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Rapid method for the microdetermination of chlorine in organic compounds involving combustion and automatic acidimetric coulometry
MICR0CHE~MlCY.L
JOURNAL.
16, 277-285
( 197 1)
Rapid Method for the Microdetermination of Chlorine in Organic Compounds Involving Combustion anld Automatic Acidimetric Coulometry’ E. DEBAL
AND R.
LEVY
Service Central de Microanalyse du Centre National Scientifique, 2 rue Henri Dunant, 94-Thiais, Received
December
de /a Recherche France
26, 1970
INTRODUCTION
This method is intended to render possible the rapid microdetermination of chlorine in organic compounds on milligram samples. Preference has been given to milligram analysis (instead of decimilligram), contrary to Walisch and Jaenicke (4), in order to avoid difficulties in weighing air-unstable substances. A 2.5-5-mg sample is instantaneously combusted in an empty silica tube of the Ingram type (3) flowed with a 60-ml/minute stream of moist oxygen at a temperature of 1000°C. A dry oxygen flow of 400 ml/ minute is then substituted for the moist oxygen flow in order to quantitatively sweep hydrochloric acid and chlorine produced through combustion into the cathodic compartment of the cell of an automatic coulometer in whose aqueous electrolyte these gases are absorbed. The apparatus is the Schoeps? CTA 5 S coulometer originally designed for sulphur determination in steel and already used in our laboratory for the determination of sulphur in organic compounds (2). Chlorine is reduced to hydrochloric acid by hydrogen peroxide in the absorbing solution of the cathodic compartment and the hydrochloric acid protons lower the pH of this solution. A glass electrode-reference electrode system transmits the pH variation to the apparatus and when the combustion is over, it starts automatically the electrolysis which neutralizes the acidic solution and thus causes the pH to rise. When the latter is back to its initial value, the electrolysis is stopped automatically. The quantity of electricity consumed is obtained by means of the discharges of an impulse condenser which controls the readings on a 1 Presented at the International Symposium on Microtechniques, VIth, tember 7-l 1, 1970, Graz, Austria. 2 Richard Schoeps, 41, Duisburg Reck, German Federal Republic. 277
Sep-
278
DEBAL
AND
LEVY
digital counter. The apparatus is calibrated through the analysis of test substances with known chlorine contents as in the case of sulphur microdetermination. The coefficient found and used is close to the theoretical one. Carbon dioxide in the combustion gases does not interfere, as it escapeswith bubbling oxygen from the electrolyte the pH of which has been deliberately fixed at 5. Acidic combustion products of nitrogen are not decomposed in the combustion chamber; they must be decomposed or retained in traps between the latter and the absorbing electrolyte. One determination within a series lasts from 12 to 13 minutes. MATERIALS
AND
METHODS
Apparatus General design, combustion chamber, traps, electrochemical
cell. See
figures 1, 2, 3, 4. Foreparts. The foreparts of the apparatus are not shown in Fig. 1. Oxygen from a steel cylinder (Air Liquide Cy) and a pressure reducer is admitted through a pressure regulator with an escape of excess gas at the lower end of a T tube about 110 cm under the level of liquid paraffin. It is purified in a glass tube (20 cm in length and 2 cm in diameter) filled half and half with soda asbestos (ascarite) and magnesium perchlorate (anhydron) and flows through a rotameter and a three-way stopcock which makes it possible to use either dry or moist oxygen during the determination. One way lets dry oxygen flow directly into the combustion tube through stopcock No. 2; another way lets it flow through a reducing valve and bubble some millimeters in depth in water at room temperature (23-25’C), in a Durand flask of about 100 ml, and then to stopcock No. 2. PROCEDURE
The procedure described below is quite similar to the one used for sulphur determination: The preparation for use of the coulometric cell, the adjustment of the coulometer, the carrying out of three preliminary combustions, and the precautionary measures for the analysis of substanceswith low chlorine contents are similar in both cases 1. Stop air cooling (see 9 below), open the combustion tube and let down the sample holder (4-6, Figs. 1 and 2) in the extension tube of the ground stopper (5, Figs. 1 and 2) on a silica rack. 2. Adjust the electrolysis time switch at 12 minutes. 3. Place the boat or the capillary with sample in the sample holder already lying in the extension which makes it necessary to use a set of two sample holders and two extensions). 4. Put the stopper with extension tube and sample on the combustion tube.
RAPID
MICRODETERMINATION
OF
CHLORINE
279
5. Push the sample holder with a ring magnet so that the sample is placed in the center of the combustion chamber (8. Figs. 1 and 2). 6. Extract the boat or capillary used for the former determination. 7. Calculate the result for the former determination. 8. Substitute the dry oxygen flow (400 ml/minute) for the moist one (60 ml/minute) by turning the three-way stopcock 3 minutes after the santple irztuoductiorz into the combustion chamber. 9. Extract the sample holder from the combustion chamber using the magnet 5 minutes after the introducfiozz of the sample: let it lie in its initial place (Fig. I) and cool it with air (from compressor or fan). 10. Weigh the sample for the next determination. 11. Turn off stopcock No. 2 (Fig. 1) 9 minutes after the introduction of the sample for a few seconds, and turn it on again (this breaking of the oxygen flow causes the absorbing-liquid level to rise in Tube 14 and wash it). When the time switch is at 0 again, it starts the electrolysis automatically, and the pH value grows again after having decreased during the gas absorption. 12. Substitute the moist oxygen flow (60 ml/minute) for the dry one. 13. Read number N on the counter. 14. Begin and follow again the former sequence of operations for the next determination.
The chlorine content is calculated after the formula
where m is the sample weight in mg, and f is the calibration coefficient in micrograms per counter round (f value is about 2.212).
FIG. 1. General design: (1) Dry oxygen inlet; (1’) Moist oxygen inlet; (2) Stopcock; (3) Iron cylinder; (4) and (6) Sample holder; (5) Extension; (7) Boat; (8) Ingram combustion chamber; (9) U-tube absorber (H,$O,); (10) Absorber (anhydrone) ; ( 11) Holes; (12) Diluting bulbs; ( 13) U-tube absorber (silica gel + H,SO,); (14) Bubbling tube in the titration cell; (15) Titration cell; and (16) 1000°C furnace.
280
DEBAL
AND
LEVY
DISCUSSION
This technique for chlorine determination was developed after testing for this purpose the one used for sulphur (2) and meeting the following difficulties. The simultaneous presence of chlorine and nitrogen in a molecule often gives high results probably correlated with the formation of nitrosyl chloride which is hydrolyzed in the cathodic compartment (the error may be important when nitrogen comes from a NO, group). The 13 lo1320°C furnace, as,is used for sulphur determination, the temperature of which is such as to get rid of acidic nitrogen compounds and their interference with the acidimetric titration, does not eliminate a possible nitrosyl chloride formation. For chlorine determination, the use of an Ingram combustion chamber at 1000°C has been chosen because it makes it possible not only, as in the former case, to carry out flash combustions, but requires a much simpler furnace with a Kanthal-wound heating element, is of a much lower cost, occupies a much smaller room, and may be kept permanently at working temperature (1000°C). When chlorine concentration in the combustion gasesis too high, it is not reduced rapidly enough by hydrogen peroxide, escapesfrom the cell,
50
i-
100
-_
120 -wo -r-
id 3 ii
dimensions : mm
60
s
od6
d6J
-od9
T id 7 L
t ,
L
%$FIG. 2. Parts of Fig. 1: (3) Iron cylinder; (4) ent silica glass); (5) Extension with ground-joint ple holder cavity for sample (transparent silica combustion chamber (transparent silica glass); dard taper.
Sample holder handle (transparstopper (Pyrex glass) ; (6) Samglass; (7) Boat; and (8) Ingram 3 = Stantemperature 1000°C.
RAPID
MICRODETERMINATION
OF
CHLORINE
281
and gives low results which are observed when dry oxygen is used. Such errors disappear with the use of moist oxygen which favors hydrochloric acid formation and lowers chlorine concentration. However, recent tests have shown that good results might be obtained with dry oxygen (with 60- and 400-ml/minute flows) provided the chlorine concentration in combustion gases is lowered and its bubbling in the absorbing solution made slower with no attention being paid to the proportion HCl:Cl,; such conditions are obtained by pressure drops in the gas flow and gas dilution by the use of three appropriate bulbs. The electrolyte and reagents in the electrochemical cell of the commercial Schoeps coulometer are convenient for the titration of sulphuric acid H+ ions but not for those of hydrochloric acid; with the latter, analytical results are no longer reproducible when hydrochloric acid concentration is growing too rapidly in the cathodic compartment. Such a drawback disappears when half of the volumes of the sodium sulphate solutions in the anodic and cathodic compartments are replaced by equal volumes of sodium chloride solutions with 15 and 5 vol % concentrations, respectively. This modified cell may be used either for chlorine or sulphur determinations. For its association with the acidimetric coulometric titration, the combustion technique described by Ingram (3) must be modified as follows : Moist oxygen is used instead of dry oxygen and the combustion chamber is heated up to 1000°C instead of 850°C to make the proportion HCl : Cl, higher. Static combustion is prohibited as it gives low results because of Ihe retrodiffusion and retention of hydrochloric acid in the back extension of the combustion tube. Solid samples and liquid ones (with low vapor pressure) in frittedalumina boats or quartz capillaries (2) are placed inside the hollow end of a sample holder (6, Figs. 1 and 2) the depth of which (30 mm only) is smaller than the one of the similar holder used for sulphur determination. This hollow carrier is fused together with a silica tube (4, Figs. 1 and 2) in which a small iron bar (3, Figs. 1 and 2) similar to Ingram’s is enclosed. Traps (the use of which is not necessary when nitrogen is absent) are described below. U tube. This absorber contains sulphuric acid which has to be renewed daily. It holds back the major part of water in the gas, reacts with nitrosyl chloride, and frees hydrochloric acid. Attention must be paid to the quality of sulphuric acid used; only newly-opened bottles should be used, otherwise bad results may be obtained (9, Figs. 1 and 3).
282
DEBAL
AND
LEVY
FIG. 3. Traps and diluting bulbs (Pyrex glass): (9) U-tube absorber with a filling of Pyrex glass beads (3 mm diameter) and sulphuric acid (1.8 ml analytical grade concentrated acid, 1.83 density); (10) Absorber with a filling of anhydrone (IO-20 mesh); (11) One mm diameter holes, (12) Diluting bulbs; and (13) U-tube absorber, wound with a heating cord (4O”C), with a filling of “sulphuric” silicagel. $ = Standard taper.
Straight tube. This absorber is filled with magnesium perchlorate which is renewed every 2 weeks. This filling brings the gasesto full dryness, thus making possible a more sweeping of the apparatus (10, Figs. 1 and 3). Bulbs. Gases are diluted in the bulbs, which favors the oxidation of nitrogen monoxide to dinitrogen tetroxide and the latter to be retained on the next filling. ( 12, Figs. 1 and 3). U tube. This absorber is filled with “sulphuric silicagel” 3 heated up to 40°C. It retains dinitrogen tetroxide. It must be renewed every 8 to 10 days (13, Figs. 1 and 3). The effect of the presence of other elements such as fluorine, bromine, iodine, sulphur, and germanium, was tested, and it was found that they all interfered. So far as sulphur is concerned, the major part of it a The preparation of sulphuric silicagel consists of three alternate impregnations, with analytical grade sulphuric acid (1.83 density), and drainings of Prolabo granulated “actigel” and of a subsequent drying in a sand bath at 200°C until white fumes have disappeared. Some silicagel batches of a yellow color had to be given up for preparing the above reagent as it gave high results when used for the analysis of nitrocompounds.
RAPID
MICRODETERMINATION
OF
CHLORINE
283
cathodic cornpad fid 2
anodic compartment
od4
\
ibore
l-1,5
dimensions:mm
FIG. 4. Coulometric cell: ( 1) Platinum anode; (2) Porcelain diaphram; (3) Platinum cathode; (4) Drain tube with stopcock; (5) Bubbling tube for combustion gases (distance of the lower end of this tube from the cell bottom: some mm); (6) Turbo-“atomizer” for gas bubbles; (7) Glass electrode; (8) Porcelain diaphragm; and (9) Reference electrode (AgCl). Filling of the anodic compartment: Calcium carbonate suspension in an electrolyte made of one part of a 15% sodium sulphate aqueous solution and one part of a 15% sodium chloride aqueous solution. Filling of the cathodic compartment: Electrolyte made of one part of a 5% sodium sulphate aqueous solution (45-50 ml), one part of a 5% sodium chloride aqueous solution and 2.8 ml of 110 vol hydrogen peroxide. Filling of the reference compartment: Five percent sodium sulphate aqueous solution saturated with sodium chloride in the presence of an excess of sodium chloride crystals.
reaches the titration cell as sulphur dioxide whose retention before the cell is being studied. No satisfactory results have been obtained until now. Phosphorus and silicium were also tested and seem not to interfere. RESULTS
The standard deviation calculated from 80 results is 6pg chlorine. It gives about the same precision as already obtained in our laboratory in-
284
DEBAL
AND
TABLE
LEVY
1
INSTANCES OF RFSULTS FOUND
Compound analyzed
oios
Sample (mg) Found
Chlorodinitrobenzene
Chloronitrobenzene
p-Chlorobenzoic acid
Dichlorodinitrobenzene
Chlorobenzene
Chloroacetamide
Tetrachloroquinone
Pentachlorophenol
Hexachlorocyclohexane
3.325 3.109 4.367 4.145 4.612 3.674 5.027 4.971 4.291 4.163 2.699 4.691 4.090 3.336 3.608 2.735 3.664 3.063 3.580 3.998 2.517 3.836 3.449 3.076 3.807 4.488 2.589 3.783 3.446 3.815 3.240 3.122 3.539 3.169 4.111 3.222 2.972 2.606 4.027 4.206 3.695 2.859 2.782 2.536 2.448
17.43 17.62 17.73 17.45 17.66 22.39 22.61 22.88 22.95 22.69 22.95 22.69 22.36 22.91 22.47 30.14 30.08 29.88 29.86 29.73 31.67 31.56 31.71 31.88 31.35 37.60 38.14 37.73 37.19 37.63 57.59 57.54 57.83 57.20 57.55 66.37 66.16 66.40 66.26 66.64 73.62 73.36 73.33 72.76 72.95
Calculated 17.50
22.50
22.65 -
31.50 -
37.91 -
_.. 57.58
66.56
73.13
RAPID
MICRODETERMINATION
OF
CHLORINE
285
volving combustion in an oxygen flask or a straight tube and a silver nitrate potentiometric titration (1) . The weighing error is negligible. The standard deviation value may be made lower by grouping the analyses of substances with similar chlorine contents or by using the simplified apparatus with no traps when nitrogen is not present. Some of the 80 results found are shown in Table 1. CONCLUSION
This paper points out the possibility of acidimetric determination for organic chlorine microanalysis. Another advantage of the method is to render it possible to carry out rapid analyses automatically and to allow the use of the same coulometer and electrochemical cell for chlorine and sulphur determination. It is all the more important as coulometer and cell make a relatively expensive apparatus, SUMMARY A method is described for the quantitative microdetermination of chlorine in the absence of any other halogen and sulphur. The sample (2.5-5 mg) is instantaneously mineralized in a stream of moist oxygen (60 ml/minute) inside an “Ingram” chamber at 1000°C with a view favoring the hydrochloric acid formation rather than the chlorine one. Hydrochloric acid and chlorine are quantitatively swept into the cathodic compartment of the cell of a coulometric electrolyzer by a dry oxygen stream (400 ml/minute) which flows through the apparatus. Chlorine is reduced into hydrochloric acid by hydrogen peroxide contained in the cathodic compartment, and total hydrochloric acid is automatically titrated by coulometric acidimetry. Interfering nitrogen-acidic-combustion compounds are decomposed and retained before the coulometric cell. Carbon dioxyde is swept by bubbling oxygen out of the absorbing solution the pH of which has an appropriate value (pH = 5). The duration of a single determination within a series is 12-13 minutes. The precision of the method is the classical one. REFERENCES 1. Debal, E. and Levy, R., Sur quelques methodes de microdosage des halogbnes (Cl, Br, I) dans les composes organiques. Etude comparative et critique. Mikrochim. Acta, 272-297 (1964). 2. Debal, E., and Levy, R., Methode rapide de microdosage du soufre dans les composes organiques par combustion et coulometrie. Bull. Sot. Chim. Fr. 426-434 (1968). 3. Ingram, G., Combustion of Organic Compounds by Their Ignition in Oxygen for the Microdetermination of Some Elements. Microchem. J. Symp. Ser. 2, 49.5-526 (1962). 4. Walisch, W., and Jaenicke, O., Messanordnung zur automatischen Ultramikrobestimmung von Halogen in organischen Verbindungen. Mikrochim. Acta 1147-1163 (1967).
JOURNAL.
16, 277-285
( 197 1)
Rapid Method for the Microdetermination of Chlorine in Organic Compounds Involving Combustion anld Automatic Acidimetric Coulometry’ E. DEBAL
AND R.
LEVY
Service Central de Microanalyse du Centre National Scientifique, 2 rue Henri Dunant, 94-Thiais, Received
December
de /a Recherche France
26, 1970
INTRODUCTION
This method is intended to render possible the rapid microdetermination of chlorine in organic compounds on milligram samples. Preference has been given to milligram analysis (instead of decimilligram), contrary to Walisch and Jaenicke (4), in order to avoid difficulties in weighing air-unstable substances. A 2.5-5-mg sample is instantaneously combusted in an empty silica tube of the Ingram type (3) flowed with a 60-ml/minute stream of moist oxygen at a temperature of 1000°C. A dry oxygen flow of 400 ml/ minute is then substituted for the moist oxygen flow in order to quantitatively sweep hydrochloric acid and chlorine produced through combustion into the cathodic compartment of the cell of an automatic coulometer in whose aqueous electrolyte these gases are absorbed. The apparatus is the Schoeps? CTA 5 S coulometer originally designed for sulphur determination in steel and already used in our laboratory for the determination of sulphur in organic compounds (2). Chlorine is reduced to hydrochloric acid by hydrogen peroxide in the absorbing solution of the cathodic compartment and the hydrochloric acid protons lower the pH of this solution. A glass electrode-reference electrode system transmits the pH variation to the apparatus and when the combustion is over, it starts automatically the electrolysis which neutralizes the acidic solution and thus causes the pH to rise. When the latter is back to its initial value, the electrolysis is stopped automatically. The quantity of electricity consumed is obtained by means of the discharges of an impulse condenser which controls the readings on a 1 Presented at the International Symposium on Microtechniques, VIth, tember 7-l 1, 1970, Graz, Austria. 2 Richard Schoeps, 41, Duisburg Reck, German Federal Republic. 277
Sep-
278
DEBAL
AND
LEVY
digital counter. The apparatus is calibrated through the analysis of test substances with known chlorine contents as in the case of sulphur microdetermination. The coefficient found and used is close to the theoretical one. Carbon dioxide in the combustion gases does not interfere, as it escapeswith bubbling oxygen from the electrolyte the pH of which has been deliberately fixed at 5. Acidic combustion products of nitrogen are not decomposed in the combustion chamber; they must be decomposed or retained in traps between the latter and the absorbing electrolyte. One determination within a series lasts from 12 to 13 minutes. MATERIALS
AND
METHODS
Apparatus General design, combustion chamber, traps, electrochemical
cell. See
figures 1, 2, 3, 4. Foreparts. The foreparts of the apparatus are not shown in Fig. 1. Oxygen from a steel cylinder (Air Liquide Cy) and a pressure reducer is admitted through a pressure regulator with an escape of excess gas at the lower end of a T tube about 110 cm under the level of liquid paraffin. It is purified in a glass tube (20 cm in length and 2 cm in diameter) filled half and half with soda asbestos (ascarite) and magnesium perchlorate (anhydron) and flows through a rotameter and a three-way stopcock which makes it possible to use either dry or moist oxygen during the determination. One way lets dry oxygen flow directly into the combustion tube through stopcock No. 2; another way lets it flow through a reducing valve and bubble some millimeters in depth in water at room temperature (23-25’C), in a Durand flask of about 100 ml, and then to stopcock No. 2. PROCEDURE
The procedure described below is quite similar to the one used for sulphur determination: The preparation for use of the coulometric cell, the adjustment of the coulometer, the carrying out of three preliminary combustions, and the precautionary measures for the analysis of substanceswith low chlorine contents are similar in both cases 1. Stop air cooling (see 9 below), open the combustion tube and let down the sample holder (4-6, Figs. 1 and 2) in the extension tube of the ground stopper (5, Figs. 1 and 2) on a silica rack. 2. Adjust the electrolysis time switch at 12 minutes. 3. Place the boat or the capillary with sample in the sample holder already lying in the extension which makes it necessary to use a set of two sample holders and two extensions). 4. Put the stopper with extension tube and sample on the combustion tube.
RAPID
MICRODETERMINATION
OF
CHLORINE
279
5. Push the sample holder with a ring magnet so that the sample is placed in the center of the combustion chamber (8. Figs. 1 and 2). 6. Extract the boat or capillary used for the former determination. 7. Calculate the result for the former determination. 8. Substitute the dry oxygen flow (400 ml/minute) for the moist one (60 ml/minute) by turning the three-way stopcock 3 minutes after the santple irztuoductiorz into the combustion chamber. 9. Extract the sample holder from the combustion chamber using the magnet 5 minutes after the introducfiozz of the sample: let it lie in its initial place (Fig. I) and cool it with air (from compressor or fan). 10. Weigh the sample for the next determination. 11. Turn off stopcock No. 2 (Fig. 1) 9 minutes after the introduction of the sample for a few seconds, and turn it on again (this breaking of the oxygen flow causes the absorbing-liquid level to rise in Tube 14 and wash it). When the time switch is at 0 again, it starts the electrolysis automatically, and the pH value grows again after having decreased during the gas absorption. 12. Substitute the moist oxygen flow (60 ml/minute) for the dry one. 13. Read number N on the counter. 14. Begin and follow again the former sequence of operations for the next determination.
The chlorine content is calculated after the formula
where m is the sample weight in mg, and f is the calibration coefficient in micrograms per counter round (f value is about 2.212).
FIG. 1. General design: (1) Dry oxygen inlet; (1’) Moist oxygen inlet; (2) Stopcock; (3) Iron cylinder; (4) and (6) Sample holder; (5) Extension; (7) Boat; (8) Ingram combustion chamber; (9) U-tube absorber (H,$O,); (10) Absorber (anhydrone) ; ( 11) Holes; (12) Diluting bulbs; ( 13) U-tube absorber (silica gel + H,SO,); (14) Bubbling tube in the titration cell; (15) Titration cell; and (16) 1000°C furnace.
280
DEBAL
AND
LEVY
DISCUSSION
This technique for chlorine determination was developed after testing for this purpose the one used for sulphur (2) and meeting the following difficulties. The simultaneous presence of chlorine and nitrogen in a molecule often gives high results probably correlated with the formation of nitrosyl chloride which is hydrolyzed in the cathodic compartment (the error may be important when nitrogen comes from a NO, group). The 13 lo1320°C furnace, as,is used for sulphur determination, the temperature of which is such as to get rid of acidic nitrogen compounds and their interference with the acidimetric titration, does not eliminate a possible nitrosyl chloride formation. For chlorine determination, the use of an Ingram combustion chamber at 1000°C has been chosen because it makes it possible not only, as in the former case, to carry out flash combustions, but requires a much simpler furnace with a Kanthal-wound heating element, is of a much lower cost, occupies a much smaller room, and may be kept permanently at working temperature (1000°C). When chlorine concentration in the combustion gasesis too high, it is not reduced rapidly enough by hydrogen peroxide, escapesfrom the cell,
50
i-
100
-_
120 -wo -r-
id 3 ii
dimensions : mm
60
s
od6
d6J
-od9
T id 7 L
t ,
L
%$FIG. 2. Parts of Fig. 1: (3) Iron cylinder; (4) ent silica glass); (5) Extension with ground-joint ple holder cavity for sample (transparent silica combustion chamber (transparent silica glass); dard taper.
Sample holder handle (transparstopper (Pyrex glass) ; (6) Samglass; (7) Boat; and (8) Ingram 3 = Stantemperature 1000°C.
RAPID
MICRODETERMINATION
OF
CHLORINE
281
and gives low results which are observed when dry oxygen is used. Such errors disappear with the use of moist oxygen which favors hydrochloric acid formation and lowers chlorine concentration. However, recent tests have shown that good results might be obtained with dry oxygen (with 60- and 400-ml/minute flows) provided the chlorine concentration in combustion gases is lowered and its bubbling in the absorbing solution made slower with no attention being paid to the proportion HCl:Cl,; such conditions are obtained by pressure drops in the gas flow and gas dilution by the use of three appropriate bulbs. The electrolyte and reagents in the electrochemical cell of the commercial Schoeps coulometer are convenient for the titration of sulphuric acid H+ ions but not for those of hydrochloric acid; with the latter, analytical results are no longer reproducible when hydrochloric acid concentration is growing too rapidly in the cathodic compartment. Such a drawback disappears when half of the volumes of the sodium sulphate solutions in the anodic and cathodic compartments are replaced by equal volumes of sodium chloride solutions with 15 and 5 vol % concentrations, respectively. This modified cell may be used either for chlorine or sulphur determinations. For its association with the acidimetric coulometric titration, the combustion technique described by Ingram (3) must be modified as follows : Moist oxygen is used instead of dry oxygen and the combustion chamber is heated up to 1000°C instead of 850°C to make the proportion HCl : Cl, higher. Static combustion is prohibited as it gives low results because of Ihe retrodiffusion and retention of hydrochloric acid in the back extension of the combustion tube. Solid samples and liquid ones (with low vapor pressure) in frittedalumina boats or quartz capillaries (2) are placed inside the hollow end of a sample holder (6, Figs. 1 and 2) the depth of which (30 mm only) is smaller than the one of the similar holder used for sulphur determination. This hollow carrier is fused together with a silica tube (4, Figs. 1 and 2) in which a small iron bar (3, Figs. 1 and 2) similar to Ingram’s is enclosed. Traps (the use of which is not necessary when nitrogen is absent) are described below. U tube. This absorber contains sulphuric acid which has to be renewed daily. It holds back the major part of water in the gas, reacts with nitrosyl chloride, and frees hydrochloric acid. Attention must be paid to the quality of sulphuric acid used; only newly-opened bottles should be used, otherwise bad results may be obtained (9, Figs. 1 and 3).
282
DEBAL
AND
LEVY
FIG. 3. Traps and diluting bulbs (Pyrex glass): (9) U-tube absorber with a filling of Pyrex glass beads (3 mm diameter) and sulphuric acid (1.8 ml analytical grade concentrated acid, 1.83 density); (10) Absorber with a filling of anhydrone (IO-20 mesh); (11) One mm diameter holes, (12) Diluting bulbs; and (13) U-tube absorber, wound with a heating cord (4O”C), with a filling of “sulphuric” silicagel. $ = Standard taper.
Straight tube. This absorber is filled with magnesium perchlorate which is renewed every 2 weeks. This filling brings the gasesto full dryness, thus making possible a more sweeping of the apparatus (10, Figs. 1 and 3). Bulbs. Gases are diluted in the bulbs, which favors the oxidation of nitrogen monoxide to dinitrogen tetroxide and the latter to be retained on the next filling. ( 12, Figs. 1 and 3). U tube. This absorber is filled with “sulphuric silicagel” 3 heated up to 40°C. It retains dinitrogen tetroxide. It must be renewed every 8 to 10 days (13, Figs. 1 and 3). The effect of the presence of other elements such as fluorine, bromine, iodine, sulphur, and germanium, was tested, and it was found that they all interfered. So far as sulphur is concerned, the major part of it a The preparation of sulphuric silicagel consists of three alternate impregnations, with analytical grade sulphuric acid (1.83 density), and drainings of Prolabo granulated “actigel” and of a subsequent drying in a sand bath at 200°C until white fumes have disappeared. Some silicagel batches of a yellow color had to be given up for preparing the above reagent as it gave high results when used for the analysis of nitrocompounds.
RAPID
MICRODETERMINATION
OF
CHLORINE
283
cathodic cornpad fid 2
anodic compartment
od4
\
ibore
l-1,5
dimensions:mm
FIG. 4. Coulometric cell: ( 1) Platinum anode; (2) Porcelain diaphram; (3) Platinum cathode; (4) Drain tube with stopcock; (5) Bubbling tube for combustion gases (distance of the lower end of this tube from the cell bottom: some mm); (6) Turbo-“atomizer” for gas bubbles; (7) Glass electrode; (8) Porcelain diaphragm; and (9) Reference electrode (AgCl). Filling of the anodic compartment: Calcium carbonate suspension in an electrolyte made of one part of a 15% sodium sulphate aqueous solution and one part of a 15% sodium chloride aqueous solution. Filling of the cathodic compartment: Electrolyte made of one part of a 5% sodium sulphate aqueous solution (45-50 ml), one part of a 5% sodium chloride aqueous solution and 2.8 ml of 110 vol hydrogen peroxide. Filling of the reference compartment: Five percent sodium sulphate aqueous solution saturated with sodium chloride in the presence of an excess of sodium chloride crystals.
reaches the titration cell as sulphur dioxide whose retention before the cell is being studied. No satisfactory results have been obtained until now. Phosphorus and silicium were also tested and seem not to interfere. RESULTS
The standard deviation calculated from 80 results is 6pg chlorine. It gives about the same precision as already obtained in our laboratory in-
284
DEBAL
AND
TABLE
LEVY
1
INSTANCES OF RFSULTS FOUND
Compound analyzed
oios
Sample (mg) Found
Chlorodinitrobenzene
Chloronitrobenzene
p-Chlorobenzoic acid
Dichlorodinitrobenzene
Chlorobenzene
Chloroacetamide
Tetrachloroquinone
Pentachlorophenol
Hexachlorocyclohexane
3.325 3.109 4.367 4.145 4.612 3.674 5.027 4.971 4.291 4.163 2.699 4.691 4.090 3.336 3.608 2.735 3.664 3.063 3.580 3.998 2.517 3.836 3.449 3.076 3.807 4.488 2.589 3.783 3.446 3.815 3.240 3.122 3.539 3.169 4.111 3.222 2.972 2.606 4.027 4.206 3.695 2.859 2.782 2.536 2.448
17.43 17.62 17.73 17.45 17.66 22.39 22.61 22.88 22.95 22.69 22.95 22.69 22.36 22.91 22.47 30.14 30.08 29.88 29.86 29.73 31.67 31.56 31.71 31.88 31.35 37.60 38.14 37.73 37.19 37.63 57.59 57.54 57.83 57.20 57.55 66.37 66.16 66.40 66.26 66.64 73.62 73.36 73.33 72.76 72.95
Calculated 17.50
22.50
22.65 -
31.50 -
37.91 -
_.. 57.58
66.56
73.13
RAPID
MICRODETERMINATION
OF
CHLORINE
285
volving combustion in an oxygen flask or a straight tube and a silver nitrate potentiometric titration (1) . The weighing error is negligible. The standard deviation value may be made lower by grouping the analyses of substances with similar chlorine contents or by using the simplified apparatus with no traps when nitrogen is not present. Some of the 80 results found are shown in Table 1. CONCLUSION
This paper points out the possibility of acidimetric determination for organic chlorine microanalysis. Another advantage of the method is to render it possible to carry out rapid analyses automatically and to allow the use of the same coulometer and electrochemical cell for chlorine and sulphur determination. It is all the more important as coulometer and cell make a relatively expensive apparatus, SUMMARY A method is described for the quantitative microdetermination of chlorine in the absence of any other halogen and sulphur. The sample (2.5-5 mg) is instantaneously mineralized in a stream of moist oxygen (60 ml/minute) inside an “Ingram” chamber at 1000°C with a view favoring the hydrochloric acid formation rather than the chlorine one. Hydrochloric acid and chlorine are quantitatively swept into the cathodic compartment of the cell of a coulometric electrolyzer by a dry oxygen stream (400 ml/minute) which flows through the apparatus. Chlorine is reduced into hydrochloric acid by hydrogen peroxide contained in the cathodic compartment, and total hydrochloric acid is automatically titrated by coulometric acidimetry. Interfering nitrogen-acidic-combustion compounds are decomposed and retained before the coulometric cell. Carbon dioxyde is swept by bubbling oxygen out of the absorbing solution the pH of which has an appropriate value (pH = 5). The duration of a single determination within a series is 12-13 minutes. The precision of the method is the classical one. REFERENCES 1. Debal, E. and Levy, R., Sur quelques methodes de microdosage des halogbnes (Cl, Br, I) dans les composes organiques. Etude comparative et critique. Mikrochim. Acta, 272-297 (1964). 2. Debal, E., and Levy, R., Methode rapide de microdosage du soufre dans les composes organiques par combustion et coulometrie. Bull. Sot. Chim. Fr. 426-434 (1968). 3. Ingram, G., Combustion of Organic Compounds by Their Ignition in Oxygen for the Microdetermination of Some Elements. Microchem. J. Symp. Ser. 2, 49.5-526 (1962). 4. Walisch, W., and Jaenicke, O., Messanordnung zur automatischen Ultramikrobestimmung von Halogen in organischen Verbindungen. Mikrochim. Acta 1147-1163 (1967).