T H E RATE OF REACTION O F AMALGAMS WITH ACIDS. I1
SODIUM AMALGAMS' W!LBUR
G. DUNNING A N D MARTIK KILPATRICK
Laboratory of Physical Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania Received J u l y $0, 1997
I n a previous paper (3) it was shown that the rate of reaction of lithium amalgams with acids in the general sense followed two different kinetic laws. For the strong acids and the weak acids, with the exception of o-chlorophenol, it was found that the rate of appearance of the alkali metal ion in the solution was given by the relationship dx - = kSC.4 dt where X is the surface and x is the number of ions of alkali metal formed a t time t by way of the reaction
M + H30+ + M+ + HzO + 1/2 Hz or
M + HOOCR -+ Mf
+ RCOO- + 1/2 Hz
The rate appeared to be independent of the concentration of the alkali metal in the amalgam, and k varied with the acid used. For the reaction with water or with buffer solutions of primary and secondary phosphate the rate equation is
- ddtA= k'SC*. dG where CMis the concentration of the alkali metal in the amalgam and the reaction presumably is
M and
M
+ HOH+M+ + OH-+
+ HZPO;
-+HPOT-
1/2
Hz
+ M+ + 1/2 Hz
1 This paper is abstracted in part from the dissertation of Wilbur G. Dunning presented to the Faculty of the Graduate School of the University of Pennsylvania in partial fulfillment of the requirements for the degree of Doctor of Philosophy, December, 1935.
215
216
WILBUR G. DUNNING AND MARTIN KILPATRICK
Equation 2 is in agreement with the results of Bronsted and Kane (2). These authors worked with dilute (0.024 molar) sodium amalgam and practically constant acid concentration. Moriguchi and Mitsukuri (ll), working with amalgams ranging from 0.6 to 1.2 molar, found the velocity of reaction in hydrochloric acid solutions to be independent of the amalgam concentration and approximately proportional to the acid concentration. Klein (8) Forked with high and practically constant concentrations of sodium in the amalgam and found that the results could be expressed by a n equation equivalent to the first two terms in equation 1 of the paper of Fletcher and Kilpatrick (3). However, the first term could not be evaluated exactly, as the results in alkaline solution were not reproducible.
FIG.1. Reaction vessel for amalgams
This lack of reproducibility for the water reaction has been observed by others. Baker and Parker (1, 12) observed that the rate of reaction of sodium with water varied with the purity of the water. They ascribed. this to the presence of traces of hydrogen peroxide. The purpose of the present work is to provide a further experimental test of equations 1 and 2, and t o attempt t o elucidate the results obtained by other workers. To do this a series of experiments has been carried out under different experimental conditions with various acids. EXPERIMEXTAL METHOD
The preparation of amalgams and the general procedure have been given by Fletcher and Kilpatrick (3). Three different reaction vessels were used and will be referred to as vessels D, F, and H. Figure 1 shows apparatus H equipped with stirrers for both solutions. In most experiments the
217
REACTION OF SODIUM AMALGAMS WITH ACIDS
stirrer for the amalgam was removed. The area of the interface was 12.3 om.* and 300 cc. of aqueous solution was used in most experiments. Apparatus D was similar in size and shape to apparatus H, the essential TABLE 1 Experiments with hydrochloric acid at d5"C. INITIAL CONCQNTRATION OF ACID
INITIAL CONCENTRATION OF AMALQAM
I 1
1
REVOLWTIONS PER YINUTE oFBT1RRER8
__
Aqueous layer
i
REMARKS
Amalgam layer
~
1
Apparatus D-area. 3.8 cm.* nioles per liter
m o l e 8 per liter
0.0150 0.0250 0.0250 0.0150 0 ,0300 0.0090 0.0100 0.1610 0,3220 0.0250 0.0250 0,0250 0.0300 0.0200 0.0260 0.0500 0.0300 0.0204 0.0204 0.0300 0.0500 0.0300 0.0500 0.0250 0.0250 0.0100 0.0150 0.0150 0.073 0.029
0.31 0.35 0.35 0.31 0.31 0.31 0.24 0.35 0.35 0.35 0.35 0.35 0.56 0.56 0.56 0.56 0.66 0.35 0.35 0.56 0.56 0.56 0.56 0.35 0.35 0.24 0.31 0.31 0.57 0.57 0.57 0.35 0.35 0.27 0.27 0.27 0.27
0.010
0.0250 0.0250 0.0540 0.0540 0.0540 0.0540
em. rnin.?
2450 2450 2450 1650 1650 1650 1650 1650 1650 1650 1650 1650 1650 1650 1650 1650 1650 1660 1660 1650 1650 1650 1650 1150 1150 980 975 520 500 500 500
500 500 500 500 500 500
0 0 0 0 0 0 0 0 0 0 0
u 0 0 0 0
0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0
2.91 1.79 2.41 1.48 1.56 1.74 1.21 1.31 1.13 1.78 1.60 1.39 1.36 1.35 1.39 1.39 1.32 1.65 1.60 1.41 1.54 1.61 1.76 1.17 1.18 1.28 1.15 0.59 0.67 0.66
T = 20°C.
T
= 20°C.
T = 30°C.
T
= 30°C.
0.60
0.69 0.65 0.65 0.87 0.89 1.03
In 0.95 M NaCl In 0.95 M NaCl In 0.95 M NaClO,
218
INITIAL CONCENTRATION '*"ID
WILBUR G. DUNNING AND MARTIN KILPATRICK
1 Lb:,I
TABLE 1-Concluded
co$&yr-
Aqueous layer
I
REMARKS
Amalgam layer
Apparatus D-area, moles per liter
ndes per liter
0.0412 0.0927 0.0927 0.0309 0.0618 0.0206 0.0399 0.0250
0.56 0.56 0.56 0.56 0.56 0.56 0.90 0.90
_____
REVOLUTIONS PER MINUTE OB mmmm
3.26 cm.2 cm. min.-1
2000 1950 1940 1900 1900 1900 800 850
100 0
97 95 95 95 75 60
1.92 1.62 1.98 2.00 1.84 2.14 1.88 2.10
In 1 M NaCl
I _ _
0,0299 0.0199 0.00997 0.0299 0.0199 0.0299 0.0399 0.0100 0.0100
0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.24 0.24
Volume = 200 cc. In 0.1 M NaCl
1500
75 75
980
0
1.28 1.37 1.44 1.02 0.81
difference being the cross section of the amalgam well, which was 3.26 in the early experiments and 3.8 in the later experiments. Apparatus F has already been described by Fletcher and Kilpatrick (3). The cross section of the well was 9.63 cm.2 The general method of carrying out an experiment has already been given. Table 1 summarizes the results with hydrochloric acid solutions. The velocity constants are calculated by the integrated form of equation 1. Table 2 gives the results with perchloric acid. Inspection of the results with aqueous solutions of hydrochloric and perchloric acids indicates that the velocity constant is independent of the concentration of sodium in the amalgam, and from the agreement of the results with the two acids the rate of reaction is approximately proportional to the hydrogen-ion concentration with the possibility of an electrolyte effect. It is also evident that the rate of reaction is proportional to the surface of the amalgam. The effect of stirring of the amalgam is small and may be due to a change in the surface exposed to the aqueous solution. These results are essentially in agreement with the results with lithium amalgam. Table 3 gives a comparison of the results with potassium and lithium amalgams.
REACTION O F SODIUM AMALGAMS WITH ACIDS
219
From these results we can conclude that the velocity constants for all three amalgams are the same under the same experimental conditions. This suggests that the rate-controlling process is independent of the alkali metal in the amalgam, and we might conclude that this process is the transTABLE 2 Ezperimenls with perchloric acid a1 26°C. INITIAL CONCENTRATION OPACrD
C I
I
RlVOLUTIONB PER MINUTE OF s T r a a B a s
rNITIAL
~
o
~
o
~
~
~
-
Aaueoua
AMALQAY
1
Amslnam
Apparatus D-area, moles par lilw
iolss per l i b
0.0400
0.56 0.56 0.56 0.67 0.60 0.60 0.60 0.60 0.60 0.60 0.60
0,0300 0.0200 0.0300 0.0050 0.051 0.010 0.030 0.030 0.030 0.030
____
3.8 om.* cm. mi%-]
1650 1660 1650 1650 500 500 500 500 500 500 500
1.45 1.31 1.30 1.29 0.83 0.85 0.82 0.80 0.88
0 0 0 0 0 0 0 0 0 0 0
1.11 0.91
In 0.17 M NaC10, In 0.34 M NaClO,
Apparatus D-area, 3.26 cm.2
0.0410 0.0308 0.0205 0.0103
0.90 0.90 0.90 0.90
0.0103 0.0205 0.0410 0.0103 0.0205 0.0308 0.0308 0.0410
0.90 0.90 0.90 0.33 0.33 0.33 0.90 0.33
~
1
700 700 665 665
60 60 45 55
1.28 1.28 1.24 1.28
1600 1600 1600 1600 1600 1600 1560 1560
80 80 80 80 80 80 78 78
1.16 1 .I1 1.20 1.20 1.16 1.14 1.07 1.13
1)
port of the acid to the amalgam surface. If this is the case, the effect of
stirring, according to Roller (13),should be in agreement with the relation koba.
90.8 = a constant
220
WILBUR G . DUNNING AND MARTIN KILPATRICK
where s is the rate of stirring. An examination of the results for apparatus D in table 1 gives an exponent which is approximately 0.7, but in the case of perchloric acid the exponent is considerably lower. Application of the equation where k a b s . is the observed velocil y constant, k the chemical velocity constant, and A an empirical constant. If A is taken as 0.005 and the chemical velocity constant as 3.00, there is fair agreement between hobs. and the TABLE 3 Comparison of velocity constants f o r alkali metal amalgams __ _-~ . i
AMALGAM
1
R.P.M.
I
k
REMARKB
Apparatus D--area, 3 . 8 em.2 ~
Sa....................
1
1650 1650 1650 500 500
K...................... Li. . . . . . . . . . . . . . . . . . . . . . Li . . . . . . . . . . . . . . . . . . . . . Ka . . . . . . . . . . . . . . . . . . . . .
cm. Inin.
Apparatus F-area,
,I
Ka.,..................
K . . . . . . . . . . . . . . . . . . . ., i Li . . . . . . . . . . . . . . . . . . . . .
i
975 975 975
i
Apparatus ~ _ _ 1980 N a . . . . . . . . . . . . . . . . .. . . . . I 1980 K. ...................... 1980 Li.. .................... ~
.I
Average of 14 experiments
Average of 7 experiments
9.63
1.06 1.01 1.07
~
i
I______
1
1 43 1 58 1 43 0 78 0.66
1
Average of 7 experiments _____H*-area, 12.3 cm.l _________ 1.43 ' 1.42 Average of 10 experiments 1.43
1
~
'
.-__
* The experiments in apparatus H were carried out by 11r. K. Hoff, t o whom due acknowledgement is made.
calculated velocity constants for the results in apparatus D. But if kaba. is the same for the three alkali metal amalgams in three different vessels, it would seem that the chemical k for lithium, sodium, hnd potassium would be the same. This seems hardly likely, so that one may conclude that an exponent of 0.67 is probably more nearly correct in the derivation of Roller. If that is the case, since
-,
h a .
is fairly constant, the conclusion
that the controlling process is the transport of the acid t o the surface of the amalgam seems justified, If one considers a mechanism similar to that proposed by Sclar and Kilpatrick (14) for the dissolution of magne-
REACTION OF SODIUM AMALGAMS WITH ACIDS
22 1
sium in alcoholic solutions of acids, the same general conclusion can be drawn. EXPERIMENTS WITH WEAK ACIDS
In the experiments with weak acids buffer solutions were used a t constant ionic strength for each series, so that the hydrogen-ion concentration TABLE 4 Experiments with acetic acid at 86°C.
,
I
Apparatus D-area.
0.0210 0.0350 0.0490
0.0210 0,0350 0.0490
moles per liter
moles per liter
0.170 0.150 0.130 0.110 0.175 0.151 0.150 0 175 0.150 0.175 0.130 0.175 0.185 0 0.05 0.15 0.15 0
0.56 0.56 0.56 0.56 0.56 0.56 0.56 0.56 0.56 0.56 0.34 0.31 0.31 0.60 0.60 0.60 0.60 0.60
I
0.0350 0.0250 0.0490 0.0250 0.0150 0.20 0.15 0.05 0.05
0 054 0.027 0 036
1
11
0.0350 0.0250 0.0490 0.0250 0.0150 0.20 0.15 0.05 0.05
1
0.0250 0.0250 0.0125 1 0.0375 0.0166 0.0334
1
0.33 0.33 0.33
3.8 cm.* :m.mi%-
1'
1650 1650 1650 1650 1650 1650 1650 1650 1650 1650 1650 1650 1650 500 500 500 500 500
1560 1600 1650
0 0 0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0
0.64 0.71 0.80 0.87 0.67 0.78 0.64 0.64 0.79 0.71 0.86 0.74 0.65 0.41 0.42 0.54 0.56 0.51
1 I 1 80 80 80
0.92 0.67 0.78
T = 20°C. T = 20°C. T = 30'C. T = 30°C.
222
WILBUR G. DUNNING AND MARTIN XILPATRICX
Tables 5 and 6 present the results with other weak acids, and table 7 summarizes the results. As in the case of lithium amalgams, the velocity constants are not very different. This can be interpreted to mean that we are dealing with a TABLE 5 Experiments with weak acids at .85°C. Apparatus D except a8 indicated I
INITIAL CONCENTRATIONOE ACID
I
T
Z
g
1-
,
I
INITIAL CONCENTRA-
INITIAL CONCENTRATIONOF CEMRIDE
~
I
INITIAL CONCENTRATIONOF AXALGAY
lk
REVOLUTION8 PER MINUTE OF IITIRREW
A
layer
moles per liter mole8 per WET moles per lzter moles per lUer
0.017 0.024 0.035
0.017 0.024 0.035
0.175 0.165 0.165
cm. min.-:
0.56 0.56 0.56
1650 1650 1650
Glycolic acid
0.034 0.024 0.015 0.040 0.040
0.100 0.100
0.034 0.024 0.015 0.040 0.040 0 0. IO0
0.165 0.175 0.185 0.160 0.160 0 0.100
0.56
0.54 0.58
0.59
____
0.31 0.31 0.31 0.60 0.60
1650 1650 1650 1650 1650 500 500
Apparatus H-area,
12.3cm.2
0.56
0 0 0
0.61 0.62 0.61 0.81 0.71 0.44 0.40
I
0.0370 0.0122 0.0244 0,0489
0.0380 0.0125
0,0260 0.0365 0.030 0.015 0.100 0.050
0.0260 0.0365 0.030 0.015 0.100 0.050
1
0.0125 0.0375
0.33 0.33 j
1680 1600
0.175 0.165 0.170 0.185 0,100 0.150
0.56 0.56 0.35 0.35
1650 1650 1650 1650 500
0.0250
1
I
84
80
I
0.62 0.72
0,0500
0.60
0.60
500
0.78 0.77 0.70 0.71 0.36 0.33
rate-controlling process involving a transport of the molecules of the carboxylic acid t o the surface of the amalgam. If this were the case, however, the order of the velocity constants should be perchloric acid, hydrochloric acid, formic acid, acetic acid (7) and if hydrochloric acid is taken
223
REACTION OF SODIUM AMALGAMS WITH ACIDS
m unity the ratios of the k's to k for hydrochloric acid should be perchloric acid 1.1,formic acid 0.26, acetic acid 0.23. The actual ratios of the veloc-
TABLE 6 Experiments with weak acids at W C .
1 I 1 1 ,
rmmu CoxmNTBATIONOF ACID
INITIAL CONCENTRA:;;T i
INITIAL CONCENTXATIONOF CHWRIDB)
I
INITIAL CONCENTRATIONOB AMALQAM
REVOLVTTONBPEE MIOF B T I R R ~ R B
Asu-us layer
1
Ay$m
Phenylacetic acid mdaa per liter
0.035
0.165 0.175
0.015 0.030 0.050 0.050
0.035 0.025 0.015 0.030 0.050 0.050
0.035 0.010 0.035 0.022 0.025 0.015
0.035 0.010 0.035 0.022 0.025 0.016
0.165 0.190 0.165 0.178 0.175 0.185
0.025
em. min.-J
moles p a Ma moles per liter moles p a liter
0.56 0.56 0.31 0.31 0.56 0.56
0.185
0.170 0.150 0.050
0 0
1650 1650 1650 1650 500 500
0.58 0.56 0.52 0.53 0.38 0.40
0 0 0
0
Prouionic acid 0.56 0.66
0.56 0.56 0.31 0.31
1650 1650 1650 1650 1650 1650
0.82 0.76 0-66 0.68 0.63 0.61
TABLE 7 Summary of experiments w'th weak acids
1
ACID
E.P.Y.
AYEBAQE
1650
0.57 0.74 0.35 0.65 0.42 0.55 0.39 0.75 0.49 0.68
k
AWBAQB DBVUTION
per cent
Mandelic. . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . Formic . . . . . . , , . , . . . . . . . . . . . , . . . . . , Glycolic. . . . . . . , . , , , , . . . . . . . . . . . . . . , . , , , , , Phenylacetic, . . . . . . , , . . . . . . . . . . . . . . . , , , . , ,
{
Acetic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , , Propionic
~
4 5 5 10 5 4 3 10 11
12
ity constants at 1600 R.P.M. are perchloric acid 0.94, formic acid 0.52, acetic acid 0.52, and at 500 R.P.M. the ratios are perchloric acid 1.3, formic acid 0.54, acetic acid 0.75. From the data of Kilpatrick and Rushton (6)
224
WILBUR G. DUNNING AND MARTIN KILPATRlClC
with magnesium, the corresponding ratios are : perchloric acid 0.74, formic acid 0.33, acetic acid 0.21, while from the data of King and Cathcart (7) we have perchloric acid 0.97, formic acid 0.34, acetic acid 0.28. Whatever the relation between the velocity constant arid the diffusion coefficient, these ratios should be the same for the dissolution of magnesium and of sodium amalgam. If we apply an equation of the type k = uDZ,the value of 1: is approximately 0.45 for sodium amalgam. These computations indicate a lack of correlation between dissolution constants and diffusion coefficients. Further experiments with weaker acids indicated that the rate law was not that of equation 1, but that the kinetic law given by equation 2 was followed. In all previous experiments when the reaction was carried to completion the hydrogen evolved was found to correspond t o the number of equivalents of sodium reacted. This was not true in the case of inonochloroacetic acid or with cacodylic acid. The latter acid TABLE 8 Experiments with phosphates AQUEOU8 BOLUTION
0.030 Jf NaH2POa.. . . . . . . . . . . . . . . . . . . . . . . . 0.030 M NazHPOa. . . . . . . . . . . . . . . . . . . . . . . 0.20 M N a C l . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 050 M KaH2POa 0 050 M NalHPOa 0.175 M KaHzPOn . . . . . . . . . . . . . . . . . . . . . . . . 2.0 M iSaH2POc . . . . . . . . . . . . . . . . . . . . . . . .
I N I T I I L CONCENTXATION OF AMALQAM
.I ~
.I
'>
.I j
0.31
' '1
R.P.Y.
1650
I
031 0.56 0.56
1650
1
~
1650 500
~
1
2.0
26 12.1. 62.0
is very easily reduced. With solutions of tertiary and secondary sodium phosphate the volume of hydrogen was always less than that calculated from the sodium reacted. In the case of the primary phosphate and mixtures of primary and secondary phosphate3, the gas evolved corresponded to the sodium reacted. Table 8 gives these results, the acid concentration being constant. The velocity constant in column 4 is the sum of the 75ater constant and the acid constant multiplied by the acid concentration. I n all cases the velocity constants have been calculated from the integrated form of cquation 2 and confirm the results of Fletcher and Kilpatrick (3) with o-chlorophenol and those of Bronsted and Kane (2) with very weak acids. THE WATER REACTIOS
With the exception of a few experiments which were followed by t>hegas evolution method, the experimental method for the water reaction was that used by Kilpatrick and Rushton (6) and by Fletcher and Kilpatrick (3). I n view of the fact that the purity of the water seemed to make a
REACTION O F SODIUM AMALGAMS WITH ACIDS
225
marked difference, all experiments were carried out with conductivity water and in most cases the water used was from a special batch having a specific conductance of 5 X lo-' mhos at 25°C. Air was not excluded from the solution except in the case of the experiments followed by gas TABLE 9 The water reaction
_____
___-
___-
INITIAL AMALQAM CONCENTRATION
180'
R.P.M. (ACJVEOUS LAYER)
moles per
liter
0.31 0.31 0.31 0.56 0.35 0.31 0.05 0.05 0.05 0.26 0.04 0.05 0.06 0.06 0.06 0.01 0.01 0.01 0.04 0.01 0.60 0.60 0.25 0.56
0.26 0.26 0.26 0.26 0.26 0.33
CONCENTRATION
x
I;,
10s
I
REMARK0
_moles p e r liter
1x 1x 1x 1x 1x 1x 1x 1x 4 x
10-Q 10-9 10-9 10-5 10-6 10-9 10-5 10-5 10-10
1x 1x 1x 1x
10-9
1x 1x 1x 1x 1x
10-5 10-5 10-5 10-6 10-4
1 x 10-9 10-7 10-5 10-5
1 x 10-6
Range 10-7 t o 10-12 1 x 10-9 I x 10-9 1 x 10-9 1 x 10-5 1 x 10-5
1 x 10-
2450 1650 1650 1650 1650 1000 1000 ROO 500 500 500 500 500 500 500 500 500 500 500 500
500 500 500 500 500 500 500 500 500 500
,
1.60 0.86 2.20 0.65 1.60 1.50 0.70 0.68 0.26 0.16 0.45 0.40 0.55 0.30 0.32 1.10 1 .00 0.71 0.36 0.89
i
18 29 17 59 0 34 0 40 0 0 0 0
Apparatus H-area,
1 ,
By gas evolution
In In In In In 1 In
'
12.3 cm.2
1 .If KC1 1 M LiCl 1 &I SaCl 0 003 .If CaCl, 1 14 M CaCL 0 33 M SrCll
evolution. The indicators used were bromophenol blue, methyl red, bromothymol blue, phenolphthalein, and thymol blue. The velocity constants were calculated from the integrated form of equation 2. CHZ0, being a constant, is included in k . The results are presented in table 9. THE JOURNAL OF PHYSICAL CHEMISTRY, YOL. 42, NO. 2
226
WILBUR G. DUNNING AND MARTIN KILPATRICK
All experiments are a t 25OC., but it should be stated that there is very little effect of temperature. A plot of the logarithm of the velocity constant against the logarithm of the stirring speed gives a line of slope of approximately 0.7. The velocity constant seems to be independent of the hydrogen-ion concentration within the reproducibility of the experiments. Since the average value of the velocity constant is smaller for the experiments carried out by the gas evolution method, the possibility of a reaction with oxygen was tested in a special series of experiments. I n addition the effect of hydrogen peroxide was studied. The results in table 10 are in qualitative agreement with those for lithium amalgams and indicate that there is a reaction with oxygen as TABLE 10 Effect of oxyge? and hydrogen peroxide Initial concentration of amalgam, 0.78; R.P.M. = 1650; temperature = 25.0"C. INITIAL CONCENTRATION OF H102
moles per liler
0 0
HYDROGEN-ION CONCENTRATION
10%
moles per liter
10-7 to 10-12 1 x 10-8
x
0.53
0.67
0
1
10-8
1 .os
0.036 0.11 0.11 0.18
1 x 10-9
2.15 3.15 3.66 6.05
1 x 10-8 1 x 10-5 1 x 10-9
B y gas evolution Open to air Oxygen bubbled through the solution 42 23 27 30
__
well as with hydrogen peroxide (9). The reduction of oxygen would result in the formation of hydrogen peroxide (4) in acid solution. Our experimental test of equation 2 was carried out a t constant acid concentration. To test the possibility that equation 2 might be applied to the acetic acid buffer we have
ix(F)
so that upon plotting -
versus Ca a straight line of slope
- k*S and intercept - k,S should result if the law is obeyed. Computations with the data for acetic acid indicate this law is not obeyed. In calculating velocity constants for the strong and weak acids, with the exception of primary phosphate, equation 1 was used and the water
REACTION OF SODIUM AMALGAMS WITH ACIDS
227
reaction was neglected in the calculation. It was realized that this neglect would lead to some discrepancies in the determination of the velocity constants. Computation by means of the equation
(where ij is the volume of amalgam in cubic centimeters, HB represents an acid such as the primary phosphate, and A represents an acid such as phenylacetic) yields information concerning the portion of the alkali metal appearing in the solution by the various reactions. dz/dt is the number of gram-ions of sodium appearing per minute in the aqueous solution. These calculations indicate that the water reaction comes in to as much as 25 per cent in the lowest concentrations of acids following the law represented by equation 1. I n the case of acids following the square-root law, the correction for the water reaction is carried out without difficulty, as shown elsewhere (2). For all the experiments with buffer solutions, computations show that the reaction with hydrogen ion is quite negligible. Equation 2 does not seem to fit any mechanism of the reaction which the present authors can offer (5, 10). One or two observations may be worth recording. In many cases an island of bubbles formed on the amalgam surface, and the larger the island the more rapid was the rate of reaction, as a rule. In cases of excess of strong acid over the number of moles of sodium in the amalgam, the reaction rate is practically constant, and just before the sodium has all reacted there is a sudden increase in rate. Here we have a reaction which is of the first order, but under the experimental conditions appears to be of zero order and completed in finite time. From the results of this paper and the preceding one (3) it is evident that the rate of reaction of alkali metal amalgams with strong and fairly weak acids follows a first-order law, in agreement with the findings of earlier workers in the field. The velocity constants are independent of the concentration of the alkali metal in the amalgam. I n solutions of weaker acids and low hydrogen-ion concentration the rate is proportional to the square root of the concentration of alkali metal in the amalgam, and the results may be interpreted as a chemically controlled rate of reaction between electrons and protons from the weak acids (2). In both cases the results can be interpreted from the point of view of the general theory of acids. In addition, one must also consider the process 8
+ oxidant -+ reductant
analogous to
8+H+-+H
228
WILBUR G . DUNNING AND MARTIN KILPATRICK SUMMARY
1. The earlier and the more recent work on the dissolution of amalgams has been confirmed. 2. For strong acids and certain weak acids the rate of solution is independent of the concentration of the alkali metal in the amalgam if Dhe experimental conditions are such that t,he water reaction is negligible. The rate is proportional t o the concentration of strong acid above 1 x 10-4 moles per liter, is proportional to the surface, and is dependent upon the stirring. For the weaker acids the increase in rate above the water reaction is proportional to the acid concentration. 3. For the water reaction and the reaction with primary phosphate the rate of solution is proport'ional to the square root of the alkali metal concentration, independent of the hydrogen-ion concentration, and proport,ional to the acid concentration and t o the surface. 4. In the presence of oxygen, hydrogen peroxide is formed and reacts with the sodium amalgam. This reaction explains in part the lack of reproducibility in the rate of reaction of amalgam with water.
The authors would like to make due acknowledgment of a research grant made to one of us (Ll. K.) by the Faculty Research Committee of the University of Pennsylvania. REFERENCES
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
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