I N D U S T R I A L A N D ENGINEERING CHEMIXTRY
April 15, 1931
desired pressure in the system. Obviously, the basic principle of this generator can be adapted to the preparation of gases other than carbon dioxide. This generator incorporates the following advantages: (1) it is compact and is constructed entirely of glass without ground-glass connections; (2) it can be warmed and pumped free of occluded gases; (3) it is sealed with mercury against contaminating atmospheric gases; (4) it has a novel automatic feeding arrangement; and ( 5 ) it Utilizes its reactants
203
completely without wasting any of the gas produced, even during long periods of standing. Literature Cited (1) Bock and Beaucourt, Mikrochemie, 6, 79 (1928). (’) Brunner* van* Chem*-Ztg.) s8r 767 (1914). (3) Hein, 2. angew. Chem., 40, 865 (1927). (4) path, IKD. END.cHIM., Anal. Ed., a, 250 (1930). (5) Pregl, “Quantitative Organische Mikroanalyse,” p. 98, Springer, 1930.
Improved Hydrogen-Electrode Cell for Determination of pH’,’ W. B. Bollens IDAHO AGRICULTURAL EXPERIMENT STATION, Moscow, IDAHO
T
HE cell illustrated, de-
The cell described, constructed from usual laboratory
Construction of Hydrogen
Electrode materials, is widely applicable for rapid and accurate veloped after experipH determinations. Novel features are (1) a PyrexThe stirrer-circulator conence w i t h numerous Alundum porous-tip calomel electrode, (2) a salt bridge sists of a small glass T-tube types of electrometric appagiving sharp, reproducible liquid junctions, and (3) a fused to a glass rod or shaft of ratus, satisfactorily meets the stirrer which centrifugally circulates through the similar diameter. An axial requirements for rapid and sample a small constant volume of hydrogen under opening, corresponding in size accurate pH determinations constant pressure. The stirrer-circulator especially to the bore of the tubing, is on a variety of solutions and adapts the hydrogen electrode to solutions containing suspensions. The hydrogen provided in the T where it dissolved carbon dioxide or other gases participating joins the shaft within the electrode, calomel electrode, in H-ion equilibria. I t also handles foaming liquids and salt bridge are readily vessel. Another opening is to advantage. Hydrogen saturation of the solution is constructed from usual laboprovided in the center of the eRected in minimum time and true H-ion equilibrium ratory materials a n d give open-end cross tube at the i s attained quickly. When side reactions interfere, service with minimum attenbottom. A hard rubber bearvoltage-time curves are useful in estimating the point tion. ing for the shaft is centered of virtual H-ion equilibrium. A centrifugal circulator or in a sulfur-free rubber stopper stirrer which circulates a small Typical data presented illustrate the rapidity, acthat has been carefully filed constant volume of hydrogen curacy, and versatility of the apparatus. or ground on the lower twothrough the liquid under conthirds of the circumference stant pressure-is a feature of the hydrogen electrode devised to remove the taper and make a tight fit with the mouth of for use with liquids containing carbon dioxide or other gases the vessel. Careful fitting of this stopper is essential; even participating in H-ion equilibria. It is especially applicable a slight taper will result in its working loose when in contact to pH determinations on solutions or suspensions of alkali with alkaline solution. The bearing should be accurately soils containing carbonates. In such cases colorimetric turned and drilled in a lathe, the upper half being drilled a methods as well as the quinhydrone electrode are often in- few tenths of a millimeter oversize, so that the shaft fits with applicable, while the use of a bubbling-type hydrogen elec- minimum play consistent with free operation. A shoulder trode results in removal of dissolved carbon dioxide with for contact with the top surface of the stopper is provided for consequent shift in acid-base equilibria and increase in pH. stability, Shaking electrode vessels of the type described by Clark (3) To hold the stirrer in place and to connect with the flexible and Snyder (6) may be used, but their construction is beyond shaft of a motor drive, a flexible coupling is made from a short the skill of ordinary glass blowers and the mechanism for piece of glass tubing fitted with a section of heavy-walled shaking is more complicated than that required for the stirrer. rubber tubing inside and projecting slightly beyond the upper Since the stirrer effects hydrogen saturation of both solution end. The glass tube is of such inside diameter that it will and electrodes in minimum time, its general application is rotate smoothly when slipped over the upper part of the hard indicated. rubber bearing; the rubber tube must fit snugly inside the By using the stirring electrode with a modification of the glass tube and also hold firmly the driving end of the flexible salt bridge designed by Waterman (7), the advantage of shaft and the driven end of the stirrer shaft. Assembly is permanent contact, together with a sharp reproducible liquid made by placing the coupling over the bearing and thrusting junction permits readings a t any time, whether the stirrer the stirrer shaft through the bearing and into the rubber tube is in operation or not. It is thus possible to follow the volt- inside the glass. To connect with the motor, the free end age change during a determination from the beginning until a of the flexible shaft is brought into proper position and inconstant or maximum value is attained This not only saves serted in the upper end of the coupling. The rubber stopper must be carefully bored to carry not time but also permits a comprehensive voltage curve for only the stirrer bearing but also a hydrogen inlet tube and the guidance in determining the point of H-ion equilibrium. hydrogen electrodes. Two of the latter are always used and Received August 8, 1930. connected to a double-throw switch so one may be checked Published with approval of the director of the Idaho Agricultural against the other during every determination. If properly Experiment Station as Scientific Paper 63. * Now associate bacteriologist, Oregon Agricultural Experiment Sta- plated with either platinum or palladium black, both should tion, Corvallis, Ore. come to equilibrium within a few seconds of each other and f
ANALYTICAL EDITION
204
agree within one millivolt. Whenever these conditions are not satisfied, both electrodes are removed, cleaned, and replated; this requires three to five minutes, far less time than will be lost by working with sluggish electrodes, and does away with a seripus source of unrertainty. Inasmuch as the hydrogen electrodes are of first importance in any hydrogen-electrode system, too much attention cannot be given to their preparation and care. The active surface is deposited according to directions given by Clark ( I ) . Palladium is used in preference to platinum because of the f+
r\
s il
Vol. 3, KO. 2
for electrical connection, two short lengths of larger glass tubing are inserted in the stopper and fitted with slip joints of rubber tubing at the lower end within the vessel. The size of these various tubes is so selected that the electrode tubes may be freely thrust through or withdrawn from below, the rubber forming tight joints. It is desirable that all holes in the stopper be bored as nearly perpendicular to the face as possible to insure proper alignment of the electrodes with respect to the stirrer and walls of the vessel. The electrodes must be firmly held where the active surface will receive a copious wash of hydrogen bubbles and solution centrifugally thrown by the stirrer, but a t the same time abrasion by contact with the vessel or stirrer must be avoided. The electrode vessel is most conveniently made from an open-top separatory funnel of about 60 cc. capacity. The stem is cut to a length of 3 em. and the bore in both plug and throat reamed out with Carborundum dust to a diameter of a t least 3 mm. to provide ample conductivity. No grease is used on the stopcock. Aside from contributing to cleanliness, the absence of grease permits determinations to be made on electrolyte solutions denser than saturated potassium chloride. In such cases, with the stopcock necessarily closed, the moist ground glass gives the conductivity required. Porous-Tip Calomel Electrode
Figure 1-Hydrogen-Electrode Cell for pH Determination
ease with which unsatisfactory deposits may be removed by electrolysis in hydrochloric acid solution. Platinum and palladium-plated electrodes were simultaneously compared in a variety of solutions, and when deposits were made under comparable conditions, excellent agreement was found. When using current under a potential of four volts for making deposits from 3 per cent hydrochloric acid solutions containing 3 per cent of either metal, 12 to 15 seconds is the optimum time for plating the electrodes. A 10-second deposit tends to give low readings, while a 20-second deposit gives a sluggish electrode. A difference of even one second in the time of plating two electrodes that are used together is reflected by difference in the rates of attaining equilibrium. For this reason the practice of electrolyzing the electrodes simultaneously in the cleaning, plating, and charging solutions is generally followed. It is important that these solutions be carefully protected from contamination, and that the sulfuric acid solution used for charging the electrodes with hydrogen be freshly prepared for each day's run. As the plating solution becomes weaker, the time of deposition must be correspondingly increased, but after a 20-second optimum has been reached, it is advisable to increase the concentration. To facilitate removal and replacement of the hydrogen electrodes, each of which consists of 3 or 4 em. of No. 20 B & S gage platinum wire suitably spiraled and sealed into one end of a 15-em. length of Pyrex tubing carrying mercury
The calomel electrode is provided with a porous tip for connection with the salt bridge. This tip also permits the half-cell to be used as a dipping electrode if desired. Parker and Dannerth (4) and Schollenberger (6) have described ground-glass plug tips, but these are difficult to construct. An equally effective and more serviceable tip can be easily prepared by sintering a 100-mesh mixture of equal parts of Pyrex glass and Alundum in one end of a Pyrex tube. A tube of appropriate size is sealed at one end and enough of the Pyrex-Alundum mixture introduced to give a column about 3 cm. long when well tamped down. This end is then placed in a blast flame and the mixture is gently tamped with a glass rod until incipient redness, to eliminate air bubbles. The rod is then withdrawn and heating continued until a temperature of approximately 950" C. has been maintained for 4 or 5 minutes. After slow cooling the tip is cut and broken off so that a plug 5 to 7 mm. long remains in the tube. The end is ground smooth with Carborundum dust and water on a glass plate, washed, dried, and finally fire-polished. The result is a neatly encased plug of sufficient porosity to permit slow liquid diffusion. With the hydrostatic head of the half-cell here described, the plugs used permit the passage of about 0.05 cc. per minute. Plugs of various porosity can be prepared by altering the size of the Pyrex and Alundum particles, and the time and intensity of heating. The main vessel of the calomel electrode consists of a small Pyrex carbon funnel having a side tube sealed to the stem for mercury connection. A short piece of platinum wire is sealed in the lower end of this side arm to prevent outside contamination of the mercury in the cell. About 3 em. of heavy-walled rubber tubing is attached to the stem and the porous-tip tube, cut to proper length, is thrust through this into the funnel; the plugged end should extend 4 or 5 em. below the stem and the open end should be well above the mercury-calomel paste but under the surface of the potassium chloride-calomel solution after the cell is made up. It is essential that the outside diameter of this tube be small enough to permit the mercury of the cell to flow around it in the stem and make contact with the platinum wire in the side arm. At the same time, the inside diameter must be sufficiently large to permit ample cross section of solution
April 15, 1931
INDUSTRIAL A N D ENGINEERING CHEMISTRY
for conductivity. The rubber slip joint is brushed with an alcoholic solution of shellac, wound tightly with a narrow strip of friction tape, and again shellacked. Since this joint must retain mercury, the section of rubber tubing must be live and fit snugly around the smaller glass tube. Shellacking and taping are effective in preventing deterioration. Several such joints have been in service over two years and show no signs of cracking or leaking. Before assembling the calomel electrode, it is advisable that the various parts, including the rubber slip joint, be thoroughly cleaned and dried. The porous-tip tube may be cleaned with cleaning solution and rinsed with distilled water under pressure. After assembly is made, the vessel is fixed in a clamp and the cell made up in the usual manner, except that precautions to prevent or eliminate air bubbles in the mercury around the tube in the stem and in the potassium chloride-calomel solution in this tube must be observed. Air bubbles in the mercury can be worked out by gently tapping the porous-tip tube on the lower end. If enough potassium chloride-calomel solution is added to cover the opening of the porous-tip tube to a depth of 1 or 2 cm., and if the tube is clean, the solution will slowly force the air down through the tip without formation of air bubbles. Bubbles inadvertently formed may be removed with a fine capillary pipet. The vessel is finally closed with a short rubber stopper fitted with a glass stopcock and a glass-stoppered separatory funnel or other similar container of about 25 cc. capacity. The stopcock is convenient as an air relief valve, while the separatory funnel is used as a reservoir for potassium chloridecalomel solution. The rubber stopper should fit tightly and should be well shellacked, as the vessel must be air-tight to prevent uncontrolled escape of solution from the tip. Assembly of Salt Bridge and Cell The salt bridge, of 7-mm. Pyrex tubing, is made in the form of a low H with inclined cross bar. A 500-cc. separatory funnel connected by 2 feet (0.6 meter) of heavy-walled rubber tubing to the lower end of the higher vertical tube, serves as a reservoir for saturated potassium chloride solution. The tip of the calomel electrode is inserted through a tight rubber connection into the upper end of this tube until it rests in line with the bore of the inclinded cross tube. The lower end of the other vertical tube is fitted with a short piece of rubber tubing and a pinchcock for use as a clean-out. Above this, closely connected by rubber, is a 3- or 4-em. length of glass tubing the same diameter as the main portion of the bridge, and carrying a smaller sealed-on wash-out tube with stopcock, bent vertically. At the top is a short section of heavy rubber tubing extending 5 mm. beyond the glass. This serves as a slip joint through which the stem of the hydrogenelectrode vessel may be freely but tightly inserted or removed. Construction details are shown in Figures 1and 2. The entire apparatus, except the hydrogen-electrode vessel, should be rigidly clamped to a solid support A large clamp for the hydrogen-electrode vessel should be securely fixed in position so that by slightly loosening the jaws, this vessel may be connected with or removed from the bridge. A view of the apparatus in working position, including the potentiometer system, is given in Figure 3. All rubber connections should be shellacked and allowed to dry before filling the bridge with saturated potassium chloride solution. If this is done, there will be no trouble with leaks or creeping of salt from the joints. The reservoir of saturated potassium chloride solution is suspended in a split ring 15 or 20 cm. higher than the liquid level in the calomel half-cell. To fill the bridge, the stopper in the reservoir is loosened and the stopcock partially opened. Air bubbles may be removed by squeezing the rubber con-
205
necting tube and manipulating the pinchcock on the cleanout. It is particularly important to eliminate air bubbles about the porous tip of the calomel electrode. If this tip is properly placed as previously described, no air bubbles interfering with conductivity will collect. After the bridge has filled and solution overflowed from the washout, the reservoir stopcock is closed. The apparatus is then ready for connection with the hydrogen electrode. It should be noted that when the potassium chloridecalomel solution reservoir on the calomel electrode has the stopper loosened and stopcock open, the solution will slowly diffuse through the porous tip into the bridge solution, prpvided the latter is not under full pressure from its reservoir. This pressure need be exerted only momentarily, as will be apparent from subsequent details of procedure, and a slight flow from the calomel electrode can be maintained as desired. Protection of the solution in this electrode is assured by allowing a short flow every hour during a series of determinations. Approximately 500 determinations may be made before the potassium chloride-calomel solution reservoir requires refilling. Method of Operation
Procedure in making a determination will vary with the amount of sample. When an ample quantity is available, an amount sufficient to fill the hydrogen-electrode vessel is poured directly into it, after first closing the stopcock and removing the stopper assembly. The latter is then inserted carefully so as to expel all air and excess liquid through the
-I
Wgure %-Hydrogen
I
Half-Cell Assembly
hydrogen inlet. The rubber tube supplying purified hydrogen a t the rate of approximately 5 cc. per second is then connected with the inlet, after which hydrogen will escape through the release valve consisting of a glass tube connected to a T in the delivery line and opening below the surface of water in a suitable container. By opening the stopcock of the vessel, a stream of liquid is allowed to run out slowly until approximately 20 cc. remain, the balance having been displaced by hydrogen. The stopcock is then closed. During this displacement the rates of the flow of hydrogen and
206
ANALYTICAL EDITION
the escape of liquid should be nearly equal. Such precaution is advisable to prevent accidental entrance of air, although the small quantity of liquid finding its way into the bearing around the stirrer shaft forms an effective seal which will hold pressure much in excess of that permitted by the hydrogenrelease valve. This liquid seal is also effective in preventing escape of hydrogen during operation of the stirrer and eliminates a mercury seal. If the period of stirring exceeds 15 or 20 minutes, the bearing may become dry and permit escape of hydrogen; this will be indicated a t once by passage of hydrogen bubbles through the washing train. It is then necessary to renew the seal by applying a few drops of water about the shaft a t the upper end of the bearing. An orifice in the glass tube of the flexible coupling is provided for this purpose.
Figure 3-Hydrogen-Electrode
Cell a n d Potentiometer S y s t e m
When the amount of sample is limited or when the solution is strongly acid or alkaline, the hydrogen-electrode vessel is first filled with distilled water, and the water then displaced with hydrogen; the sample, in a shallow container, is then drawn into the vessel through the stem by suction applied at a T in the hydrogen supply line, a small quantity thus introduced being first used as a rinse. This T is fitted with two stopcocks, as shown in Figures 1 and 4; for the obvious manipulations required. Suction is conveniently obtained by controlled siphoning of water from a large bottle. To avoid entrance of air into the vessel, the hydrogen flow is not shut off at the T until the stem is submerged in the sample. Care in using the suction is also essential to prevent drawing in air bubbles with the last traces of liquid. A volume of 8 to 10 cc. is the minimum on which a determination can be made with the size of apparatus described. Figure 4 shows the position of the hydrogen-release valve and suction bottle in relation to the hydrogen-purifying train. After the sample has been introduced, the vessel is lowered through its supporting clamp and the stem inserted through the short rubber connection into the bridge with the stopcock of the washout tube open. The clamp is then tightened. Connection with the potentiometer system is made by inserting the proper wires into the mercury leading to the hydrogen electrodes, The stopcock of the vessel is opened to permit about 1 GC. of liquid to escape over the saturated potassium chloride solution into the washout tube. It is then closed, and the saturated potassium chloride-reservoir stopcock opened slightly to allow potassium chloride solution to sweep out the mixture through the washout tube. While potassium chloride solution is still flowing the stopcock on the washout tube is closed, followed by closing of the potassium chloridereservoir stopcock. Owing to flexibility of the rubber tube connecting this reservoir with the bridge and to cushioning
Vol. 3, No. 2
effect of air pockets in the system, the solution is retained under sufficient pressure so that when the stopcock of the hydrogen-electrode vessel is now opened, the junction between saturated potassium chloride solution and the liquid in the vessel rises 4 or 5 mm. into the stem, This junction is plainly visible and perfectly sharp, and may be reproduced a t will. The flexible drive shaft is connected with the stirrer by swinging the drive shaft bearing into proper position and inserting the driving end into the flexible coupling. After connection has been made, the bearing is raised slightly, adjusted so that the stirrer rotates smoothly, and the motor started. The speed should be so regulated that the stirrer, as it circulates the confined hydrogen, throws a stream of both liquid and gas about the electrodes, and thus rapidly brings about equilibrium. Following a determination, the stopcock on the hydrogenelectrode vessel is closed, the stopcock on the washout tube opened, and with necessary disconnections made, the vessel is lifted from the bridge, washed with distilled water, and then rinsed with a portion of the next sample preparatory to another run, as previously described. It is unnecessary to remove the stirrer and electrodes from the stopper between each determination for washing. Operation of the stirrer has no disturbing effect on the liquid junction. This is clearly shown by adding phenolphthalein to sodium hydroxide solution in the hydrogenelectrode vessel and then starting the stirrer; the main body of solution becomes pink almost instantly, but even after prolonged stirring the color will not extend past the open stopcock to the junction in the stem. Pulsation of the junction is avoided by placing the hydrogen-release valve before the wash bottles in the hydrogen supply line. The stirrer is especially efficacious with solutions which tend to foam badly. As soon as foam rises to the level of the upper orifice in the shaft, it is drawn down through this into the solution again. Changes due to surface phenomena are thus reduced to a minimum. Five minutes is the average time required for a routine determination, including all incident manipulations.
ZiPTun e e//
Q
Figure 4-Hydrogen-Purifying Train with Suction Bottle a n d Release Valve
Use of Standard Half-Cell
A saturated potassium chloride-calomel half-cell was used in all determinations and was checked against a 0.1 N potassium chloride-calomel half-cell from time to time, the average values a t various cell temperatures being within 0.5 millivolt of the values given by Clark ( 2 ) . Readings were made with a Leeds &' Northrup Type K potentiometer and a No. 2420-c enclosed lamp and scale galvanometer. The entire apparatus was appropriately shielded and housed in an air bath, the temperature of which varied not more than 1' C. during a given set of determinations.
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
April 15, 1931
207
The 0.1 N potassium chloride-calomel half-cell used for checking may be temporarily introduced a t the slip joint provided for the hydrogen-electrode vessel. As soon as readings have been made, it is removed and clamped out of the way. The porous tip is washed and flushed, and then kept immersed in some of the half-cell solution in a small protecting test tube held in place by a rubber sleeve above the tip. It is preferable, however, to place the comparison electrode in the system permanently. The tip is inserted
utle
’C 1
r .3
Ori
-
:e
Hydrogen Etecfrode
Figure 5-Bubbling
Hydrogen Electrode
through a tight rubber connection to a level slightly below the outlet of a T provided near the middle of the bridge. By opening the stopcock on the outlet tube and allowing saturated potassium chloride solution to rise about the porous tip, contact can be established as desired without disturbing the half-cell. Contact should be broken as soon as comparison has been made. This is done by draining off some bridge solution and allowing a protective air bubble to form about the porous tip. The tip may then be flushed in the usual manner to prevent contamination. The total volume of the various air bubbles or pockets in the complete bridge system should be kept a t a minimum, or else the combined cushioning action may produce too high a rise of the liquid junction in the stem of the hydrogen-electrode vessel. Some practice in the various manipulations is required before the desired results may be obtained. Attainment of H-Ion Equilibrium
As soon as the stirrer is started, the voltage rises rapidly and usually reaches a constant maximum in 1 to 2 minutes. A drift due to junction potential may develop, but by renewing the liquid junction, the original value can be restored. I n observations made on 0.05 M potassium acid phthalate solutions during stirring periods of one hour, the voltage varied not more th8n 0.5 millivolt from the initial maximum attained in less than 2 minutes; with junction renewals the variation was reduced to 0.1 millivolt. Similar results were obtained with various solutions of carbonates and sulfates. I n determinations made on nearly one thousand soil extracts, equilibrium was generally attained within 2 to 3 minutes. Occasionally there occurred serious drifts which could not be obviated by junction renewals. Often the cause was obscure, but in some cases was due to deposition of certain cations on the electrodes. With a series of soil samples carrying traces of zinc, lead, and copper it was found necessary
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Drifts which cannot be eliminated by junction renewal may be attributed to reaction between certain sample constituents and hydrogen or the electrode. When these are extensive, the point of H-ion equilibrium is often masked. If the e. m. f . change in such cases be followed closely, there may be obtained a voltage-time curve valuable for interpreting results A rapid, more or less asymptotic rise during the first 2 or 3 minutes of stirring indicates the progress of hydrogen saturation and reduction of the solution. I n the absence of pronounced side reactions in the solution or a t the electrodes, the maximum value is subsequently maintained indefinitely and represents the point of true H-ion equilibrium. When incidental reactions alter the original pH, a point of inflection usually occurs in the curve at or,near the maximum. I n general, the e. m. f . then either falls more or less rapidly or continues to rise more slowly and indefinitely,
A N A L Y T I C A L EDITION
208 Table I-Potentiometer MATERIAL
Vol. 3, No. 2
-
Readines durinr? TYDical DH Determinations w i t h Circulating Hvdroaen Electrode -I
DETN.
0
0.5
1
2
3
Volts
Volts
Volts
Volts
Volts
0.7759 0.7765 0.7756 0.7769 0.5202 0.5190 0.5135 0.5153 0.4586 0.1995" 0.4705 0.2480C 0.5093 0.5076 0.5015 0.4957 0.4450 0 4457 0.5124 0,5125 0.4808 0.4900 0.5004 0.5014 0.3082 0.3086 0.3260 0.3265 1.1735 1.1687 0.4394 0.4410
0.7974 0.7975 0.8003 0.8001 0.5710 0..5720 0.5690 0.6701 0.7117 0.7120 0.7120 0.7120 0.5397 0.5370 0.5315 0.5313 0.4870 0.4876 0.5329" 0.53206 0.51770 0.5240" 0.532Oa 0.6324Q 0.3531 0.3530 0.3463 0.3463 1.1767 1.1774 0.4785 0.4788
0.8184 0.8193 0.8219' 0.8219" 0.5717" 0.5720" 0.5722 0.5721 0.7140" 0.7140" 0.7142 0.7145 0.5455" 0.5450" 0.5321 0.5316 0.4949 0.4980 0.5315 0.5313 0.5150 0.5127 0.5308 0.5311 0.353Z5 0.3532a 0.3515" 0.3515a 1.1773 1.1775 0.4787 0.4788
0.8200a 0.8203" 0.8215 0.8211 0.5717 0.5720 O.572Za 0.5722" 0.7140 0.7140 0.7143" 0.7145" 0.5427 0.5426 0.5319 0.5315 0.5010 0.5076 0.5293 0.5300 0.5140 0.5116 0.5295 0.5298 0.3532 0.3533 0.3515 0.3515 1.1782" 1.1783" 0.4788" 0.4788a
4 ~
Soils:b
c10
1 2
L4
1 2
I
M2
2 Apple-leaf press juice, fresh; not sprayed Preservedd sprayed
1 2 1
1ced.e sprayedf
2
3 4 0 . 0 2 N H2SO4
1
0.02 N H2SOc saturated with Cog
1
Saturated NaOH soln., COrfree
1
0.05 M KH phthalateo
2
%&?$:
MINUTES FROM START OF STIRRING ~
Volts
0.8191 0 8188 0.8193 0.8189 0.8214 0 8205 0.8205 0 8201 0.5711 0.5710 0.5717 0.5713 0.7135 0.7132 0.7142 0.7141 0.5421 0.6416 0.5420 0.5413 0.5319 0.5321" 0.5317 0.5321" 0.5056 0.5066 0.5090 0.5102 0.6283 0.5293 0.5281 0.5286 0.3532 0.3532 0.3514 0.3514 1.1778 1.1797 0.4789 0.4788
5
0.3531 0.3532 0.3512 0.3512 1 1778 1 1791 0 4788 0 4788
__
Volts
0 8191 0 8185
0 5319 0.5319 0.5075'1 0.6100"
1.1767 1 1776 0.4788 0 4784
PING
STIRRER
Volts
0.8286 0.8256 0.8257 0.8255 0.5774 0.6772 0.5776 0.5776 0.7297 0.7297 0.7235 0.7235 0.5496 0.5498 0.5362 0,5360 0.5302 0.5318 0.5385 0.5410 0.5404 0.5424 0.5414 0.5411 0.3529 0.3630 0.3512 0.3512 1.1769 1.1776 0.4790 0.4790
Av.
ACCEPTED VALUES"
OF
VOllS
DH
0.8202
9 72
0.SZ"l6
9.75
0.5719
5.53
0 5722
5.53
0 7140
7.93
0.7144
7.94
0.6453
5.08
0.5321
4.85
0.5088
4.46
0.5325
4.86
0.5209
4.66
0.5322
4.85
0.3532
1.84
0.3615
1.80
1.1783
15.80
0.4788
3.95
Values accepted as most nearly representing equilibrium with original H-ion concentration. b Determinations made on solutions decanted after shaking 20 grams of air-dry soil with 100 cc. distilled water intermittently for 30 minutes. A second 20-gram portion of each soil was similarly treated for the duplicate determinations. In all other determinations both hydrogen electrodes were approximately 4 cm. in C Second hydrogen electrode was 2 cm. in length; the first, 4 cm. length. d Sample reserved with 0.1 per cent formalin for 30 days Leaves sprayed once with lead arsenate. e Sample frozen as soon as collected; thawed 4 days later And tested when at room temperature. Leaves sprayed 3 times with lead arsenate. f Determinations made on separate portions of same sample. Electrodes used for first determination had just previously been used on two other samples from heavily sprayed leaves. Electrodes replated and checked for second and fourth determinations. a Determination made immediately following preceding determination on saturated NaOH solution after rinsing vessel and electrode5 with 3 portions of the phthalate solution, stopper assembly remaining in place. O
according to the nature of the incidental reactions. The pH corresponding to the e. m. f. a t such inflections is regarded as a closer approach to the original than a pH based on socalled constant e. m. f. values which may appear later. If a constant value is not attained within 2 or 3 minutes, or if subsequent changes are due to causes other than junction potential, an apparently constant value obtained much later will usually be revealed by extended observation as a point in a retarded phase of drift. This is because electrodepoisoning phenomena and alteration of the original pH of the solution by contact with hydrogen or the hydrogen electrode are often relatively slow reactions. Rigid limits of tolerance for variation between duplicate determinations can be satisfied in cases of rapidly attained constant readings. The limits need be little less rigid when inflections in voltagetime curves are used. Discussion of Results
Typical examples of voltage changes during determinations on various samples are presented in Table I. The general course of e. m. f. during a determination with the stirring electrode is shown to be a rapid rise during the first 0.5 minute, after which the rate of increase falls off sharply until a maximum value is reached. This maximum e. m. f. is regarded as concurrent with the desired hydrion equilibrium and is used for calculating the original p H of the solution. It usually persists for less than 1 minute, generally being promptly followed by a more or less rapid drop. Exceptions occur with certain biological materials, of which seminal fluid is a striking example. As illustrated in Table 11, this gives an e. m. f.-time curve showing a rapid rise and approach to a nearly maximum value, after which a gradual
but persistent rise continues indefinitely. The point of inflection marking the beginning of the gradual rise is selected as the value most nearly coincident with equilibrium involving the original H-ion concentration. Side reactions during pH determinations with the hydrogen electrode are of general occurrence. Their inevitable influence on the original pH can be reduced to a minimum by a most rapid hydrogen saturation to effect H-ion equilibrium, and on this basis is justified the calculation of pH from the rapidly attained maximum or inflection e. m. f. value as just described. A consta,nt e. m. f. or static equilibrium was rarely found except with certain simple solutions, such as 0.05 M potassium acid phthalate and 0.02 N sulfuric acid. This is not surprising in view of the high catalytic activity of the hydrogen electrode, and with the stirring electrode a constant e. m. f. need not be required as a criterion of reliability. In each determination where the hydrogen electrodes functioned normally, they agreed within less than 1millivolt a t the promptly attained equilibrium. Greater variation than 1millivolt between the two at equi!ibrium, or slow attainment of equilibrium, or both, generally was associated with electrode poisoning. This is well illustrated by the data given for apple-leaf press juice. Equilibrium between normally functioning electrodes and 0.05 M potassium acid phthalate was invariably attained after 1 to 2 minutes of stirring. Values of 0.4780 to 0.4820 volt were obtained under temperature and pressure conditions ranging from 20' to 32' C. and 670 to 720 mm. mercury. The time required for equilibrium with other solutions varied from 0.5 to 4 minutes. An observation period of not more than 5 minutes was sufficient, therefore, to determine the point of H-ion equilibrium in any case. Additional data in Table I1 include results obtained with
INDUSTRIAL A N D ENGIXEERING CHEMISTRY
April 15, 1931 Table 11-Comparison
209
of Voltage Change during pH Determinations by Bubbling a n d Circulating- HydrorLen . - Electrodes ~
SAXPLE 0
Volt Soils a ClOb Clod E7 E7d
E7 e F22 F22d F22e
Seminal Biiid Duroc boar 1 Duroc boar I /
ONE MINUTE
MINUTESFROM START OF STIRRING OR BUBBLIKG 0 5
3
4
5
6
YOll
VOll
Volt
VOll
Volt
0 7148 0 7169 0 6812 0 0819
0.7677 0.8240 0.8238 0.5752 0.6675 0.6678 0.6508 0.6508 0.6867 0.7433Q 0.7435" 0.7377 0.7380
0.8162 0.8231 0.8225 0.6290 0.6700 0.6702 0.6535 0.6538 0.7244 0.7433 0.7435 0.7392 0.7394
0.8241 0.8222 0.8221 0.6501 0.07110 0.6712" 0.6551 0.0555 0.7365 0.7430 0.7431 0.73940 0.73950
0 6624 0 6664 0 6005
0 6813 0 6845 0.6016
0.6855 0.6863 0.6730
0.68605 0.6862 0.68700. 0.6875 0.6872 0.7005
0 8030 0 8017
0 0 0 0
6050 6051 5830 5780
Volt
2
0.7000 0.8245" 0.8245" 0.5052 0.6580 0.6584 0.6404 0.6408 0.3101 0.7424 0.7425 0.7358 0.7362
e
Volt
1
AFTER
0.8270 0.8281 0.8220 0.8215 0.6590 0.0644 0.6711 0.6707 0.0711 0 0708 0 6554" 0.6551 0.6555" 0.6555 0.7388 0 7415
0 7395 0 7395
0.7390 0.7389
0.6865 0.6880 0.7120
0.0809 0,6888 0.7190
10
Volt
0.8283" 0.8278 0.6083 0.6740 0.6701 0.0703 0.6544 0.0548 0,74470 0.7432
0.6871 0.0889 0.7222
0.6880 0.6899 0.7290
~-
~~
STOP~PING
STIRRING OR BUBBLIKG
Av.
OF
ACCEPTED
VALUESa
Volt
VOll
9H
0.8301 0.8238 0.8258 0.6802 0.6718 0.6718 0.6563 0.6571 0.7442 0.7438 0.7438 0.7434 0.7439
0.8283
9.80
0.8245 ?
9.80 ?
0.6884 0.6904 0 7325
0.6712
7 21
0.6555 0.7447
6.94 8.45
0.7434
8.43
0.7395
8.30
0.0865 ?
7.47 ?
Values accepted as most nearly representing equilibrium with original H-ion concentration. Compare with data for C10 in Table I. Only one hydrogen electrode used in bubbling vessel. d Solution used in preceding determination transferred to stirring-electrode vessel. 6 Determination on separate portion of Same soil extract used for two preceding runs I Sample from stirring-electrode vessel transferred to bubbling vessel. I For analysis of soils see Table 111. 0
b c
B bubbling hydrogen electrode constructed as shown in Figure 5 and used in the stirring-electrode vessel. More time was always required for the bubbling cell to bring about equilibrium, although hydrogen was bubbled a t a maximum rate and efficient circulation of solution was obtained. In no case was equilibrium attained in less than 6 minutes, and in numerous instances it was not reached in 10 minutes. Not infrequently soil extracts and biological fluids were encountered that apparently could not be brought to equilibrium with the bubbling electrode, the e. m. f. continuing to rise during periods of bubbling as long as 4 hours. Since junction potentials and electrode poisoning were eliminated as contributing factors to such pronounced drifts, they can be attributed to slow side reactions. With potassium acidphthalate solutions, prepared with ordinary distilled water, the valuw obtained were approximately 1 millivolt higher than those given by the stirring electrode under the same conditions. This increase was due to removal of dissolved carbon dioxide by the escaping hydrogen. Values obtained on carbon dioxide-free phthalate solution in the stirring cell were identical with values secured on either carbon dioxidefree or carbon dioxide-saturated phthalate solutions with the bubbling hydrogen electrode. Similar results were obtained with other acid solutions. The different behavior of the stirring and bubbling hydrogen electrodes is shown graphically in Figure 6. Only when solutions are free of carbon dioxide do both methods give like values; the greater the carbon dioxide content the greater the divergence in results. The greater rapidity of the stirring electrode in all cases is also illustrated. Most significant, however, is the evidence that when appreciabIe amounts of carbon dioxide are present in solution, it is impossible even to approximate the original pH value with the bubbling hydrogen electrode. If bubbling were continued long enough to remove all dissolved carbon dioxide, and if no irreversible reactions were involved, the pH finally reached would be the same as that of the original carbon dioxide-free solution. An attempt was made to realize this experimentally with the carbon dioxidesaturated carbonate solutions. As the curves indicate, sufficient carbon dioxide was removed in 10 minutes to increase the pH approximately 2 units, but further observation showed that the rate of increase became gradually slower so that after 2 hours the e. m. f., although apparently constant, had not yet reached the maximum. The effect of the bubbling electrode is further emphasized
by differences obtained when a determination is repeated on the same portion of sample by the stirring electrode, and vice versa. This is illustrated with each sample in Table I1 Table 111-Analyses
SOIL
of 1:s Water Extracts of Soils Used for Data in Table I1
EQUIVALENT PARTSPER MILLION OF SOLUTION"
-
sios
COa HCOJ SO4 NO3 CI
Ell 'F22
6 6 0 6 0 6
9 6 0 0 0 0
C10
188 1 0 0 9
Pe
AI
2 0 0 1 0 7 0 7 0 0 0 1 0 2 0 0 0 4 0 3 0 1 0 1 0 0 0 0 0 0
Ca
Mg N a
K
0 0 00123121 0 4 0 1 0 1 0 5 0 8 0 2 0 6 0 2
1 Equivalent p. p. m. = p. p. m X equivalent weight* For discussion of this method of expression see Bollen and Neidig, Soil Science, 24, 69-70 (1927). Q
Values recorded under zero time from start of stirring or bubbling indicate that during introduction of a sample into the stirring-electrode vessel, there occurs Sufficient contact with hydrogen to give an e. m. f. not much less than the equilibrium value. This preliminary e. m. f. is more or less unstable, tending to increase slowly and fluctuating in proportion to current withdrawal during potentiometric balancing. The bubbling electrode gave no e. m. f. before hydrogen was allowed to flow. On discontinuing either stirring or bubbling, however, there usually developed an e. m. f. distinctly higher than any preceding value. This generally reached a relatively constant maximum in about 1 minute, although the extent and duration of increase were found to vary with the nature of the solution and condition of the electrodes It is noteworthy that the length of the hydrogen electrode influenced only preliminary e. m. f., as is shown by the data for soil M2 in Table I. The initial and final readings in the various examples are presented as a matter of interest. In routine determinations it is sufficient to follow the e. m. f. closely only in the region of equilibrium, Literature Cited (1) Clark, "Determination of Hydrogen Ions," p. 280, Williams & Wilkins, 1928. (2) Clark, I b i d . , p. 488. (3) Clark, J. Biol. Chem., 23, 475-86 (1915). (4) Parker and Dannerth, IND. ENG.CHBM.,17, 637-9 (1925). (5) Scbollenberger, Ibid., 17, 649 (1925). (6) Snyder, U. S. Dept. Agr., Circ. 66 (1928). (7) Watermsn, J . Assocn. Oficial Agr. Chem., 10, 390-6 (1927).
