columns which have very low capacities for the sample to be injected. It is important to note that the comppsition of the gas sample will rarely be the same as the liquid, because of differences in relative vapor pressures of the components. This fact may be used t o advantage if a small amount of a relatively volatile component is being sought. This sampling technique differs somewhat from that of Pitkethly ( I O ) , in which a small volume of liquid is completely volatilized into a large volume of inert gas before sampling the gas phase. However, both techniques accomplish the goal of decreasing the total amount of solute in the sample.
LITERATURE CITED
(1) Bohemen, J., Purnell, J. H., “Gas
Chromatography, 1958,” D. H. Desty, ed., p. 6, Academic Press, New York, 1958. (2) Deemter, J. J. van, Zuiderweg, F. J., Klinkenberg, A., Chem. Eng. Sei. 5, 271 (1956). (3) Golay, M. J. E., “Gas Chromatography, 1958,” p. 36, Academic Press, New York, 1958. (4) James, A. T., Martin, A. J. P., Biochem. J . (London) 52, 238 (1952). (5) Keulemans, A. I. hl., “Gas Chromatography,” Reinhold, l i e w York, 1957. (6) Keulemans, A. I. M., Kwantes, A,, “Vapour Phase Chromatography,” D. H. Desty, ed., p. 15, Academic Press, New York, 1957. (7) Kieselbach, R., “Gas Chromatog-
raphy, 1958,” p. 123, Academic Press, New York, 1957. (8) Littlewood, A. B., Ibid., p. 23. (9) London Symposium 1956 Nomenclature Recommendations, “Vapour Phase Chromatography,” p. xiii, Academic Press, New York, 1958. (10) Pitkethly, R. C., ANAL.CHEY.30, 1309 (1958). (11) Purnell, J. H., Ann. N . Y . Acad. Sci. 72,592 (1959). (12) Union Carbide Chemicals,Fo., “Carbowax-Polyethylene Glycols, 1957. RECEIVED for review September 14, 1959. Accepted December 9, 1959. Work supported in part by the U. s. Atomic Energy Commission under Contract AT(30-1)905.
Response of Thermal-Conductivity Cells in Gas Chromatography L. J. SCHMAUCH
and R. A. DINERSTEIN
Research and Development Department, Standard Oil Co. (Indiana), Whiting, Ind.
To understand better the performance of thermal-conductivity cells as detectors in gas chromatography, theoretical and experimental studies were made of the response of cells to changes in gas composition and operating temperatures. An equation for response was derived for cells in which most of the heat is conducted through the gas. Response was considered as the product of two factors: one, a cell factor, depends on operating conditions including electrical parameters of the cell and bridge; the other, a thermal-conductivity factor, depends on the difference in conductivity of the carrier gas and the gas mixture passing through the cell when a component is eluted. Experimental results show that when there is a large difference in conductivity, as in the case of organic components in helium carrier gas, response is approximately linear and only a single calibration factor is required for quantitative analysis a t a given operating temperature. When the difference is small, as in the case of organic components in nitrogen, or hydrogen in helium, response is less linear, and calibrations covering the concentration range are necessary.
D
for gas chromatography must measure the changes in composition of a flowing gas. If the chromatogram is to be readily usable for quantitative analysis, the response of the detector should be linear with concenETECTORS
tration, and the sensitivity-response per unit of concentration-should be the same for all compounds. Of the many types of detectors, the theimal-conductivity cell is most commonly used: i t is inexpensive, sturdy, and easy to construct and operate. However, its response t o changes in gas composition or operating conditions-especially temperature-has not been well enough understood to define its performance. Although some reports suggest that the response of a cell is proportional to weight (6, 9, 16, 61) or mole (11, 28) concentration, others state that no such simple relationship exists (14, 64) and that calibration is essential. Prediction of calibration factors (6) has given only fair accuracy. For some mixtures, the serious nonlinearities t h a t have been reported (10, 1.2, 21) make calibration very difficult. Explanations for these nonlinearities have been based on decomposition ( I d ) , maxima in the thermal-conductivity isotherms of gaseous mixtures (10, 1 4 , and a combination of heat-capacity and thermal-conductivity effects (6). T o define the performance of thermal-conductivity cells, we have employed both theoretical and experimental approaches. .4n equation for the response of a cell was derived having two major parts: a cell factor that depends on electrical parameters of the cell and bridge, and a thermal-conductivity factor t h a t depends on the thermal conductivity of the gases passing through the cell. Cell response was
measured for a series of binary gas mixtures under a variety of temperatures of the cell and sensing element; either hot wires or thermistors were used as sensing elements. The gas mixtures consisted of components of high or low conductivity and carrier gases of either high or low conductivity. The equation has been used t o explain observed effects of changing carrier gas, composition of the gas mixtures, and temperatures. DERIVATION OF RESPONSE EQUATION
A convenient form of thermal-conductivity cell employs a temperature-sensitive element having a resistance RM, mounted within a chamber that contains the gas. The element is usually a thin wire or a small thermistor. When an electric current, I, is passed through the element, the power, 12R.,I,developed causes its temperature to rise. The heat from the element flows to the chamber nalls by thermal conduction through the gas, by convection, by radiation, and also by conduction along the element mountings; if the cell is flow-sensitive (a, 26)>part of the heat is carried away in the flowing gas. Cells can be directly tested for flow sensitivity and for the amount of heat conducted through the gas. Flow sensitivity can be measured by observing the response of different gas mixtures known to have the same thermal conductivity but different heat capacities, or by other means (66). Heat losses can be measured quantitatively (18). I n a well designed cell, most of the VOL. 32, NO. 3, MARCH 1960
343
2
-H O T - W I R E C E k L - -THERMISTOR C E L L
Figure 1. circuit
I
,
Simplified bridge
4 Figure 2. Effect of temperature on cell factor 0
IO0
300
200
T E M P E R A T U R E OF S E N S I N G E L E M E N T , 8 R ,'C.
heat is conducted through the gas. In this case, the temperature of the element, and therefore its resistance, depends primarily on the thermal conductivity of the gas, and also on the current. When the rate of conduction through the gas changes, as by a change in composition, the heat loss, and hence the temperature of the element and its resistance, must change. By connecting the element in a bridge circuit, such as shown in Figure 1, these changes may be measured as electrical response, EO. In practice, a reference chamber containing carrier gas and a resistor similar t o RM replaces R1 or Rz to compensate partially for drifts in current and ambient temperature. However, for simplicity in deriving an expression for EO,resistances R1, R2, and RI, as well as the bridge voltage, EB, are assumed to be constant for a small change in composition. An expression for Eois derived by considering the change in resistance within the circuit, and then relating this change to that in conductivity of the gas. In the bridge circuit, Eo is the difference between the voltages across RM and Rz. With carrier gas around RM,these voltages are balanced by adjusting slider S to give zero output or base line. Then,
With Equation 1 and the expression, RIM), Equation 2 reduces to :
EB = I (R1
+
The quantity (Rw' - R,5f)depends on the temperature of thc sensing element and how its resistance changes with temperature. Response for Hot-wire Cell. For hot n-ires : RM = Ra ( I RM' = Ro(1
ANALYTICAL CHEMISTRY
(4)
+ OR -'r aA@R)
(5)
where Ro is the resistance a t 0' C., a is the temperature coefficient of resistance, & is the temperature of the sensing element, and A& is the temperature change accompanying a change in conductivity of the gas. From Equations 4 and 5 : RM' - RM = Ro~AOR
Eo
Z~RIRORM~ d(R1 RM')
+
=
dK,( 8 ~ Ow) K
- Ow
(@EM
Rw' and I' can be eliminated by substituting from Equation 3 and from the relationship between I' and I : EB = Z(Ri
+ R M )= I'(R1 + RM')
to obtain an expression for response:
(6)
The temperature change, AOR, depends on the conductivity of the carrier gas, Kz, and of the mixture, K M ; it is derived from the heat-balance equations for the cell:
I12Ru' = ~
344
QSR)
Substituting for AeZ in Equation 6 and then using the resultant expression for R M ' - R M in Equation 3 gives:
and
PRv
When gas composition is changed, RM assumes a new value, RM',and response becomes :
+
lower conductivity than Kz, BR has increased by an amount AOR; RM is thereby increased to RM' and I is reduced to I f . From Equations 7 and 8:
(7)
+ A@R) (8)
where a is a cell constant that depends on the area and length through which t h e heat passes; J is Joule's mechanical equivalent of heat; and Kz and K M are the conductivities a t the mean temperature between that of the sensing element, 8R, and that of the cell wall, em. I n Equation 8, written for a gas of
With this expression, Eo can be predicted and different cells can be compared a t equal current. Such a comparison involves different levels of OR and different conductivities, as well as differences in the cells. On the other hand, comparison a t the same O R involves only the differences in the cells. For this purpose, the response equation is rewritten in terms of temperature by substituting for I from Equation 7 to give :
0.15
I
100,
I
I
I
I
1
I
HELIUM
n-HEXANE
C Y CLO H E X A N E BENZENE 0.10
K 0
c V 4
>
METHANOL
0.05
ul
-c c > 0
a P
z 0
0
1
0
I
I
I-
I
I
i
1
COMPONENT CC
i
0 2 4 :ENTRATION, MOL%
6
8
Figure 4. Response vs. composition for different components in helium or nitrogen TEMPERATURE OF G A S ,
Figure
3.
Effect of
temperature
nhere C,, cailed the cell factor, is:
The ccll constant ( I S ) , a, for coaxial cylinders of n chani!)er arid a nire is: a =
2iTL _rv In TR
nhere L is the length of the wire, and r R and rw are the radii of the wire and n all, respectively. Equation 12, defining response of a hot-wire cell, can be considered as the product of two factors: C,, depending on geometry, temperature, and electrical parameters; and the last bracketed portion, depending on conductivity. Because EO is usually much smaller than IRrf orIR1, a n approximate statement can be written:
OC,
on conductivity factor
is eluted. If KP> KAM, response is positive; if K M > K z , response is negative. If means of heat transfer other than conduction are important, other terms nould be added. For example, a term for the heat capacity of the gas must be added if heat transfer occurs by means other than conduction-eg., by natural convection-or if the cell is flow-sensitive, whether in streamlined or turbulent flow ( 9 ) . These two factors were negligible for the cells used in the experimental portion of the present study. Response for Thermistor Cell. T h e response for thermistor sensing elements can be derived in a manner similar t o t h a t for hot wires. I n this case, t h e resistance-temperature relationship is given (3) by: B
T
-'273. )
R here B is nearly constant and depends on the material of which the thermistor IS made, and T is the thermistor temperature in O K. The solution for R u t - R.,*, analogous to Equation 6, is given by:
R,r'
This expression s h o m t h a t the conductivity factor depends on the difference in thermal conductivity of the carrier gas and of the binary gas mixture passing through the cell whcn a component
(A -
R.w = Roe
- R,u (e
E
RM X
- 1)
-
RO
i t decreases markedly as eR increases, whereas the coefficient for a wire is almost constant. For thermistors:
E O = C t (1
-
2)
X
(12a)
which is the same as the response equation derived for hot wires except that the cell factor is now defined by:
cz
=
Because the thermistor geometry is usually not cylindrical, the cell constant, a, will also differ from that shown in Equation 12. If both t h e thermistor and chamber were spherical, a R-ould equal (29):
An approximate statement, similar to t h a t for a hot-wire cell, can be written for a thermistor cell:
(- 3) AoR
For small values of 6 8 R . The quantity BR,,*, ROT2is the temperature coefficient of resistance for the thermistor;
The response again depends on the product of the cell and conductivity factors. VOL. 32, NO. 3, MARCH 1960
0
345
Thermal Conductivity of Gas Mixture. T h e conductivity factors in
Equations 12 and 12a depend on the conductivity, ICdf,of the mixture of component and carrier gas. For t h e conductivity of such mixtures, a n equation has been proposed (50) ; Ku = I
I
I
5
10
15
20
C O N C E N T R A T I O N OF H Y D R O G E N , M O L %
where K1 and K I are the conductivities of the pure component and carrier gas, respectively, x is the mole fraction of component, and -41.2 and A2.1 depend on physical properties of the gases. The best agreement of calculated with observed K.w values was obtained with A values calculatcd by an equation (17) involving viscosities, molecular weights, and Sutherland constants of the components, and the temperature of the gas mixture. Although Equation 13 is adequate for calculating K.w for some purposes, it and other approaches (4, 90) have serious limitations when applied to predicting values of EOfrom Equation 12, for two reasons. First, results in calculating K N are accurate only to nithin a few per cent (17, 20, 25); small errors in calculated values of KAfcan be a large fraction of K I - Knf and lead t o large errors in predicted values of Eo. Second, the masima or minima sometimes observed in the relationship between KM and x are not predicted. Such maxima or minima would be expected when the slopes, d K M / d x , at x = 0 and x = 1 are opposite in sign. The derivative of Equation 13 shows these slopes to be:
and
The signs of the slopes are determined by the magnitudes of the product A , ,and the ratios of the conductivities. For a maximum, both K1/K2 and K 2 / K l must be great,er than A1.2 AZJ; for a minimum, the opposite conditions Az,l must esist. In both cases, must be less than unity. Conditions for a maximum may be met when IC1 and K z are close ( 7 ) . Maxima have been reported for many mixtures, including carbon dioxide in nitrogen (25), methanol in nitrogen (IO),acetylene in nitrogen (14, and methanol in argon ( 1 ) . On the other hand, with gases having a large difference in conductivity, the values of AI,*-42,1(15) are slightly greater than unity and the conditions for a iiitlsimuni or minimum cannot be met.
346
ANALYTICAL CHEMISTRY
Figure
5.
Response vs. composition for hydrogen in helium I
I
I
I
n - H E X A N E CONCENTRATION, M O L %
Figure 6.
Response for n-hexane in helium a t different temperatures
Because K 1 , Kz, AI,?,and A2,1 change with temperature, both K,v and the appearance of maxima or minima depend on temperature. For gas mixtures where Kl and K 2 are close and converge with increasing temperature, a maximum may be observed a t a higher temperature but not a lower one. With gases differing widely in conductivity, a maximum will not be observed even a t a n elevated temperature, although the conductivities may tend to converge.
so
=
a
(%)= o = c [= =
--
1
Kz (14)
where C,
= cYRo(6R
- ow)(RN - R1)
RN (Ri
or
+ RN)
Ct
DEPENDENCE OF RESPONSE ON TEMPERATURE
The effect of temperature on response may be predicted after evaluating separately the temperature dependence of the cell and the conductivity factors. Equations 12 and 13 could be used for this purpose by selecting a single value for conccntration, 2. The factors may be evaluated more conveniently, however, for a small concentration, equivalent t o d.c. Then the factors and the sensitivity, So,for low concentrations are obtained by combining derivatives of Equations 12 and 13 t o give:
for the wire and thermistor, respectively. The bracketed quantity in Equation 14 is the conductivity factor for x = 1. For the case that RI = RAW,C equals C , or Ct. Values of EO could be estimated from the product of So and the concentration if response us. concentration were linear. Effect on Cell Factor. How the cell factor, C, changes with temperature of the sensing element is shown in Figure 2 , computed for cell-wall temperatures, eip, of 25’, looo, and 150’ C.
The solid curves are for a hot-wire cell in which R I ,in a reference chamber, equals Rni, Ro is 51 ohms, CY is 0004 per degree, and a is 1.61; the dashed curves are for a thermistor cell in nhich RL,in a reference chamber, equals RM, R1 is 2000 ohms, I?, is 4896 ohms, B is 3278 degrees, and a is 0.52. Both sets of conditions have been observed experimentally. For a given ew, the factor for the hot wire increases as OR - ew increases, whereas that for the thermistor goes through a maximum. As a result, the factor for the thermistor may drop below that is raised. For a for the hot wire as given eR-eN, on the other hand, the factor for the hot wire decreases somewhat as ero is raised, whereas that for the thermistor decreases more rapidly. Effect on Thermal-Conductivity Factor. How t h e conductivity factor changes n i t h temperature of t h e gas is shonii in Figure 3, computed for nine pairs of gases. Literature values wcre used for viscosity ( 5 ) and conductivity (15); examples of values for conductivity and calculated values for A for four mixtures are shoKn in Table I. Temperature dependence is determined principally by the ratio K I / K 1 . When the ratio is small, as with helium, differences in changes of Zil and K z with temperature have little effect; therefore the effect of temperature is small and almost the same for the four compounds shown. When the ratio is large, as with nitrogen or ethane, differences in the effect of tempcrature on I
-
Table 1.
Temp. of Gas, "C. 0
50 100 150 200
Values of Conductivity and
K2
A1,2
- 4 1 , A2.1 ~
K1/K2
A for Four M',xt ures K2
A1.z
A1,2 A2.1
Ki/Kz
Benzene in K1 2.16 3.00 4.11 5.44 6.89
34.1 37.7 40.8 43.8 46.8
Helium 0.253 1.538 0.262 1,600 0.271 1.617 0 278 1.633 0.285 1.648
4.33 5.87 7.68 9.63
Ethane 0.844 0.710 0.857 0.727 0.872 0 743 0.888 0.758
0.0635 0.0796 0.1009
0.1241 0.1471
5.74 6.59 7.41 8.24 9.02
Nitrogen 0.433 0.964 0,442 0,971 0.453 0.976 0.463 0.981 0.472 0.985
0.377 0,455 0.555 0.661 0.764
Nitrogen 0.570 0.670 0,590 0.689 0.610 0.705 0.629 0.720
0.586 0.604 0 653 0 710
Methanol i n 0
50 100 150
3.36 3 98 4.84 5.85
0.777 0.679 0.630 0.607 cal.
Values of KI and K2 given in units of
T
I
I
I
x 10-5.
(em.) (sec.) (' C.)
K1 and Kt are more significant; temperature has a large effect t h a t is different for the four compounds. With nitrogen, K1 increases faster with temperature than Kt and the factor becomes smaller with increasing temperature. With methanol in ethane, however, K 2 increases faster than K 1 , and the temperature dependence is opposite in slope. The product dl,z Az,l is affected little by temperature and contributes little to the net effect. Figure 3 also shows the effect of carrier gas on the conductivity factor. For mixtures with helium, the factor is large because Kt is larger than K1; for mixtures with nitrogen or ethane, the factor is small because K 2 is close to Kl.
I
5.74 6.59 7.41 8.24
' 1
Figure 7. Response for n-hexane in nitrogen
Thus, carrier gases of high thermal conductivity are preferred for maximum sensitivity in the detection of organic vapors (27). Because the conductivity of methanol is closer to that of the carrier gases, the factors for methanol mixtures are lower than for those with the hydrocarbons. Total Effect. How response changes with temperature may be predicted from the product of t h e two factors from Figures 2 and 3. T h e significant temperature for selecting a value of the condufitivity factor is the average of ex and ew. At a given ew, response for hot-wire cells will increase with eR-eW for gas mixtures with conductivity factors that increase with temperature (components in helium; methanol in ethane). Response will go through a maximum for mixtures with conductivity factors t h a t decrease with temperature (components innitrogen). With thermistor cells, response will go through a maximum in all cases because the cell factor goes through a maximum. At a given B R - ~ ~ response , for hot-
C /nROMTIOORAPHC i
1
I
0AND
RESPONSE
IS
CONCENTRATIOb
I
I
-
'
4
MILLIVOLTS
BAND ~
-2
0
2 n-HEXANE
JCONCENTRATION, MOL%
-
. ...... ..
I Figure 8. Effect of cell response on chromatographic band VOL. 32, NO. 3, MARCH 1960
e
347
Table
II.
Effect of Flow Rate on Response
(Hotwire cell) Response, Mv., at Flow Rate of Mole yo in Nitrogen 20 ml./min. 100 ml./niin. Benzene a t 300" OR and 150" Ow 7.85 9.32 9.31 4.51 4.63 4.60 2.56 1.85 1.85 1.03 0.11 0.12 0.55 -0 19 -0.20 0.30 -0 24 -0 24 0.16 -0 19 -0.21 Methanol at 225" eR and 25" ew 7.80 3.00 3.00 0.60 3.76 0.60 2.03 -0.04 -0.04 0.95 -1.41 -1.37 0.45 -0.07 -0.07 -
I
wire cells will decrease with increasing ew for mixtures with conductivity factors that decrease with temperature. On the other hand, response may increase or decrease with iiicreasing ea for mixtures with conductivity factors that increase n ith temperature, as governed by the relative rates of change of the tn-o factors; for the helium mixtures, response decreases with increasing Bw, but the opposite is true for methanol in ethane. Similar trends would be espectcd with thermistor cells. EXPERIMENTAL
Response of thermal-conductivity cells was measured for selected gas mixtures consisting of minor amounts of a component in major amounts of the carrier gas. T o study the effect of composition, the concentration of component was varied over the range usually found in gas chromatography. The caiI ier gases had either a high or a low conductivity. Two cells were used: one with a hot wire as the sensing element, the other with a thermistor; most measurements were made n ith the hot Lvire. T o study the effect of temperature, the cells were operated at, selected teniperatures of the \r-all and of the sensing element. The characteristics of the cells w r e measured to establish the teniperatures and to compute the cell factors. Each cell consisted of a brass block containing two 0.6-ml. chambers in which the sensing element and reference resistor were mounted. The block was heated by electrical tape connected to a variable transformer. An iron-constantan thermocouple, inserted in a well in the block, was used to determine wall tvmperature, Ow. The cells \\ere shielded from drafts with thermal insulation. Both cells have a short response time and low sensitivity to changes in the flow rate of the carrier gas (26). Evidence of low flow sensitivity was obtained 348
e
ANALYTICAL CHEMISTRY
I
I
I
I
-
I
0
2
4
6
8
M E T H A N O L CONCENTRATION, M O L %
Figure 9.
Response for methanol in nitrogen
by inensuring response of t'he hot-wire cell for various mixtures at two flow rates. .4s shown in Table 11, response for a given gas mixture and operating temperature was not affected significantly by a large change in flow rate, even for these gas mixtures, which have a greater heat capacity than nitrogen. Flow sensitivity decreases as flow rate drops (26); hence. no change in response would be expected below 20 d. per minute. The sensing element and reference resistor for the hot-wire cell were tungsten coils; the element and resistor for the thermistor cell were matched glass-coated thermistors (Fenwal GC 32L1). Resistances of the elements, mounted in the cells, were measured by means of a Wheatstone bridge a t 5 to 10 ambient temperatures in the range of 25" to 200". From these nieasurements, element characteristics needed for the response equation were calculated: For the hot wire, Ro is 51 ohms and a is 0.004 per degree; for the thermistor, Ro is 4896 ohms and B is 3278 degrees. From these data, the resistances of both elements n-ere caIculated for values of BR used in the experimental program. The current required to heat each element to n-as then determined for various levels of Ow and a given carrier gas by adjusting Z until the calculated resistance was obtained. Because cell geometry was not known accurately, the cell constants, a, were calculated from Equation 7 rather than from cell dimensions. To measure cell response, the elements were connected in separate bridge circuits similar to that in Figure 1. The hot-wire elements were connected in a series unbalanced bridge (8) as RdWand
RI, with fixed resistances Rz and R3 of 100 ohms each; the thermistor elements were in a parallel unbalanced bridge as R.~Iand Rz,with fixed resistances R1and R3 of 2000 ohms each. The circuits contained additional means for zero adjustment, equivalent to positioning slider S in Figure 1. The predetermined values of I for the desired OR were established by adjusting the total current to the bridge for the required voltage (IR.31) across RM. Response, EO, was measured potentiometrically. 3lixtures were prepared by admitting the constituents to selected prmsures in an evacuated vessel and stirring. Concentration of the Component in the em-rier gas was calculated from the pressure readings. Concentrations below S mole 70w r e prepared by dilution of higher concentrations. T o establish response CIS. composition for a given operating condition, measurements were made for four or more concentrations in the range of 0 to 8 mole %. By displacement .with mercury, each mixture was introduced into the thermal-conductivity cell. A fixed flow rate: usually 25 ml. per minute, was maintained until the cell was purged and a steady value of Eo was observed. Khen the effect of flov sensitivity was studied, flow rates of 20 and 100 ml. pcr minute were used. I n all cases, the steady-state operation of the cell and a zero Eu had previously been established with carrier gas at a flow ratorted, there has been no comprehensive investigation of the polarographic reduetion process. Among others, Pech ( 2 5 ) and Heyrovskf ( 5 ) reported the deposition potential of the ammonium ion and found that the reduction wave -vas well-defined. Podrouzek (26) and F’1”ek (17) pointed out that the wave n-as shifted in basic solution to more negative potentials. The latter author carried out oscillographic studies and concluded that the ammonium ion-ammonium amalgam half cell was reversible in basic solutions. An additional report that the polarographic reduction process was reversible was made by Kalousek (71 He employed a system whereby a m m u t a t o r alternately switched (5 LTHOUGII
352
e
ANALYTICAL CHEMISTRY
times per second) from a voltage a t which the reduction product was formed to a voltage a t mThich the product was oxidized. I n reversible electrolysis, the anodic process should cause a current of the same magnitude as in the cathodic process, but opposite in direction. By this criterion, ammonium ion reduction was considered by Kalousek to have been a reversible process. The product of reduction of ammonium ion a t mercury cathodes has been reported by numerous investigators (4, 12, 17) to be ammonium amalgam, and Johnson and Ubbelohde (6) were Lthle to show that ar:iillgnm formation , a t room ‘wuld he mad? to o c c u ~ ?:’en temper:i.ture. a t zi more rapid rate than the decomposition of the amalgam into ammonia and hydrogen. A systematic investigation iyas undertaken to determine the polarographic characteristics of ammcniuni ion and to determine, if possible, tht: electrode reaction :it the dropping njcrcury electrode. EXPERIMENTAL
Ammonium chloride solutions were prepared from Merck reagent grade material after recrystallization from water. Tetramethylammonium iodide n.as obtained from comnicrci>il sourcm and recrystnllized four times from water. This procedure was necessary to get a residual current less than i pa. a t -2.1 volts us S.C.E. .I0.2.11 dock sblution I( as prepared by diluting neighed nmounT:: to volume. Tetr.imethy1ammoniur.i hydroxide (?as prc.pared fro;n the iodi,ii;. I7reshly prepared and thoroughly washed silver Reagents,
ovide was added in slight excess to a saturated jca. 0.26M) solution of unrecrystallized iodide. The slurry was allowed to stand in darkness for 24 hours, then heated briefly to 70” C., cooled, and filtered two or three times through a sintered-glass funnel. The solution was then diluted to 0.2M and estimated by titration with standard perchloric acid solution. The hydroxide solution gave a negative test for iodide ion. The water used to prepare the solutions was distilled twice, the second distillation being from a solution of potassium hydroxide and permanganate. As a n added precaution, the tetramethylammonium iodide and hydroxide solutions were stored in polyethylene bottles. Equipment. The polarographic cell employed was an H-cell with a bulb-type saturated caloniel electrode (S.C.E.) (8, 1 1 ) . The dropping mercury electrode conipartment had a volume of approximatel!. 20 ml. to keep the volume of reagents a t a minimum. To prevent contamination of the solution in this compartment n ith potassium the agar bridge of ion from the S.C.E., the H-cell was made with C , l M tetramethylammonium iodide solution. The lower half of the vertical agar bridge to the S.C.E. was similarlj- prepared, n hile the upper half of the vertical agar bridge was made with 0.1M potassium chloride. The junction between the bridge of the H-cell and the bridge of the S.C.E. was made \vith 0.1X tetramethylammonium iodide solution, which was replaced with each succeeding run. The dropping mercury electrode had values of m2’3t1 of 0.837 mg.? sec.-1’2 a t -2.35 volts us. S.C.E. and 0.915 mg.213 set.-' a t -2.20 witq, the
