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Indirect polarographic method for the microdetermination of sulfur in organic compounds
hlICROCHF~MICAI.
JOUKNAL
indirect Microdetermination
15,
21
1-217
(1970)
Polarographic of SAFWAT
Method
Sulfur W.
in Organic
for
the Compounds
BISHARA
INTRODUCTION From the different methods available for the microdetermination of sulfur, the titrimetric (1-6) and gravimetric (7,8) finishes predominate. Usually, the organic substance is ignited in an O-filled flask (Z-6, 8-10). In the titrimetric determination of sulfur, a general trend can be figured out. This is to absorb the products of combustion in an oxidizing absorbent such as hydrogen peroxide ( I ,4.5), bromine water (2), or fuming nitric acid (2) and titration of the resuling sulfate with barium perchlorate (1,4-6)) chloride (2), or nitrate (3). Some authors (/,3-5) recommended addition of sufficient ethanol or isopropanol to give 80% alcoholic reaction medium. Thorin (1,4,5), tetrahydroxyquinone (2 ), carboxyarsenazo (3 ), and sulfonazo III (6) are suggestedas visual indicators. However, in spite of the well-known simplicity of the titrimetric finish, there seems to be some difficulty in detecting the color change of most of the above-mentioned indicators (69). Gildenberg (10) determined sulfur amperometrically using lead nitrate solution. Both of the amperometric and titrimetric finishes have the disadvantage that phosphorous if present interferes (7, ZO). Ogg (7) reported that for compounds containing phosphorous, sulfur should be determined gravimetrically. Lysyj and Zarembo (8) applied a gravimetric procedure and eliminated the interference due to phosphorous simply by employing an acidified reaction medium prior to precipitation. Unfortunately, however, the gravimetric finish happens to be somewhat tedious and time-consuming. A method that would be simple and rapid is therefore desired for the microdetermination of sulfur in organic compounds containing phosphorous. In the proposed procedure, the solution containing the sulfate ion, produced from combustion of the organic sample, is acidified, and
211
212
BISHARA
then quantitatively precipitated ride; the unreacted barium ion ciple, the method depends on amount of barium before and the organic sulfur compound.
using a known excess of barium chlois measured polarographically. In prinrecording the polarogram of a known after precipitating a certain weight of
EXPERIMENTAL
METHODS
Apparatus Radiometer polarograph, type P03f with accessories. Universal U-shaped polarographic cell; the anode compartment is filled with mercury and saturated potassium chloride solution. The capillary used has a drop time of 3-4 seconds under an open head of 60 cm of mercury. Reagents All reagents employed are of A.R. grade except M.A.R. p-toluenesulfonic acid. Acetic acid. 0.2 N solution. Barium chloride. ca. 0.2 N solution was prepared by dissolving 11.7688 g of the dihydrate in 500 ml of water. Calcium chloride. 0.5 M solution. Gelatine. 1% solution. Saturated bromine water. Recommended procedure The sample (3-9 mg) is burned in a 300-ml O-filled flask using the Schoniger method. Products of combustion are absorbed in 7 ml of water. Rinse stopper with another 2 ml followed by 1 ml of saturated bromine solution. Excess bromine is removed by heating the solution until almost colorless. While solution is hot, add 0.5 ml of 0.2 N acetic acid, then 1 ml of ca. 0.2 N barium chloride solution. Digest under reflux condenser for 20 minutes on hot plate; then allow it to cool for 15 minutes. Transfer quantitatively to 50-ml measuring flask, then add 2.5 ml of 0.5 M calcium chloride, 0.15 ml of 1% gelatine solution, and water up to the mark. Shake mixture well, and transfer it to the polarographic cell. Record the barium wave starting from - 1.80 V applied potential and using a sensitivity of 150 (0.2 PA/mm), damping 3, voltage multiplier 0.8, and compensation of condenser current 2. Under exactly similar conditions, except for the sample, record the polarogram corresponding to the total amount of barium (standard solution) . Percentage sulfur is calculated from the equation:
O?$ s=
16.033 X A X N X (B BXW
C) X 100,
SULFUR
IN
ORGANIC
COMPOUNDS
213
where, A :m- volume of barium chloride solution added (1 ml); N -:normality of the barium solution; B :: wave height of the standard solution (mm): C :: wave height of the sample solution (mm); and W = weight of sample (mg). The standard deviation v is computed from the formula (1):
RESULTS
AND
DISCUSSTON
As in any indirect method, maximal precision is attained when the amount of barium added in excess is kept small compared to that precipitated. The results in Table 1 show that the use of 1 ml of ca. 0.2 N barium chloride solution (slightly more than required for quantitative precipitation of 5 mg of a compound containing 50% sulfur) proved quite satisfactory for the analysis of all the available 12 sulfur compounds. The average error for 28 determinations is 20.3%; the maximum error amounted to -i 0.63% (Table 1 ). Comparison between direct and indirect (or comparativej polarographic methods of analysis indicate that the latter eliminates many of the experimental difficulties (I 1 j. The limiting factors are the reproducibility of the waves and the precision with which comparison of wave heights may be made. The latter is nearly independent of the method of measurement provided similar polarographic conditions are applied for both the unknown and the standard solution. Taylor (12) recommended the use of indirect methods not only in routine work but where analytical results of high precision are desired. It is already known that barium gives well-developed normal waves in 0.05 A4 calcium chloride supporting electrolyte (13). In the present work. a 0.025 M solution was found equally well. Plot of the barium ion concentration versus the wave height (mm) gave a straight line passing by the origin showing that the system satisfies the Ilkovic equation (14) over the range of I .34 X IO-” to 3.66 >: lo-” N (Fig. 1). This encouraged the use of barium, as a pilot ion, in the indirect polarographic microdetermination of sulfur; a method which proved simple, accurate. and fairly rapid. Absorbing medium. Bromine water has been preferred, as absorbent, over hydrogen peroxide because of the ease with which it can be expelled. The use of I ml of saturated bromine solution, added after ignition of the sample, was found sufficient for quantitative oxidation of the sulfur oxides to sulfate. The slight cxccss is removed by heating for a few minutes. Rerno\~al of yhosphotwus. Unfortunately, only two phosphorouscontaining organic compounds were available (Table 1). Weakly acidic
214
BISHARA TABLE 1 ANALYSIS OF ORGANIC
Sample p-Toluenesulfonic acid
Sample w bxi9
SULFUR COMPOUNDS Sulfur (%)
Calc
3.845 5.030 8.230 3..500 6.265
16.86
Diphenylthiocarbazone (Dithizone)
4.125 5.105
12.51
4-(p-Nitrophenylazo) chromotropic acid (Na salt)
6.412 7.079
2-Nitroso-l-naphthol-4-sulfonicacid
Found
Error (%I
SD c
17.25 17.12 16.71 17.24 16.91
f0.39 +0.26 -0.15 $0.38 +0.05
12.35 13.08
-0.16 +0.57
12.49
12.35 11.89
-0.14 -0.60
f0.32
8.561 3.792
12.66
12.60 12.83
-0.06 +0.17
10.16
6-Chloro-5-nitrotoluene-3sulfonicacid
7.213 7.748
11.72
11.50 12.30
-0.22 +0.5t?
zto.57
Tetrabromophenol sulfophthalein
6.134 9.437
4.79
4.69 4.33
-0.10
z!zO.25
-0.46
Dibenzoyldisulfide
5.829 6.742
23.31
23.73 23.43
$0.36 +0.06
+0.21
Nitroso-R-salt
7.515 6.075
17.00
17.54 17.63
$0.54 +0.63
10.06
I-Cystine
7.220 3.836
26.69
26.15 27.26
-0.54 +0.57
zkO.78
o-Mercaptobenzoic acid
3.932 6.787
20.80
20.66 20.71
-0.14 -0.09
rto.03
Diphenylphosphine dithioic acid
4.457 3.896 8.392
25.62
25.14 26.07 25.33
-0.48 +0.45 -0.29
11.01 10.66
+0.12 -0.23
Triphenylphosphine sulfide
4.450 7.638
i 10.89
zlzo.30
10.52
zto.49 ho.25
medium, ca. 0.01 N in acetic acid, prevented coprecipitation of phosphate provided the organic sample contains as much as 1 mg of phosphorous (in case of 8.392 mg of diphenylphosphine dithioic acid; Table 1). The acetic acid can be dispensed with if phosphorous is known to be absent. Precipitation of sulfate. Complete and rapid precipitation of sulfate as the barium salt has been achieved following the method of Ogg (7)
SULFUR
IN
ORGANIC COMPOUNDS
(8) 70t
I
I
03
06 MI
1~d 09
0 1927
N-Ba
FIG. 1A. Cathodic waves of barium ion in 0.025M calcium chloride: concentration of barium ion was (I) 1.34 X IIFN; (2) 2.10 X IO-“N; (3) 2.88 X 1WN; and (4) 3.66 X lo-3N. (B) Variation of wave height with concentration of barium ion (calibration curve).
who advocated digestion of the reaction mixture (total volume not more than 11 ml) for at least 15 minutes. In the present work, digestion for 20 minutes gave accurate values for all the compounds analyzed. The procedure (7) is much less time-consuming compared with that of Lysyj and Zarembo (8) ; they recommended leaving the precipitate undisturbed for 2 hours. Determination of burium. (i) After precipitation, the unreacted barium ions were determined polarographically in the same solution
216
BISHARA
containing the barium sulfate. The small amount of the precipitate, a few milligrams, present in 50-ml total volume exerted but negligible influence on the limiting diffusion current (or wave height). This, besides saving time, eliminates many of the difficulties associated with the filtration process. (ii) Barium, like the other alkaline earth metals, is characterized by very negative reduction potential (13). Zlotowski and Kolthoff (15) reported that the cathodic reduction wave of barium has a halfwave potential of - 1.94 V vs. SCE. This suggested the possibility of recording the polarograms even without bubbling nitrogen; the oxygen wave has a half-wave potential of -0.94 V vs. SCE. (13)) i.e., well separated from that of barium. Interference. The available organic compounds contain carbon, hydrogen, nitrogen, chlorine, bromine, phosphorous, oxygen, and sodium. None of them interfered with the procedure; carbon dioxide is polarographically inactive, nitrogen oxides present in amounts arising from organic compounds did not interfere, chloride and bromide give anodic oxidation waves, phosphate is kept in solution by using 0.01 N acetic acid medium, and sodium (or potassium) in quantities not exceeding that of barium should not interfere (13). Iodine, if present, would be oxidized to iodate by the bromine water. The half-wave potential of the iodate wave falls between -0.50 and -0.65 V vs. SCE (16), i.e., would not interfere with that of barium. An advantage of the proposed method worth mentioning lies in the fact that after recording the polarogram, the amount of barium present in solution can be redetermined by any other method, e.g., titration with 0.02 M EDTA solution (17). In this way, one can easily carry out two determinations on the same sample weight. SUMMARY An indirect polarographic method has been developed for the microdetermination of organically-bound sulfur using barium as pilot ion. The analysis of 12 organic compounds gave accurate results with an average error of *0.3%. Nitrogen, chlorine, bromine, and sodium do not interfere. Interference due to phosphorous is simply eliminated by using an acidic medium prior to precipitation. Iodine, if present, should not interfere. The method is simple, accurate, and relatively rapid. One determination takes ca. 40 to 45 minutes. ACKNOWLEDGMENT I express my thanks to Mr. George Toma for his help in a part of the experimental work. REFERENCES 1.
Wagner,
H., Microdetermination
of sulphur
in organic
substances. M&o-
SULt:LiR
IN
ORGANIC
217
COMPOI[NDS
2. Steyermark, A., Bass, E. A., Johnston, C. C., and Dell, J. C.. Microdetermination of sulphur in organic compounds utilizing the Schoniger combustion. Micuoc/~e,n. I. 4, 55 ( 1960). .3 Novikova, K. F., Hasargin, N. N., and Tsyganova, M. F., Microdetermination of sulphur in organic substances. with a new indicator (carboxyarsenazo) for the titration of sulphate. %/I. .4n~I. K/rim. 16, 348 (1961). 4. White, D. C.. Simultaneous determination of zulphur and chlorine in organic compounds. MiXroclrh. Acta 1962, X07. 5. Colson, A. F.. Removal of phosphate in the barium perchlorate titration of sulphate. Application of the method to oxygen-flask combustion technique. Ar~n(,vyt (Londm) 88, 26 C 1963 ). 6. Hudesinsky, R.. Modification of Schoniger flask method of organic sulphur determination. Determination of sulphate with sulphonazo 111. Anal. Chon. 37, 115Y (1965). 7. Ogg. c‘. L.. Report on the gravimetric misrodetermination of sulphur. 1. Ass. Oflic. Ajir-. (‘he/?!. 38. 377 (19.55 ). 8. Lysyj, 1.. and Zarembo, .I. E.. Rapid quantitative determination of sulphur in organic compounds. A&. Chcrrr. 30. 428 (1958). Y. MacDonald. A. M. G.. Oxygen flask review. Anmlysl (Lo&on) 86, 3 t.1961). IO. Gildenherg, I.., Rapid determination of \ulphur in organic compounds. Microclw/fr. J. 3, 167 (1959). I I. Bishara, S. W.. Studies on the microdetermination of some nitrogen functions in organic compounds. Ph.D. thesis, University of A’in Shams (1967); Mikrorhim. Acta 1968, 499, 1129: 1969, 44. I-7. Taylor, J. K.. Examination of absolute and comparative methods of polarographic analysis. Aml. Chr~. 19, 368 ( 1947). /J. Kolthoff, 1. M.. and Lingane, J. J.. “Polarography.” Vol. 2, 2nd ed.. pp. 433, 558. Interscience, New York, 1952. 14. Ilkovic, D.. Dependence 07 limiting currents on the diffusion constant, on the
rate
of
dropping
and
on
the
size
of
drops.
Collect.
Cc~~r~rr~trn. 6, 49X ( 1934). IS. Zlotowski. I.. and Kolthoff, I. M., Polarographic behaviour metals. I. Barium and Strontium. .I. Amer. Clrenz. Sm. 16. Habashy, G. M.. Gawargious. Y. A., and Faltaoos, B. microdetermination of halogens in organic compounds combustion. Tr~icl~rltr 15, 403 (196X). 17.
Vetter,
G..
pounds (1963).
Microdetermination
of
aulphur
content
of up
sulphur to 70%
in
solid C’horl.
Czcrfr.
of alkaline 66,
1431
Clan.
earth (1944).
N.. Polarographic after oxygen flask
and liquid organic Teclr. (Leipzig)
15,
corn43
JOUKNAL
indirect Microdetermination
15,
21
1-217
(1970)
Polarographic of SAFWAT
Method
Sulfur W.
in Organic
for
the Compounds
BISHARA
INTRODUCTION From the different methods available for the microdetermination of sulfur, the titrimetric (1-6) and gravimetric (7,8) finishes predominate. Usually, the organic substance is ignited in an O-filled flask (Z-6, 8-10). In the titrimetric determination of sulfur, a general trend can be figured out. This is to absorb the products of combustion in an oxidizing absorbent such as hydrogen peroxide ( I ,4.5), bromine water (2), or fuming nitric acid (2) and titration of the resuling sulfate with barium perchlorate (1,4-6)) chloride (2), or nitrate (3). Some authors (/,3-5) recommended addition of sufficient ethanol or isopropanol to give 80% alcoholic reaction medium. Thorin (1,4,5), tetrahydroxyquinone (2 ), carboxyarsenazo (3 ), and sulfonazo III (6) are suggestedas visual indicators. However, in spite of the well-known simplicity of the titrimetric finish, there seems to be some difficulty in detecting the color change of most of the above-mentioned indicators (69). Gildenberg (10) determined sulfur amperometrically using lead nitrate solution. Both of the amperometric and titrimetric finishes have the disadvantage that phosphorous if present interferes (7, ZO). Ogg (7) reported that for compounds containing phosphorous, sulfur should be determined gravimetrically. Lysyj and Zarembo (8) applied a gravimetric procedure and eliminated the interference due to phosphorous simply by employing an acidified reaction medium prior to precipitation. Unfortunately, however, the gravimetric finish happens to be somewhat tedious and time-consuming. A method that would be simple and rapid is therefore desired for the microdetermination of sulfur in organic compounds containing phosphorous. In the proposed procedure, the solution containing the sulfate ion, produced from combustion of the organic sample, is acidified, and
211
212
BISHARA
then quantitatively precipitated ride; the unreacted barium ion ciple, the method depends on amount of barium before and the organic sulfur compound.
using a known excess of barium chlois measured polarographically. In prinrecording the polarogram of a known after precipitating a certain weight of
EXPERIMENTAL
METHODS
Apparatus Radiometer polarograph, type P03f with accessories. Universal U-shaped polarographic cell; the anode compartment is filled with mercury and saturated potassium chloride solution. The capillary used has a drop time of 3-4 seconds under an open head of 60 cm of mercury. Reagents All reagents employed are of A.R. grade except M.A.R. p-toluenesulfonic acid. Acetic acid. 0.2 N solution. Barium chloride. ca. 0.2 N solution was prepared by dissolving 11.7688 g of the dihydrate in 500 ml of water. Calcium chloride. 0.5 M solution. Gelatine. 1% solution. Saturated bromine water. Recommended procedure The sample (3-9 mg) is burned in a 300-ml O-filled flask using the Schoniger method. Products of combustion are absorbed in 7 ml of water. Rinse stopper with another 2 ml followed by 1 ml of saturated bromine solution. Excess bromine is removed by heating the solution until almost colorless. While solution is hot, add 0.5 ml of 0.2 N acetic acid, then 1 ml of ca. 0.2 N barium chloride solution. Digest under reflux condenser for 20 minutes on hot plate; then allow it to cool for 15 minutes. Transfer quantitatively to 50-ml measuring flask, then add 2.5 ml of 0.5 M calcium chloride, 0.15 ml of 1% gelatine solution, and water up to the mark. Shake mixture well, and transfer it to the polarographic cell. Record the barium wave starting from - 1.80 V applied potential and using a sensitivity of 150 (0.2 PA/mm), damping 3, voltage multiplier 0.8, and compensation of condenser current 2. Under exactly similar conditions, except for the sample, record the polarogram corresponding to the total amount of barium (standard solution) . Percentage sulfur is calculated from the equation:
O?$ s=
16.033 X A X N X (B BXW
C) X 100,
SULFUR
IN
ORGANIC
COMPOUNDS
213
where, A :m- volume of barium chloride solution added (1 ml); N -:normality of the barium solution; B :: wave height of the standard solution (mm): C :: wave height of the sample solution (mm); and W = weight of sample (mg). The standard deviation v is computed from the formula (1):
RESULTS
AND
DISCUSSTON
As in any indirect method, maximal precision is attained when the amount of barium added in excess is kept small compared to that precipitated. The results in Table 1 show that the use of 1 ml of ca. 0.2 N barium chloride solution (slightly more than required for quantitative precipitation of 5 mg of a compound containing 50% sulfur) proved quite satisfactory for the analysis of all the available 12 sulfur compounds. The average error for 28 determinations is 20.3%; the maximum error amounted to -i 0.63% (Table 1 ). Comparison between direct and indirect (or comparativej polarographic methods of analysis indicate that the latter eliminates many of the experimental difficulties (I 1 j. The limiting factors are the reproducibility of the waves and the precision with which comparison of wave heights may be made. The latter is nearly independent of the method of measurement provided similar polarographic conditions are applied for both the unknown and the standard solution. Taylor (12) recommended the use of indirect methods not only in routine work but where analytical results of high precision are desired. It is already known that barium gives well-developed normal waves in 0.05 A4 calcium chloride supporting electrolyte (13). In the present work. a 0.025 M solution was found equally well. Plot of the barium ion concentration versus the wave height (mm) gave a straight line passing by the origin showing that the system satisfies the Ilkovic equation (14) over the range of I .34 X IO-” to 3.66 >: lo-” N (Fig. 1). This encouraged the use of barium, as a pilot ion, in the indirect polarographic microdetermination of sulfur; a method which proved simple, accurate. and fairly rapid. Absorbing medium. Bromine water has been preferred, as absorbent, over hydrogen peroxide because of the ease with which it can be expelled. The use of I ml of saturated bromine solution, added after ignition of the sample, was found sufficient for quantitative oxidation of the sulfur oxides to sulfate. The slight cxccss is removed by heating for a few minutes. Rerno\~al of yhosphotwus. Unfortunately, only two phosphorouscontaining organic compounds were available (Table 1). Weakly acidic
214
BISHARA TABLE 1 ANALYSIS OF ORGANIC
Sample p-Toluenesulfonic acid
Sample w bxi9
SULFUR COMPOUNDS Sulfur (%)
Calc
3.845 5.030 8.230 3..500 6.265
16.86
Diphenylthiocarbazone (Dithizone)
4.125 5.105
12.51
4-(p-Nitrophenylazo) chromotropic acid (Na salt)
6.412 7.079
2-Nitroso-l-naphthol-4-sulfonicacid
Found
Error (%I
SD c
17.25 17.12 16.71 17.24 16.91
f0.39 +0.26 -0.15 $0.38 +0.05
12.35 13.08
-0.16 +0.57
12.49
12.35 11.89
-0.14 -0.60
f0.32
8.561 3.792
12.66
12.60 12.83
-0.06 +0.17
10.16
6-Chloro-5-nitrotoluene-3sulfonicacid
7.213 7.748
11.72
11.50 12.30
-0.22 +0.5t?
zto.57
Tetrabromophenol sulfophthalein
6.134 9.437
4.79
4.69 4.33
-0.10
z!zO.25
-0.46
Dibenzoyldisulfide
5.829 6.742
23.31
23.73 23.43
$0.36 +0.06
+0.21
Nitroso-R-salt
7.515 6.075
17.00
17.54 17.63
$0.54 +0.63
10.06
I-Cystine
7.220 3.836
26.69
26.15 27.26
-0.54 +0.57
zkO.78
o-Mercaptobenzoic acid
3.932 6.787
20.80
20.66 20.71
-0.14 -0.09
rto.03
Diphenylphosphine dithioic acid
4.457 3.896 8.392
25.62
25.14 26.07 25.33
-0.48 +0.45 -0.29
11.01 10.66
+0.12 -0.23
Triphenylphosphine sulfide
4.450 7.638
i 10.89
zlzo.30
10.52
zto.49 ho.25
medium, ca. 0.01 N in acetic acid, prevented coprecipitation of phosphate provided the organic sample contains as much as 1 mg of phosphorous (in case of 8.392 mg of diphenylphosphine dithioic acid; Table 1). The acetic acid can be dispensed with if phosphorous is known to be absent. Precipitation of sulfate. Complete and rapid precipitation of sulfate as the barium salt has been achieved following the method of Ogg (7)
SULFUR
IN
ORGANIC COMPOUNDS
(8) 70t
I
I
03
06 MI
1~d 09
0 1927
N-Ba
FIG. 1A. Cathodic waves of barium ion in 0.025M calcium chloride: concentration of barium ion was (I) 1.34 X IIFN; (2) 2.10 X IO-“N; (3) 2.88 X 1WN; and (4) 3.66 X lo-3N. (B) Variation of wave height with concentration of barium ion (calibration curve).
who advocated digestion of the reaction mixture (total volume not more than 11 ml) for at least 15 minutes. In the present work, digestion for 20 minutes gave accurate values for all the compounds analyzed. The procedure (7) is much less time-consuming compared with that of Lysyj and Zarembo (8) ; they recommended leaving the precipitate undisturbed for 2 hours. Determination of burium. (i) After precipitation, the unreacted barium ions were determined polarographically in the same solution
216
BISHARA
containing the barium sulfate. The small amount of the precipitate, a few milligrams, present in 50-ml total volume exerted but negligible influence on the limiting diffusion current (or wave height). This, besides saving time, eliminates many of the difficulties associated with the filtration process. (ii) Barium, like the other alkaline earth metals, is characterized by very negative reduction potential (13). Zlotowski and Kolthoff (15) reported that the cathodic reduction wave of barium has a halfwave potential of - 1.94 V vs. SCE. This suggested the possibility of recording the polarograms even without bubbling nitrogen; the oxygen wave has a half-wave potential of -0.94 V vs. SCE. (13)) i.e., well separated from that of barium. Interference. The available organic compounds contain carbon, hydrogen, nitrogen, chlorine, bromine, phosphorous, oxygen, and sodium. None of them interfered with the procedure; carbon dioxide is polarographically inactive, nitrogen oxides present in amounts arising from organic compounds did not interfere, chloride and bromide give anodic oxidation waves, phosphate is kept in solution by using 0.01 N acetic acid medium, and sodium (or potassium) in quantities not exceeding that of barium should not interfere (13). Iodine, if present, would be oxidized to iodate by the bromine water. The half-wave potential of the iodate wave falls between -0.50 and -0.65 V vs. SCE (16), i.e., would not interfere with that of barium. An advantage of the proposed method worth mentioning lies in the fact that after recording the polarogram, the amount of barium present in solution can be redetermined by any other method, e.g., titration with 0.02 M EDTA solution (17). In this way, one can easily carry out two determinations on the same sample weight. SUMMARY An indirect polarographic method has been developed for the microdetermination of organically-bound sulfur using barium as pilot ion. The analysis of 12 organic compounds gave accurate results with an average error of *0.3%. Nitrogen, chlorine, bromine, and sodium do not interfere. Interference due to phosphorous is simply eliminated by using an acidic medium prior to precipitation. Iodine, if present, should not interfere. The method is simple, accurate, and relatively rapid. One determination takes ca. 40 to 45 minutes. ACKNOWLEDGMENT I express my thanks to Mr. George Toma for his help in a part of the experimental work. REFERENCES 1.
Wagner,
H., Microdetermination
of sulphur
in organic
substances. M&o-
SULt:LiR
IN
ORGANIC
217
COMPOI[NDS
2. Steyermark, A., Bass, E. A., Johnston, C. C., and Dell, J. C.. Microdetermination of sulphur in organic compounds utilizing the Schoniger combustion. Micuoc/~e,n. I. 4, 55 ( 1960). .3 Novikova, K. F., Hasargin, N. N., and Tsyganova, M. F., Microdetermination of sulphur in organic substances. with a new indicator (carboxyarsenazo) for the titration of sulphate. %/I. .4n~I. K/rim. 16, 348 (1961). 4. White, D. C.. Simultaneous determination of zulphur and chlorine in organic compounds. MiXroclrh. Acta 1962, X07. 5. Colson, A. F.. Removal of phosphate in the barium perchlorate titration of sulphate. Application of the method to oxygen-flask combustion technique. Ar~n(,vyt (Londm) 88, 26 C 1963 ). 6. Hudesinsky, R.. Modification of Schoniger flask method of organic sulphur determination. Determination of sulphate with sulphonazo 111. Anal. Chon. 37, 115Y (1965). 7. Ogg. c‘. L.. Report on the gravimetric misrodetermination of sulphur. 1. Ass. Oflic. Ajir-. (‘he/?!. 38. 377 (19.55 ). 8. Lysyj, 1.. and Zarembo, .I. E.. Rapid quantitative determination of sulphur in organic compounds. A&. Chcrrr. 30. 428 (1958). Y. MacDonald. A. M. G.. Oxygen flask review. Anmlysl (Lo&on) 86, 3 t.1961). IO. Gildenherg, I.., Rapid determination of \ulphur in organic compounds. Microclw/fr. J. 3, 167 (1959). I I. Bishara, S. W.. Studies on the microdetermination of some nitrogen functions in organic compounds. Ph.D. thesis, University of A’in Shams (1967); Mikrorhim. Acta 1968, 499, 1129: 1969, 44. I-7. Taylor, J. K.. Examination of absolute and comparative methods of polarographic analysis. Aml. Chr~. 19, 368 ( 1947). /J. Kolthoff, 1. M.. and Lingane, J. J.. “Polarography.” Vol. 2, 2nd ed.. pp. 433, 558. Interscience, New York, 1952. 14. Ilkovic, D.. Dependence 07 limiting currents on the diffusion constant, on the
rate
of
dropping
and
on
the
size
of
drops.
Collect.
Cc~~r~rr~trn. 6, 49X ( 1934). IS. Zlotowski. I.. and Kolthoff, I. M., Polarographic behaviour metals. I. Barium and Strontium. .I. Amer. Clrenz. Sm. 16. Habashy, G. M.. Gawargious. Y. A., and Faltaoos, B. microdetermination of halogens in organic compounds combustion. Tr~icl~rltr 15, 403 (196X). 17.
Vetter,
G..
pounds (1963).
Microdetermination
of
aulphur
content
of up
sulphur to 70%
in
solid C’horl.
Czcrfr.
of alkaline 66,
1431
Clan.
earth (1944).
N.. Polarographic after oxygen flask
and liquid organic Teclr. (Leipzig)
15,
corn43