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Determination of mercaptans by titration with lead tetraacetate
236
Atrdyticu 4.1 .* Eisevicr Scientific
SHORT
Publishing Company,
Cltindccc Acta. 65 ( 1973) 236-239
Amsterdam
- Printed in The Ncthertnnds
COMMUNICATION
Determination
KRISHNA
of mcrcslptans
K. VERMA
by titration
and SAMEER
Depurtnwtl of Posr-Grchutc
with lead tctraacctate
BOSE
Stttclics uttd Rcsecrrdr bl ClretmM-y,
Ut~itwrsity qf’ .Iahrripm, .lcdx~iprrr, M.P.
(Idicz)
(Reccivcd
24th October
1972)
A rapid and precise titrimetric method for the determination described. In anhydrous acetic acid medium, mercaptans are tetraacetate to disulfrdes’, according to the reaction: 2RSH + Pb(OAc),
+ RSSR + Pb(OAc),
of mercaptans is oxidized by lead
+ 2HOAc
Tomicek and Valcha2 have already referred to this reaction, and Suchomelova and Zyka3 used it for titration of a few mercaptans. The reaction is immediate and its end-point may be determined visually withquinalizarin as indicator2, thecolorchanging from red to blue. In potentiometric titrations, the equivalence point is shown by a large potential change (about 250 mV for 0.05 ml of a 0.05 M lead acetate solution); the potential at the inflexion is in the region of 400 mV vs. S.C.E. A large number of mercaptans can be determined by this method. Experirnert to 1 Reagents
and apparatus. Lead tetraacetate solution (0.05 M) was prepared by dissolving ca. 23.0 g of acetic acid-soaked tetraacetate salt4 in 1 1 of anhydrous iodimetricaily 5. Most of the mercaptans acetic’acid; it was standardized were gifts from Evans Chemetics, New York, U.S.A. Some samples were prepared and purifiedG by the authors. Potentiometric titrations were performed with Toshniwal titration potentiometer type CL 06, equipped with a fiber-type saturated calomel electrode and a bright platinum electrode. For visual indication 5-10 drops of a 0.2% solution of quinalizarin (I, 2, 5, %tetrahydroxyanthraquinone) in anhydrous acetic acid was used. Vi&ml procedure. Weigh the sample containing 0. I-1.0 mmole of mercaptan into a loo-ml Erlenmeyer flask containing 15 ml of solvent and add about 0.5 ml of quinalizarin Solution to make the solution orange-red. Titrate with 0.05 M’ lead tetraacetate solution from a IO-ml microburet, to a blue color. Potentiometric proceciwe. Weigh the sample containing 0.1-1.0 mmole of mercaptan into a loo-ml beaker containing 40 ml of solvent and titrate potentiometrically with 0.05 M lead tetraacetate, using a platinum indicating electrode and a
SHORT
COMMUNICATION
237
saturated caiomel reference electrode. The point of equivalence is shown by a large potential jump. Choice of titration soluents. Anhydrous acetic acid is the medium of choice for oxidimetry with lead tetraacetate; also, it is an excellent solvent for all types of mercaptans and their disulfides. Ethanol reacts with tetraacetate and gives high results. A mixed solvent of acetic acid with methanol can be used in all proportions. Acetic acid-benzene, acetic acid-toluene and acetic acid-petroleum ether solvents containing less than 60% (v/v) of hydrocarbon may be used without causing precipitation. Water, if present, should not exceed 6% of the total solvent. Results Quantitative results for the visual titration of several mercaptans are given in Table 1. As a check, mercaptans were determined by independent methods and the values agreed within analytical precision with those obtained bv lead(N) titration. The results obtained by ~ot~n~ornetri~ titration agreed with ihe v&al indicator results within *0.20/,.
TABLE
I
DETERMINATION
OF
o-Mercaptobenzoic acid 3-Mercaptopropionic acid 2-Mercaptopropionic acid 2-Diethylarninocthanethiol 2-Butanethiol Tolucne-a-thioi Mercaptoacetic acid I-Butanethiol Allylthiol Mercaptosuccinic acid I-Pentanethiol 2-Mercaptobcnzoxazole
MERCAPTANS
hydrochloride
WITH
QUINALIZARIN
99.8 100.0 96.4 98.1 97.2 98.8 80.1 90.5 93.2 99.3 80.0 98.1
0.3 0.0 0.3 0.1 0.2 0.3 0.2 0.3 0.4 0.5 0.4 0.0
INDICATOR
99.5 99.7 96.6 98.1 97.1 98.5 80.3 90.5 93.0 99.1 88.2 98.0
Hy2 + titration’ Hg2 + titration’ Iodimetrys Iodimetrys Iodimetrye Jodimetry* Iodimetrys Iodimetrys Acctylation’ Hg2+ titration’ Iodimetry” Titrated as ecid with NaOH
Q Six determinations.
In the titration ofsome mercaptans, e.g., mercaptoacetic and mercaptosuccinic acid, a white precipitate appears. It dissolves rapidly near the equivalence point and does not affect the titration or the end-point. In such cases, addition of an extra 10 ml of the solvent is recommended. Under the described conditions of * titration, mercaptosuccinic acid yields low results, but this is prevented by using 3% hydrochloric acid solution in anhydrous acetic acid as the medium. Hydrochloric acid and the mercaptan should be present in a molar ratio of about 2:l.
238 TABLE
SHORT
COMMUNICATION
II
INTERFERENCES Mercaptun
Conrpolottl
titrotd
Molar ndtletl RSH
NIIIId
ratio contptl./ -.-
Toluene-tr-thiol
l-Pcntanethiof 1-Butancthiol
3-Mercnptopropionic
acid
2-Mer~aptopropionic
acid
o-Mercaptobcnzoic
” Average of sample (Table * Molar ratio c Molar ratio J Molar ratio F Molar ratio
acid
Sulfur Potassium cyanide* Sulfosalicylic acid Bromobenzcne Mcthylacrylatc Thiophcnc Acctonc Isobutyl methyl ketone Diethylsulfonc Diphcnyl disulfidc Diacetonc alcohol“ Diphenyi suliidc Malcic acid Sodium chIoride Carbon disulfide Formic acid o-Dichlorobcnzenc Acrylonitrilcf. Cinnamic acid Urea
2 determin~ltions; ‘x, recovery I). 25:l gives l.S*/, high results. 4O:l gives 3’%, high results. S7:l gives 2’%, high results. 321:l gives 1.5% low results.
takes
into
account
29:1 6:l 4:l 53:1 9:t 18:l 31:l 3O:l 15:l 36:I 7:1 28:l 3:l 3:l 77: 1 158:l 81:l 138:l 6:l 15:l .- the previously
RSH recovi~ry (W”
100.0 100.2 99.7 100.2
100.0 100.2 99.7 100.0 99.7 99.8 100.4 100.1 100.2 100.1 99.2 100.0 100.0 100.0 100.3 100.3 dctcrmincd
purity
of
Interferences
In Table II, results for the determination of mercaptans in the presence of several interfering compounds are given. The interferences studied included compounds that interfere in existing methods, and compounds that contain other sulfurcontaining functional groups. The visual indicator method was used in the determinations. Iodide, bromide, sulfide ions and thiocarbonyl compounds interfere seriously. The color change observed was indistinct for samples containing, in addition to the merdaptan function, groups sensitive to oxidation with lead tetraacetate; examples are 2-mercaptoethylammonium chloride, cysteine, 2-mercaptobenzimidazole, etc. We are grateful most of the mercaptan
to Evans Chemetics; New York, N-Y., for generous samples and to Dr. B. P. Sinha for timely help.
REFERENCES 1 L. Field and .I. E. Lawson. 2 0. TomiEck and .I. Valcha.
J. Arrter. C/tent. SW.. 80 (1958) Cfrrrn. List?: 44 (1950) 283.
838.
gifts of
SHORT 3 4 5 6
COMMUNICATION
239
L. Suchomelova and J. Zyka, J. Elecirouttal. Chettr., 5 (1963) 57. R. E. Ocsper and C. L. Dcasy. J. Amer. C/tent. Sm., 61 (1939) 972. A. Berka, J. Vultcrin and J. Zyka, Newer Redox Titrattts, Pcrgamon Press, Oxford, 1965, p_ 78. A. I. Vogel, A Test-book o/ Practical Orgatric Chctnistry, Longmans, London, E.L.B.S. Ed.. 1968, p. 197. 7 .I. S. Fritz and T. A. Palmer. Atral. Chern,, 33 (1961) 98. 8 J. W. Kimball, R. L. Kramer and E. E. Reid, J. Chettt. Sot., 43 (1921) 1199. 9 G. H. Schenk and J. S. Fritz, Anal. Clrettz.. 32 (1960) 987.
Atrdyticu 4.1 .* Eisevicr Scientific
SHORT
Publishing Company,
Cltindccc Acta. 65 ( 1973) 236-239
Amsterdam
- Printed in The Ncthertnnds
COMMUNICATION
Determination
KRISHNA
of mcrcslptans
K. VERMA
by titration
and SAMEER
Depurtnwtl of Posr-Grchutc
with lead tctraacctate
BOSE
Stttclics uttd Rcsecrrdr bl ClretmM-y,
Ut~itwrsity qf’ .Iahrripm, .lcdx~iprrr, M.P.
(Idicz)
(Reccivcd
24th October
1972)
A rapid and precise titrimetric method for the determination described. In anhydrous acetic acid medium, mercaptans are tetraacetate to disulfrdes’, according to the reaction: 2RSH + Pb(OAc),
+ RSSR + Pb(OAc),
of mercaptans is oxidized by lead
+ 2HOAc
Tomicek and Valcha2 have already referred to this reaction, and Suchomelova and Zyka3 used it for titration of a few mercaptans. The reaction is immediate and its end-point may be determined visually withquinalizarin as indicator2, thecolorchanging from red to blue. In potentiometric titrations, the equivalence point is shown by a large potential change (about 250 mV for 0.05 ml of a 0.05 M lead acetate solution); the potential at the inflexion is in the region of 400 mV vs. S.C.E. A large number of mercaptans can be determined by this method. Experirnert to 1 Reagents
and apparatus. Lead tetraacetate solution (0.05 M) was prepared by dissolving ca. 23.0 g of acetic acid-soaked tetraacetate salt4 in 1 1 of anhydrous iodimetricaily 5. Most of the mercaptans acetic’acid; it was standardized were gifts from Evans Chemetics, New York, U.S.A. Some samples were prepared and purifiedG by the authors. Potentiometric titrations were performed with Toshniwal titration potentiometer type CL 06, equipped with a fiber-type saturated calomel electrode and a bright platinum electrode. For visual indication 5-10 drops of a 0.2% solution of quinalizarin (I, 2, 5, %tetrahydroxyanthraquinone) in anhydrous acetic acid was used. Vi&ml procedure. Weigh the sample containing 0. I-1.0 mmole of mercaptan into a loo-ml Erlenmeyer flask containing 15 ml of solvent and add about 0.5 ml of quinalizarin Solution to make the solution orange-red. Titrate with 0.05 M’ lead tetraacetate solution from a IO-ml microburet, to a blue color. Potentiometric proceciwe. Weigh the sample containing 0.1-1.0 mmole of mercaptan into a loo-ml beaker containing 40 ml of solvent and titrate potentiometrically with 0.05 M lead tetraacetate, using a platinum indicating electrode and a
SHORT
COMMUNICATION
237
saturated caiomel reference electrode. The point of equivalence is shown by a large potential jump. Choice of titration soluents. Anhydrous acetic acid is the medium of choice for oxidimetry with lead tetraacetate; also, it is an excellent solvent for all types of mercaptans and their disulfides. Ethanol reacts with tetraacetate and gives high results. A mixed solvent of acetic acid with methanol can be used in all proportions. Acetic acid-benzene, acetic acid-toluene and acetic acid-petroleum ether solvents containing less than 60% (v/v) of hydrocarbon may be used without causing precipitation. Water, if present, should not exceed 6% of the total solvent. Results Quantitative results for the visual titration of several mercaptans are given in Table 1. As a check, mercaptans were determined by independent methods and the values agreed within analytical precision with those obtained bv lead(N) titration. The results obtained by ~ot~n~ornetri~ titration agreed with ihe v&al indicator results within *0.20/,.
TABLE
I
DETERMINATION
OF
o-Mercaptobenzoic acid 3-Mercaptopropionic acid 2-Mercaptopropionic acid 2-Diethylarninocthanethiol 2-Butanethiol Tolucne-a-thioi Mercaptoacetic acid I-Butanethiol Allylthiol Mercaptosuccinic acid I-Pentanethiol 2-Mercaptobcnzoxazole
MERCAPTANS
hydrochloride
WITH
QUINALIZARIN
99.8 100.0 96.4 98.1 97.2 98.8 80.1 90.5 93.2 99.3 80.0 98.1
0.3 0.0 0.3 0.1 0.2 0.3 0.2 0.3 0.4 0.5 0.4 0.0
INDICATOR
99.5 99.7 96.6 98.1 97.1 98.5 80.3 90.5 93.0 99.1 88.2 98.0
Hy2 + titration’ Hg2 + titration’ Iodimetrys Iodimetrys Iodimetrye Jodimetry* Iodimetrys Iodimetrys Acctylation’ Hg2+ titration’ Iodimetry” Titrated as ecid with NaOH
Q Six determinations.
In the titration ofsome mercaptans, e.g., mercaptoacetic and mercaptosuccinic acid, a white precipitate appears. It dissolves rapidly near the equivalence point and does not affect the titration or the end-point. In such cases, addition of an extra 10 ml of the solvent is recommended. Under the described conditions of * titration, mercaptosuccinic acid yields low results, but this is prevented by using 3% hydrochloric acid solution in anhydrous acetic acid as the medium. Hydrochloric acid and the mercaptan should be present in a molar ratio of about 2:l.
238 TABLE
SHORT
COMMUNICATION
II
INTERFERENCES Mercaptun
Conrpolottl
titrotd
Molar ndtletl RSH
NIIIId
ratio contptl./ -.-
Toluene-tr-thiol
l-Pcntanethiof 1-Butancthiol
3-Mercnptopropionic
acid
2-Mer~aptopropionic
acid
o-Mercaptobcnzoic
” Average of sample (Table * Molar ratio c Molar ratio J Molar ratio F Molar ratio
acid
Sulfur Potassium cyanide* Sulfosalicylic acid Bromobenzcne Mcthylacrylatc Thiophcnc Acctonc Isobutyl methyl ketone Diethylsulfonc Diphcnyl disulfidc Diacetonc alcohol“ Diphenyi suliidc Malcic acid Sodium chIoride Carbon disulfide Formic acid o-Dichlorobcnzenc Acrylonitrilcf. Cinnamic acid Urea
2 determin~ltions; ‘x, recovery I). 25:l gives l.S*/, high results. 4O:l gives 3’%, high results. S7:l gives 2’%, high results. 321:l gives 1.5% low results.
takes
into
account
29:1 6:l 4:l 53:1 9:t 18:l 31:l 3O:l 15:l 36:I 7:1 28:l 3:l 3:l 77: 1 158:l 81:l 138:l 6:l 15:l .- the previously
RSH recovi~ry (W”
100.0 100.2 99.7 100.2
100.0 100.2 99.7 100.0 99.7 99.8 100.4 100.1 100.2 100.1 99.2 100.0 100.0 100.0 100.3 100.3 dctcrmincd
purity
of
Interferences
In Table II, results for the determination of mercaptans in the presence of several interfering compounds are given. The interferences studied included compounds that interfere in existing methods, and compounds that contain other sulfurcontaining functional groups. The visual indicator method was used in the determinations. Iodide, bromide, sulfide ions and thiocarbonyl compounds interfere seriously. The color change observed was indistinct for samples containing, in addition to the merdaptan function, groups sensitive to oxidation with lead tetraacetate; examples are 2-mercaptoethylammonium chloride, cysteine, 2-mercaptobenzimidazole, etc. We are grateful most of the mercaptan
to Evans Chemetics; New York, N-Y., for generous samples and to Dr. B. P. Sinha for timely help.
REFERENCES 1 L. Field and .I. E. Lawson. 2 0. TomiEck and .I. Valcha.
J. Arrter. C/tent. SW.. 80 (1958) Cfrrrn. List?: 44 (1950) 283.
838.
gifts of
SHORT 3 4 5 6
COMMUNICATION
239
L. Suchomelova and J. Zyka, J. Elecirouttal. Chettr., 5 (1963) 57. R. E. Ocsper and C. L. Dcasy. J. Amer. C/tent. Sm., 61 (1939) 972. A. Berka, J. Vultcrin and J. Zyka, Newer Redox Titrattts, Pcrgamon Press, Oxford, 1965, p_ 78. A. I. Vogel, A Test-book o/ Practical Orgatric Chctnistry, Longmans, London, E.L.B.S. Ed.. 1968, p. 197. 7 .I. S. Fritz and T. A. Palmer. Atral. Chern,, 33 (1961) 98. 8 J. W. Kimball, R. L. Kramer and E. E. Reid, J. Chettt. Sot., 43 (1921) 1199. 9 G. H. Schenk and J. S. Fritz, Anal. Clrettz.. 32 (1960) 987.