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An automatic recording constant pressure gas coulometer
JOdRXAL
8
AN
AUTOMATIC
OF
ELECTROXNALYTIC-4,X,
RECORDING
CHEhIISTRY
CONSTANT
VOL.
PRESSURE
1
(rg5g16o)
GAS
COULOMETER P.
Depavlnzent
L.
BOURGAULT
of Chemistry.
.tND
B
E.
CON\VAY
U12zvevsrty of Oltawa. Ottawa (Caizada)
In a number of applications, the necessity arises for measuring and continuously recording the vohume of gas evolved in a reaction, under conditions of constant pressure. In the course of studies on the rate of self-discharge of nickel-nickel o-side electrodesl, on the efficiency of charging such electrodes and on the rates of evolution of gaseous products of anodic electrolJ-sis of solutions of carb0.qlate salts”, we ha\-e developed an automatic recording coulometer for measuring rates of electrochemical gas evolution at constant pressure. It is the purpose of this paper to describe the construction and operation of this device. DESIGN
OF
THE
_.PPXRATUS
The apparatus shown in Fig. I consists of the electrochemical cell C (or any other vessel in which the gas evolution may be occurring) provided with a capillaq e-tit tube T wh.icL is joined by means of further capillary tubing to the measuring burette B and to a manometer hI with electrical contacts E and E’ which form part of a feed-back circuit for maintenance of constant pressure. The right-hand side of the
Fig. References
p_
12
I.
Schematic
diagram
of the recording .
gas coulometer.
. VOL.
1
(Igsg/6o)
AUTOBIATIC
RECORDING
GAS
COULOBIETER
9
manometer RI is closed at a known pressure with the mercury just making contact with the platinnm probe P. A circuit is made between the probe P and the electrical contact E’ through an electronic relay R, Fig. 2. Upon closin g the circuit by a rise of the mercury on the right-hand side of hI owing to build up of pressure in the apparatus C, the contact at P is closed and the relay R is activated and supplies current to a
5000
a= ‘:: R'
l-r
FJg.
2.
Electrical
clrcult.
powerful small motor MO which drives a screwed shaft S at a rate of about 1: r.p.m. The pitch of the thread on this shaft is about 0.5 mm. The shaft ;_; provided ~-ith an accurately machined nut which carries between guides a clamp holding a small reservoir of mercy connected to the lowx end of the gas burerte B by means of small diameter pressure tubing. So long as there is excess gas pressure in the burette B, the manostatic circuit between the manometer and the motor provides a drive which decreases the height of mercury in B until a constant pressure is restored in B equal to that in the closed right-hand limb of &I_The gas is therefore collected continuously at a constant pressure in B. The whole apparatus except the motor, shaft, and recorder is maintained m an air thermostat at 25 -J= 0.05~. Automatic recording of the volume of gas in B is achieved as follows. A fine platinum wire (0.001 in. diameter) is stretched longitudinally down the centre of the burette B and sealed at each end to the glass Z&~LOZL~ coxfact with the glass at arty poirrt along ifs Zengfh except at the ends. The upper end is connected to about 0.5 in. of thick platinum wire of negligible resistance which completes the seal through the glass and eliminates end-effects. The fine platinum wire is connected in series wirh a 12-V battery and a 5000 L2 variable resistance. The fine platinum wire has a resistance of about 15 S2, i.e. very small compared with the 5000 Q with which it is in series. An electronic recording potentiometer is connected as shown in Fig. z in parallel across the fine platinum wire in the burette. This wire then acts as a potential divider giving a potential difference proportional to tk amount of wire exposed above the level of the meniscus of mercury in the burette. I’Jhen the menisciis is moved automatically by the manostatic feed-back circuit as more gas is collected at constant pressure, the output potential difference across the wire gives a trace on the recorder. Calibration of the burette and adjustment of the current in the circuit by the variable resistance R’ then enables the full-scale displacement (20 mV) on the recorder to be made to correspond to the volume of the burette between the meniscus of mercury at its lowest level and the upper thick platinum contact. References
p.
12
P. L. BOURGXULT,
IO
SEKSIlIVITb-
B. E. CONAVAY AND
VOL.
1 (Ig5g/6o)
ACCUR%CS
The sensitivity of the instrument depends upon (i) the diameter of the burette, i.e. to upon the ratio LIZ/A 3. where 4.4 is the length of wire in the burette corresponding a volume .A V ; (ii) the ap p li e d voltage ZIor the current passing in the circuit from the battev; (iii) the channel sensIti\ity on the recorder, i.e., the p-d. required to cause fu&scale deflection of the pen ; and (iv) the dead space in the apparatus. Magnification of any- range of volumes to be measured In the burette can be readrly achieved b> increase of the voltage z or of the channel sensitivity. In the apparatus actually constructed, 2 ml was the maGrrmm volume measurable in the burette at full-scale on the recorder and changes in volume of 0.01 ml could be detected and recorded. Burettes larger than z ml could obviously be used in other applications. The dead space varied ikom about 4 to 6 ml dependmg on the volume of gas cohected. The accuracy of the instrument depends mainly on the closeness with which the pressure can be followed by the manostat and motor drive. The feed-back differential in the apparatus constructed w-as equivalent to about 0.005 to 0.01: ml but could be for e-sample, of the decreased b)- use of a more sensitive magnifyin, = manometer, ~.ith L-anous resistance wires, e.g. fine nichrome, showed inclined type a_ Experiments that linearity of response was very much dependent on the quality of xire used. Wire of precisely constant cross-section is desirable. Satisfactory results were only achieved by use of fine platinum
wire. RESULTS
Some
typical
records
of volume measurements and of the recorder response are of a recorder tracing, shows shown in Figs. 3 and 4. Fig. 3. which is a reproduction when there is a continuous that very satisfactoT linearity of response i, ac hieved linear mcrease in volume with time. This test was made by running the drive motor in B fell at a constant rate. disconnected from the manostat so that the mercury
rate of change of Fig. 3. Recorder trace showmg linearly of response in the case of a -form Upper line. increasmg volume with time. Lower line: decreasmg volume of gas in the burette volume xvlth time. Full-scale deflechon IS equivalent to 2 ml. Time is measured lmearly along References
p. I2
1 (rgsg;6o)
VOL.
AUTOIMATIC
RECORDING
GAS
COULOMETER
II
Fig. 4 shows the response during an actual kinetic run in which oxygen was tieing exrolved during self-discharge of a nickel-nickel o-side electrode on open circuitl. The halts in the response correspond to changes of volume somewhat -less than 0.01 ml.
i i --I
:: Fzg
4
Recorder
trace
of
osygen trode
evolution dumng at ~5~ Full-scale
self-&scharge of deflectlon is 2 ml
a
nickel-nickel
i ‘I oxide
’
+_
elec-
The instrument can therefore record changes as low as c 31 ml of o_xygen, hydrogen or other gases e\-olx ed. This is equix-alent to about o.o_cz 2 coulombs, xi-here 2 is the number of equivalenrs per mole of gas. Comparison v&h the sensitivity. for example, of the silver coulometer is impressi\-e since for the same nwnber of cou.Iombs it would be necessav to detect Sy weight changes of about $3.1 2 ,ug of silver. Pre-saturation of the solution lx-ith the gas whose \-olume is being measured is of course necessq if for the most accurate results and w-as carried out in the p. _ sent work. Howe\-er, of evol\-ed gases is small the electrolyte is concentrated (> I or 2 117) solubilit> owing
to salting-out. ACKXO\ZEDGEXIESTS
from the One of us (P.L.B.) wishes to a&now-ledge the a\\-ard of a Fellowship Consolidated Alining and Smelting Company held during the esecution of this work. \Ve are also indebted to the Defence Research Board, Department of National Defence, Canada, for support of this w-ork zm Grant NO. S&O-IP.
The construction and operation of a constant-pressure automatic recording gas coulometer for measuring the rates of electrochemical gas evolution in certain reaccons is described. The apparatus consists of a manostat which operates through a relay
and
Referezzces
automatically p. 12
controls
the pressure
at which ,
the gas is collected. Auto-
P. L. BOURGAULT,
12
VOL.
B. E. CONWAY
1
(Ig5g/6o)
smatic recording is achieved by using a fine wire in a mercury pipette as a potential divider, the height of mercury in the gas collecting vessel determining the potential drop which is fed to a recording potentiometer. RlkuN& Les auteurs d&-ivent la construction et le mode op&2toire d’un coulom&e 8 gaz Q pression con&ante avec enregistrement automatique pour la mesure de; la vitesse d’evolution de gaz electrochimique au tour< de certaines reactions. L’appareil consiste en un manostat qui opere par l’intermediaire d’un relais et qui regle autoautomatique ma tiquement la pression B laquelle le gaz est collect& L’enregistrement est r&li& par l’emploi d’un fil fin dans une pipette de mercure comme diviseur de potentiel, la hauteur de mercure dans le vase collecteur de gaz determinant la chute de potcntiel qui cst en&gist&e. ZUSKMMENFASSUNG
Aufbau und Arbeitsweise eines automatisch registierenden Gascoulometers fttr konstanten Druck zur Nessung der Geschwindigkeit der elektrochemischen Gasentwicklung bei gewissen Reaktionen werden beschrieben. Der Apparat besteht aus einem Manostat der von einem Relais bedient wird und welcher automatisch den Druck, bei dem das Gas aufgefangen wird, regelt. Die automat&he Registierung wird folgendermassen erzitlt : ein dtmner Draht in einer Quecksilberpipette dient als Potentialteiler, die Hohe des Quecksilbers in dem Gasauffanggefass bestimmt den Potentialsturz, der dann regisfxiert wird. REFERENCES 1 B. E. CONW_~X= AND 2 B. E. CON\\-_a~ AND 3 BI
BURTON,
Ind.
P. L. BOU~G_~ULT, 31. DZIECIUCH,
Eng.
CJIern , AszaC.
Cmr. J_ CAern..
~11 course
Ed
37 (1959) 292.
of publ~caiaon-
, g (1937) 335-
Received
April zznd,
1959
8
AN
AUTOMATIC
OF
ELECTROXNALYTIC-4,X,
RECORDING
CHEhIISTRY
CONSTANT
VOL.
PRESSURE
1
(rg5g16o)
GAS
COULOMETER P.
Depavlnzent
L.
BOURGAULT
of Chemistry.
.tND
B
E.
CON\VAY
U12zvevsrty of Oltawa. Ottawa (Caizada)
In a number of applications, the necessity arises for measuring and continuously recording the vohume of gas evolved in a reaction, under conditions of constant pressure. In the course of studies on the rate of self-discharge of nickel-nickel o-side electrodesl, on the efficiency of charging such electrodes and on the rates of evolution of gaseous products of anodic electrolJ-sis of solutions of carb0.qlate salts”, we ha\-e developed an automatic recording coulometer for measuring rates of electrochemical gas evolution at constant pressure. It is the purpose of this paper to describe the construction and operation of this device. DESIGN
OF
THE
_.PPXRATUS
The apparatus shown in Fig. I consists of the electrochemical cell C (or any other vessel in which the gas evolution may be occurring) provided with a capillaq e-tit tube T wh.icL is joined by means of further capillary tubing to the measuring burette B and to a manometer hI with electrical contacts E and E’ which form part of a feed-back circuit for maintenance of constant pressure. The right-hand side of the
Fig. References
p_
12
I.
Schematic
diagram
of the recording .
gas coulometer.
. VOL.
1
(Igsg/6o)
AUTOBIATIC
RECORDING
GAS
COULOBIETER
9
manometer RI is closed at a known pressure with the mercury just making contact with the platinnm probe P. A circuit is made between the probe P and the electrical contact E’ through an electronic relay R, Fig. 2. Upon closin g the circuit by a rise of the mercury on the right-hand side of hI owing to build up of pressure in the apparatus C, the contact at P is closed and the relay R is activated and supplies current to a
5000
a= ‘:: R'
l-r
FJg.
2.
Electrical
clrcult.
powerful small motor MO which drives a screwed shaft S at a rate of about 1: r.p.m. The pitch of the thread on this shaft is about 0.5 mm. The shaft ;_; provided ~-ith an accurately machined nut which carries between guides a clamp holding a small reservoir of mercy connected to the lowx end of the gas burerte B by means of small diameter pressure tubing. So long as there is excess gas pressure in the burette B, the manostatic circuit between the manometer and the motor provides a drive which decreases the height of mercury in B until a constant pressure is restored in B equal to that in the closed right-hand limb of &I_The gas is therefore collected continuously at a constant pressure in B. The whole apparatus except the motor, shaft, and recorder is maintained m an air thermostat at 25 -J= 0.05~. Automatic recording of the volume of gas in B is achieved as follows. A fine platinum wire (0.001 in. diameter) is stretched longitudinally down the centre of the burette B and sealed at each end to the glass Z&~LOZL~ coxfact with the glass at arty poirrt along ifs Zengfh except at the ends. The upper end is connected to about 0.5 in. of thick platinum wire of negligible resistance which completes the seal through the glass and eliminates end-effects. The fine platinum wire is connected in series wirh a 12-V battery and a 5000 L2 variable resistance. The fine platinum wire has a resistance of about 15 S2, i.e. very small compared with the 5000 Q with which it is in series. An electronic recording potentiometer is connected as shown in Fig. z in parallel across the fine platinum wire in the burette. This wire then acts as a potential divider giving a potential difference proportional to tk amount of wire exposed above the level of the meniscus of mercury in the burette. I’Jhen the menisciis is moved automatically by the manostatic feed-back circuit as more gas is collected at constant pressure, the output potential difference across the wire gives a trace on the recorder. Calibration of the burette and adjustment of the current in the circuit by the variable resistance R’ then enables the full-scale displacement (20 mV) on the recorder to be made to correspond to the volume of the burette between the meniscus of mercury at its lowest level and the upper thick platinum contact. References
p.
12
P. L. BOURGXULT,
IO
SEKSIlIVITb-
B. E. CONAVAY AND
VOL.
1 (Ig5g/6o)
ACCUR%CS
The sensitivity of the instrument depends upon (i) the diameter of the burette, i.e. to upon the ratio LIZ/A 3. where 4.4 is the length of wire in the burette corresponding a volume .A V ; (ii) the ap p li e d voltage ZIor the current passing in the circuit from the battev; (iii) the channel sensIti\ity on the recorder, i.e., the p-d. required to cause fu&scale deflection of the pen ; and (iv) the dead space in the apparatus. Magnification of any- range of volumes to be measured In the burette can be readrly achieved b> increase of the voltage z or of the channel sensitivity. In the apparatus actually constructed, 2 ml was the maGrrmm volume measurable in the burette at full-scale on the recorder and changes in volume of 0.01 ml could be detected and recorded. Burettes larger than z ml could obviously be used in other applications. The dead space varied ikom about 4 to 6 ml dependmg on the volume of gas cohected. The accuracy of the instrument depends mainly on the closeness with which the pressure can be followed by the manostat and motor drive. The feed-back differential in the apparatus constructed w-as equivalent to about 0.005 to 0.01: ml but could be for e-sample, of the decreased b)- use of a more sensitive magnifyin, = manometer, ~.ith L-anous resistance wires, e.g. fine nichrome, showed inclined type a_ Experiments that linearity of response was very much dependent on the quality of xire used. Wire of precisely constant cross-section is desirable. Satisfactory results were only achieved by use of fine platinum
wire. RESULTS
Some
typical
records
of volume measurements and of the recorder response are of a recorder tracing, shows shown in Figs. 3 and 4. Fig. 3. which is a reproduction when there is a continuous that very satisfactoT linearity of response i, ac hieved linear mcrease in volume with time. This test was made by running the drive motor in B fell at a constant rate. disconnected from the manostat so that the mercury
rate of change of Fig. 3. Recorder trace showmg linearly of response in the case of a -form Upper line. increasmg volume with time. Lower line: decreasmg volume of gas in the burette volume xvlth time. Full-scale deflechon IS equivalent to 2 ml. Time is measured lmearly along References
p. I2
1 (rgsg;6o)
VOL.
AUTOIMATIC
RECORDING
GAS
COULOMETER
II
Fig. 4 shows the response during an actual kinetic run in which oxygen was tieing exrolved during self-discharge of a nickel-nickel o-side electrode on open circuitl. The halts in the response correspond to changes of volume somewhat -less than 0.01 ml.
i i --I
:: Fzg
4
Recorder
trace
of
osygen trode
evolution dumng at ~5~ Full-scale
self-&scharge of deflectlon is 2 ml
a
nickel-nickel
i ‘I oxide
’
+_
elec-
The instrument can therefore record changes as low as c 31 ml of o_xygen, hydrogen or other gases e\-olx ed. This is equix-alent to about o.o_cz 2 coulombs, xi-here 2 is the number of equivalenrs per mole of gas. Comparison v&h the sensitivity. for example, of the silver coulometer is impressi\-e since for the same nwnber of cou.Iombs it would be necessav to detect Sy weight changes of about $3.1 2 ,ug of silver. Pre-saturation of the solution lx-ith the gas whose \-olume is being measured is of course necessq if for the most accurate results and w-as carried out in the p. _ sent work. Howe\-er, of evol\-ed gases is small the electrolyte is concentrated (> I or 2 117) solubilit> owing
to salting-out. ACKXO\ZEDGEXIESTS
from the One of us (P.L.B.) wishes to a&now-ledge the a\\-ard of a Fellowship Consolidated Alining and Smelting Company held during the esecution of this work. \Ve are also indebted to the Defence Research Board, Department of National Defence, Canada, for support of this w-ork zm Grant NO. S&O-IP.
The construction and operation of a constant-pressure automatic recording gas coulometer for measuring the rates of electrochemical gas evolution in certain reaccons is described. The apparatus consists of a manostat which operates through a relay
and
Referezzces
automatically p. 12
controls
the pressure
at which ,
the gas is collected. Auto-
P. L. BOURGAULT,
12
VOL.
B. E. CONWAY
1
(Ig5g/6o)
smatic recording is achieved by using a fine wire in a mercury pipette as a potential divider, the height of mercury in the gas collecting vessel determining the potential drop which is fed to a recording potentiometer. RlkuN& Les auteurs d&-ivent la construction et le mode op&2toire d’un coulom&e 8 gaz Q pression con&ante avec enregistrement automatique pour la mesure de; la vitesse d’evolution de gaz electrochimique au tour< de certaines reactions. L’appareil consiste en un manostat qui opere par l’intermediaire d’un relais et qui regle autoautomatique ma tiquement la pression B laquelle le gaz est collect& L’enregistrement est r&li& par l’emploi d’un fil fin dans une pipette de mercure comme diviseur de potentiel, la hauteur de mercure dans le vase collecteur de gaz determinant la chute de potcntiel qui cst en&gist&e. ZUSKMMENFASSUNG
Aufbau und Arbeitsweise eines automatisch registierenden Gascoulometers fttr konstanten Druck zur Nessung der Geschwindigkeit der elektrochemischen Gasentwicklung bei gewissen Reaktionen werden beschrieben. Der Apparat besteht aus einem Manostat der von einem Relais bedient wird und welcher automatisch den Druck, bei dem das Gas aufgefangen wird, regelt. Die automat&he Registierung wird folgendermassen erzitlt : ein dtmner Draht in einer Quecksilberpipette dient als Potentialteiler, die Hohe des Quecksilbers in dem Gasauffanggefass bestimmt den Potentialsturz, der dann regisfxiert wird. REFERENCES 1 B. E. CONW_~X= AND 2 B. E. CON\\-_a~ AND 3 BI
BURTON,
Ind.
P. L. BOU~G_~ULT, 31. DZIECIUCH,
Eng.
CJIern , AszaC.
Cmr. J_ CAern..
~11 course
Ed
37 (1959) 292.
of publ~caiaon-
, g (1937) 335-
Received
April zznd,
1959