107
FEBRUARY, 1950
quent competitive reaction with methyl alcohol and n-butyl alcohol, is also of doubt,ful value for student instruction. HERBERT C. WOHLERS
To the Editor: I think that the approach to the hydrolysis of NHGN taken bv Eisemann in vow November JOURN.~L (n. 607) is unnecessarily obscnie. The problem is really one in algebra, not chemi~t~ry.After giving equations (I), (2), and (3), he needs a fourth independent relation in order to solve for four variables. This fourth relationfihin ~ - - - is not,hine more than the fundamental nrincinle that the sum of positive and negative charges shall be equal in any reasonably large volume unit. Thus, we have, ~A
L~~~
-
~
IFI+]
+ [NH,+] = [CN-1 + [OH-]
(1)
which he finally arrives at in equation (7). I think it, desirable to solve these problems by straightforward application of the mles of algebra; the solutions are vulnerable enough when one considers activities. The exact equation (14) can be arrived a t by substit,uting t,he value of [OH-] from (4) above into equation (3) and eliminating [NHI+] and [CN-I by combining his equations (1) and (2) with the relationships: [NHnl
+ IN&+]
=
0.1 = (CN-1
+ [HCN]
which are obvious from stoichiometry. I solved his reduced equation (15) graphically and got the same answer as he did, using the approximation formula (16). The large table a t the bottom of page 608 is good only for that particular system. It couldn't be used at all for ammonium formate, but equation (14) can always be reduced by a consideration of the coefficients for particular values of the ionization constants of t,he weak acid and weak base.. . . ROBERT EPPLE
T o the Editor: In the experiment, "A Demonstration of the Couductivities of Dilute and Concentrated Sulfuric Acids," reported by S. Porter Miller in THIS JOURNAL, 26, 317 (1949), the voltage of the electric mains supplying the current was not stated. As this power supply varies in some localities of the world we had to ascertain from available sources the fact that the voltage used for tho experiment in the locality mentioned was 110 v. a. c. This demonstration was included in our items for the science exhibition in our laboratory held this year. The apparatus set up according to the design given, when connected to our mains (220 v. a. 0.) failed in a rehearsal
to give any noticeable difference in the brightness of the two (100-watt) bulbs. Interposing various resistances in the circuit before it divided in parallel to connect the bulbs did not appreciably improve the experiment. But when the copper electrodes were kept 13 inches apart in each liquid the dim and the bright glows in the bulbs were readily obtained. The acids may be kept in long, shallow porcelain trays with hemispherical edges a t the ends, or in glass tubes of required length with the ends bent upwards and immersed in troughs of cold water. AND ARCOTVISWANATHAN SUNDAREASAN SOUNDRARAJAN
M.;~',"~cr;;&"p',"h""
T o the Editor: Various laboratory ozone generators have been described in the literature. The author has found though. that a simple generator suitable for lecture demonstrations can be easily constructed using no special materials or technique in twenty minutes from a 12inch reflux condenser. This is wrapped with aluminum foil and fastened with copper wire for the outer electrode; the center tube is corked tightly and is filled with mercury for the inner electrode. A typical generator of this type yielded ll/rvolume per cent of ozone when dry oxygen was used a t a flow rate of 7 liters per hour with a potential across the electrodes of 6,000 volts, and l'/a. per cent of ozone was . ohtained a t 4 liters per hour. Absolutely dry oxygen must be used and drying may he done hy bubbling the oxygen from an oxygen cylinder through concentrated sulfuric acid before it is led to the generator. The bubble count in the drying acid may be used to estimate a low flow rate. Occasionally a somewhat higher potential may be necessary if the electrodes are farther apart due to individual differences in the outside diameters of condenser jackets. If it is desired to check the operation of the ozonizer, the ozone which is produced may he bubbled through a potassium iodide solution where free iodine is liberated quantitatively, and this can he t,itrated with standard sodium thiosulfate solution.
To the Editor: In a recent paper by T. J. 'Phillips (THISJOURNAL, 26, 593 (1949)) the aut,hor states, "In most elementary physical chemistry textbooks the formulas for calculating the junction potential and the total potential of without proof or . cells with transference are either the student is referred to a text on electrochemistry." I disagree with this statement since Glasstone, in his
JOURNAL OF CHEMICAL EDUCATION
108
,
[Hf1 [CN-I = "Elements of Physical Chemistry," D. van Nostrand [NH4+1 [OH-] = 1,8 fHCN] [NHdOHl Co., Inc., pp. 466467,1946, has already given the proofs . cle&ly a& simply. Substituting X for [CN-] and [NH4+]in these equaI would like to call attention to the author's state- tions and for [HCN] and [NH,OH], the equations ment that y+ is the mean activity coefficient. This is become emoneons, since the mean activity coefficientis defined = 7 X 10-10, X1 = 1.8 X 10-6 by Y V y*
= (Y+"+
. y_v-)l/"
where v+ and v- are the number of positive and negative ions, respectively, and v = v+ v-. The inclusion of the term Eo in the paper referred to is also wrong, since the cell involved is a simple concentration cell. For the past several years in my conrse in physical chemist~y,I have given the proofs according to Glasstone. In addition I have let the students develop formulas for the cell
+
Ag, AgCI, HCI (a) I HCI (ad AgC1, Ag
where the electrode is reversible with respect to the anion instead of to t,he cation. The formulas in this case of course now become RT l n mril Ei = (L - .+I.+) F
mnrr
and E , = - 2 % - lRT nF
mm
mm
The students are reminded t,hatthroughout the developments the assumption is made that the transference numbers are independent of concentration and this is only >vhena, and are not very different from each other. NORWNC. LI ST.LOUISUNIVERSITY ST.Loms, M~ssormr
Substituting
lo-'& for [OH-] these may be com[H+l bined and reduced to an equation containing only a single unknown:
g=7x Y
[Hf1
- 1.8 x 10-5 [Hf] 1.10-"
-
From this [H+]'
10-'o 10-L4, [HC] = 6.2 1.8 X 10-6
=
x
10-LO
Using this value in the equation for the ionization constant of HCN 6.2 X 10-lo [CN-I =
[HCNI
Rearranging [CN-1 - 7 X 10-" [HCNI 6.2 X 10-lo
- 61 .2
From this one may calculate the extent of hydrolysis as 6.2 IHCNI = [CN-I [HCN] 7 6.2
+
+
-
- 47%
The fact that in the answer obtained the concentrations of H + (6.2 X 10-lo) and OH- (1.6 X lo-') are both low may be taken as evidence that the net reaction may be used properly for all concentrations of NH4CN down to the limit where the concentration of NHI+ is no longer large as compared with 1.6 X lo-'. I n such an extremely dilute solution the NH4+becomes more comparable to Na+ or K+.
To the Editor: R. K. MCALPINE The paper by Kurt Eisemann in the November, 1949, OF CHEMICAL EDUCATION on pH issue of the JOURNU UNIVERslrY M~~.~~~~ ANN ARBOR, MICHIGAN and the hydrolysis of a doubly weak salt presents a method of attack which is unnecessarily complex. In contrast to this it seems worth while to call attention to another approach which makes use of the principle of To the Editor: the "net reaction!' Several years ago an article appeared from this In the hydrolysis of NH4CN the equation of the net laboratory entitled "Utilization of a Classroom Waste reaction may be daitt,en Product"' in which the author showed how discarded pieces of chalk from the classroom might be employed NH,+ + CN- + HzO = NH,OH + HCN in increasing interest in chemistry teaching. This equation assumes that the two reactions of hyBy employing the chromatographic adsorption prindrolysisl CN- + = HCN OH- and NH4+ ciple it is possible to use these discarded pieces of chalk HtO = NH40H + H + ~produce sufficient amounts of in other ways and increase interest in laboratory work H+ and OH-so that the resulting neutralization carries as well as increase interest in the technique mentioned, both of these reactions along to equal extents. On that Discarded pieces of chak were ground in a mortar basis, since NH&N supplies equal concentrations of and washed, The material as a sludge was placed in N B + and CN-, when equilibrium has been reached discarded buret tubes about 250 ml, in length and m,+] = [CN-] = X and [NH40HI = [HCNI = u ml. in diameter. To this sludge, which was previously IJ. CHEM. EDUC.,16, 495 (1939). From the ionization constants of HCN and N K O H
