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Coulometric titration of carbon dioxide on the micro scale
Talanta. 1966. Vol. 13. pp. 1303 to 1311.
Per~amon Press Ltd. Printed in Northern Ireland
COULOMETRIC TITRATION OF CARBON DIOXIDE ON THE MICRO SCALE D. C.
WHITE
Distillers Chemicals and Plastics Ltd., Research and Development Great Burgh, Epsom, Surrey, England.
Division,
(Received 7 March 1966. Accepted 2 May 1966) Summary-A method is described for the coulometric titration of milligram and submilligram amounts of carbon dioxide. Carbon dioxide is absorbed and titrated in dry acetone containing about 0.5 % of methanol and saturated with potassium iodide, in a two-compartment cell. Thymol Blue is used as visual indicator. Current integration is carried out by means of a low inertia motor or a solid-state integrator. The generation efficiency is 99400%. The method has been applied to the determination of small amounts of carbon (O-l mg) in organic and inorganic materials and in aqueous solution. INTRODUCTION
THE determination of carbon dioxide, produced by oxidation of organic carbon compounds in a micro combustion furnace, by absorption in dry acetone and continuous titration with sodium methoxide in dry methanol/pyridine, using Thymol Blue as indicator, has already been fully described. l A brief description was given of a technique for the coulometric generation of alkali, with current integration, which eliminated the need for the preparation and frequent standardisation of titrant. This technique has since been further investigated, and shown to be extremely reliable as a routine method for the determination of carbon in aqueous solutions, small organic samples etc., over a wide range of concentrations. The method has been in constant routine use for over a year. The carbon dioxide is absorbed in ‘dry’ acetone, in one half of a two-compartment cell with platinum electrodes (Fig. I), each half normally having a capacity of about 30 ml. The electrode in the absorption half is made the cathode at which alkali is generated on passage of current through the cell. Carbon dioxide may then be continuously titrated to the Thymol Blue end-point by passage of current. The current is integrated by a suitable device, such as a low inertia motor or an electronic integrator. A smaller size of cell is used for the determination of very small amounts of carbon dioxide. An alternative form of titration cell was previously described1 which employed a silver gauze anode and thus obviated the need for a double-compartment cell. Although this had the advantages of simplicity and low resistance, it was noticed that slow spontaneous generation of alkali took place in certain conditions, and this design was therefore abandoned in favour of the double-compartment cell which is more suitable for routine use. EXPERIMENTAL Apparatus Electrical supply. Any d.c. to 100 volts is suitable.
supply capable of supplyingup to 50 mA at a variable potential up 1303
D. C. WHRE
1304
Integrator. Two types are described under Development of Method. The circuit used with the low inertia motor is shown in Fig. 2. Titration cells. For the determination of carbon in the range O-l mg, a cell with a capacity of about 30 ml (to the top of the electrode in the absorption side) is used. The connections to the eiectrodes are suitably guarded so that the d.c. voltage may be connected via a safety plug. The cell is shown in Fig. 1 (larger dimensions). For smaller amounts of carbon in the range O-100 pg. a cell of one third the capacity is used, with a ceramic plug between the two halves instead of the sintered disc; Fig. 1 (smaller dimensions). Combustion furnace. A standard “empty tube” carbon-hydrogen furnace is used. Any other type of micro combustion furnace could be used, although the “empty tube” variety would seem to be the most suitable for analysing aqueous solutions.
A. B. C. D. E.
FIG. 1.-Titration cells Details for smaller cell are given in brackets. Sintered disc (ceramic plug), 10 mm. Number 1. Platinum spiral. 1 Sintered disc. 20 mm (10 mm). Number 1. B24 Socket. ’ . ’ Gas inlet tube, 60 mm (50 mm) long.
Reagents Dry acetone. The acetone used should preferably have a water content between 0.01 and 0.02 %. The recently introduced Dow desiccant 812 is a great improvement over Drierite or the molecular sieves which were used previously. This material, an ion-exchange resin in the potassium form, is very clean, effective and &adily regenerated. Analytical reagent grade acetone is passed slowly down an 18 x 3 cm column of the resin at cu. 12 ml/min. The dried acetone is kept in a screw-capued bottle over some of the resin (dry acetone is hygroscopic). The water content after several days-is usually 0.01%. The resin is regenerated by drying at 13.5-160” for 3 hr in open dishes. The acetone is saturated with potassium iodide by shaking with the crystals in a stoppered Bask in small quantities as required. Thvmol Blue indicator. A O*lO’/, solution in drv methanol. Should the water content of the acetone be below 0+010°~, sufficient-water should b;? added to the indicator solution to make the water content of the absorbent 0.010 to O-015%. Discard the solution after 1 week. Procedure General The combustion technique for solid and aqueous samples has already been described.’ Solid samples are weighed into a porcelain or platinum boat. Aqueous solutions are injected on to a silica wool plug from a micrometer syringe. When aualysing aqueous solutions it is preferable, especially when they contain volatile organic compounds, to close the combustion tube with a rubber injection
Coulometric titration of carbon dioxide on the micro scale
1305
septum through which the sample is injected. Solutions containing alkali metal carbonate may be analysed for total carbon by transferring them to a porcelain boat filled with ground silica. The titration cell is separated from the combustion furnace by a Flaschentrtiger tube filled with magnesium perchlorate or silica gel to remove water, and also by a manganese dioxide tube@if nitrogen is present in the sample. Silica gel is used when analysing aqueous solutions; when exhausted it should be thrown away and the tube refilled with fresh material. It has been found that if the exhausted silica gel is regenerated by heating, subsequent results may be high, repeated regeneration aggravating this behaviour. At the start of work the complete train is conditioned by cornbusting an unweighed quantity of benzoic acid, or a portion of an aqueous solution. Failure to do this may result in a faulty first result. Aqueous solutions containing more than about 1% of carbon may be more satisfactorily analysed by first diluting with boiled distilled water. Range O-l mg of carbon Use the larger titration cell, with the low inertia motor in parallel with a lkn resistor. Adjust the oxygen flow to about 30 ml/min. Fill both sides of the titration cell to the same level with acetone saturated with potassium iodide, and add 0.15 ml of indicator solution. Pass current at about 10 mA until the indicator reaches the lirst full blue colour. Turn off one of the taps of the drying tube (to prevent the acetone running back down the sinter when the combustion tube is opened), introduce the sample and restore the oxygen flow. Take up to 100 ~1 of aqueous solution or enough sample to contain about 5OOpg of carbon. Readjust the indicator colour if necessary. Zero the integrator and start a timer. Start the combustion and titrate the carbon dioxide continuously by passage of a suitable current, increasing it as necessav to 20-25 mA. The colour of the absorption solution should Should the carbon dioxide enter the not be allowed to go completely yellow except momentarily. cell too fast, the titration may be maintained at the desired rate by temporarily reducing the oxygen flow. When all the carbon dioxide has entered the cell (usually after 15 min) finally adjust the endpoint colour, and read the integrator and timer. Allow the oxygen to flow for a further 5-10 min, depending on the precision required, readjust the end-point colour, and read the integrator and timer. Calculate the drift and subtract it from the first reading to give the net integral equivalent to the carbon in the sample. Determine the value of the integral per mg of carbon (F) by combustion of known amounts of organic carbon, either as a solid or in aqueous solution. net integral X 100 % Carbon in sample = F x mgofsample’ Range O-100 pg of carbon The same method is used, but with the smaller cell. A larger resistor (2.2 kR) is used in parallel with the low inertia motor to provide greater sensitivity. Lack of linearity when using the motor at low current levels is overcome by titrating at a fixed current (2 mA). The oxygen flow and amount of indicator are adjusted accordingly. DISCUSSION
Reactions involved in the titration process Alkaliformation. Generation of alkali, which is virtually instantaneous on passage of current, takes place both in the presence and absence of 0.5% of methanol. Benzoic acid can be quantitatively directly titrated with 100% current efficiency in both cases. The overall efficiency for titration of carbon dioxide is 99.1%. Furthermore only traces of water (0.01 to 0.02%) are required. No evolution of gas is observable at the cathode during alkali generation and the acetone solution of the base is colourless. In view of this it is almost certain that formation of alkali takes place by direct reaction of potassium with acetone. It seems most likely that the main course of the reaction is direct reduction of acetone by potassium to form the basic potassium pinacolate, 2(CH3),C==0 + 2K - (CH&C-C(CH,),
& & 2K+
D. C. WHITJX
1306
Since acetone can exist in the enolic form, direct reaction with potassium to yield a potassium derivative with the formation of hydrogen is also possible, 2CH,=C-CHs bH
+ 2K --t 2CHFC-CH,
+ H,
&)K+
Although no evolution of gas can be observed at the cathode during formation of alkali, the hydrogen produced could directly reduce acetone to isopropanol or pinacol and so fail to appear in the gaseous form.2*3 The possibility of ketyl formation can be discounted as this is always associated with aromatic or heavily substituted and branched side chain aliphatic ketones, and all those described are coloured. 4*5,6 The ketyls of methyl and ethyl t-butyl ketones are stated to have only a transient existence.’ A further minor reaction is probably direct electrolytic reduction of acetone to isopropanol and pinacol. This would result in a complete loss of current as regards alkali formation. However, in view of the near 100% current efficiency of the alkali formation process, the direct reaction of potassium with acetone to form a base must play the major part and electrolytic reduction of acetone can only take place to a maximum of 1 y0 of the total. Titration of carbon dioxide. In the absence of methanol the solution does not become acid when carbon dioxide is passed into it, as shown by the failure of the indicator to change to more than a slightly paler blue, and the titration cannot be carried out. With methanol present the indicator changes to the yellow (acid) colour and titration proceeds normally. The effect may be demonstrated by adding a little solid Thymol Blue to acetone/potassium iodide in the cell and titrating to the blue end-point. Pass carbon dioxide into the solution until a slight fading of the indicator takes place. Now add one or two drops of dry methanol. The solution becomes yellow. The effect is not due to acidity in the methanol. When a similar amount of ethanol replaces methanol, carbon dioxide may still be titrated, although to a somewhat less definite end-point, and the recovery of carbon dioxide is too high. The need for a small amount of methanol for the successful titration of carbon dioxide may be because carbon dioxide reacts with methanol to form the methyl carbonate ion :* 0 CH,---OH + CO, + CH3-O-&H’ The methyl carbonate ion would then react with potassium ions from any of the bases suggested above, to form insoluble potassium methyl carbonate: 0
0
CH3-O-&H+
+ RzjK+ -+ CH3-O-$-OK
+ ROH
E$ect of water
Since water in other than very small amounts has an adverse effect on the titration by reducing the sharpness of the end-point and increasing the end-point drift, its concentration in the absorbent is kept to a minimum, preferably <0_02%.l When this is achieved, the end-point drift, for which allowance is made in accurate work,
Coulometric titration of carbon dioxide on the micro scale
1307
is equivalent to about 1 ‘A of the titration integral per 10 min. Very satisfactory results were obtained with batches of acetone dried to a final water content of CO15 %. When a drier acetone was used, with a water content of about O-005%, the recoveries of carbon from 1 mg amounts of bromobenzoic acid were variable and up to 14 % high. The magnitude of this effect depended largely on the humidity of the air (pick-up of water during cell filling) and was most noticeable when the humidity was low. It was demonstrated in a series of experiments (Table I) and was overcome by adding a small amount of water to the acetone in the cell, equivalent to O-01%. The effect is probably due to depression of ionisation. The magnitude of the end-point drift is dependent on the water concentration in the absorbent, and it can be seen that very low drift values (<5 rug carbon/IO min) are numerically related to faulty high values for carbon found. An abnormally low drift value, that is, less than 1% of the total titration integral, is therefore indicative of too low a water concentration. This source of error is easily avoided by adding sufficient water with the indicator to ensure that the minimum concentration in the absorbent (0*010-0~015%) is maintained. Integrator
A low inertia motor was initially used because of its ability to integrate varying currents over a fair range, and because of comparatively low price. The type chosen was the Electromethods Electrical Integrator Type 923/3, 24 volt, 10,000 cph. The integrator was connected as shown in Fig. 2. The integral is displayed on a clock-type dial with hands that can be reset. Since initial experiments indicated that the maximum current to be integrated would be about 30 mA, the integrator was operated in parallel with a 1 kSZ resistor in the circuit; the calibration was acceptably linear (&O.lSo/,) from 10 to 30 mA. The linearity fell off at currents below 10 mA and especially below 5 mA. The counter registered 109.60 units per coulomb at 20 mA. A solid-state electronic current integrator (Kent ‘Transdata’ d.c. electronic integrator) was used later as an alternative to the low inertia motor. This integrator can accept currents up to 30 mA, the integral being displayed on a counter unit that can be reset. The one used was linear within &O-2% between 25 and 1 mA, and &-O-l% between 20 and 5 mA, 798.0 counts per coulomb being obtained at 20 mA. Twelve of the values referred to under Precision for the titration of carbon dioxide from the combustion of bromobenzoic acid were obtained using both integrators in series. TABLE I.-EFFECT OF USING VERY DRY ACETONE AS ABSORBENT (WATER CONTIZNT ON THE RECOVERY OF CARBON, AND OF THE ADDITION OF WATER
Atmospheric condition Very dry
Carbon taken,
Carbon found,
Error,
to absorbent,
mg
v
%
%
0456 0.572 0599 0.673 0.575 0.616 0.635 0.632 0.659 0.598 0.552
0.467 0.650 0.628 0.669 0.578 0.616 0.633 0.670 0.662 0608 0.543
<0905%)
Water added
+2.6 t-13.7 t4.9 -0.6 +0.4 0.0 -0.3 +5.9 +0.4 +1-7 -1.6
-
Drift at end-point pg C/10 min 3.8 1.8
0.017 OTlO
2.3 9.5 7.0 9.0 6.7
0.010
3.4 5.6
0.017
4.3 7.4
1308
D. C. WHITE
Precision of the coulometric titration
As the sole interest in this technique lay in the possibility of determining carbon dioxide after combustion of organic carbon in various forms, the process was investigated by the combustion of l-2 mg quantities of bromobenzoic acid (B.D.H. Organic Analytical Standard) in an empty tube carbon-hydrogen furnace... To about 30 ml of dry acetone saturated with potassium iodide in the absorption side of the two-compartment cell was added O-15 ml of O-1% methanolic Thymol Blue. Accurately weighed amounts of bromobenzoic acid were introduced into the combustion tube and the initial acidity of the acetone titrated to the end-point. The samples were then cornbusted and the carbon dioxide titrated continuously using a generating current of 10 to 20 mA. Allowance was made for the small end-point drift, determined at the end of the titration, and the net integral per mg of carbon was calculated. Thirty such determinations were carried out at intervals over 12 months covering the period before and after the method was put into routine use.
FIG. 2.-Integrator circuit for low inertia motor R, = 2 kQ 25 W variable resistor. R, = 1 kR, 5 W dampingresistor. R, = 1 kQ, 5 W wire wound resistor. Other values may be switchedin for increased sensitivityat lower currents. S = Integrator on/off switch. I = Electromethodselectricalintegrator, type 923/3. M = Milliammeter,O-5mA and O-50mA ranges. With the low inertia motor the mean integral per mg of carbon was 889 units. After allowance for the weighing error the relative standard deviation of the titration and integration process was 0.47%. The standard deviation for carbon within the range taken, i.e., 0447 mg, after allowing for weighing error, was thus 3-O,ug. One determination of 0.9 mg of carbon (error $-O-4 pg) and one of l-25 mg of carbon (error +5*2 pg) were included. A slight improvement in the net precision is obtained using the electronic integrator. Thus, for the 12 determinations where the comparison was made, the relative standard deviation was 0.45 y0 with the low inertia motor and 0.36 % with the electronic integrator. Two series of experiments were also carried out using 100 ,ul aliquots of aqueous solutions of n-butanol (containing about 0.5% of carbon). The butanol had been fractionated and its water content determined. Six determinations were carried out on each solution (see Table II). The low inertia motor was used. The mean integral values/mg of carbon were 889 and 890 and in each case the relative standard deviation was 0.20%; the standard deviation of the weight of carbon found was 1 ,ug. It would appear therefore that the precision of the procedure is actualiy higher than that indicated by the experiments using bromobenzoic acid, and is about fO-2 %. This is possibly because the weighing error was larger than expected, though the bromobenzoic acid was slightly less than 100 % pure (for which allowance was made). The same precision is obtained in the direct titration of benzoic acid.
Coulometrictitration of carbon dioxide on the micro scale TABLIX II.-A~.uvsrs OFSYNTHBTIC carbon
Solution
*en.
Acetaldehyde
0293
Acetaldehyde
0258
Butanol
0553
Butanol
0516
%
AQUEOUS soLuTIoNs
Carbon found, % 0.296 0.296 0.297 0.297 0.263 0.261 0.261 0552 0552 0.554 0.554 0.554 0.552 0.518 0.517 0.516 0.516 0.516 0.518
Standard deviation
0.2964
O*OOl
0.2614
0.5527
O*ool
0.5169
OGOl
A further estimate of the precision was obtained as a result of the analyses, carried out over a period of time, of 12 aqueous solutions of organic materials which included some fairly volatile components. Each solution was diluted 5-fold and duplicate lOO+l samples taken for analysis. The amounts of carbon determined were between 0.2 and O-9mg, with the majority about O-5 mg; the standard deviation was 2-Opg. Possible loss of volatile organic compounds from aqueous samples was tested for by analysing acetaldehyde solutions prepared by weight from recently made acetaldehyde that was considered to be 100% pure. The results, corrected for the blank on the distilled water are shown in Table II. The positive error may be due to the volatility of the acetaldehyde causing its solutions to be slightly more concentrated than was calculated, because of displacement of air by acetaldehyde vapour before the solution was reweighed. Range of estimation The upper limit to the amount of carbon dioxide which can be titrated in a particular size of cell is determined by the fact that the indicator is adsorbed on the surface of the precipitate, thus making observation of the end-point more difhcult. The maximum amount of carbon which can be conveniently estimated in the 30-ml cell is about 1 mg. The smaller cell used in conjunction with a 2.2 IGR resistor and the low inertia motor for carbon in the range O-100 pg was tested by delivering various volumes of a solution of butanol(67 lug of carbon/100 ~1) into the combustion tube. (A submicro balance was not available for direct weighing of solid samples.) Allowance was made for the blank on the distilled water. The results are shown in Table III. Reversibility of ceN Although the cell is not truly reversible in the sense that iodine and not acid is liberated at the anode, in this particular non-aqueous system excess of iodine or alkali
1310
D. C. WHITE TABLEIII.-DETERMINATION OF CARBON,RANGE O-100 pg Carbon taken,
Carbon found,
Serifs
rg
I%
A
54.2 54.2 50.8 40.7 33.9 33.9 27.1 20.3 10.2 6.8 50.5 47.1 33.6 33.6 33.6 20.2 20.2 6.7 6.7
53.3 54.2 51.5 40.1 34.3 34.4 27.4 21.0 9.5 7.2 51.1 47.6 34.6 34.1 34.7 21.3 21.1 7.5 7.2
B
Error, Pg -0.9 +aO.; -0.6 +0.4 +0.5 $0.3 +0*7 -0.7 +0*4 +0.6 $0.5 +I.0 +0*5 il.1 +1*1 +0.9 +0.8 t-o.5
generated in one half of the cell may be back-titrated to the Thymol Blue end-point, over a limited range, by simply reversing the current flow. Although it does not occur in aqueous solution, it is supposed that in dry acetone the reaction 2RO-
+
+I, + OI- + RsO
goes to completion and the first excess of alkali over iodine is marked by complete disappearance of the iodine colour and a sharp rise in alkalinity as shown by the Thymol Blue indicator. This fact is of use if the carbon dioxide is overtitrated. After titration of the absorbent, with oxygen flowing, to the Thymol Blue endpoint, the electrodes were reversed and iodine was generated in the absorption half of the cell to an amount equivalent to about 113 pg of carbon (100 counter units). The direction of the current was again reversed and alkali generated till the end-point was reached. (The reversibility of the low inertia motor facilitates this.) The experiment was carried out both before and after titration of carbon dioxide, and in every case (5 experiments) the amount of alkali required to titrate the iodine back to the endpoint was correct to within the equivalent of 1 ,ug of carbon (1 counter unit). When excess of alkali is generated in the absorption side and then back-titrated with iodine by current reversal the results are not so good because some of the excess of alkali is lost, possibly due to a reaction such as 0 ROK + CH,--O-&-OK
-+ CH,--O-R
+ K&O3
analogous to KOH + KHCOa + K&Oa + H,O. (The same reaction may be the cause of the slow drift at the end-point.) The loss may amount to the equivalent of 24 pg of carbon. Despite this the reverse titration may be used to save an estimation which has been accidentally overtitrated, provided the excess of alkali is not too great (not more than
Coulometric titration of carbon dioxide on the micro scale
1311
TABLE IV.-BACK-TITRATION OF ALKALI EbKessof alkali generated (as rg of carbon)
36 34 45 z 37
First end-point, carbon found, pg
Second end-point after back-titrating excess alkali, carbon found, rg
552 463 185 607 479 493
555 462 185 604 481 499
The cell is reversed the equivalent of 50 ,ug of carbon) and is not long left u&rated. and iodine generated until the absorbent just returns to the colour of the acidic form. The current flow is then restored and the absorbent titrated to the proper end-point. The current reversal switch must be a point in the circuit that suits the integrator used. The procedure was tested by first locating the correct end-point and then generating excess of alkali and back-titrating it. The results are given in Table IV. Zusammenfamum-Fiir die coulometrische Titration von Milligramm- und Submilligrammengen von Kohlendioxyd wird eine Methode bescbrieben. Kohlendioxyd wird in trockenem Aceton, das ungefiihr O,S% Methanol enthlllt und mit Kaliumjodid gesitttigt ist, in einer zweigeteilten Zelle absorbiert und titriert. Thymolblau wird als visueller Indikator verwendet. Die Stromintegration wird mit Hilfe eines tragheitsarmen Motors oder eines festen Integrators ausgefti. Die Stromausbeute betragt 99-100%. Die Methode wurde auf die Bestimmung kleiner Mengen Kohlenstoff (O-l mg) in organischen und anorganischen Materialien und in wtiriger Lijsung angewandt. R&aun&Gn d&it une m&ode de dosage coulometrique du gax carbonioue aux echelles du milligmmme et de la fraction de milligramme: Le gax carbonique est &so&$ et dose dans L&tone s&he contenant environ 0,5 % de m&than01 et satur& d’iodure de potassium, dans une cellule a deux compartiments. On utilise le bleu de thymol comme indicateur visuel. L’integration du courant est met&e au moyen d’un moteur ii faible inertie ou d’un integrateur transistorid. Le rendement de l’ensemble est de 99-100%. On a applique la methode au dosage de petites quantites de carbone (O-l mg) dans des substances organiques et inorganiques et en solution aqueuse. REFERENCES 1. D. C. White, Tukmtu, 1963, 10, 727. 2. Beilstein’s Handbook of Organic Chemistry, Vol. 1. p. 639; Vol. 1, 1st supplement p. 340; Vol. 1, 3rd supplement p. 2720. 3. J. Schmidt, Organic Chemistry, 7th Ed., p. 564. Oliver and Boyd, London, 1955. 4. H. Noboru and S. I. Weissman, J. Am. Chem. Sot., 1960,82,4424. 5. Ann. Reports, Chem. Sot., 1925, 22, 124. 6. Ibid., 1934, 31,248, 250. 7. I. B. Nararov, Ann. Leningrad Stute Univ., Chem. Ser., 1935,1, 123 (Chem. Abs., 1937,31,6617). 8. A Review of M.E.A. Chemistry, p. 31, U.S. Naval Research Lab., June 1962. 9. R B&her and G. Ingram, Anal. Chim. Actu, 1950,4, 118.
Per~amon Press Ltd. Printed in Northern Ireland
COULOMETRIC TITRATION OF CARBON DIOXIDE ON THE MICRO SCALE D. C.
WHITE
Distillers Chemicals and Plastics Ltd., Research and Development Great Burgh, Epsom, Surrey, England.
Division,
(Received 7 March 1966. Accepted 2 May 1966) Summary-A method is described for the coulometric titration of milligram and submilligram amounts of carbon dioxide. Carbon dioxide is absorbed and titrated in dry acetone containing about 0.5 % of methanol and saturated with potassium iodide, in a two-compartment cell. Thymol Blue is used as visual indicator. Current integration is carried out by means of a low inertia motor or a solid-state integrator. The generation efficiency is 99400%. The method has been applied to the determination of small amounts of carbon (O-l mg) in organic and inorganic materials and in aqueous solution. INTRODUCTION
THE determination of carbon dioxide, produced by oxidation of organic carbon compounds in a micro combustion furnace, by absorption in dry acetone and continuous titration with sodium methoxide in dry methanol/pyridine, using Thymol Blue as indicator, has already been fully described. l A brief description was given of a technique for the coulometric generation of alkali, with current integration, which eliminated the need for the preparation and frequent standardisation of titrant. This technique has since been further investigated, and shown to be extremely reliable as a routine method for the determination of carbon in aqueous solutions, small organic samples etc., over a wide range of concentrations. The method has been in constant routine use for over a year. The carbon dioxide is absorbed in ‘dry’ acetone, in one half of a two-compartment cell with platinum electrodes (Fig. I), each half normally having a capacity of about 30 ml. The electrode in the absorption half is made the cathode at which alkali is generated on passage of current through the cell. Carbon dioxide may then be continuously titrated to the Thymol Blue end-point by passage of current. The current is integrated by a suitable device, such as a low inertia motor or an electronic integrator. A smaller size of cell is used for the determination of very small amounts of carbon dioxide. An alternative form of titration cell was previously described1 which employed a silver gauze anode and thus obviated the need for a double-compartment cell. Although this had the advantages of simplicity and low resistance, it was noticed that slow spontaneous generation of alkali took place in certain conditions, and this design was therefore abandoned in favour of the double-compartment cell which is more suitable for routine use. EXPERIMENTAL Apparatus Electrical supply. Any d.c. to 100 volts is suitable.
supply capable of supplyingup to 50 mA at a variable potential up 1303
D. C. WHRE
1304
Integrator. Two types are described under Development of Method. The circuit used with the low inertia motor is shown in Fig. 2. Titration cells. For the determination of carbon in the range O-l mg, a cell with a capacity of about 30 ml (to the top of the electrode in the absorption side) is used. The connections to the eiectrodes are suitably guarded so that the d.c. voltage may be connected via a safety plug. The cell is shown in Fig. 1 (larger dimensions). For smaller amounts of carbon in the range O-100 pg. a cell of one third the capacity is used, with a ceramic plug between the two halves instead of the sintered disc; Fig. 1 (smaller dimensions). Combustion furnace. A standard “empty tube” carbon-hydrogen furnace is used. Any other type of micro combustion furnace could be used, although the “empty tube” variety would seem to be the most suitable for analysing aqueous solutions.
A. B. C. D. E.
FIG. 1.-Titration cells Details for smaller cell are given in brackets. Sintered disc (ceramic plug), 10 mm. Number 1. Platinum spiral. 1 Sintered disc. 20 mm (10 mm). Number 1. B24 Socket. ’ . ’ Gas inlet tube, 60 mm (50 mm) long.
Reagents Dry acetone. The acetone used should preferably have a water content between 0.01 and 0.02 %. The recently introduced Dow desiccant 812 is a great improvement over Drierite or the molecular sieves which were used previously. This material, an ion-exchange resin in the potassium form, is very clean, effective and &adily regenerated. Analytical reagent grade acetone is passed slowly down an 18 x 3 cm column of the resin at cu. 12 ml/min. The dried acetone is kept in a screw-capued bottle over some of the resin (dry acetone is hygroscopic). The water content after several days-is usually 0.01%. The resin is regenerated by drying at 13.5-160” for 3 hr in open dishes. The acetone is saturated with potassium iodide by shaking with the crystals in a stoppered Bask in small quantities as required. Thvmol Blue indicator. A O*lO’/, solution in drv methanol. Should the water content of the acetone be below 0+010°~, sufficient-water should b;? added to the indicator solution to make the water content of the absorbent 0.010 to O-015%. Discard the solution after 1 week. Procedure General The combustion technique for solid and aqueous samples has already been described.’ Solid samples are weighed into a porcelain or platinum boat. Aqueous solutions are injected on to a silica wool plug from a micrometer syringe. When aualysing aqueous solutions it is preferable, especially when they contain volatile organic compounds, to close the combustion tube with a rubber injection
Coulometric titration of carbon dioxide on the micro scale
1305
septum through which the sample is injected. Solutions containing alkali metal carbonate may be analysed for total carbon by transferring them to a porcelain boat filled with ground silica. The titration cell is separated from the combustion furnace by a Flaschentrtiger tube filled with magnesium perchlorate or silica gel to remove water, and also by a manganese dioxide tube@if nitrogen is present in the sample. Silica gel is used when analysing aqueous solutions; when exhausted it should be thrown away and the tube refilled with fresh material. It has been found that if the exhausted silica gel is regenerated by heating, subsequent results may be high, repeated regeneration aggravating this behaviour. At the start of work the complete train is conditioned by cornbusting an unweighed quantity of benzoic acid, or a portion of an aqueous solution. Failure to do this may result in a faulty first result. Aqueous solutions containing more than about 1% of carbon may be more satisfactorily analysed by first diluting with boiled distilled water. Range O-l mg of carbon Use the larger titration cell, with the low inertia motor in parallel with a lkn resistor. Adjust the oxygen flow to about 30 ml/min. Fill both sides of the titration cell to the same level with acetone saturated with potassium iodide, and add 0.15 ml of indicator solution. Pass current at about 10 mA until the indicator reaches the lirst full blue colour. Turn off one of the taps of the drying tube (to prevent the acetone running back down the sinter when the combustion tube is opened), introduce the sample and restore the oxygen flow. Take up to 100 ~1 of aqueous solution or enough sample to contain about 5OOpg of carbon. Readjust the indicator colour if necessary. Zero the integrator and start a timer. Start the combustion and titrate the carbon dioxide continuously by passage of a suitable current, increasing it as necessav to 20-25 mA. The colour of the absorption solution should Should the carbon dioxide enter the not be allowed to go completely yellow except momentarily. cell too fast, the titration may be maintained at the desired rate by temporarily reducing the oxygen flow. When all the carbon dioxide has entered the cell (usually after 15 min) finally adjust the endpoint colour, and read the integrator and timer. Allow the oxygen to flow for a further 5-10 min, depending on the precision required, readjust the end-point colour, and read the integrator and timer. Calculate the drift and subtract it from the first reading to give the net integral equivalent to the carbon in the sample. Determine the value of the integral per mg of carbon (F) by combustion of known amounts of organic carbon, either as a solid or in aqueous solution. net integral X 100 % Carbon in sample = F x mgofsample’ Range O-100 pg of carbon The same method is used, but with the smaller cell. A larger resistor (2.2 kR) is used in parallel with the low inertia motor to provide greater sensitivity. Lack of linearity when using the motor at low current levels is overcome by titrating at a fixed current (2 mA). The oxygen flow and amount of indicator are adjusted accordingly. DISCUSSION
Reactions involved in the titration process Alkaliformation. Generation of alkali, which is virtually instantaneous on passage of current, takes place both in the presence and absence of 0.5% of methanol. Benzoic acid can be quantitatively directly titrated with 100% current efficiency in both cases. The overall efficiency for titration of carbon dioxide is 99.1%. Furthermore only traces of water (0.01 to 0.02%) are required. No evolution of gas is observable at the cathode during alkali generation and the acetone solution of the base is colourless. In view of this it is almost certain that formation of alkali takes place by direct reaction of potassium with acetone. It seems most likely that the main course of the reaction is direct reduction of acetone by potassium to form the basic potassium pinacolate, 2(CH3),C==0 + 2K - (CH&C-C(CH,),
& & 2K+
D. C. WHITJX
1306
Since acetone can exist in the enolic form, direct reaction with potassium to yield a potassium derivative with the formation of hydrogen is also possible, 2CH,=C-CHs bH
+ 2K --t 2CHFC-CH,
+ H,
&)K+
Although no evolution of gas can be observed at the cathode during formation of alkali, the hydrogen produced could directly reduce acetone to isopropanol or pinacol and so fail to appear in the gaseous form.2*3 The possibility of ketyl formation can be discounted as this is always associated with aromatic or heavily substituted and branched side chain aliphatic ketones, and all those described are coloured. 4*5,6 The ketyls of methyl and ethyl t-butyl ketones are stated to have only a transient existence.’ A further minor reaction is probably direct electrolytic reduction of acetone to isopropanol and pinacol. This would result in a complete loss of current as regards alkali formation. However, in view of the near 100% current efficiency of the alkali formation process, the direct reaction of potassium with acetone to form a base must play the major part and electrolytic reduction of acetone can only take place to a maximum of 1 y0 of the total. Titration of carbon dioxide. In the absence of methanol the solution does not become acid when carbon dioxide is passed into it, as shown by the failure of the indicator to change to more than a slightly paler blue, and the titration cannot be carried out. With methanol present the indicator changes to the yellow (acid) colour and titration proceeds normally. The effect may be demonstrated by adding a little solid Thymol Blue to acetone/potassium iodide in the cell and titrating to the blue end-point. Pass carbon dioxide into the solution until a slight fading of the indicator takes place. Now add one or two drops of dry methanol. The solution becomes yellow. The effect is not due to acidity in the methanol. When a similar amount of ethanol replaces methanol, carbon dioxide may still be titrated, although to a somewhat less definite end-point, and the recovery of carbon dioxide is too high. The need for a small amount of methanol for the successful titration of carbon dioxide may be because carbon dioxide reacts with methanol to form the methyl carbonate ion :* 0 CH,---OH + CO, + CH3-O-&H’ The methyl carbonate ion would then react with potassium ions from any of the bases suggested above, to form insoluble potassium methyl carbonate: 0
0
CH3-O-&H+
+ RzjK+ -+ CH3-O-$-OK
+ ROH
E$ect of water
Since water in other than very small amounts has an adverse effect on the titration by reducing the sharpness of the end-point and increasing the end-point drift, its concentration in the absorbent is kept to a minimum, preferably <0_02%.l When this is achieved, the end-point drift, for which allowance is made in accurate work,
Coulometric titration of carbon dioxide on the micro scale
1307
is equivalent to about 1 ‘A of the titration integral per 10 min. Very satisfactory results were obtained with batches of acetone dried to a final water content of CO15 %. When a drier acetone was used, with a water content of about O-005%, the recoveries of carbon from 1 mg amounts of bromobenzoic acid were variable and up to 14 % high. The magnitude of this effect depended largely on the humidity of the air (pick-up of water during cell filling) and was most noticeable when the humidity was low. It was demonstrated in a series of experiments (Table I) and was overcome by adding a small amount of water to the acetone in the cell, equivalent to O-01%. The effect is probably due to depression of ionisation. The magnitude of the end-point drift is dependent on the water concentration in the absorbent, and it can be seen that very low drift values (<5 rug carbon/IO min) are numerically related to faulty high values for carbon found. An abnormally low drift value, that is, less than 1% of the total titration integral, is therefore indicative of too low a water concentration. This source of error is easily avoided by adding sufficient water with the indicator to ensure that the minimum concentration in the absorbent (0*010-0~015%) is maintained. Integrator
A low inertia motor was initially used because of its ability to integrate varying currents over a fair range, and because of comparatively low price. The type chosen was the Electromethods Electrical Integrator Type 923/3, 24 volt, 10,000 cph. The integrator was connected as shown in Fig. 2. The integral is displayed on a clock-type dial with hands that can be reset. Since initial experiments indicated that the maximum current to be integrated would be about 30 mA, the integrator was operated in parallel with a 1 kSZ resistor in the circuit; the calibration was acceptably linear (&O.lSo/,) from 10 to 30 mA. The linearity fell off at currents below 10 mA and especially below 5 mA. The counter registered 109.60 units per coulomb at 20 mA. A solid-state electronic current integrator (Kent ‘Transdata’ d.c. electronic integrator) was used later as an alternative to the low inertia motor. This integrator can accept currents up to 30 mA, the integral being displayed on a counter unit that can be reset. The one used was linear within &O-2% between 25 and 1 mA, and &-O-l% between 20 and 5 mA, 798.0 counts per coulomb being obtained at 20 mA. Twelve of the values referred to under Precision for the titration of carbon dioxide from the combustion of bromobenzoic acid were obtained using both integrators in series. TABLE I.-EFFECT OF USING VERY DRY ACETONE AS ABSORBENT (WATER CONTIZNT ON THE RECOVERY OF CARBON, AND OF THE ADDITION OF WATER
Atmospheric condition Very dry
Carbon taken,
Carbon found,
Error,
to absorbent,
mg
v
%
%
0456 0.572 0599 0.673 0.575 0.616 0.635 0.632 0.659 0.598 0.552
0.467 0.650 0.628 0.669 0.578 0.616 0.633 0.670 0.662 0608 0.543
<0905%)
Water added
+2.6 t-13.7 t4.9 -0.6 +0.4 0.0 -0.3 +5.9 +0.4 +1-7 -1.6
-
Drift at end-point pg C/10 min 3.8 1.8
0.017 OTlO
2.3 9.5 7.0 9.0 6.7
0.010
3.4 5.6
0.017
4.3 7.4
1308
D. C. WHITE
Precision of the coulometric titration
As the sole interest in this technique lay in the possibility of determining carbon dioxide after combustion of organic carbon in various forms, the process was investigated by the combustion of l-2 mg quantities of bromobenzoic acid (B.D.H. Organic Analytical Standard) in an empty tube carbon-hydrogen furnace... To about 30 ml of dry acetone saturated with potassium iodide in the absorption side of the two-compartment cell was added O-15 ml of O-1% methanolic Thymol Blue. Accurately weighed amounts of bromobenzoic acid were introduced into the combustion tube and the initial acidity of the acetone titrated to the end-point. The samples were then cornbusted and the carbon dioxide titrated continuously using a generating current of 10 to 20 mA. Allowance was made for the small end-point drift, determined at the end of the titration, and the net integral per mg of carbon was calculated. Thirty such determinations were carried out at intervals over 12 months covering the period before and after the method was put into routine use.
FIG. 2.-Integrator circuit for low inertia motor R, = 2 kQ 25 W variable resistor. R, = 1 kR, 5 W dampingresistor. R, = 1 kQ, 5 W wire wound resistor. Other values may be switchedin for increased sensitivityat lower currents. S = Integrator on/off switch. I = Electromethodselectricalintegrator, type 923/3. M = Milliammeter,O-5mA and O-50mA ranges. With the low inertia motor the mean integral per mg of carbon was 889 units. After allowance for the weighing error the relative standard deviation of the titration and integration process was 0.47%. The standard deviation for carbon within the range taken, i.e., 0447 mg, after allowing for weighing error, was thus 3-O,ug. One determination of 0.9 mg of carbon (error $-O-4 pg) and one of l-25 mg of carbon (error +5*2 pg) were included. A slight improvement in the net precision is obtained using the electronic integrator. Thus, for the 12 determinations where the comparison was made, the relative standard deviation was 0.45 y0 with the low inertia motor and 0.36 % with the electronic integrator. Two series of experiments were also carried out using 100 ,ul aliquots of aqueous solutions of n-butanol (containing about 0.5% of carbon). The butanol had been fractionated and its water content determined. Six determinations were carried out on each solution (see Table II). The low inertia motor was used. The mean integral values/mg of carbon were 889 and 890 and in each case the relative standard deviation was 0.20%; the standard deviation of the weight of carbon found was 1 ,ug. It would appear therefore that the precision of the procedure is actualiy higher than that indicated by the experiments using bromobenzoic acid, and is about fO-2 %. This is possibly because the weighing error was larger than expected, though the bromobenzoic acid was slightly less than 100 % pure (for which allowance was made). The same precision is obtained in the direct titration of benzoic acid.
Coulometrictitration of carbon dioxide on the micro scale TABLIX II.-A~.uvsrs OFSYNTHBTIC carbon
Solution
*en.
Acetaldehyde
0293
Acetaldehyde
0258
Butanol
0553
Butanol
0516
%
AQUEOUS soLuTIoNs
Carbon found, % 0.296 0.296 0.297 0.297 0.263 0.261 0.261 0552 0552 0.554 0.554 0.554 0.552 0.518 0.517 0.516 0.516 0.516 0.518
Standard deviation
0.2964
O*OOl
0.2614
0.5527
O*ool
0.5169
OGOl
A further estimate of the precision was obtained as a result of the analyses, carried out over a period of time, of 12 aqueous solutions of organic materials which included some fairly volatile components. Each solution was diluted 5-fold and duplicate lOO+l samples taken for analysis. The amounts of carbon determined were between 0.2 and O-9mg, with the majority about O-5 mg; the standard deviation was 2-Opg. Possible loss of volatile organic compounds from aqueous samples was tested for by analysing acetaldehyde solutions prepared by weight from recently made acetaldehyde that was considered to be 100% pure. The results, corrected for the blank on the distilled water are shown in Table II. The positive error may be due to the volatility of the acetaldehyde causing its solutions to be slightly more concentrated than was calculated, because of displacement of air by acetaldehyde vapour before the solution was reweighed. Range of estimation The upper limit to the amount of carbon dioxide which can be titrated in a particular size of cell is determined by the fact that the indicator is adsorbed on the surface of the precipitate, thus making observation of the end-point more difhcult. The maximum amount of carbon which can be conveniently estimated in the 30-ml cell is about 1 mg. The smaller cell used in conjunction with a 2.2 IGR resistor and the low inertia motor for carbon in the range O-100 pg was tested by delivering various volumes of a solution of butanol(67 lug of carbon/100 ~1) into the combustion tube. (A submicro balance was not available for direct weighing of solid samples.) Allowance was made for the blank on the distilled water. The results are shown in Table III. Reversibility of ceN Although the cell is not truly reversible in the sense that iodine and not acid is liberated at the anode, in this particular non-aqueous system excess of iodine or alkali
1310
D. C. WHITE TABLEIII.-DETERMINATION OF CARBON,RANGE O-100 pg Carbon taken,
Carbon found,
Serifs
rg
I%
A
54.2 54.2 50.8 40.7 33.9 33.9 27.1 20.3 10.2 6.8 50.5 47.1 33.6 33.6 33.6 20.2 20.2 6.7 6.7
53.3 54.2 51.5 40.1 34.3 34.4 27.4 21.0 9.5 7.2 51.1 47.6 34.6 34.1 34.7 21.3 21.1 7.5 7.2
B
Error, Pg -0.9 +aO.; -0.6 +0.4 +0.5 $0.3 +0*7 -0.7 +0*4 +0.6 $0.5 +I.0 +0*5 il.1 +1*1 +0.9 +0.8 t-o.5
generated in one half of the cell may be back-titrated to the Thymol Blue end-point, over a limited range, by simply reversing the current flow. Although it does not occur in aqueous solution, it is supposed that in dry acetone the reaction 2RO-
+
+I, + OI- + RsO
goes to completion and the first excess of alkali over iodine is marked by complete disappearance of the iodine colour and a sharp rise in alkalinity as shown by the Thymol Blue indicator. This fact is of use if the carbon dioxide is overtitrated. After titration of the absorbent, with oxygen flowing, to the Thymol Blue endpoint, the electrodes were reversed and iodine was generated in the absorption half of the cell to an amount equivalent to about 113 pg of carbon (100 counter units). The direction of the current was again reversed and alkali generated till the end-point was reached. (The reversibility of the low inertia motor facilitates this.) The experiment was carried out both before and after titration of carbon dioxide, and in every case (5 experiments) the amount of alkali required to titrate the iodine back to the endpoint was correct to within the equivalent of 1 ,ug of carbon (1 counter unit). When excess of alkali is generated in the absorption side and then back-titrated with iodine by current reversal the results are not so good because some of the excess of alkali is lost, possibly due to a reaction such as 0 ROK + CH,--O-&-OK
-+ CH,--O-R
+ K&O3
analogous to KOH + KHCOa + K&Oa + H,O. (The same reaction may be the cause of the slow drift at the end-point.) The loss may amount to the equivalent of 24 pg of carbon. Despite this the reverse titration may be used to save an estimation which has been accidentally overtitrated, provided the excess of alkali is not too great (not more than
Coulometric titration of carbon dioxide on the micro scale
1311
TABLE IV.-BACK-TITRATION OF ALKALI EbKessof alkali generated (as rg of carbon)
36 34 45 z 37
First end-point, carbon found, pg
Second end-point after back-titrating excess alkali, carbon found, rg
552 463 185 607 479 493
555 462 185 604 481 499
The cell is reversed the equivalent of 50 ,ug of carbon) and is not long left u&rated. and iodine generated until the absorbent just returns to the colour of the acidic form. The current flow is then restored and the absorbent titrated to the proper end-point. The current reversal switch must be a point in the circuit that suits the integrator used. The procedure was tested by first locating the correct end-point and then generating excess of alkali and back-titrating it. The results are given in Table IV. Zusammenfamum-Fiir die coulometrische Titration von Milligramm- und Submilligrammengen von Kohlendioxyd wird eine Methode bescbrieben. Kohlendioxyd wird in trockenem Aceton, das ungefiihr O,S% Methanol enthlllt und mit Kaliumjodid gesitttigt ist, in einer zweigeteilten Zelle absorbiert und titriert. Thymolblau wird als visueller Indikator verwendet. Die Stromintegration wird mit Hilfe eines tragheitsarmen Motors oder eines festen Integrators ausgefti. Die Stromausbeute betragt 99-100%. Die Methode wurde auf die Bestimmung kleiner Mengen Kohlenstoff (O-l mg) in organischen und anorganischen Materialien und in wtiriger Lijsung angewandt. R&aun&Gn d&it une m&ode de dosage coulometrique du gax carbonioue aux echelles du milligmmme et de la fraction de milligramme: Le gax carbonique est &so&$ et dose dans L&tone s&he contenant environ 0,5 % de m&than01 et satur& d’iodure de potassium, dans une cellule a deux compartiments. On utilise le bleu de thymol comme indicateur visuel. L’integration du courant est met&e au moyen d’un moteur ii faible inertie ou d’un integrateur transistorid. Le rendement de l’ensemble est de 99-100%. On a applique la methode au dosage de petites quantites de carbone (O-l mg) dans des substances organiques et inorganiques et en solution aqueuse. REFERENCES 1. D. C. White, Tukmtu, 1963, 10, 727. 2. Beilstein’s Handbook of Organic Chemistry, Vol. 1. p. 639; Vol. 1, 1st supplement p. 340; Vol. 1, 3rd supplement p. 2720. 3. J. Schmidt, Organic Chemistry, 7th Ed., p. 564. Oliver and Boyd, London, 1955. 4. H. Noboru and S. I. Weissman, J. Am. Chem. Sot., 1960,82,4424. 5. Ann. Reports, Chem. Sot., 1925, 22, 124. 6. Ibid., 1934, 31,248, 250. 7. I. B. Nararov, Ann. Leningrad Stute Univ., Chem. Ser., 1935,1, 123 (Chem. Abs., 1937,31,6617). 8. A Review of M.E.A. Chemistry, p. 31, U.S. Naval Research Lab., June 1962. 9. R B&her and G. Ingram, Anal. Chim. Actu, 1950,4, 118.