson and Revlnson, Anal. Chem., 22, 1272, (1950)

Bein, Ibid, 26,909, (1954)Bladel and Malmstadt. Ibid, 22, 732, (1950)

Cooke, Reilley and Furman, Ibid., 23, 945, (1951)

DeFord, Pitts, and Johns, Ibid., 23, 938, (1951)

Gemmell, Masters Thesis, University of Arizona, (1955)Hall, and Gibson, Anal. Chem., 23, 966, (1951)Kowalowski, Kennedy, and Farrington, Ibid., 26, 626, (1954) Kurshan, and Lohrman, Review of Scientific Instruments, 24,

334, (1953)Leisey, Anal. Chem., 26, 1607, (1954)Lingane, J. Am. Chem. Soc., 67, 1916, (1945)Nankano, Kara, and Yashiro, Anal. Chem., 26, 636, (1954) Reilley, Furman, and Adams, ibid., 24, 1044, (1952)

Sease, Niemann, and Swift, Ibid., 19, 197, (1947) Szebelledy, and Smogyi, Z. Anal. Chem., 112, 313, (1938)

Wagner and Kaufmann, Anal. Chem., 25, 538, (1953)Wise, Gillis and Reynolds, Ibid. 25, 1344, (1953)