- Email: [email protected]
Microdetermination of organic chlorine or bromine by combustion and direct coulometric titration in a modified oxygen flask
MICROCHEMICAL
JOURNAL
Microdetermination by Combustion
11,
139-145 (1966)
of Organic Chlorine and Direct Coulometric
in a Modified D. F. KETCHUM Research Labroatories,
Eastman
AND Kodak
Oxygen
or Bromine Titration
Flask
H. E. PRAGLE JOHNSON Company,
Rochester,
New
York
14650
Received August 27, 1965
The oxygen flask method of combustion (5) of organic halides is universally accepted as the method of choice for routine analysis (3). Several methods for estimation of chloride following oxygen flask combustion have been proposed. The method of Haslam et al. (2) utilizes a single unit for electrical ignition and potentiometric titration. The method of Olson and Krivis (4) uses a specially constructed coulometer to titrate chloride following a normal oxygen flask combustion. The Aminco-Cotlove (1) chloride titrator operates on the principle of coulometric, constantcurrent generation of silver ions. Although all methods cited work satisfactorily, a unit was desired which would incorporate the important features of each, such as the simplicity of an oxygen flask combustion followed by the extremely accurate coulometric, constant-current silver titration of chloride in the same oxygen combustion flask. It was also decided to extend the method to the determination of bromine as well as chlorine. An apparatus with these features has been constructed and full details of the apparatus, together with a working procedure, are given in the next section. The method described can be used advantageously for routine analyses. The automatic coulometric titration is fast and accurate and standard solutions are not required for titrations. The halide is titrated directly in the oxygen flask following combustion, thereby eliminating any error due to transfer of solution. And finally, the modified oxygen flask and auxiliary titration head are relatively simple to construct and use. EXPERIMENTAL
The development of the procedure involved three phases of experimental work. First, the usual oxygen flask (5) was modified so that, after com139
140
D. F. KETCHUM
AND
H. E. PRAGLE
JOHNSON
bustion, the halide could be titrated on the Aminco-Cotlove coulometric chloride titrator (1) without transfer of combustion products. As shown in Fig. 1, a cylindrical section, 30 mm wide and 60 mm high, was added to the bottom of a normal oxygen flask to facilitate the coulometric titration. Exact dimensions are not critical, although it is advantageous to standardize the dimensions if several modified flasks are to be used with the same titration apparatus. Second, the Aminco-Cotlove coulometric chloride titrator was modified to accept an auxiliary titration head and circuit. The head and circuit were prepared so that the electrodes could be positioned and operated in the cylindrical section of the modified oxygen flask, as shown in Fig. 2. The electrode system (Fig. 3) is simple in design and easily constructed. Four silver wires, 1.5 mm in diameter and about 36 cm long, are encased in a
FIG. 1.
Modified
oxygen
flask.
MICRODETERMINATION
OF
ORGANIC
CHLORINE
141
thin aluminum tube which acts as a grounded shield. The electrode wires are covered with Teflon tubing and are held in place by snug-fitting Teflon plugs, one on each end of the aluminum tube. Twenty-four-inch flexible leads from the electrodes are encased in a flexible grounded shield and terminate in a four-pronged, one-position plug. The titrator was modified TO A~~~xw~Y WING NUT *Q SUPPLY
ELECTRODE
FIG. 2. Titration
SPOOL
ASSEMBLY
MODIFIED
OXYGEN
STIRRING
BAR
FLASK
assembly.
to accept the four-pronged plug by means of a four-socket, one-way plug, which is attached to its housing and connected to the original titration head at the corresponding terminals just inside the instrument panel. A rough sketch of the wiring is shown in Fig. 4. Finally, a third phase concerned the application of the procedure to the determination of organic bromine. When organic chlorine samples are burned in an oxygen flask, all the chlorine is converted to ionic chloride, which can be titrated with silver. Similarly, the bromine in some organic compounds such as bromoacetanilide is converted stoichiometrically to ionic bromide. However, when many organic bromine compounds, such as bromobenzoic acid, are burned in an oxygen flask, practically all of the bromine is converted to the bromate, which must be reduced to the bromide before the titration with silver is carried out. In the past, sodium bisulfite in acid solution has been used to reduce
142
D. F. KETCHUM
AND
H. E. PRAGLE
JOHNSON
FIG. 3. Titration head. Note: 1. The front generator supply electrode fits snugly only at the bottom % inch of the lower Teflon plug. 2. The remaining three electrodes fit snugly in the entire lengths of the two Teflon plugs.
/
INSTRUMENT
HOUSIN(
ORIGINAL -TITRATION HEAD TERMINALS
FOUR-SOCKET FOURAUXILIARY
PRONGED
HEAD
FIG. 4.
PLUG PLUG
LEADS
Auxiliary
head wiring
diagram.
bromate to bromide. However, in the coulometric silver titration, SO:! seriously interferes and cannot be used. Experiment has shown that the presence of a small amount of sodium borohydride in an alkaline-absorbing
MICRODETERMINATION
OF ORGANIC
CHLORINE
143
solution resulted in complete absorption as the bromide. The solution is then acidified and titrated on the modified Aminco-Cotlove Coulometric Titrator. Reagents 1. Sodium hydroxide, 0.04’5 aqueous solution. 2. Sodium borohydride, 98f per cent purity
(Metal Hydrides, Inc.,
12-24 Congress St., Beverly, Mass.). 3. Sodium hydroxide-sodium borohydride solution: Dissolve 100 mg of NaBH4 in 100 ml of 2% aqueous NaOH. (Make fresh daily.) 4. Nitric-acetic acid mixture: (Approximately 0.8 N HNOs-80% HAc) : Add 50 ml of concentrated nitric acid to 800 ml of glacial acetic acid and dilute to 1 liter. 5. Gelatin reagent, obtained from American Instrument Co., in bottles containing 6.2 g of the dry mixture: Add 1 liter of hot distilled water to 6.2 g of the dry mixture in a ls-liter flask and heat gently with continuous swirling until the solution is clear. Cover the mouth of the flask with a beaker and store it in the refrigerator. In use, remove a portion sufficient for one day’s analyses. The dissolved reagent can be used for at least 6 months, provided it is kept under refrigeration. 6. Silver wire, 0.051 inch in diameter and 25 feet long. The Aminco-Cotlove Automatic Chloride Titrator was obtained from the American Instrument Co. under catalogue No. 4-4420. The rates of titration time may be expressed in terms of milligrams of halide per second; the high titration rate is equal to 0.01 mg of chlorine per second, the medium rate equals 0.0025, and the low rate equals 0.0005. The bromine rates are about double these values. The titration rate required depends upon the total amount of halide to be titrated. Experience has shown that the most reliable values are obtained when the total time of titration lies between 100 and 200 seconds. With a given sample size and a certain expected halide content, that titration rate is chosen which will give a titration of about 150 seconds. The milligrams of sample required for a titration of this magnitude may be calculated from one of the following formulas: For the high titration rate, the milligrams of sample required equals 1SOjper cent chlorine expected; the medium rate requires SO/per cent; the low rate requires lO/ per cent expected. The required amounts for the bromine are about double these estimates.
144
D. F. KETCHUM
AND
H. E. PRAGLE
JOHNSON
Procedure The sample is prepared for combustion in the modified oxygen flask in the same manner as that for the usual oxygen flask. One of the following solutions is used for the absorption of combustion products: for chlorine, about 10 ml of 0.04% aqueous sodium hydroxide; for bromine, about 8 ml of the sodium hydroxide-sodium borohydride solution. After combustion, allow a one-half hour absorption period. Remove the stopper; rinse it and the sides of the flask with lo-15 ml of halogen-free water. To the chloride solution, add 1.2 ml of the nitric-acetic acid mixture and 8 drops of gelatin reagent; to the bromide solution add 8 ml of the acid mixture and 11 drops of the gelatin reagent. The final volume is 30 + 2 ml. Insert the titration head into the modified flask, as shown in Fig. 2. Titrate the halide according to the instructions given in the AmincoCotlove Titrator Manual, No. 751A. A magnetic stirrer is used at constant rate throughout all of the titrations. The ratio of milligrams of halide per second is estimated by means of standard runs on pure organic compounds, with the above identical procedure being used. Blank runs are determined in the same manner. It is advisable to run one or two standards each day to be certain this ratio is constant. CALCULATIONS
Per cent X = (mg X/set) (A - B) (lOO)/mg sample, where A = total titration time in seconds; B = reagent blank in seconds; and X = chlorine or bromine. TYPICAL
RESULTS
TABLE 1 (HIGH TITRATION
Sample
Found
Chlorobenzoic acid (Theory = 22.65% Cl)
22.35 22.45 22.72 22.55 22.88 39.98 40.02 39.64 39.60 39.85 37.23 37.39 37.35 37.54 37.11
Bromobenzoic acid (Theory = 39.75% Br)
Bromoacetanilide (Theory = 37.33% Br)
RATE)
?C = 22.59 a=
0.21
x = 39.82 0=
0.19
?? = 37.32 (5 =
0.16
MICRODETERMINATION
OF
ORGANIC
CHLORINE
145
ACKNOWLEDGMENT The authors wish to express their appreciation to Elizabeth A. Von Bacho, who performed some of the early experimental work involving the determination of bromine. SUMMARY -4 method is described for the determination of chlorine or bromine in organic samples. The method combines the convenience of the oxygen flask combustion with the inherent simplicity and accuracy of the coulometric, constant-current silver titration procedure. By the use of a modified combustion flask and electrode system, the combustion and the titration can be carried out in the same flask, thereby avoiding quantitative transfers or aliquoting of the sample solution. The relative standard deviation of the method is 0.20. REFERENCES 1.
2. 3. 4.
5.
H. V., AND BOWMAN, R. L., An instrument and method for automatic, rapid and sensitive titration of chloride in biological samples. J. Lab. C&z. Med. 51, 461-468 (1958). HASLAM, J., HAMILTON, J. B., AND SQUIRRELL, D. C. M., The determination of chlorine by the oxygen flask combustion method. Analyst 85, 556-560 (1960). MACNNALD, A. M. G., The oxygen flask method. A review. Analyst 86, 3-12 (1961). OLSON, E. C., AND KRMS, A. F., Rapid micro methods of elemental analyses. I. Determination of chlorine in organic compounds. Micro&em. J. 4, 181-186 (1960). SCHONIGER, W., Mikrochim. Acta 1955, 123-129; 1956, 869-876; 1957, 545-552; 1959, 670-692. COTLOVE, E., TRANTHAM,
JOURNAL
Microdetermination by Combustion
11,
139-145 (1966)
of Organic Chlorine and Direct Coulometric
in a Modified D. F. KETCHUM Research Labroatories,
Eastman
AND Kodak
Oxygen
or Bromine Titration
Flask
H. E. PRAGLE JOHNSON Company,
Rochester,
New
York
14650
Received August 27, 1965
The oxygen flask method of combustion (5) of organic halides is universally accepted as the method of choice for routine analysis (3). Several methods for estimation of chloride following oxygen flask combustion have been proposed. The method of Haslam et al. (2) utilizes a single unit for electrical ignition and potentiometric titration. The method of Olson and Krivis (4) uses a specially constructed coulometer to titrate chloride following a normal oxygen flask combustion. The Aminco-Cotlove (1) chloride titrator operates on the principle of coulometric, constantcurrent generation of silver ions. Although all methods cited work satisfactorily, a unit was desired which would incorporate the important features of each, such as the simplicity of an oxygen flask combustion followed by the extremely accurate coulometric, constant-current silver titration of chloride in the same oxygen combustion flask. It was also decided to extend the method to the determination of bromine as well as chlorine. An apparatus with these features has been constructed and full details of the apparatus, together with a working procedure, are given in the next section. The method described can be used advantageously for routine analyses. The automatic coulometric titration is fast and accurate and standard solutions are not required for titrations. The halide is titrated directly in the oxygen flask following combustion, thereby eliminating any error due to transfer of solution. And finally, the modified oxygen flask and auxiliary titration head are relatively simple to construct and use. EXPERIMENTAL
The development of the procedure involved three phases of experimental work. First, the usual oxygen flask (5) was modified so that, after com139
140
D. F. KETCHUM
AND
H. E. PRAGLE
JOHNSON
bustion, the halide could be titrated on the Aminco-Cotlove coulometric chloride titrator (1) without transfer of combustion products. As shown in Fig. 1, a cylindrical section, 30 mm wide and 60 mm high, was added to the bottom of a normal oxygen flask to facilitate the coulometric titration. Exact dimensions are not critical, although it is advantageous to standardize the dimensions if several modified flasks are to be used with the same titration apparatus. Second, the Aminco-Cotlove coulometric chloride titrator was modified to accept an auxiliary titration head and circuit. The head and circuit were prepared so that the electrodes could be positioned and operated in the cylindrical section of the modified oxygen flask, as shown in Fig. 2. The electrode system (Fig. 3) is simple in design and easily constructed. Four silver wires, 1.5 mm in diameter and about 36 cm long, are encased in a
FIG. 1.
Modified
oxygen
flask.
MICRODETERMINATION
OF
ORGANIC
CHLORINE
141
thin aluminum tube which acts as a grounded shield. The electrode wires are covered with Teflon tubing and are held in place by snug-fitting Teflon plugs, one on each end of the aluminum tube. Twenty-four-inch flexible leads from the electrodes are encased in a flexible grounded shield and terminate in a four-pronged, one-position plug. The titrator was modified TO A~~~xw~Y WING NUT *Q SUPPLY
ELECTRODE
FIG. 2. Titration
SPOOL
ASSEMBLY
MODIFIED
OXYGEN
STIRRING
BAR
FLASK
assembly.
to accept the four-pronged plug by means of a four-socket, one-way plug, which is attached to its housing and connected to the original titration head at the corresponding terminals just inside the instrument panel. A rough sketch of the wiring is shown in Fig. 4. Finally, a third phase concerned the application of the procedure to the determination of organic bromine. When organic chlorine samples are burned in an oxygen flask, all the chlorine is converted to ionic chloride, which can be titrated with silver. Similarly, the bromine in some organic compounds such as bromoacetanilide is converted stoichiometrically to ionic bromide. However, when many organic bromine compounds, such as bromobenzoic acid, are burned in an oxygen flask, practically all of the bromine is converted to the bromate, which must be reduced to the bromide before the titration with silver is carried out. In the past, sodium bisulfite in acid solution has been used to reduce
142
D. F. KETCHUM
AND
H. E. PRAGLE
JOHNSON
FIG. 3. Titration head. Note: 1. The front generator supply electrode fits snugly only at the bottom % inch of the lower Teflon plug. 2. The remaining three electrodes fit snugly in the entire lengths of the two Teflon plugs.
/
INSTRUMENT
HOUSIN(
ORIGINAL -TITRATION HEAD TERMINALS
FOUR-SOCKET FOURAUXILIARY
PRONGED
HEAD
FIG. 4.
PLUG PLUG
LEADS
Auxiliary
head wiring
diagram.
bromate to bromide. However, in the coulometric silver titration, SO:! seriously interferes and cannot be used. Experiment has shown that the presence of a small amount of sodium borohydride in an alkaline-absorbing
MICRODETERMINATION
OF ORGANIC
CHLORINE
143
solution resulted in complete absorption as the bromide. The solution is then acidified and titrated on the modified Aminco-Cotlove Coulometric Titrator. Reagents 1. Sodium hydroxide, 0.04’5 aqueous solution. 2. Sodium borohydride, 98f per cent purity
(Metal Hydrides, Inc.,
12-24 Congress St., Beverly, Mass.). 3. Sodium hydroxide-sodium borohydride solution: Dissolve 100 mg of NaBH4 in 100 ml of 2% aqueous NaOH. (Make fresh daily.) 4. Nitric-acetic acid mixture: (Approximately 0.8 N HNOs-80% HAc) : Add 50 ml of concentrated nitric acid to 800 ml of glacial acetic acid and dilute to 1 liter. 5. Gelatin reagent, obtained from American Instrument Co., in bottles containing 6.2 g of the dry mixture: Add 1 liter of hot distilled water to 6.2 g of the dry mixture in a ls-liter flask and heat gently with continuous swirling until the solution is clear. Cover the mouth of the flask with a beaker and store it in the refrigerator. In use, remove a portion sufficient for one day’s analyses. The dissolved reagent can be used for at least 6 months, provided it is kept under refrigeration. 6. Silver wire, 0.051 inch in diameter and 25 feet long. The Aminco-Cotlove Automatic Chloride Titrator was obtained from the American Instrument Co. under catalogue No. 4-4420. The rates of titration time may be expressed in terms of milligrams of halide per second; the high titration rate is equal to 0.01 mg of chlorine per second, the medium rate equals 0.0025, and the low rate equals 0.0005. The bromine rates are about double these values. The titration rate required depends upon the total amount of halide to be titrated. Experience has shown that the most reliable values are obtained when the total time of titration lies between 100 and 200 seconds. With a given sample size and a certain expected halide content, that titration rate is chosen which will give a titration of about 150 seconds. The milligrams of sample required for a titration of this magnitude may be calculated from one of the following formulas: For the high titration rate, the milligrams of sample required equals 1SOjper cent chlorine expected; the medium rate requires SO/per cent; the low rate requires lO/ per cent expected. The required amounts for the bromine are about double these estimates.
144
D. F. KETCHUM
AND
H. E. PRAGLE
JOHNSON
Procedure The sample is prepared for combustion in the modified oxygen flask in the same manner as that for the usual oxygen flask. One of the following solutions is used for the absorption of combustion products: for chlorine, about 10 ml of 0.04% aqueous sodium hydroxide; for bromine, about 8 ml of the sodium hydroxide-sodium borohydride solution. After combustion, allow a one-half hour absorption period. Remove the stopper; rinse it and the sides of the flask with lo-15 ml of halogen-free water. To the chloride solution, add 1.2 ml of the nitric-acetic acid mixture and 8 drops of gelatin reagent; to the bromide solution add 8 ml of the acid mixture and 11 drops of the gelatin reagent. The final volume is 30 + 2 ml. Insert the titration head into the modified flask, as shown in Fig. 2. Titrate the halide according to the instructions given in the AmincoCotlove Titrator Manual, No. 751A. A magnetic stirrer is used at constant rate throughout all of the titrations. The ratio of milligrams of halide per second is estimated by means of standard runs on pure organic compounds, with the above identical procedure being used. Blank runs are determined in the same manner. It is advisable to run one or two standards each day to be certain this ratio is constant. CALCULATIONS
Per cent X = (mg X/set) (A - B) (lOO)/mg sample, where A = total titration time in seconds; B = reagent blank in seconds; and X = chlorine or bromine. TYPICAL
RESULTS
TABLE 1 (HIGH TITRATION
Sample
Found
Chlorobenzoic acid (Theory = 22.65% Cl)
22.35 22.45 22.72 22.55 22.88 39.98 40.02 39.64 39.60 39.85 37.23 37.39 37.35 37.54 37.11
Bromobenzoic acid (Theory = 39.75% Br)
Bromoacetanilide (Theory = 37.33% Br)
RATE)
?C = 22.59 a=
0.21
x = 39.82 0=
0.19
?? = 37.32 (5 =
0.16
MICRODETERMINATION
OF
ORGANIC
CHLORINE
145
ACKNOWLEDGMENT The authors wish to express their appreciation to Elizabeth A. Von Bacho, who performed some of the early experimental work involving the determination of bromine. SUMMARY -4 method is described for the determination of chlorine or bromine in organic samples. The method combines the convenience of the oxygen flask combustion with the inherent simplicity and accuracy of the coulometric, constant-current silver titration procedure. By the use of a modified combustion flask and electrode system, the combustion and the titration can be carried out in the same flask, thereby avoiding quantitative transfers or aliquoting of the sample solution. The relative standard deviation of the method is 0.20. REFERENCES 1.
2. 3. 4.
5.
H. V., AND BOWMAN, R. L., An instrument and method for automatic, rapid and sensitive titration of chloride in biological samples. J. Lab. C&z. Med. 51, 461-468 (1958). HASLAM, J., HAMILTON, J. B., AND SQUIRRELL, D. C. M., The determination of chlorine by the oxygen flask combustion method. Analyst 85, 556-560 (1960). MACNNALD, A. M. G., The oxygen flask method. A review. Analyst 86, 3-12 (1961). OLSON, E. C., AND KRMS, A. F., Rapid micro methods of elemental analyses. I. Determination of chlorine in organic compounds. Micro&em. J. 4, 181-186 (1960). SCHONIGER, W., Mikrochim. Acta 1955, 123-129; 1956, 869-876; 1957, 545-552; 1959, 670-692. COTLOVE, E., TRANTHAM,