INDUSTRIAL AND ENGINEERING CHEMISTRY
January 15, 1931
contents of the tube from the air. Just enough mercury is used to seal the tubes. The tube for the absorption of water was filled with Anhydrone. Ascarite, followed by 3 to 4 cm. of Anhydrone, was
Figure 1-Position
Figure 2-Tube
of Tube during Combustion
Rotated 180° and Sealed
used to absorb the carbon dioxide. Both tubes were kept cool during the combustion by wrapping them in wet flannel. No disadvantage resulted from having one of the sealed glass stoppers next (4) to the electric furnace. ~ ~ l preglrs l ~directions ~ h emctly, ~ but using the new micro-absorption tube, very satisfactory carbon and hydrogen determinations have been made in this laboratory for a period
6
of 6 weeks. Practically theoretical figures can be obtained. One illustration might be of interest: 4.570 mg. (pyrogallol); 1.95 mg. HzO, 9.583 mg. Con. Calcd.: H, 4.79 (8), C, 57.12. Found: H, 4.79 (2); C, 57.19. The advantages of the micro-absorption tube here described over those already recorded are: (1) Since both ends can be removed, the cleaning of the tube is greatly facilitated. (2) While standing on the rack and during the weighing, the tube is sealed-yet, because of the shape it can be wiped before each weighing and handled in the manner so carefully worked out by Professor Pregl. (3) The tube can be weighed when iilled with oxygen or air. Literature Cited (1) Remmerer and Hallett, IND. END.CAEM.,19, 173 (1927). (2) Pregl, “Quantitative Organic Microanalysis,” translated by Fyleman, (3) Ibid., p. 42, p. 63. Blakiston, 1924. (4) Ibid., p. 87.
Hydrogen-Ion Determinations with Low-Resistance Glass Electrodes’ G. Ross Robertson UNIV~RSITY OF CALIFORNIA AT Los ANGELES, CALIF.
HE estimation of hy-
A description is given of glass electrodes which have may accordingly be connected resistances of but 2 t o 3 megohms, and are made of drogen ion by standand used in a glass electrode a commercially available glass. Such electrodes permit ard methods becomes apparatus in the same manner the use of a d’Arsonva1 galvanometer in hydrogen-ion difficult or impossible when which one would employ with estimation with fair accuracy. Their use requires little the solution under examinaa q u i n h y d r o n e electrode. more skill than is needed with an industrial quinhytion contains certain active Furthermore, since the resistdrone apparatus. Electrometers are eliminated, ajld oxidizing or reducing agents. ance of the glass electrode is electrostatic shielding becomes of little importance. I n such situations the glass so low, static charges do not Directions for construction and use of the apparatus electrode invites consideraaccumulate and disturb the tion. Although this device are given. galvanometer as they do an has been known for over electrometer which by its very twenty years (2), it has been generally overlooked in Ameri- nature has no current “leak.” can laboratories. Thanks largely to the recent work of The sharpness of estimation of the null point is of course MacInnes and Dole (4, 5 ) , and Elder and Wright (I), it is not so great as in a common potentiometer circuit in a hydronow attracting considerable attention. gen-ion determination. It is, however, sufficient to serve orA serious drawback of the glass electrode has been its very dinary purposes, as seen in the following illustration: large electrical resistance. Values of 20 to 100 megohms are Assume 3 megohms resistance, and the normal galvanometer commonly reported in the literature. When such a device is deflection of 1 mrn. for 10-lp amperes current. Suppose the placed in the ordinary potentiometer circuit in place of the potentiometer adjustment has come within just one millivolt common hydrogen or quinhydrone electrode, the current of the null point. The current which is to actuate the galvanomebecomes too small for detection on a common galvanometer ter upon depression of the contact key is, therefore, by Ohm’s law at adjustments near the null point. A quadrant electrometer, or vacuum-tube potentiometer ( I ) , 3 O.x Ool1 0 6 = 3 x 10-10amperes is accordingly used instead of the galvanometer. More or Such a current deflects the No. 2500-f galvanometer marker less electrostatic shielding and special insulation are necessitated in such apparatus, I n any case the exacting technic 3 mm., and thus one may locate the null point to the accuracy required in the use of these physical instruments is discourag- of one millivolt with ease. This indicates a maximum error of ing to an ordinary chemist who is seeking only an approxi- *0.02 pH in this part of the operation. On account of the relatively long period of the galvanometer any greater premate estimation of pH value, I n the present work the resistance of the electrodes was cision in estimation requires more time than one would care cut to values from 2 to 3 megohms, the bulbs being blown to take in rapid commercial work. thin. It was then found that Leeds and Northrup type R Preparation of Electrode galvanometer, model No. 2500-f, an instrument of moderate cost, became useful. This galvanometer, while not of preCorning No. 015 glass, as suggested by MacInnes and Dole cision grade, has a sensitivity of 0.0001 microampere. It ( 5 ) , proved to be suitable for the purpose. It is obtainable from the manufacturer. A piece of 10-mm. tubing of this 1 Received May 23, 1930.
T
Vol. 3, No. 1
ANALYTICAL EDIY’ION
6
glass, about 8 cm. long, is drawn to a tapering point in a flame. With the aid of a needle-pointed blast flame a small lump of molten glass of about 100 to 150 mg. weight is allowed to accumulate. Care is taken not to heat the glass to a temperature where gas bubbles are formed. While the tip is at a bright red heat the tube is quickly removed from the flame and held point down. A bulb of 8 to 10 cc. volume is now blown. Experience alone will show how the air pressure may be controlled to prevent a blowout and give a very thin bulb of good mechanical strength. The writer has success when he restrains the blowing until the bulb, half blown, is getting almost too cool, and then by one hard blow stretches it to full size. This trick involves the
II
I A p p a r a t u s f o r Hydrogen-Ion D e t e r m i n a t i o n s
“setting” of portions of the bulb which are already as thin as they should be, and the further spreading of other portions which are still hot enough to spread. A large part of the bulb thus becomes thin enough to conduct the electric current freely in the application to follow. Such technic will sometimes give slightly lopsided bulbs, which are nevertheless serviceable. After the electrode is cool, its open end is dipped momentarily into melted paraffin as deeply as may be necessary to protect the section which is to be held by a clamp on the supporting stand. The following measurements were made of one particularly good electrode of 2 megohms resistance :
..
Volume of bulb rection of electrode Surface area of thin, or useful part of bulb, approx Average thickness of wall Variation in thickness
.
.
8 5 cc 14 sq cm 0 03 mm 0 015 to 0 05 mm.
Arrangement of Apparatus The whole cell and stirrer assembly, as shown in the right section of the figure, is mounted securely on one large ringstand to insure rigidity and protection of the bulb. Calomel electrode C1dips into a test tube filled with dilute potassium chloride solution. The slender side stem of this test tube is filled with potassium chloride-agar jelly, and dips into the reference solution of hydrochloric acid (approx. 1 N ) in the glass electrode. Calomel electrode CZ,presumably identical with C1, dips into the solution under examination, placed in the main vessel. A mechanical stirrer is included. GI is the type R galvanometer provided with a special tapping key used to stop the motion of the coil. This key merely serves to short-circuit the galvanometer temporarily. Some workers may prefer to include a damping resistance of from 2000 to 10,000 ohms in the key circuit, in case a retardation of the coil is desired instead of instant stoppage. A convex lens is placed directly in front of the galvanometer mirror, and the image of a straight-line-filament incandescent lamp thus projected on an external scale.
The rest of the apparatus is Leeds and Northrup’s student potentiometer outfit with slight changes in wiring to accommodate a second galvanometer. Manipulation Assuming that 1 N hydrochloric acid is in the bulb, the solution of unknown pH value is placed in the main vessel. The operator then proceeds to determine the potential just as though a hydrogen or quinhydrone electrode were in place of the glass-calomel electrode. The damping key is used freely to stop impending wide deflections and bring the galvanometer to rest promptly. It is not advisable to attempt a calculation of pH vaIue simply from the concentration of the hydrochloric acid and the known potential-a procedure which is correct in simple theory. Unfortunately such calculations are vitiated by glass surface potentials. The surface potential is not predictable, will change during the day, but will remain fairly constant over the few minutes required for a determination, It is thus advisable to ignore the numerical value of hydrogen-ion activity in the bulb. The plan then is to determine the potentials with three solutions: the unknown, one buffer solution of higher pH value, and one of lower. Direct linear interpolation gives a value sufficiently accurate for many industrial uses. The following example, taken from a rapid determination without thermostat or special refinement of technic, illustrates results obtainable in this manner: A solution of commercial acid potassium phthalate (approx. 0.2 M),and a standard buffer solution (pH 5) were taken as unknowns. The pH value of each was estimated in reference to two standard buffers, pH 3 and pH 7. The potentials a t 29’ C. were as follows:
. . . .. .. . . , . .. .. .. . , . , . .. .. .. , . . , ..... . . .
Standard buffer p H 3 . , Acid potassium phthalate., Buffer p H 5 Standard bu
I . . . .
....
Volt 0.097 0.164 0.218 0.331
Interpolation between standard buffers 3 and 7 assigns a pH value of 4.1 to the phthalate, and of 5.0 to the intermediate buffer. The apparent potential interval per pH unit between 3 and 5 was 0.059, and between 5 and 7 was 0.058. Precautions The three determinations of potential required in the estimation of a pH value should be run off in rapid succession. I n such a case the glass potential will be constant, and can be ignored in calculation, as only the concentration potential will enter into the numerical values gotten by interpolation. Furthermore, the electrode should not be subjected to any unusual disturbance at or near the time of the potential measurements. This advice means constancy in temperature as a prime requirement. Apparently sudden changes in temperature not only cause the small changes in potential called for by electrometric theory, but also may stimulate glass potential drift of unknown magnitude. It is not advisable to permit the electrode to come in contact with a very concentrated acid or alkali at a time near that of a regular series of three measurements. Undoubtedly the most accurate performance of this apparatus will come in a situation where pH measurements are made continuously in one fairly narrow region; where the electrode does not come into contact with a changing variety of electrolytes; and where temperatures are constant within a half degree or better. Polarization
It was feared that an attempt to draw enough current from a glass electrode to actuate an energy-consuming instrument such as a galvanometer would polarize the electrode and throw
January 15, 1931
INDUSTRIAL A N D ENGINEERING CHEMISTRY
potential readings astray. Experiments have shown, however, that no measurable error is recorded even after the contact key has been closed for a full minute, unless the potentiometer is set much more than 20 millivolts off the null point. Even when the setting was as much as 100 millivolts off the null point the potential returned to normal within a few seconds of rest. Such abuse should not occur in practice; in fact, it would actually send the galvanometer marker entirely off the scale, twist the suspension, and require a new zero setting. It is likely that the increase in conductivity provided in these electrodes has brought an increase in resistance to polarization with a given current flow. It was noted, however, that in cases where the electrode was polarized with large currents it did not return to normal potential as rapidly as would a quinhydrone electrode. Obviously one should avoid long periods of key contact. Silver-Silver Chloride Electrodes
some workers may prefer to use the silver chloride electrode
in place of the calomel electrodes here df%csibed. In this way one might eliminate the potassium chloride-agar bridge
7
which, though simple and easy to prepare, is not permanent, The silver electrode would be placed directly in the glass electrode bulb. Details of the silver electrodes have been given by MacInnes and Beattie (3) and MacInnes and Dole (4). Acknowledgment
The writer wishes to thank James B. Conant, and also the Corning Glass Company, for suggestions and materials in connection with this work. Note-Since the writing of the foregoing account, an abstract [C. A , , 24, 4188 (1930)l has been published giving a report of the work of C.Morton with glass electrodes of character similar to those described above, but with different electrical accessories and technic.
Literature Cited (1) Elder and Wright, Proc. Nail. Acad. Sci., 14, 936 (1928). (2) and Klemensiewicz, 2. fihys. Chem., 67, 385 (1909). (3) MacInnes and Beattie, J. A m . Chem. SOC.,41, 1118 (1920). (4) MacInnes and Dole, IND.END.CHEM.,Anal. Ed., 1, 57 (1929). ( 5 ) MacInnes and Dole, J . A m . Chem. S O L , 52, 29 (1930).
Methods for Determining the Solubilities of Some Fluosilicates' Katherine K. Worthington with Malcolm M. Haring UNIVERSITY OF MARYLAND, COLLEGE PARK,MD.
H E fluosilicates have recentiy attracted considerable attention due to their use as insecticides and fungicides, in waterproofing and hardening concrete, in electroplating, etc. However, quantitative data in connection with these substances are not numerous and usually of questionable accuracy. Carter (3) has recently published the solubility curves for barium,, potassium, and sodium fluosilicates, together with a compilation of all available solubility figures on the fluosilicates. Methods of analysis, likewise, are not all that might be desired. This paper presents the solubilities and densities, both a t 20' C., of the fluosilicates of sodium, magnesium, zinc, lead, and copper. It includes also a brief discussion of the analytical methods.
joints covered with rubber tubing and was completely immersed in an automatic thermostat adjusted to h0.5' C. No leakage through the submerged joints was observed a t any time. A small filter of packed asbestos fiber was formed in a conical tube inserted between saturator and pycnometer. The solutions, after passing the filter, were always absolutely clear. A Sprengel pycnometer of about 6.8 cc. capacity was used. All weights and volumetric apparatus were carefully calibrated. Kearly saturated solutions were made in a flask before placing in the saturator. At least 2 hours were allowed to elapse before the first sample was removed from the saturator. Successive samples never showed any trend in analysis.
Apparatus
The fluosilicates have the general formula Met(SiF6). The water of crystallization varies from 0 to 6 molecules. They are efflorescent and also decompose slowly, especially those of the heavy metals. They are all decomposed rapidly by hot sulfuric acid and by hot sodium hydroxide as follows :
T
The apparatus deemed most suitable for the purpose was that of Bahr (1). The air used for stirring was first passed through a mercury pressure regulator, by which the rate of flow was maintained constant a t 4 bubbles per second. Thence it passed in succession through three DreFhsel gas-washing bottles (equipped with petticoat bubblers) containing sodium hydroxide solution, water, and the solution being saturated. Tests showed that the air current produced no decomposition of the fluosilicates above that taking place when the air stream was stationary. Saturator and pycnometer were arranged exactly as described by Bahr, save that a single three-way cock was used in place of two ordinary cocks. A small test tube equipped with an air vent was attached to the pycnometer so as to catch the overflow produced during filling. The whole apparatus was connected by means of ground-glass 1 Received August 29, 1930. Abstracted from a thesis submitted by Katherine K. Worthington in partial fulfilment of the requirements for the degree of master of science in the Graduate School of the University of Maryland.
Analytical Methods
MetSiF6 MetSiFe
+ HLSO4= MetSOc + SiF4t + 2HF t + 4NaOH = 4NaF + 2Hz0 + Si02 4 + MetFz (1) (2)
The first reaction is quantitative and the second may or may not be, depending on the extent of hydrolysis, etc. Hydrolysis is very slight in the case of the alkali fluosilicates, but more marked with'the others-e. g., MgSiFs.
++
++ +
MgSiFs 2H20 = Mg(0H)z 4 2HaSiFa HnSiFa 6NaOH = 6NaF Si02 J. 4Hz0
(3) (4)
Since the methods of analysis used are based on Reactions 1 and 2 , it is obvious that hydrolysis will occasion an error in the results when Reaction 2 is used. Reaction 2 requires 4 mols of alkali per mol of fluosilicate whereas Reaction 4 requires 6 mols. Hence volumetric methods of estimation
