Potentiometric Titration Behavior of Sodium Sulfide, Methyl Mercaptan, Dimethyl Sulfide, Dimethyl Disulfide and Polysulfides in Mixed Alkaline Solutions and Sulfate Pulping Black Liquors S-t Chh Paclflc Reslns, A Dlvlslon of Borden Chemlcal Co., New Westmlnster, B.C.
L. Paszner The Unlversltyof Brltlsh Columbla, Faculty of Forestry, Vancouver,B.C.
Potentlometrlc tltratlon of mercaptan fortlfled sulfate black llquors ylelds up to three lnflectlon polnts correspondlng to potential drops due to monosulflde (A), organlc/lnorganlc polysulflde (bound mercaptan) (E), and lonlzed free mercaptan (C). In normal mlll run black Ilquor, most of the mercaptlde Ion may be bound by traces of elemental sulfur to form organo-polysulflde usually present In oxldlred black IIquors. Thus, determlnatlon of methyl mercaptan by potentlometrlc tltratlon Is very dlfflcult. Correct lnterpretatlon of the tltratlon curves becomes thus cruclal In the sulflde analysls of black Ilquors. Thlollgnln was found to have no measurable effect on the outcome of potentlometric tltratlon of sulfides In BL.
Current public and governmental regulations on water and air pollution abatement demand accurate and rapid analytical techniques for estimating treatment requirements on discharged pollutants. Measurement of reduced sulfur compounds in industrial effluents is made difficult by the extremely low levels at which they must be analyzed. Only recent advances in instrumentation allow determinations with desired accuracy and precision, making most of the work reported prior to 1966 inadequate and questionable (11. A review of the pertinent literature on sulfide analysis makes two points become clear: First, the analytical techniques hitherto proposed are by far not quantitative, and second, for complete analysis of sulfides and mercaptans in aqueous (alkali) solutions, at least two different methods have to be used. The pulping industry is not the first one to run into problems relating to sulfide analysis. Early papers originate from the petroleum industry dealing with “sweetening” or desulfurization of crude oil distillates. Alkaline extracts of gasoline were found to contain sodium sulfide (NazS), mercaptans, and other organic sulfides and thus created analytical problems very similar to those encountered with sulfate pulping liquors. Considerable difficulties are met in direct reduced sulfur analyses in black liquor. While H2S and methyl mercaptan (CH3SH) ionize readily in alkaline media, dimethyl sulfide (CHsSCH3) and dimethyl disulfide (CH3SSCH3) are present in a free state. Thus the latter are readily extracted with neutral solvents such as ether and carbon tetrachloride. Although H2S and CH3SH can be readily driven off on acidification of the black liquor (2-6) and are subsequently absorbed in a neutral solvent, or identified directly ,from the vapor phase by gas chromatography, for their direct estimation in aqueous solution, potentiometric titration has been used extensively (4, 7,8). Silver Nitrate Potentiometric Titration. First efforts 1910
8
of potentiometric precipitation of heavy metal sulfides were reported by Dutoil and Weisse (9). The use of silver nitrate as titrant was first indicated by Treadwell and Weiss (10) in 1919 and applied to pulping liquors by Borlew and Pascoe (11) in 1946. The silver nitrate method was substantially improved by Tamele and coworkers (12-14) for hydrogen sulfide in mixtures with mercaptans and was further refined for the analysis of dissolved sulfides in pulp black liquors by Bilberg ( 1 5 ) and Collins (16). The widespread popularity of the silver nitrate method is due to the insolubility and easy flocculation of silver sulfide in ammoniacal alkaline solution and the accuracy by which changes in drops of the particular potentials can be followed. For classical titrations, the analysis is carried out in sodium acetate-buffered solutions using a potential calibrated, sulfide coated silver-silver (Ag/AgzS) electrode (12). To date, difficulties were reported in locating the proper end points in sulfide ion titrations by the classical methods. Since the titration curve is asymmetrical, it shows an unusually large and rapid drop in potential, without a readily detectable inflection point, just prior to reading the equivalence point. To further aggrevate the problem, satisfactory results are obtained only with 0.0001N or greater sulfide concentrations. Large errors were obtained below this limiting concentration of silver sulfide ( I 7). Theory of Silver Nitrate Potentiometric Titration. The solubility product or ion product constant of silver sulfide is extremely low (Ksp = 1.6 X 25 OC). The titration cell consists of a silver sulfide-silver electrode and a high pH glass reference electrode. The cell may be written as: AgIAgClIO.1N HClIl Sample Solution // Ag2S 1 Ag
(1)
The potential difference between the two electrodes when both are immersed in a solution containing sulfide ions ( S 2 - ) can be expressed as:
E =K =K
(z)”’
+ RT In a A g + F
+7 R T In
RT
El=K~--lnass(2) 2F where E1 = potential, K1 = constant, R = gas constant, T absolute temperature, and F = Faraday constant. It should be noted that the sulfide ion activity (asz-) in Equation 2 being a strong base, will contribute to acidlbase balances in accordance with the following schemes:
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
+ H20 H2S + H 2 0
S2-+ H3O+ * HSHS-
+ H30+
&
(3) (4)
Table I. Effect of Inflection Points A and B on Calculation of Hydrogen Sulfide in Black Liquor
-450
H2S calculation, g/l.
-350 MI. -250
-150
20 20
Liquor
20
BL 1-1 BL 1-2 BL 1-3 BL 1-4 BL 2-1 BL 3-1 BL4-1L BL 4-2V
20 >
=
20 20
-50
ri
20
5 +50
.I 0
ti50
1250 t350
.
1
0
1
250
II
500
305N
. II I
750
AGQ.
IV
v
VI
000
vii
vi11
Inflect. pt
Inflect. pt
A
B
4.25” 0.95 3.42 2.42 2.63 1.71 8.80 0.81
4.76 1.50 3.84 2.86 3.19 2.83 9.41 1.72
Difference, g/l.
Error, 4,
0.50 11.8 0.55 57.3 0.42 12.3 0.44 18.2 0.56 21.3 1.12 65.2 0.61 7 .O 0.91 111.7 0 Numbers are averages of two determinations ( n = 2).
1250
ML
Potentiometric titration curves of sulfate and polysulfide black liquors. (Numbers in parenthesis, e.g. (1-2), are codes to particular liquor sample used for the analysis) Equation 2 also indicates that the potential difference ( E l )is dependent on the sulfide ion activity (asn-).Tamele et al. (12), studying the behavior of silver electrode in the potentiometric titration of sulfide ions in cases of extreme dilution indicated that, if the solution contains hydrosulfide ion (HS-) concentrations below 10-5N, the high negative potential drops abruptly on further dilution and approaches a low constant value. Thus, the limiting concentration for normal sulfide ion response in potentiometric titration becomes lO+N, or greater, if spurious behavior is to be avoided. T o resolve some of these difficulties, several modifications of the silver nitrate method were proposed. The Tappi Standard T-625 ts-64 ( 1 8 ) gives the closest procedure to that proposed by Borlew and Pascoe ( 2 1 ) . Cashen and Bauman (7) determined sulfide and mercaptans in black liquor by employing a vacuum tube circuit and a “magic eye” or tuning control tube for detecting the sharp potential inflection occurring at the equivalence points. Olsson and Samuelson (19) used an anion exchange resin to separate the sulfide from the organic (non-sulfur) constituents of black liquor. The sulfides retained on the resin are sequentially eluted and quantitatively determined by patentiometric titration with mercuric chloride (HgC12) in the presence of sodium hydroxide. Swartz and Light (20) studied the sulfide ion-selective electrode as an analytical tool for the analysis of sulfide in black liquor and claimed that silver reduction also occurred. Bilberg (21), comparing silver nitrate potentiometric titration of sulfide in black liquor with procedures using mercuric chloride and cadmium sulfate (CdSOI), found that the method based on mercuric chloride titration gave the least effect of components in black liquors other than the sulfides. Frant and Ross (22) recently proposed the use of anti-oxidant solution for storing black liquor. Further, a known cadmium activity solution is added to the black liquor and the sulfide is determined quantitatively by measuring the decrease of cadmium activity in the solution with a cadmium activity electrode. Other methods, such as the one proposed by Bethge et al. (23) for the colorimetric determination of hydrogen sulfide and mercaptans in industrial effluents, were proposed recently. Although the precision and accuracy of these methods were satisfactory for hydrogen sulfide, inaccuracies were reported only in simultaneous determinations with methyl mercaptan because of the expected short life of mercaptan complexes formed in the presence of hydrogen sulfide, and Figure 1.
sources
some interference by oxygen. Thus, the potentiometric titration with silver nitrate is accepted as the easiest and most accurate sulfide determination method, if correct interpretation of the titration curve is made. Typical potentiometric titration curves, derived from six sulfate and two polysulfide black liquors (Figure l),show obviously two inflection points (A and B) as the sulfide end points. According to the older convention, the inflection point B is taken as the end point of sulfide and no existence of mercaptan was assumed. The difference of reading between points A and B can amount to 100% but, on the average, lies between 10 to 20% (Table I). Validity of these assumptions was checked with aqueous alkaline model solutions. Although discrepancies originating from using inflection B for calculating the sulfide content were reported by Felicetta et al. (24),recently published figures by Murray et al. (25) and Papp (26, 27) support the need for discriminating readings of inflections A and B. Data obtained on model solutions and black liquor now clearly indicate correctness of the new interpretation of titration curves.
EXPERIMENTAL Potentiometric titration apparatus was manufactured by the Radiometer Corporation, Copenhagen, Denmark, and consisted of a Model TTT-11 Automatic Titrator, Type ABU-1 Autobourette unit coupled with an X-Y recorder for automatically recording titrant volume and associated change in potential. The electrodes were Corning triple-purpose glass electrode (No. 476024), and Radiometer type silver-silver electrode (P4011, KT). The silver sulfide electrode was prepared by electrolytically coating a layer of silver sulfide (AgZS) on the silver electrode (11). To a 150-ml beaker equipped with a magnetic stirrer, 20 ml of 20% sodium hydroxide together with 5 ml of about 30% aqueous ammonium hydroxide solution, 75 ml of distilled water, and a layer of paraffin oil were added. Under continued stirring, an aliquot of the sulfide sample containing a t least 0.005 g of sodium sulfide (equivalent to about 2 ml of strong black liquor) was injected into the solution by using a 5-ml syringe. The sample was then titrated with 0.05 or 0.1N silver nitrate solution. The volume of titrant and potential readout were recorded automatically on the recorder in the X-Y mode. The end point, determined by inspection, for the monosulfide was read a t the first inflection of the titration curve. The reactions for the titration are as follows:
+ 2Ag+ AgzS (black ppt) CH3S- + Ag+ --.*CH3S Ag (yellow ppt) S2-
.-+
(5) (6)
And the following calculations give sulfide and mercaptan in grams per liter (gh.): NazS = 1.951 X ml of AgN03 (0.05N) (7) ml of liquor sample or ml of AgN03 (0.05N) HzS = 0.852 X ml of liquor sample
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
1911
LL ?I
+IS0
-
I I
\
'.
1 '
t350
-
0
I1
I
3.75
150
111
11.25
t
I 3.75
0
II
750
O.IN AGNO,,
111 I125
I 0
3.75
II 7.50
111 11.25
ML
Figure 2. Potentiometric titration curves of sodium sulfide solution, sulfate white liquor (WL, 1-3) and black liquor (BL, 1-2) and added methyl mercaptan
CH3SNa = 3.505 X
ml of AgN03 (0.05N) ml of liquor sample
(9)
Titration of Dimethyl Sulfide and Dimethyl Disulfide in Alkaline Solution. In order to test the interference caused by di-
or CH3SH = 2.405 X
ml of AgN03 (0.05N) ml of liquor sample
were each mixed with 4 ml of 16.6 g/l. sodium mercaptan solution. The mixed solutions were then titrated with 0.1N silver nitrate.
(10)
Effect of ,Mercaptide Ion Concentrations on titration. In order to demonstrate the effect of the presence of mercaptide ion on the outcome of sulfide determination, a mixture of known volume of commercial pulp mill white (WL 1-3) and black liquor (BL 1-2) was pipetted into the titration cell. In a series of titrations (using the above mixture as stock solution), various quantities of sodium mercaptan stock solution (16.6 g/l.) were added, and the solutions titrated immediately with 0.1N silver nitrate. Titration of Inorganic Polysulfide Solutions. A polysulfide sample was prepared by dissolving 0.164 g of elemental sulfur in 20 ml of 10 g/l. sodium sulfide alkaline solution under a nitrogen atmosphere. The solution was shaken for three days on an automatic shaker a t room temperature (20 "C). A golden yellow polysulfide solution was obtained. For titration, 0.25-ml samples of the polysulfide solution were transferred with a microsyringe to the titration cell and titrated with 0.1N silver nitrate. For a further series of titrations, varying amounts of sodium mercaptide were added to 0.25-ml aliquots of polysulfide solution. Titration of Oxidized Sulfide Liquors. Black liquor oxidation was conducted in a 500-ml jar equipped with a rubber stopper and inlet and outlet tubes. A 400-ml sample containing about 10 g/l. sodium sulfide in 1 N sodium hydroxide, with and without adding l% thiolignin, was purged with oxygen through a porous glass gas-dispersion tube immersed in the solution. The oxygen flow rate (44 ml/min) was controlled by means of a reducing valve and measured with a calibrated rotameter. Simultaneously to oxygen purging, the solution was stirred by a magnetic stirrer. The foam was reduced by adding a few drops of octanol. Changes in sodium sulfide concentration were determined from time to time by withdrawing 1-ml portions of the solution. These
methyl sulfide, an experiment was conducted by mixing 1.0 ml of dimethyl sulfide with 100 ml of 1 N sodium hydroxide solution. The solution was titrated with silver nitrate after having been stirred for 30 minutes a t room temperature. The same procedure was applied to dimethyl disulfide.
Titration of Mercaptan in the Presence of Elemental Sulfur. In order to confirm the hypothesis that in black liquor the mercaptan reacts readily with elemental sulfur to form organopolysulfide, different amounts of elemental sulfur were dissolved in aliquot portions of 20 ml of sodium methyl mercaptan-alkaline solution (6.3 g/l.) by shaking for three days under a nitrogen atmosphere at room temperature. The silver nitrate potentiometric titration was conducted on 4.0-ml samples of the solutions prepared.
RESULTS AND DISCUSSION The potentiometric titration curves of four sulfate and two polysulfide black liquors are shown in Figure 1. Obviously, two inflection points A and B can be identified as sulfide titration end points. According to these curves, none of the black liquors shows existence of mercaptan. Conventionally (20, 2 4 ) , inflection point B is taken as the end point for calculating the sulfide content of the solutions. The difference of reading at inflection points A and B can obviously affect the sulfide calculation as shown in Table I. This indicates that an appreciable error may be caused by reading inflection B for calculation of monosulfide in solution. Interpretation of Titration Curves in the Presence of Polysulfides. The behavior of sulfide ions in black liquor is complicated. Trace quantities of elemental sulfur may exist in both white and black liquor. The elemental
Table 11. Accuracy of Potentiometric Titration of Sulfide in Alkaline Solution in the Presence of Methyl Mercaptan Found, mg Added, mg Some
Na,S solutiona
White
Na2S
CHgSiYa
10.0 10.0 19.9 22.6 19.9
66.5 67.7 56 .a 55.1 57.8
Thiolignin
... 18
... ... ...
Bound h'amercaptan inflect. A-B
Free Namercaptan
XaaSpt A inflect.
NaaS accounted by A, % for
Total S compounds found, mg
a?or, counted %
19.1 21.2 23.1 40.3 37.7
45.1 44.5 35.7 16.1 20.1
9.3b 9.8 19.9 18.7 19.5
93 .O 98.0 100.0 82.7 98 .O
64.2 65.7 58.8 56.4 57.8
96.5 97.0 103.5 102.3 100.0
liquor (1-3) Reagent grade NazS in 1N NaOH solution. * Numbers are averages of two determinations ( n = 2). 1912
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
S
SAMPLE,
rable 111. Effect of Excess Methyl Mercaptan and Added Thiolignin on Interpretation and Accuracy of Titration Curves Recorded for Alkaline Sodium Sulfide Solutions
II
as
to
1V 0.5
-2w
YL
30
Found Added NaZS inflection NaZS mg Thiolignin, pt A, mg
,
10.0 10.0 10.0 10.0 0
%
... ... 1.0 1.0
Error, 96
-7
9.3" 10.7 9.8
+7
9.8
-2
-2
NaZS inflection pt B, mg
22.1 22.9 22.0 22.4
Error,
%
rmo
+120.5 +129.1 +119.7 +124.4
I o 0
Numbers are averages of two determinations ( n = 2 ) .
sulfur may also be formed by oxidation of black liquor monosulfides on contact with air. The secondary reaction with methyl mercaptan may be considered to occur as follows (28):
+ So ---*CH3SS-
CH3S-
2CH3SSAg Sn2-
IV
mm
m, N
Figure 2 clearly indicates that alkaline sulfide solutions, as well as kraft white liquor, give similar potentiometric traces to those obtained earlier with black liquor. This also indicates that inflection B is caused by the presence of the mercaptide ion coupled with polysulfide. Titration curves of sulfide and mercaptan mixtures having no inflection a t B were recorded by Tamele et al. (13) as well as Cashen and Bauman (7).However, Felicetta et al. (24)reported that inflection B was present indeed for the mixtures of sulfide and mercaptan. Their line of reasoning in arriving a t a decision to calculate the sulfide content from the inflection point B was unclear. For quantitative demonstration, a 10 gh. sodium sulfide solution was prepared by dissolving the appropriate amount of reagent grade sodium sulfide in 1N NaOH solution with and without 1%thiolignin additive. One ml of the sodium sulfide solution and excess sodium mercaptide solution were mixed. The mixed solution gave a titration curve typical for the excess mercaptan. The inflection points at A and B were taken for calculation of sodium sulfide and the results were tabulated in Table 111. The data indicate that the sodium sulfide calculated from inflection point A agrees well with the original quantity of sulfide added. However, great error was found if sodium sulfide was calculated from inflection B. Bilberg (15) proposed that the presence of aromatic polyhydroxy compounds (such as lignin or thiolignin) in black liquor will also reduce the silver ion to precipitate silver metal. This may contribute to errors of the silver nitrate titration. From Table IV, it is obvious that there is little, if any, difference in titration responses of alkaline sulfide liquor samples with and without added thiolignin. Elemental sulfur interferes with the titration procedure as it reacts with the mercaptide and its product (Equation 11) and imparts to the electrode a potential close to that observed for the monosulfide ion. The titration curves reproduced in Figure 3 show clearly the case of excess elemental sulfur and its effect on mercaptan, whereby all the
+ 2Ag+ -* 2CH3SSAg Ag2S 4 + CH3SSSCH3 + 2Ag+ -..* Ag2Sn
IN1
1i.m
Flgure 3. Effect of methyl mercaptan addition on potentlometrlc titration curves of elemental sulfur containing sulfate white (WL, 1-3) and black liquors (BL, 1-2)
(11)
(13)
-+
II
7.w
O.IN
The effect of the polysulfide formed on determination of sulfide by silver nit.rate potentiometric titration is proposed as (6): 2CH3SS-
sn
Ag&, AgzS 1 + S,-lo (14) Titration curves of sodium sulfide in alkaline solution of white liquor (WL 1-3) and black liquor (BL 1-2), with and without the addition of CHSSNa solution, are shown in Figure 2. Similarly, Table I1 shows the accuracy of sulfide determination of known quantities of hydrogen sulfide and methyl mercaptan in alkaline solution with and without 1% thiolignin as an additive. On the titration curve, inflection point A is taken for calculation of the actual sulfide present, A to B for bound mercaptan (organiclinorganic polysulfide), B to C for free mercaptan. Effect of Mercaptide Ion Concentration on Titration. The potentiometric titration of sulfide and mercaptide ion mixtures is based on silver sulfide being precipitated first at a high negative potential, followed by a considerable change in potential after all of the sulfide ion has reacted. The precipitation of silver mercaptide commences at a lower potential and, on completion of mercaptide ion precipitation, another sharp change in potential occurs. The results support the observation that inflection point A is due to monosulfide, and inflection B originates from bound mercaptan. This also indicates that one mole of bound mercaptan is precipitated by an equivalent mole of silver nitrate in a stoichiometric ratio, as shown by calculation in Equation 13. The addition of 1%thiolignin did not affect the titration results. ---+
Table IV. Determination of Sulfide and Bound Mercaptan by Addition of Excess Na-Mercaptan to Oxidized Alkaline Sulfide Solution with or without Thiolignin, as Additive Oxidationb time, min
0
5
10
9.8" 9.3 8.8 Na,S, g/l. 21.2 37.6 27.4 Bound Nam e r c a p t a n g/l. B. Without Na,S, g/l. 12.4 10.1 1% thioBound Na22.8 29.4 lignin m e r c a p t a n , g/l. Numbers are averages of two determinations ( n = 2 ) . 0 2 flow rate = 44 ml/min.
A. With 1% thiolignin
... ...
20
50
70
80
6.2 20.5
1.2 17.8
0.4 17.4
... ...
9 .o 20.9
5.5 19.5
... ...
4 .O 10.9
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
1913
-4%
SAMPLE,
d
SAMPLE,
ML
NA-POLYSULFIDE NA-MERCAPTAN 0.25 I 025 00 0.0 0 25 I1 025 IO 1.0
-350
025 0 25 0.25 025
ill IV
-250
-350
-
I II
ML
1.0 (CH,SW,) 100 (IN NaOH) I O (CH$SCbl 100 (IN NeOH)
40 4.0 IO 0 10.0
- IM p
-M
i
5
+50
+I50
+2x
t3%
I
II
5 00
2 50
0
IV
111
Z50
O.IN k N 4 ,
ML
Flgure 4. Potentiometric titration curves of sodium polysulfide in
\ '.
presence of methyl mercaptan -450
-350
SAMPLE,NbSCH ML I
I
6
CONTROL (0MIN OXIDATION)
YY IO
-350
0
40 b-MERCAPTAN
-250
0
ML
4.0
40
40 40
-150
+350
-.
1
II
. Ill
I
11'
I' +2M
~~~
0
250
5w
750
OIN
1000
&%,
I2 50
1500
ML
tu0
Flgure 5. Potentiometric titration curves of sodium sulfide in 1% thiolignin containing alkaline solution with and without oxidation and addition of sodium methyl mercaptan
250
5 00
OIN PIGNO,
mercaptan ions are converted to organo-polysulfide (CHsSS-). The excess elemental sulfur slowly dissolves in alkaline solution to give S2-, &Os2-, and Sn2-. By increasing the quantity of mercaptan solution to react with the excess elemental sulfur, a free mercaptan inflection can be obtained on the titration curve (C). Thus, it is certain that there are few, if any, methyl mercaptide ions existing in black liquor, because most of the mercaptide ions are converted into organo-polysulfide in black liquor. Titration of Inorganic Polysulfide Solution. As shown in Equation 14, polysulfide-sulfide (silver sulfide) is precipitated and elemental sulfur is released by titration of polysulfide solutions with silver nitrate. In Figure 4,again two inflection points are found on the titration curves of inorganic polysulfide solutions. A similar pattern was reported earlier by Bilberg (15), and recently by Murray et al. ( 2 5 ) and Papp (26, 27) for titration of solutions containing sulfide and polysulfide. Bilberg (15)and Papp (26, 27) further showed that, when the polysulfide solution was pretreated with sodium sulfite, the inflection a t B disappeared. The following chemical reaction was found to be involved: 1914
~
750 ML
Flgure 7. Potentiometric titration curves of methyl mercaptan with and without added elemental sulfur in alkaline solution
Sn-iz-
+ (n - 1)S0s2- - * (n - 1) S 2 0 3 ' - + S2-
(15)
By addition of varying amounts of sodium mercaptide, location of the inflection point B was gradually pushed out until it reached a constant value B (Figure 4). This is further taken as proof that inflection B of the black liquor titration curve is due to the presence of organic and inorganic polysulfides in the solution rather than to the hitherto assumed monosulfide ions. Titration of Oxidized Sulfide Liquors. Changes in sulfide and bound mercaptan concentrations due to oxidation are shown in Table IV. As expected, apparent quantities of the dissolved sulfide and bound mercaptan were gradually decreased as oxidation times were progressively increased. The potentiometric titration of oxidized sodium sulfide (irrespective of 1%added thiolignin) in alkaline solution, in the absence of sodium mercaptan, shows a well defined sulfide end point (A) (Figure 5 ) . The effect of added sodium mercaptan (C) is also quite readily observable. Since oxida-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
Table V. Potentiometric Titration of Methyl Mercaptan in Alkaline Solution in the Presence of Elemental Sulfur Calculated CH,SNa 11.22b 11.22 11.22 11.22 11.22 11.22 S in Elemental sulfur 0 4 .O 8.O 20.0 38.4 457.0 sample, Approx. mercaptan 0 2.8 1.4 0.6 0.3 0.02 mga s/so 1.11 1.41 0 0.51 0 1.01 Titrated Apparent sulfide S (found) Bound S2 0 3.62 6.83 10.42 11.24 11.22 0 4.13 12.35 12.63 6.83 11.43 mg Total Accounted for, % ' 0 103.3 85.4 59.1 32.1 2.8 0 3.62 Titrated Bound 4.42 11.22 11.24 11.22 mercaptan S mercaptan3 (found), mg Apparent 11.22 7.21 6.83 0 0 0 mercaptan4 Total 11.22 11.22 11.24 11 -22 10.83 1!.25 Accounted for, % 100.0 96.5 100.3 100.0 103.7 100.0 0 pH = 13.3-13.6.n - 2. b Numbers are averages of two determinations ( n = 2). 1 From inflection point A; 2 From inflection point A to B; 3 From inflection point A to B; From inflection point B to C (Figure 7). tion of sodium sulfide solutions produces polysulfide and elemental sulfur (29, 30), the appearance of inflection B is readily explained by the observation made in the foregoing sections, i.e., B is due to the organic and inorganic polysulfides. Thiolignin content of the solution does not seem to influence the oxidation rate of sodium sulfide and methyl mercaptan to polysulfide by any measurable degree. Titration of Dimethyl Sulfide and Dimethyl Disulfide in Alkaline Solution. Black liquor usually contains small quantities of dimethyl sulfide (CH3SCH3) (0.5 to 1.1 X gh.) and dimethyl disulfide (CH3SSCH3) (2 to 7 X gh.). Swartz and Light (20) claimed that the endpoint break of silver nitrate potentiometric titration of black liquor is caused by the presence of organo-sulfide compounds such as methyl mercaptan and dimethyl sulfide, which may supply sulfide ions to the system. It is agreed that there is a considerable effect of mercaptide ions on the titration procedure. Results of the present experiment show that alkaline solutions of dimethyl sulfide give no sulfide ion potentials but, as expected, dimethyl disulfide solutions gave organopolysulfide potential. For comparison, potentiometric titration curves of dimethyl sulfide, dimethyl disulfide, elemental sulfur-mercaptan and methyl mercaptan alkaline solutions are shown in Figure 6. The potentials of disulfide (Curve 11) and sulfur-mercaptan (Curve 111) in alkaline solution are very close. Accordingly, the reaction of dimethyl disulfide in alkaline solution may be proposed as follows:
.:'
nu-
CH3SSCH3
CH3SS-
+ CH3OH
Titration of Mercaptan in the Presence of Elemental Sulfur. Potentiometric titration of mercaptan in black liquor results in poorly defined titration curves (Figure 1). As previously discussed, this is explained by the presence of elemental sulfur in the solution, whereby mercaptan is capable of further reacting with elemental sulfur to produce organo-polysulfide, Equation 11. The titration curves of added elemental sulfur containing mercaptan solutions are reproduced in Figure 7 . The bound sulfur and bound mercaptan are calculated from Equation 9. The calculated values are tabulated in Table V. The potentiometric titration of mercaptan in the presence of elemental sulfur is dependent on the ratio of mercaptan to elemental sulfur. As shown in Table V and Figure 3, as long as the amount of elemental sulfur is greater than the corresponding mercaptan concentration, no free mercaptan inflection point is detectable on the titration curves. The excess elemental sulfur remains unreacted or dissolved
in the strongly alkaline solution in the form of sulfide (A) (29, 30). This is also indicated in Table V, as well as Curves I and I11 in Figure 7. Conversely, when the amount of mercaptan present is in excess of that needed to consume the elemental sulfur, organo-polysulfide (B) and mercaptan (C) appear on the titration curves such as reproduced in Curve I1 on Figure 7. It is proposed to keep the ratio of elemental sulfur and mercaptan sulfur greater than one during sulfate cooking by adding small amounts of elemental sulfur, whereby the methyl mercaptide ions will react with sulfur to form organo-polysulfide, before they react further to form dimethyl sulfide and dimethyl disulfide. The organo-polysulfide is considered to be less volatile than methyl mercaptan. Part of the air pollution problem associated with sulfate pulp cooking may thus be abated by reducing methyl mercaptan, dimethyl sulfide, and dimethyl disulfide to the less volatile organo-polysulfides.
CONCLUSIONS Sulfate black liquor potentiometric titration gives two inflection points: the first break (A) is considered to be monosulfide; the second inflection (B) is due to the presence of organo-polysulfide in unoxidized and organiclinorganic polysulfide in oxidized black liquors. A third inflection point (C)is also evident in solutions containing ionized free mercaptan. Most of the mercaptide ions in black liquor may be bound by traces of elemental sulfur present in black liquor to form organo-polysulfide, which also contributes to the potentiometric titration. Determination of methyl mercaptan by potentiometric titration in sulfate black liquor with any degree of sensitivity, is most difficult. However, correct interpretation of the titration curve gives considerable promise of accurate determination of sulfides in oxidized black liquor. It is proposed that partial abatement of air pollution originating with the sulfate pulping process can be obtained by periodical replenishment of the excess elemental sulfur to keep the ratio of elemental sulfur to mercaptide sulfur greater than one throughout the cooking stage. Thereby, the mercaptide will be bound by elemental sulfur to form an organopolysulfide which is considerably less volatile than methyl mercaptan. ACKNOWLEDGMENT The authors thank J. W. Wilson for his valuable guidance and arrangement of funding for the project, of which this paper was a part.
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LITERATURE CITED ( 1 ) K. V. Sarkanen, 6. F. Hrutflord, L. N. Johanson, and H. S. Gardner. Tappl, 53, 766 (1970). (2) K. Anderson, Sven. Papperstid., 73, l(1970). (3) B. F. Hrutfiord and J. L. McCarthy, Tappi, 50, 82 (1967). (4) M. J. Matteson, L. N. Johanson, and J. L. McCarthy, Tappi, 50, 86
(1967).
(5) W. T. McKean, Jr., 6.F. Hrutfiord, and K. V. Sarkanen, Tappi, 48, 699 (1965). (6) T. T. C. Shih, B. F. Hrutfiord, K. V. Sarkanen, and L. N. Johanson, Tappi, 50, 630 (1967). (7) R. F. Cashen and H. D. Bauman. Tappi, 46, 509 (1963). (8) V. F. Felicetta and J. L. McCarthy, Tappi, 42, 162A (1959). (9) P. Dutoll and 0.V. Welsse, J. Chem. Phys., 9, 608 (1911). (IO) W. D. Treadwell and L. Welss, Helv. Chim. Acta, 2, 680 (1919). ( 1 1 ) P. B. Borlew and T. A. Pascoe. Pap. TradeJ., 122, 31 (1946). (12) M. W. Tamele, V. C. Imine, and L. B. Ryland, Anal. Cbem., 32, 1002 (1960). (13) M. W. Tamele, L. B. Ryland, and R. N. McCoy, Anal. Chem., 32, 1007 (1960). (14) M. W. Tamele, L. 6.Ryland, and V. 0.Imine, hd. Eng. Chem., Anal. Ed., 13, 618 (1941). (15) E. Bllberg and P. Landmark, Nor. Skoglnd., 15, 221 (1961). (16) T. T. Collins, Jr., Pap. TradeJ., 129, 23 (1949). (17) I. M. Kolthoff and N. H. Furman, “Potentiometric Titrations”, John Wlley and Sons Inc.. New York, 1931, p 110. (18) Anon., Tappl Standard T-625 ts-64 (1964), 5 pp.
(19) J. E. Olsson and 0. Samuelson, Sven. Papperstidn., 68, 179 (1965). (20) J. L. Swartz and T. S. Light, Teppl, 53, 90 (1970). (21) E. Bllberg, Nor. Skogind., 13, 307 (1959). (22) M. S. Frant and J. W. Ross, Jr., Tappl, 53, 1753 (1970). (23) P. 0. Bethge, M. Carlson. and R. Radestrom, Sven. Papperstidn., 71, 864 (1968). (24) V. F. Feilcetta. Q. P. Penlston, and J. L. McCarthy, Tappi, 36, 425 (1953). (25) F. E. Murray, L. G. Tench, and C. D. Eamer, 6th Paper Ind. Air and Stream Improvement Conference, April 13-15, 1971, Quebec City, Canada. (26) J. Papp. Cellul. Chem. Techno/., 5, 147 (1971). (27) J. Papp. Sven. Papperstidn., 74, 310 (1971). (28) J. H. Karchmer, Anal. Chem., 29, 425 (1957). (29) F. E. Murray, Tappi, 42, 761 (1959). (30) F. E. Murray, Pulp Pap. Mag. Can., 89, T26 (1968).
RECEIVEDfor review November 25, 1974. Accepted May 13, 1975. This paper is dedicated to the 60th birthday of Dr. Karl Kratzl, Professor, Institute of Organic Chemistry, Vienna, Austria. The financial assistance of the National Research Council of Canada to the senior author during various phases of these and related experiments is gratefully acknowledged.
Simultaneous Potentiometric Determination of Precise Equivalence Points and p K Values of Two- and Three-pK Systems 1.N. Briggs and J. E. Stuehr Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44 106
Equations are developed for the preclse and slmultaneous determlnatlon of pK values, equivalence polnts, and proton purlty In the potentlometrlc determlnatlon of dl- and trl-bask weak aclds. A computer technlque based upon nonlinear regresslon Is described which automatlcally finds the optlmum parameters. The procedure Is based on exact mole balance relatlonshlps and takes Into account the presence of any strong acld or base, as well as the changes In lonlc strength which occur during the course of a tltratlon. Analysls of several representatlve systems lndlcates that experlmental pH-volume curves can be reproduced to withln a standard devlatlon of 0.003 pH unlt or better.
In a multi-pK system, the individual pK’s will be effectively independent of each other if they are separated by a t least 2.7 units (I).In such cases, each can be evaluated individually by methods developed for single pK systems. However, very often successive pK’s are not sufficiently separated for such procedures to be applicable. Some current methods of obtaining pK’s in such situations are based on the fact that m titration points in a system yield m simultaneous equations. If there are n pK’s, then n titration points will yield a single value for each of the n pK’s. Several combinations can be used from the set of m titration points and the average results can be taken as the most probable values. For example, if there are two pK’s in a system, pairs of simultaneous equations (2) can be solved for pK1 and pK2. Hendrickson (3) has suggested a method by which this principle can be used to obtain any number of pK’s. However, in practice, difficulties may arise when 1916
such methods are used, primarily because random errors can cause the equations to be inconsistent. An alternative approach is to cast the equation into the form of a straight line which yields the constants from its slope and intercept. This approach was used by Speakman ( 4 ) to evaluate the pK’s of dibasic acids. I t was extended to three pK systems by Tate et al. (5). They assumed that the third pK had little effect on the lower regions of the titration curve, and obtained first estimates of the lower pK’s from these points. These estimates were then used with points in the upper region to obtain the third pK. The last value was then substituted into a rearranged form of the functional equation to obtain a better value of the first pK. The process was repeated until the values of the pK’s showed no further changes. A key feature to the above methods is that they all involve the assumption that the concentration of the substance under study has been previously determined. We have already discussed the difficulties involved in such determinations (6). These difficulties become even greater as the separation between pK’s decreases. The method which we present here does not require such prior knowledge. We have shown (6) that if each point in the titration curve is used to calculate the pK of a monoprotic weak acid, the resulting values will be constant only if the correct equivalence volume (V,) is used. If V , is incorrect, the resulting values show a systematic drift with increasing pH. More correctly, the proper criterion (7) for the best choice of the titration parameters is the closeness with which calculated pH values agree with the measured values. In either case, the relevant equations are relatively simple and
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