JULY 15, 1939
ANALYTICAL EDITION
a greater accuracy for the determination. N hydrochloric acid and diethanolamine are recommended as the extractive reagents, since it was shown that 0.1 N hydrochloric acid and piperidine do not extract equivalent amounts of nonuric acid nitrogen from materials similar to chicken excrement free of uric acid.
Acknowledgment The authors wish to thank I. J. Duncan of the Depart-
383
ment of Agricultural Chemistry for valuable suggestions made in connection with this investigation.
Literature Cited (1) Fritz, J. C., IND.ENG. CHEM.,Anal. Ed., 7, 123 (1935). (2) St. John, J. L., and Johnson, O., J. Biol. Chern., 92, 41 (1931). P R E S ~ N Tbefore E D the Division of Biological Chemistry a t the 97th Meeting of the American Chemical Society, Baltimore, Md. Published with the approval of the Director of the West Virginia Agricultural Experiment Station as Scientifio Paper No. 218.
Determination of Electrometric Equivalence Points JOHN R. GAY Geological Survey, Washington, D. C.
T
HE staff of Research Project No. 4 of the American Pe-
troleum Institute, in their study (9) of organic matter included in sedimentary rocks, employed a modification of the analytical method of Schollenberger (8). This method of analysis involves the oxidation of the powdered sediment with a predetermined amount of 0.4 N chromic acid, in excess, and the titration of the unreduced acid with 0.2 N ferrous ammonium sulfate solution. The end point in this titration has been determined with the aid of an internal indicator, diphenylamine.
R
rnkchanicol stirrer
FIGURE 1. SCHEMATIC-PICTORIAL DIAGRAM OF TITRIMETER a, b. Poles for two circuits controlled b y switch S
E.
Cell composed of electrodes and electrolyte
F. Dry cell (1.5 volts)
M . Milliammeter, 0 t o 1 milliampere, 1000 ohms per volt sensitivity (Triplett model 221 or 321) Pt. Platinum electrode 22. 1500-ohm, 1-watt, rheostat-type control (wire wound) 5. Single-pole double-throw knife switch W. Tungsten electrode
However, difficulty is encountered in detecting the end point of this particular titration. The suspended particles in the titration vessel may mask or even prevent the observation of the color change, especially with iron-rich sediments. The color of the trivalent chromium is an intense blue, affecting the normal purple hue of the oxidized indicator. More-
over, the indicator in the oxidized condition is a dye, which decomposes gradually and forms a dense, black, tarry deposit around the edges of the solution in the beaker; also, a strong source of light is needed in order to observe the gradual change of the color of the indicator. The indicator has to be added carefully, and a beginner usually requires several days of experience before he becomes adept in detecting the endpoint color changes. These disadvantages are eliminated by electrometric titration. Present methods of detecting equivalence points resolve themselves into two main groups. The first and oldest method involves the use of a sensitive potentiometer. The practice is to balance the potential of cell E (Figure 1) with a known potential, and, in this way, to determine the greatest potential change for the smallest volume of titrant added (1). It is necessary to plot the potential changes throughout the entire reaction, in order to ascertain the equivalence point. The second method employs the vacuum-tube voltmeter, and is a comparatively recent development. I n this method, the cell potential is measured directly, without drawing any current from the cell. The cell potential is impressed in static charge on the control grid of the vacuum tube, and has a control or Thyratron action on the flow of plate current of the tube. Variations in the plate current are either detected in the first stage or amplified further by a cascade amplifier. The main point is that in both methods no current is drawn from the cell, so that polarization of the cell is prevented. Polarization causes the formation of gases on both of the electrodes, and halts the action of the cell. The potentiometric method involves continual manipulation throughout the titration. The vacuum-tube voltmeter, like electron-tube titrimeters, requires a power line source of alternating current of a very good degree of regulation. Moreover, the apparatus needed for both methods is costly. Consequently, an electrometric method of titration that will overcome these disadvantages is desirable. An inexpensive apparatus was developed which mas found to be applicable to electrometric titrimetry to the same extent as the more expensive equipment. The object of this paper is to explain the operation and applications of this titrimeter, especially its application to simplifying the Schollenberger method of determining organic matter, as employed in the laboratory of Project 4 of the American Petroleum Institute. The instrument gives promise of being useful in other types of titrations, and three others are described in detail in this paper-namely, ferrous
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iron-potassium permanganate, ceric cerium-ferrous tetravalent vanadium-potassium permanganate.
VOL. 11, NO. 7
iron,
600
Description of Method
500
APPARATUS. The apparatus employed in this device can be purchased for about $4. The parts used are schematically represented in Figure 1.
400
Platinum indicator and tungsten reference electrodes (6, 7 , 10) are employed, the surface area of each in contact with the liquid being 15 and 3.7 sq. cm., respectively. Different electrode contact areas will probably prove equally efficient. The meter, M, has a sensitivity of 1000 ohms per volt, and requires a 1500-ohm resistor in series with it and a dry cell to drop the current to the proper value of full-scale deflection. Cell F is an ordinary 1.5volt flashlight battery. When the e. m. f. of the cell drops below 1.2 volts, it should be replaced. OPERATION.Since the meter is calibrated in tenths and hundredths milliampere, all readings will be expressed in units of milliamperes times 100. The first step is to throw the single-pole double-throw switch, S, to position a. This provides a complete circuit including M, R, and F . The resistance, R, is then varied until meter M shows full deflection of 1 milliampere. Switch S prevents accidental shorting of cell E. Next, the connections from the electrodes to a and b are arraneed so that the e. m. f. of cell E will be otmosed to that of F. In che average oxidimetric reaction, the plat&m electrode may be considered the anode and the tungsten electrode the cathode. The switch is then thrown to position b. At this point, a reductant electrolyte will give approximately full-scale deflection (90 to 100 units), because the cell potential will be low, and the needle will drop at the equivalence point, as the cell potential rises when the solution is titrated. For an oxidant electrolyte, the needle will read about half scale (50 units), owing to the fact that the cell e. m. f. resists that of the dry cell, and the needle will rise a t the equivalence point, as the potential drops.
Pot.
1 E FIGURE 2. ABSOLUTE E. M. F. MEASUREMENT E. Same as cell E in Figure 1 Pot. Calibrated potentiometer (0 t o 0.8 volt) Sw. Double-pole double-throw switch (ceramic base knife switch) V T V M . Vacuum-tube voltmeter (0 t o 0.8 volt)
PRECAUTIONS. The solutions must be stirred continually throughout titration. The meter circuit should be fused to prevent accidental overloads. The electrodes must be periodically cleaned and sensitized. I n order t o clean the electrodes, they should be placed in chromic acid cleaning solution, and rinsed with distilled water until free from acid. The platinum electrode should then be ignited in an alcohol flame (not gas), in order to prevent the formation of platinum carbides. The tungsten electrode may be cleaned by polishing with an emery cloth, or better, by dipping momentarily in a bath of molten sodium nitrite and quenching with water before the reaction becomes too vigorous. The electrodes should be left in concentrated chromic acid cleaning solution when not in use, in order to keep them in a sensitive condition. If the meter swings violently off-scale, when the switch is thrown to position b, it is an indication that the cells are acting in series. The leads from cell E to contacts a and b must be reversed.
; ; f ._ 0 1 ._ -
I
300
5
c
Q r c
a
200
s
100
3
FIGURE3. TITRATION OF 0.4 N POTASSIUM DICHROMATE WITH 0.2 N FERROUS AMMONIUM SULFATE A. Potential variations of cell E throughout reaction B . Titrimeter variations of cell E throughout reaction
MEASUREhIEKT O F POTENTIAL VARIATION IN CELL. In order to measure the actual potential of any particular cell throughout a titration, the apparatus shown in Figure 2 was employed.
The double-pole double-throw switch, Sw, is thrown to the left. This impresses the cell e. m. f. on the vacuum-tube voltmeter, VITVM. The reading should be noted. The switch is then thrown to the right, and the potentiometer, Pot., adjusted so that the reading of the vacuum-tube voltmeter is the same as that obtained with the cell. The cell e. m. f. is then read directly or by interpolation, on the potentiometer. In this manner, the cell e. m. f. is accurately obtained without drawing any current that would polarize the cell. PROCEDURE IN OXIDIMETRIC TITRIMETRY. After the titrimeter has been adjusted to full-scale deflection, as described above, the titrant is added gradually, until the needle of the meter flickers perceptibly (about 5 units) from the constant deflection determined by the original cell potential. This shifting of the needle indicates that the equivalence point is within 2 ml. Since the addition of a large amount of titrant, with inadequate stirring of the cell liquid, will cause a sudden deflection regardless of the state of the reaction, the liquid should be continually and rapidly stirred, preferably by a glass propeller as illustrated in Figure 1. The titrant should then be added dropwise; as the equivalence point is approached, each drop will cause a perceptible flicker of the needle, until, a t the exact point, a slight excess of the titrant will cause a sudden, large deflection. For example, when dichromate is titrated with ferrous ion (Figure 3, curve B), the deflection is from a reading of 50 units to one of 70 units, for a positive increment of 0.05 ml. of ferrous sulfate solution. At the equivalence point in the reverse reaction, the needle returns to 50 units. The titration of potassium dichromate with ferrous ammonium sulfate is illustrated in Figure 3. The reaction may be represented schematically as follows: Pt, CrzO?--
- Cr3(0.4N)//Fe2 - Fe3(0.2N ) , W
This reaction is involved in the Schollenberger method for determining organic content of soil. I n this reaction, ferrous ammonium sulfate is the titrant. The acid concentration in all cases is 5 per cent by volume of concentrated sulfuric acid (sp. gr. 1.83).
JULY 15, 1939
ANALYTICAL EDITION
The equivalence point in this reaction is attended by a large potential drop. For a positive increment in volume (delta V) of 0.05 ml. a t the equivalence point, the drop in potential is .- 200 millivolts and the corresponding titrimeter variation is +29 units. Since a deflection of 5 units is easily noted, 0.01-ml. increment in ferrous ammonium sulfate can be readily detected. Hence, the sensitivity is greater than is experimentally possible to measure with normal volumetric apparatus. The end points obtained with the use of diphenylamine indicator were observed to differ slightly from the equivalence points, as determined electrometrically. This discrepancy is due to the fact that a concentration of 4 mg. of chromic anhydride per liter of solution is necessary to keep the diphenylamine in the oxidized condition. However, since the control is affected in the same way, the results of titrations are not changed, as is shown by the accordance between the results of titrating 8 samples in the laboratory of Project 4, with diphenylamine and with the electrometric titrimeter (Table I).
385
Other Applications INDIVIDUAL OXIDIMETRICREACTIONS.I n order to ascertain whether the titrimeter would find greater applications in the field of electrometric titrations, three reactions were studied : (1) potassium permanganate-ferrous ammonium sulfate, (2) ceric cerium-ferrous ammonium sulfate, and (3) tetravalent vanadium-potassium permanganate.
FIGURES FOR EIGHT SEDIVEXTS TABLE I. TITRATION Sample
Diphenylamine 3.00 0.73 0.30 1.01 0.75 1.95
1.67 1.42
Titrimeter 3.07 0.75 0.28
1.00 0.72 1.95 1.67 1.41
SENSITIVITY.The sensitivity of the titrimeter varies with individual reactions. With regard to the examples of oxidimetry studied, the maximum sensitivity was found to be 33 millivolts. I n order to calculate this maximum sensitivity, the reaction between ferrous ammonium sulfate and potassium dichromate was studied (Figure 3), because the potential changes a t the equivalence point were the most pronounced of all those encountered. For an addition (delta V) of 0.05 ml. of ferrous sulfate a t the equivalence point (26.50 to 26.55 ml), the potential changes from 400 to 200 millivolts, or -200 millivolts (curve A ) . For this same delta V, by projecting the abscissas onto curve B , the titrimeter reading is seen to vary from 50 to 79 units, Since the smallest deflection of the titrimeter that can be readily noted is 5 units, about one sixth of the delta V, or less than 0.01 ml., could be detected in this case. This corresponds to 200/6 or 33 millivolts potential change. If the potential change is very gradual, it will be necessary to plot titrimeter deflections per delta V against delta V (6). The maximum peak of the curve will represent the equivalence point. DISCUSSIOXS.The tungsten-platinum bimetallic electrode system has been found to be the most sensitive system which is resistant to the conditions of acidity and oxidation encountered in oxidimetry (3, 10, 11). The platinum electrode is the anode or indicator electrode, and the tungsten is the cathode or reference electrode. The opposing e. m. f. of the dry cell against that of cell E renders the system sensitive by preventing desensitizing polarization of E. This has been the objection to methods that employ a moving coil meter to indicate potential change. All these methods polarize the cell, even when considerable resistance is inserted in series with the meter. These methods also employ a galvanometer costing more than $20. Measurement of the cell potential with a 100-millivolt moving coil meter and multiplying resistor was attempted, but polarization occurred a t once, and the potential dropped to zero. Change in conductance ( 2 ) of the cell is not great enough to affect the measurement of the equivalence point with the device described.
ml. OIN KMn04 OF 0.2 N FERROUS AMMONIUM SULFATE FIGURE4. TITRATION WITH 0.1 N POTASSIUM PERMANGANATE
A . Potential variations of cell E throughout reaction B . Titrimeter variations throughout reaction
The first reaction was chosen as one very commonly employed in permanganimetry ; the second, because there is no very satisfactory color indicator for use with ceric cerium; and the third, because of the very small potential change a t the equivalence point, which would be a strict test of the sensitivity of the titrimeter. The purpose of all the titrations was to plot the potential changes throughout the reaction against those of the titrimeter for the identical reaction, and thus determine the sensitivity and applicability of the instrument. The acid concentration in all the titrations was 5 per cent by volume of concentrated sulfuric acid (sp. gr. 1.83). The reaction between 0.1 N potassium permanganate and 0.2 N ferrous ammonium sulfate is illustrated in Figure 4. Schematically, the reaction may be represented as follows: Pt, Fez - Fe3 (0.2 N)//Mn04--
- Mn2 (0.1 N ) , W
Potassium permanganate is the titrant. The equivalence point in this reaction is represented by a potential rise of 210 millivolts a t the 0.05-ml. increment between 12.50 and 12.55 ml., and is accompanied by a titrimeter variation of -12 units. Thus, an accuracy of 0.02 ml. is possible. The potential drops slightly as the equivalence point is approached, and a t the same time the titrimeter reading increases slightly for a proportional variation in the volume of permanganate added. Deflection and potential maxima coincide for given delta V a t the equivalence point.
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VOL. 11, NO. 7
The reaction between 0.1 N ceric sulfate and 0.2 N ferrous ammonium sulfate is illustrated in Figure 5. Schematically, the reaction may be represented as follows: Pt, Ce4 - Ce3 (0.1 N)//FeZ - Fe3 (0.2 N), W I n this reaction, ceric sulfate is the titrant. Maximum titrimeter and potential changes occur a t the same point-namely, after the addition of 20.45 ml. of ceric sulfate. The potential rise of 180 millivolts is equivalent to a titrimeter deflection of -7 units. Since deflections of 5 units are readily discernible, it follows that an accuracy of 0.04 ml. of titrant is readily obtainable. The cell potential changes are closely followed by proportional meter deflections. The reaction between 0.1 N tetravalent vanadium and 0.1 N potassium permanganate is illustrated in Figure 6. Schematically, the reaction may be represented as follows:
Pt, V4
- V6 (0.1 N)//MnO4-- - Mn2 (0.1 N), W
I n this reaction, potassium permanganate is the titrant. The equivalence point is accompanied by a relatively small potential change (Q), and occurs between 26.15 and 26.20 ml. The potential change is $90 millivolts. The corresponding titrimeter variation is -4 units. Potential and deflection maxima again coincide for minimum delta V (0.05 ml.), The volumetric accuracy is lower in this titration, but the same could be said of any electrometric system. An accuracy of 0.06 ml. is possible.
250
I
I
I
25 4
258
26 2
lo 26 6
27 0
m l OIN K M n 0 4
FIGURE6. TITRATION OF TETRAVALENT VANADIUM(0.1 N ) WITH POTASSIUM PERMANGANATE (0.1 N ) A . Potential variations of oell E throughout resotion
E. Titrimeter variations throughout reaction
Summary
A titrimeter is described for application to potentiometric titrimetry which is comparable in sensitivity to other electrometric methods.
700
300
500
-(0
c
0 >
.-.-
too 5 0
Its advantages over other electrometric systems may be summarized as follows: 1. Desensitizing polarization of the cell (E, Figure 1) is eliminated. 2. Maximum sensitivity is retained throughout the titration. 3. Inexpensive apparatus is used, which is readily obtainable, 4. Simplicity of operation; no adjustment is necessary for individual reactions. 5, There is positive and immediate indication of potential changes, which are roughly proportional to the actual potential variations of E. 6. Since the needle deflections are roughly proportional to the actual potential changes taking place in the cell, an inference can be made as to the potential variations taking place throughout any specific titration. 7. Reverse titration (back-titration) is easy and accurate. 8. Sensitivity is great enough for common electrometric titrations. 9. Adequate warning of the approach of the equivalence point is given.
._ c al c c
0
n
500 = 0
!OO
00
1
i ml. 01 N C e ( S 0 4 ) ~ OF 0.2 N FERROUS AMMONIUM SULFATE FIGURE5. TITRATION WITH 0.1 N CERICSULFATE
A . Potential variations of oell E throughout reaotion B . Titrimeter variations throughout reaotion
Literature Cited (1) Bottger, W., 2. physik. Chem., 24, 253 (1899); “Die Anwendung der Elektrometers als Indikator beim Titrieren von Sauren und Basen”. dissertation. Lei~sie.1897. (2) Britton, H. T. S., “Conductometric Analysis”, New York, D. Van Nostrand Co., 1934. (3) Furman, N. H., and Wilson, E. B., J. Am. Chem. Soc., 60, 277 (1928). (4) Gustavson, R. G., and Knudson, C. M., Ihid., 44, 2756 (1922). (5) Hostetter, J. C., and Roberta, H. S., Ihid., 41, 1337 (1919). (6) Kolthoff, I. M., and Furman, N. H., “Potentiometric Titrations”, New York, John Wiley & Sons, 1926. (7) Muller, E., “Die electrometrische (Potentiometrische) Massanalyse”, 4th ed., Dresden, Th. Steinkopff, 1926. (8) Sohollenberger, C. J., Soil Sci., 24, 65, 68 (1927); 31, 483-6 (1931). (9) Trask, P. D., and Hammar, H. E., “Organic Content of Sediments”, Drilling and Production Practice, American Petroleum Institute, 1934; pp. 117-30, 1935. (10) Willard, H. H., and Fenwiok, Florence, J . Am. Chem. Soc., 44, 2504 (1922). (11) Ibid.,44, 2516 (1922).
