INDUSTRIAL A N D ENGINEERING CHEMISTRY
June, 1927
749
Polarized Platinum Electrode in Neutralization Reactions'*' By A. H. Wright and F. H. Gibson W A S H I N G T O N A N D JEPFERSON COLLEGE, W A S H I N G T O N , P A .
I
3 A study of the hydrogen electrode and other electrode systems used in electrometric analysis, a number of titrations involving neutralization reactions were performed with a polarized platinum electrode system similar to that described by Willard and Fenwick3 and used by them in oxidation and reduction reactions. Apparatus and Procedure
Figure 1 illustrates the arrangement of the apparatus used. Two dry cells connected through a slide-wire resistance to give 2 volts, then through a high resistance of 460,000 ohms, gave a polarizing current of 2.3 X amperes at the platinum electrodes. The potential of the cathodically polarized platinum electrode was measured with a potentiometer, a normal calomel halfcell being used as the r e f e r e n c e electrode. The change of e. m. f . of t h e cathodically Lpolarized p l a t i n u m electrode was followed during the progress of a titration where the hydrogen-ion concentration of the solution was changing. The rate of change of the e. m. f. a t the end point for strong acids was found to be large and t h e t i t r a t i o n Figure 1-Polarized P l a t i n u m Electrode curves gave good flex System p o i n t s . F o r weak E-Platinum electrodes acids the curves had PC-Polarizing circuit
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base the change occurs very close to the neutral point, or a hydrogen-ion concentration of lo-'. *
Results
The following data are obtained when 20 cc. of 0.5 iV hydrochloric acid were titrated with sodium hydroxide: Cc. of N a O H 0.0
Vdts
+O. 200
5.0 10.0
f O . 140
15.0 20.0 21.0 21.5
+O. 106
+0.122 f0.120
f0.094 f0.0SlMethyl orange end point
Cc. of NaOH
21.6 21.05 21.75 21.8 22.0 23.0 25.0
Volls f0.055 -0,023
- 0,100
Phenolphthalein end point
-0.140 -0.159 -0.180 -0.190
These data are plotted in Curve A , Figure 2. This is a typical curve for strong acids and bases. The methyl orange indicator changed while the electrode was still slightly positive, but the electrode was negative with respect to the calomel half-cell when the phenolphthalein indicator color appeared. A very good flex point is produced which, together with the change of polarity, makes it possible to determine the end point within 0.05 cc. I n this titration the total volume of solution at the end point was approximately 75
P-To potentiometer V-Voltmeter R-High resistance C-Calomel half-cell Mechanical stirrer not shown
the reference curves for the hydrogen electrode given by Davis, Oakes! and Sa1isbu1-y.~ The end point breaks in successive titrations do not always occur at exactly the same potential in every c:ase, but they do occur at the same hydrogen-ion concentration. Frequent treatment with cleaning solution and ignition to white heat before each titration helps give more consistent curves. The electrode reaches equilibrium in from 1 to 2 minutes and titrations can be completed much faster than with a hydrogen electrode. The electrode is positive with respect to the calomel halfcell while in an acid solut,ion and becomes negative when in an alkaline solution. When titrating a strong acid with a
-0.08
2
e
0.00
d.04
\
\L
750
I-VD USTRIAL A,YD ENGINEERING CHEMISTRY
cc. and but 0.3 cc. of titrating solution was required to give all the end-point phenomena. From a similar procedure Curve B was produced by titrating dilute phosphoric acid with sodium hydroxide. This curve has the two characteristic flex points. Curve C resulted when sodium carbonate was titrated with hydrochloric acid and gives two flex points.
Vol. 19, No. G
Perhaps the greatest advantage of this electrode system is the fact that titrations involving oxidation and reduction reactions may be performed in the same apparatus. Hence, any laboratory having electrometric apparatus may, by adding an auxiliary electrode with the polarizing circuit, perform a number of titrations ordinarily requiring the hydrogen electrode.
Comparison of Sensitivity of Various Tests for Methanol' By Leslie 0. Wright U. S. INTERNAL REVENUEBUREAU, BRANCH LABORATORY, BUFFALO, N. Y.
EARLY all of the numerous proposed tests for Reagents which were used to condense the formaldehyde methanol depend upon the same principle-oxidation to a color, with reasons for their rejection, are as follows: of the methanol present to formaldehyde, to formic REAGENTS WHY DISCARDED acid, and in some of them to carbon dioxide and water. Less sensitive than rosaniline Fuchsin bisulfite Georgia and Morale9 report that the oxidation to formic Can be used only in concentrated acid Morphine sulfate Not sensitive acid or carbon dioxide is unsatisfactory, so that the generally Poor keeping qualities z$&,hine accepted method is by oxidation to formaldehyde and the Rosaniline was found to be best suited for the detection subsequent detection of this product by a suitable reagent. of formaldehyde because it The- adequacy of the test may be made into a stable, implies, fir& the efficiency A survey has been made of the various methods procolorless (or nearly so) reof the oxidation of methanol, posed for detection of methanol. Oxidation with poagent. It is very sensitive, and second, the use of a tassium permanganate to form formaldehyde, which a n d a n easily recognized sensitive reagent t o detect in turn is detected by reduced rosaniline solution, is violet color is produced with formaldehyde by producing the method adopted. formaldehyde. a compound giving an easily I t is recommended for standard work, where comThus, after trying various recognized color. For this parisons are desired, that solutions of 5 per cent alcohol reagents, both for oxidation second reaction a rosanilinebelused, and that other standard conditions, as outand for the condensation of sulfurous acid reagent was lined, prevail. For qualitative work, samples may be the formaldehyde to a color, used. run as received. If a positive test is found, glycerol t h o s e given below, based The purpose of this work or pectin may be present and the sample must then upon the work of C h a ~ i n , ~ was to study the degree of be distilled and again tested. E l v o ~ e and , ~ Deniges5 were sensitivity, best concentrafound to be best suited. tion of alcohol, other reagents for the oxidation of Reagents the methanol, and the elimination of various interfering substances such as glycerol. (1) Potussium Permangunate. Dissolve 3 grams of potassium
N
Preliminary Experiments
As oxidizing agents, hydrogen peroxide, chromic acid, ammonium persulfate, hot copper spiral, and potassium permanganate were tried, but all except potassium permanganate were rejected as unsatisfactory. The chromic acid came next in the order of desirable oxidizing agents, but gave a green color when no methanol was present (due to reduction of chromium by any organic matter present), and this color is undesirable because a violet color produced by small amounts of methanol may be obscured. While oxidation of methanol takes place more rapidly with chromic acid as the oxidizing agent, the test is not so sensitive and hence not 60 desirable as oxidation with potassium permanganate. The copper spiral was discarded because a positive test was obtained with ethyl alcohol. Hydrogen peroxide apparently has no effect on methanol and was therefore rejected. Ammonium persulfate was not sufficiently sensitive and gave a color for a negative result. These experiments confirmed those of Georgia and hlorales.* 1 Received February 28, 1927. Printed by permission of the Prohibition Unit, U. s. Internal Revenue Bureau. * THISJOURNAL, 18, 304 (1926).
permanganate in 100 cc. of water previously distilled over potassium permanganate, and containing 15 cc. of phosphoric acid. By using water previously distilled from potassium permanganate solution, this reagent can be kept for a long time. (2) Oxalic Acid Solution. Dissolve 5 grams of oxalic acid in a solution made by diluting 50 cc. of sulfuric acid (1.84) to 100 cc. (3) Schiff's Reagent (modified). Dissolve 0.2 gram of rosaniline, or a n equivalent weight of its salt, in 120 cc. of hot water, cool, and add this to a solution of 2 grams sodium bisulfite in 20 cc. of water. Finally add 2 cc. of concentrated hydrochloric acid and dilute the whole t o 200 cc. This solution should become colorless or nearly so after standing. If it is protected from the air no deterioration results. A very pure rosaniline base was used in these tests, but a product of lesser purity may be used, giving a somewhat colored reagent, which is, nevertheless, satisfactory.
Determination Place 2 cc. of the sample in a test tube and add 1 cc. of potassium permanganate solution. Allow t o stand for 10 minutes; decolorize by adding 1 cc. of the oxalic solution, followed by 2 cc. of the modified Schiff's reagent and mix. The solution must be mixed thoroughly. If methanol is present a violet color is developed. With traces of methanol the color is not 4
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THIS ] O E R N A L , 13, 543 (1921). I b i d . , 9, 295 (1917). C o n 9 t . r e n d . , 160, 823 (1910).
