Potentiometric Titrations with the GermaniumGermanium Dioxide Electrode RI. L. NICHOLS
AND
S. R . COOPER, D e p a r t m e n t of Chemistry, Cornel1 University, Ithaca, Y. Y.
I
T HAS BEEN shown in a previous paper (4) that the germanium-germanium dioxide electrode shows a potential difference against a saturated calomel electrode which is somewhat dependent upon the pH value of the solution. The electromotive force values obtained with the present electrode were not sufficiently constant nor reproducible to allow it t o be used for accurate pH determinations but it might be satisfactory for potentiometric titrations where only the equivalence point was desired. Therefore, an investigation was made to determine its applicability for the titration of acids, bases, and salts.
(curve A) and absence (curve B) of the insoluble germanium dioxide. The curves plotted from the data of these two titrations are given in Figure 2. In these titrations the time required for the titration was shortened from 5 t o G hours to 2.25 hours without affecting the accuracy of the titration. Also the insoluble germanium dioxide did not have to be present in order to obtain the correct equivalence point for the titration. Titrations of 0.1 N, 0.05 N , and 0.01 iV solutions of bydrochloric acid and sodium hydroxide offered no difficulties. The end points of all the germanium electrode titrations agreed exactly with the titrations using the hydrogen elec-
Experimental Titrations of 0.2 N hydrochloric acid with 0.2 N sodium hydroxide were made by the same procedure as given in the previous paper (4) using both mechanical stirring (curve B) and stirring with air (curve C), freed from carbon dioxide. A check titration was made also using a Bunker-type hydrogen electrode (curve A). The curves plotted from the data of these titrations are given in Figure 1. These experiments show that the germanium-germanium dioxide electrode gives a titration curve similar to that given by the hydrogen elec-
0.2
0
IO
20
nl.
30 0.2
40
50
N. NnOH
FIGURE 2
-/,
trode. As the strength of the solutions decreased, the sluggishness of the germanium electrode tended to make the end point less pronounced, but it was located easily for all of the strengths employed. Having secured satisfactory results in the above titration without germanium dioxide, the same procedure, except that the time of stirring was shortened from 3 t o 2 minutes, was used in all subsequent titrations. In this manner titrations were performed of monobasic acids, dibasic acids, weak bases, and salts. Kone of the solutions used in this part of the investigation were accurately standardized, but all of the titrations were checked with either a hydrogen or a quinhydrone electrode. A summary of the titrations performed and the precision obtained is given in Table I. All the values given are the result of a t least two check titrations with each electrode. The equivalent point volume was determined in each case by computing the maximum slope of the curve from the data obtained. In addition to the above titrations, titrations of ammonium hydroxide with hydrochloric acid and the reverse
o.zN N A O H
FIGURE1 trode. In addition the equivalence points of the germanium and the hydrogen-electrode titrations, obtained by calculation of the maximum slope of the curves, agree within 0.1 ml. This indicates that the germanium electrode will give the equivalence point of an acid-base titration accurately. The solution may be stirred mechanically but more conveniently by using air, freed from carbon dioxide. As the previous titrations were too time-consuming, they were repeated by a more rapid procedure both in the presence 353
INDUSTRIAL AND ENGINEERING CHEMISTRY
354
VOL. 7, NO. 5
-
TABLE 1. Solution Titrated
Approx.
Normality
"0s HCzHsOz HC104 CsHiCOzH Lactic acid CHzClCOzH HCOiH HzS04 HzCa04 Tartaric acid Malonic acid
0.1 0.1
Titrating Solution
SUMMARY OF TITRATIONS
.4pprox.
Normality
0.1 0.1 0.1 0.1 0.1 0 1
NaOH NaOH NaOH NaOH NaOH NaOH NaOH NaOH NaOH NaOH NaOH
0.1 0.1 0.1 0.02 0.1 0.1 0.1 0.1 0.1
Succinic acid o-Phthalic acid
0.1 0.05
NaOH NaOH
0.1 0.05
Salicylic acid Maleic acid
0.01 0.1
NaOH NaOH
0.01 0.1
Fumaric acid HsCrOr
0.1 0.05
NaOH NaOH
0.1 0.05
HzCrOr
0.05
NaOH
0.05
NaOH HsPO4
0.05 0.1
HzCr04 NaOH
0.05 0 1
HsBOs Citric acid HCl NazCOs
0.1 0.1 0.05 0.05
NaOH NaOH Aniline HC1
0.1 0.1 0.05 0.1
0.1 0.02 0.1
with solutions ranging from 0.1 N to 0.01 N , titrations of 0.1 N and 0.05 N solutions of ethylamine with hydrochloric acid and the reverse, titrations of 0.05 N solutions of telluric acid with sodium hydroxide, titrations of 0.1 N solutions of sodium trinitride and sodium sulfite, and the back-titration of a solution of sodium sulfide were attempted, but in none of these cases could satisfactory results be obtained. In a previous investigation (4) it was shown that although the germanium-germanium dioxide electrode shows a potential difference against a calomel electrode which is somewhat dependent, upon the pH of the solution, constant and reproducible values could not be obtained. Titrations with this electrode make this characteristic more evident, for when several titrations were performed successively the initial electromotive force for each succeeding one was always higher than the former. The electrode, without any insoluble dioxide in the solution to be measured, functions well as a hydrogen-electrode substitute in potentiometric titrations when the solution is stirred with purified air. Characteristic curves were obtained for monobasic acids, dibasic acids, tribasic acids, a weak base, and sodium carbonate. The results obtained in these cases are comparable to those which have been reported for the antimony electrode. In addition the electrode is satisfactory for certain titrations in which the hydrogen electrode is useless; its use in solutions of perchloric and nitric acids are examples of its action in solutions of strong oxidizing agents. No plausible explanation can be offered for the fact that in some cases the slope of the germanium electrode titration curve on the alkaline side is very much greater than that for a similar hydrogen electrode curve. Kolthoff and Hartong (3) obtained abnormal values of the electromotive force in titrations of tartaric acid with the antimony electrode, which they attributed to the formation of antimonyl tartrate, while Barnes and Simon (1) reported that this electrode did not function well in large quantities of hydroxy acids. Neither reason seems t o offer a sattisfactory explanation of the present case, and the difficulty may be due to the solubility of the germanium dioxide in sodium hydroxide. The deportment of the germanium electrode in chromic acid solutions when the acid was titrated with a base was similar to that shown by the air electrode in that the electrode was first positive to a saturated calomel electrode, passed
0.1 0.1
Equivalent Point Obtained M1. 23.1 23.5 27.7 23.55 25.1 23.6 23.2 23.5 23.3 26.0 13.0 23.7 24.5 12.0 24.1 22.6 11.9 23.4 23.5 11.6 23.7 11.6 23.7 26.0 11.3 22.5 24.0 25.4 24.1 5.5 11.7
Checking Electrode Quinhydrone Hydrogen Quinhydrone Quinhydrone Hydrogen Hydrogen Hydrogen Hydrogen Hydrogen Hydrogen Hydrogen Hydrogen
Diff. Checking from Electrode M1. -0.05 0.0 0.0 -0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Hydrogen Quinhydrone
0.0 -0.1
Quinhydrone Hydrogen
-0.1 -0.1 $0.1 0.0 +0.2
Air Hydidgen Hydrogen Hydrogen Hydrogen Hydrogen
0.0
...
0.0 0.0 1 0.0 0.0 0.0
+o.
through zero, and was finally negative a t the end. This is probably due to the high oxidizing potential of chromic acid. The antimony electrode showed a similar action in permanganate solutions (S), which was explained upon the assump tion that the trioxide present was oxidized to the pentoxide. The germanium electrode gave lower electromotive force values in solutions in which the insoluble dioxide was present, but the difference was not of the magnitude of that displayed between chromic acid and other comparable acids. Doubtless some of the dioxide was formed in the chromic acid solution, but this does not explain the abnormal readings obtained. As no higher oxides of germanium are known, it is impossible to explain this action upon the basis of their formation. Titrations of salts with this electrode gave poor results generally. Among those which were chosen, sodium carbonate alone was satisfactory, although difficulty was experienced in titrating it with both the hydrogen and germanium electrodes. The antimony electrode has been used for backtitrations of salts of hydronitric and sulfurous (2) acids, while the germanium electrode was found to be unsuitable. The results obtained by using this electrode for titrations are very good, although a t present no means has been perfected for stabilizing it so that itrwill give reproducible values satisfactory for determining absolute hydrogen-ion concentrations. The electrode, when once prepared, is rugged enough to last indefinitely; a mounted one has been in almost daily use for about a year. Then too, the ease with which nongas electrodes can be operated offers a great advantage of this electrode over the hydrogen electrode. Although most of the titrations which have been performed with the germanium electrode have consumed over 2 hours, it is possible that more rapid titrations can also be performed.
Summary The germanium-germanium dioxide electrode, without the addition of the insoluble dioxide, functions well in potentiometric titrations of acids, bases, and salts. It has been used for the titration of hydrochloric, acetic, benzoic, lactic, monochloroacetic, formic, sulfuric, oxalic, tartaric, malonic, succinic, phthalic, salicylic, phosphoric, boric, and lactic acids with sodium hydroxide. Titrations of weak bases and salts with this electrode were unsatisfactory except in the case of
SEPTEMBER 15, 1935
ANALYTICAL EDITION
aniline and sodium carbonate. It has also been used in solutions of nitric, perchloric, maleic, and fumaric acids where the hydrogen electrode does not work satisfactorily. The electrode was positive to a saturated calomel electrode in a 0.05 1V solution of chromic acid, and as the acid was titrated with sodium hydroxide it passed through gero and finally became negative. The germanium titration curves generally show a greater slope on the alkaline side than on the acid side. The results of t,he titrations compare favorably with those
355
obtained with the antimony electrode. The data for a titration curve of a monobasic acid can be obtained in 2.25 hours.
Literature Cited (1) Barnes and Simon, J. Am. SOC.Agron., 24, 156 (1932). (2) Britton and Robinson, J . Chem. SOC., 1931, 45% (3) Kolthoff and Hartong, Rec. trav. chim., 44, 113 (1925).
(4) Nichols and Cooper, IND. ENG. CHEM.,Anal. Ed., 7, 350 (1935).
RECEIVED April 20, 1936. Based upon the thesis presented t o the Faculty of the Graduate School of Correll University by S. R. Cooper in partial fulfillment of the requirements for the degree of doctor of philosophy.
Constant-Flow Orifice Meters of Low Capacity R. T. PAGE, Department of Industrial Hygiene, Harvard School of Piiblic Health, Boston, Mass.
M
AXY devices, differing widely in principle of opera-
tion and in design, are employed for measuring the rate of flow of gases. The advantages and limitations of each device are well defined. The usual laboratory methods possess features which render them unsuitable for the metering of small amounts of gases, particularly when a constant rate of flow is required. The displacement type of meter, using the receiver method of measuring gas (typified by the ‘ldry” meter used in the commercial metering of illuminating gas, the more accurate “wet” meter, and the spirometer) doesnot give an instantaneous reading of the rate of flow, but requires the measurement of the time interval during which a definite volume is passed. Anemometers and float meters are not only delicate but also inaccurate a t low rates of flow. The method of measuring change in temperature of the gas due to the addition to it of a known quantity of heat has the disadvantage of not giving a direct reading of the rate of flow without complicated apparatus. Pitot tubes and venturi meters, while excellent in principle because of the comparatively small resistance they introduce into the line, are difficult and expensive to make, and unsuitable for measuring gases a t rates of the order of 50 liters per minute or less. The resistance-tube meters developed by Muster ( I $ ) , and described by Guye and Schneider (Q),were extensively used by the Chemical Warfare Service. In these meters the loss of head due to passing the gas through a capillary tube is a function of the rate of flow. Benton ( 2 ) gives detailed specifications for the calculation of dimensions and the construction of meters ranging from 500 cc. to 200 liters per minute in capacity. These meters are difficult to duplicate accurately and each one must be calibrated separately. They are fragile and not ideal for field use, but are unexcelled for metering pure gases a t rates lower than 4 liters per minute. When the gas is contaminated with particulate matter, an appreciable error may be caused by the deposition of dust a t the mouth of the capillary tube. Frequent cleaning becomes necessary and often proves difficult. The orifice compares favorably with the resistance-tube meter. Orifices can be constructed in noncorrosive materials, are easily cleaned, and are satisfactory at fairly low rates of flow. At low pressure differentials, the rate of flow of gas through an orifice is a function of both the pressure above and the pressure below the orifice. This is also true in the case of the resistance-tube meter and the venturi meter. Both pressures must remain constant, or the rate of change of one must be a definite function of the rate of change of the other, to obtain a constant rate of flow. This result is very difficult to obtain without complicated and expensive compensating apparatus.
Constant-Flow Orifices The problem of securing small constant rates of flow of gas has been solved practicably in this laboratory by the use of small orifices operating a t pressure differentials greater than the critical, the pressure above the orifice being equal to or less than atmospheric pressure, and the downstream pressure being less than 0.53 times the upstream pressure. When an orifice is operated a t a pressure differential equal to or greater than this definite critical value, the pressure below the orifice has no effect on the rate of flow (11); and for constant upstream pressures, a constant flow can be obtained through the orifice irrespective of wide variations in the downstream pressure. When a suction pump or ejector is used to create a less than critical pressure below the orifice, the only factors affecting the constancy of flow will be the atmospheric pressure and the resistance in the line above the orifice. In practically all cases these will remain constant within the allowable limits of error of the problem, but for accurate work a resistance compensator can easily be located above the orifice to insure constant rates of flow. A constant-flow orifice was developed in this laboratory (IO) which passed approximately 28.3 liters per minute (1 cubic foot per minute) of air when it was placed in the suction line below a modified Greenburg-Smith impinger, a dustsampling instrument. The satisfactory performance of this and similar orifices which have been put to a wide variety of uses leads the author to believe that a description of this useful laboratory tool will be of interest.
Orifice Characteristics The rate of flow of a gas through an orifice is a function of both the pressure above the orifice, P I , and the pressure in the vena contracta, P. For an orifice with a smoothly rounded approach, P is the pressure in the throat. Coefficients for use with the theoretical discharge formulas for certain types of orifice operating under certain definite conditions have been determined accurately enough to allow the calculation of the rate of gas flow with high precision, without experimental calibration of each orifice. This holds for large orifices, which, if properly designed and installed with properly located pressure taps, serve as primary measuring instruments for low pressure differentials. The value of the coefficient decreases as the size of the orifice decreases, since the effects of skin friction and any irregularities due to construction vary inversely as the hydraulic radius. Unless very small orifices are made with extreme precision, the coefficient for each orifice should be calculated from an experimental calibration if the orifice is t o be used for precise measurements.
