Tliis serpience of reactions is also a d d u c d to account for the positive tcqtq for ester with inorganic iodide, ioriatc, and periodate salts and their acids;. With iodide, the peroxyacetic arid forms hydrogen iodide and then osidims thc acid to iodine.
0 (,4r21)+
-*
0
(Ar2I-0)
+
+
-
I1
ACKNOWLEDGMENT
(.hI-+O) CH,,C-0-H H-0OCCH3
+-
[
ArzI/OH
O
1’
+H
+ CHjC-0-0
2HI
+ CH3&--O-0-H
tetrachloride or by reduction with dilute solution of neutral aqueous permanganate.
‘OOCCH,
1’
-+
0
I? + HnO
I + CHaC-0-H
OOCCHs -
I n common with other peroxides, peroxyacetic acid is also a reductant and
‘OOCCH,
its reaction nith periodic and iodic acids > ields elementary iodine which forms the trkrtate.
CONCLUSIONS
0
I
+ HI04
CHaC-0-0-H
+
0
I‘
+ HI03 + O n
CEI,C-O-H 0
5c€r~~~-o-o-H
+ 2HI0, +
0
I1
5CH3C-0-H
+ + 502 + H20 12
Diaryliodonium cations are Lcn is bases and thrir ready formation of esters with peroxyacetic acid ma! be postulated t o occur through the formation of a n iodyl ration that add. acetic acid to produce a monohydro then uiidergoes eqterification t o yield thc diacctate.
1
+ €I20
The qualitative dc%ection of iodine in organic and inorganic compounds by formation of acetate esters with 407, peroxyacetic acid employs a new principle for discriniinating this element from bromide and chloride and avoids the difficulties in methods that depeiid on the differences in the ease of oxidation of the halide anions. Other funct’ional groups that give positive hydroxarnic acid tests are readily differentiated by using appropriate group tests. The possibility of interference from acid derivatives is eliminated by oarrying out) the hydroxamic acid test before reaction with peracetic acid; alcohols are similarly detected by first converting to esters. Dinitrophenylhydrazine will similarly detecht a carbonyl group, and olefins may be establishccl either the decolorization of bromine in carbon
The authors express their appreciation to the Buffalo Electrocheiiiical Corp. for the 407, peroxyacetic acid used in this study, to 11. Behringer for samples of iodonium compounds. and to Theodore Sulzberg for assisting with some of the tests. LITERATURE CITED
(1) Becco Chemical Division, Food Machinery & Chemical Corp., Buffalo 7, S . I-., Bull. 4, “Peracetic Acid 40%,” Kovember 1957. (2) Cheronis, N. D., “Semimicro Experimental Organic Chemistry,” p. 116, John de Graff, Kew York, 1958. (32, Cheronis, N. D., Entriken, J. B., Semimicro Qualitative Organic Analysis,’’ 2nd ed., p. 178, Interscience, Xeiv York, 1957. (4) Davidson, T),, J . Chern. Educ. 17, 81 (1940). (5) Davidson, D., Perlman, D., “,4Guide to Qualitative Organic Analysis,” 2nd ed., p. 15, Brooklyn College Press, Brooklvn. S . Y.. 1958. (6) Fichtkr,’ F., Stern, S., Helv. Chinz. Acta 11, 1256 (1928). (7) Latimer, W. RX., “Oxidation Potentials,” 2nd ed., pp. 51-69, Prentice-Hall, Kew York, 1952. 181 ~, Pausacher. K. H.. J . Chem. Soc. 1953. 107. (9) Gidgwick, S . V., Barkworth, X. n. P., Ibzd., 1931, 807. (10) Willgerodt, C., Ber. 25, 3594 (1892).
RECEIVEDfor review October 5, 1959. Accepted March 22, 1960. Division of Analytical Chemistry, 136th Meeting, ACS, Atlantic City, K. J.,September 1959.
Cha racte rizatio n
of Petroleum Nitrogen Co m po unds by Selective Acetylation and Nonaqueous Titration
S. W. NICKSIC and S. H. JUDD California Research Corp., Richmond, Calif.
b Acetylation of nitrogen bases with titration in glacial acetic acid and acetic anhydride is described. This combination can b e used to determine nitrogen compounds in petroleum streams without the isolation of the nitrogen compound from petroleum and without any separations from the reaction medium. Classes of compounds, such as tertiary amines, aromatic primary and secondary, and aliphatic primary and secondary, can b e distinguished one from another. Mixtures of known amines can be analyzed after reaction rates have been measured. The determination of compounds, such as pyrroles, di998
0
ANALYTICAL CHEMISTRY
phenylamines, and amides, is discussed and further characterization possibilities are described.
T
ITRATION in glacial acetic acid was first described by Conant and Hall ( 1 ) and reviewed by Pifei and Wollish (6). The proceduie is very useful in the petrolcum industry because acetic acid is a good solvent for hydrocarbons and because the titration is sensitive mough for the low level of nitrogen bases that must be carefully controlled in catalytic processes. Although the system was used by Hall ( 3 ) to measure the basic dissociation constants of oiganic compounds,
the usual titiation is nondifferentiating -i.e., both strong and weak bases give appioximately the same end point. Hon-eve1, a combination of acetylation n-ith titration in acetic acid differentiates one nitrogen base from another, making it possible to characterize complex mixtures and t o determine combinations of t v o or more amines quantitatively. Further differentiation is des c r i b d using acetic anhydride as the solvent in place of acetic acid. K h r n acetic anhydride is added to a sample of nitrogen bases in acetic acid, acetylation occurs and the primary and secondary amines are converted to substituted acetamides, which are no
I
ACETIC ANHYDRIDE
/?
THEORETICAL END P O I N T
I\
ANILINE
0 1
0
20
40
60
80
I00
2C
C3
60
80
TIME IN MINUTES
Figure 1.
Acetylation rates in acetic acid
longcr titratable under the conditions of the test. The rate of acetylation can be follomd by periodically withdrawing an aliquot from the reaction medium and titrating it with perchloric acid to a visual end point'. Tertiary aniinrs do not acetylate, so t'hat tlir titration remaining after completion of the acetylation reaction is a dircct mcasurcx of' the trrtiary amine content. This assumes that the amine is sufficiently hasic to be titrated in glacial acetic acid t o begin with and that the conditions for acetylat'ion are drastic enough to take care of slow rcacting amines. Kagner: Brown! and Petrrs (9) have, described the dcterniination of trrtiary amines based on this bcliavior. The rate of acetj.lation is a function of both basic: dissociation constant aiid steric factors. The difference in acetylation rate pe!,mit,s further characterizat,ioii of amines in their mixturc.s in addition to the tertiary aminc separation. Except for Kolb and Tocnnics' n-ork on amino acids ( d ) , there wem to have h e n no rate st'udies of acetylation in glacial acetic acid. These authors wew interested chiefly in comparing reactivity wit'li position of the amino group and the structure of the cmtire niolecdc. Sakami and Toennies (8) studied the inhibition of acetylation of amino groups in work on tliffercntial awtylation of hydroxyl groups in the presence of amino groups. Others ( 2 , 6! 7') h a r e also described t'he t,itr,ation of nitrogen comyountls in nonaqueous solvents. EXPERIMENTAL
T h e c'xpci imental n-ork is similar to that of Kolh and Toennits, ewept that changes i r e r e made to study rates on a broad spectrum of nitrogen compounds and to emphasize the analytical iniplication of the results. All work was done at fairly constant temperature near 23" C. to minimize the high coefficient of expansion of acetic acid. Reagents. Glacial acetic acid, C.P. -4cetic anhydride solution, 1M in acetic acid. Primary standard, 0.1000~4'sodium carbonate in acetic acid. Perchloric acid, 0.1.Y, accurately standardized against sodium
A00
1
---
~ _ _ _
Figure 2. amides
T - ~
,
N. N
- D I BEN Z Y L A C ETA U I DE
N,N'-DIETHYLACETAUIDE
1
-__-__--d
2
M L O F 0 100 N HC104
Solvent comparison in titration of two
cmbonate. Stock solution of amines (usually Eastman), 1J1 in acetic acid. Indicator, 1% crystal violet in acetic acid. Procedure. TITRAYIOX. Add one drop of intlicatoi t o 100 ml. of acetic acid in a 250-nil. Eilenmeyer flask. Titrate n i t h percliloiic acid reagent t o t h e color change of ~ i o l e t o calcar blue. This is t h r blank titration. Add a .uitable aliquot of uample and titrate again to the same coloi change. ACETILATION.N i y an aliquot of the amine solution nith an appropriate amount of acctic anhydride solution and dilute immcdiatc~lyto exactly 100 ml. uith acetic acid. For complete :iertylation in the tertiai y amine tlcterniination, hcating on a steam liath may he necrssar? prior to tlic final dilution t o volume. Othernisc, a\-oid teniperature changrs h a u s e they change reaction rates. R ~ T ESTCDI, , A t suitable intervals, pipet a n aliquot of the acetylation mixture into the titration solvent, whose blank has been neutralized previously. dgain titrate to the same end point. The aliquot size will depend on the acctylation rate and on the normality of the perchloric acid used. The time interval is selected on the basis of the number of points desired for the curve :ml on the rate of change of the titration for successive alicluots. RESULTS
Katc studics showed large diff(wiicc's in the rate of accxtylation of individual amines. Figurr 1 sliom the change in hasic nitrogen contciit with time for four representativc amines, n-butylamine, bcnzylaminc. dibrnzylamiiic, and anilint,. For convcnicncc. the concentrabion of amine is esprcssctl as normality. and the data arc' otitaincd with appi,oximatcly cxquiniolar solut'ions tic anhydritlc. Thc ic> is alinost complete within 2 minutes. whcreas 71-hutylamine has scarccly rmctcti to any measurable rstent after 20 niiiiutc3a. Colorimetric titmtioii was usecl to advantage in this study. as the fast rractions with aromatic amirirs require data n-it'hin 1 minute after the start of the waction. Conductivity ineasurcment,s may be
takcn on the reaction mixture, hut the interpietation of the data is not SO easy as in the direct titration. Experiments with a large number of amines n-ere then made and the. results are given in Table I, which lists the order
Table I. Approximate Order of Reactivity of Various Amines with Acetic Anhydride
00%
Reaction Time for 0.LY Amine in LV .4nhydride
CompouIltl
2-Saphthylamiiie p-Phenylenediaminc Aniline 2-tert-Bu t ylanilinea 2-Methylaniline 3,3'-Dimethylbenzitline 1-Kaphthylamine n-blethylaniline 2,6-Jlimethylaniline 2-( 1-blethylpropj 1)-6ethylanilinea
1 minute
1-5 minutes
2-Ethyl-l-naphthylaminea
Ethylenediamine n--Hydros2-ethyIethj lenediamine Dibenzylamine ?ti-( Bisaminomethj-1)bcnzene Benz) lamine p-( Bis-p-aminoethy1)benzene 0-(Bis-p-aminoethyl)benzene ni-( Bis-p-aminoethy1)benzene p-( Bifiaminometh?.l)benzene Ethanolamine Piperidine 11-Propylamine n-Butylamine Iliethanolaniine Tallowamine Isopropylamine IX-n-butylamine Dime thylamine Di-n-prop ylamine
1-2 hours
2-6 hours
5-30 hours
One week or more
1,1,3,3-'retramethylbutyl-
amine Diisopropylamine Courtesy of Ethyl Corp.
VOL. 32,
NO. 8, JULY
1960
999
of reactivity by decreasing acetylation rate. The data of this table should be taken only as a guide because some adjacent members of the series may h a r e their older reversed b y very careful measurement. Kevertheless, the approximate position in the table can be used to choose binary mixtures which lend themselves to easy analysis on the basis of reaction rates. For example, diisopropylamine is not acetylated appreciably a t loom temperature, even after sever a1 days ; whereas m-xylylenediamine is about 50% acetylated in 30 minutes with only one tenth the concentration of acetic anhydride. I n this case, the 30-minute decrease in basic nitrogen content is a measure of mxylylenediamine content, with no correction iequired unless high accuracy is desired. Determination of Tertiary Amines. Tertiary amines do not acetylate, so t h e determination of this class of amines is based on the acetylation of all other baqic amines. Both alkylaryl amines and nonhindered alkyl amines a i e readily acetylated n i t h a large excess of acetic anhydride. Heating is required for certain amines, such as fert-butylamine, which d o not acetylate Ieadily. Conditions for acetylation can he judged on the basis of reactivity as given in Table I. I n application to most samples, it is generally desirable to make two or three deteiminations a t increasing severity. If no significant decrease in basic nitiogen content is observed, acetylation is assumed complete. Caution is iecommended as some types n ith n hich no expel ience was had may not acetylate, even under diastic conditions. Teitiary amine is calculated from the total basic nitrogen content of the material after reaction with acetic anhydride is complete. The unique advantage of this procedure is that no separation of reactants 01 products is necessary. Pyrroles and other nonbasic compounds such as diphenylamine are not determined nor do they interfere with the acetylation of other amines. I n any questionable case, the titration of the original amine in glacial acetic acid should be verified. For example, pylazine. n hich contains t n o nitrogen atoms in a six-membered ring. will titrate; but only one equivalent of perchloric acid will be required for the t\vo nitlogen atoms. Apparently, the formation of a perchlolate salt with one nitrogen atom makes the other nitrogen atom nonbasic. Some highly hindered primary and secondary aromatic aminesfor example. some 2,6-disubstituted anilines-may be difficult to acetylate. KO anomalies were found among the compounds available for this study. The titration and acetylation behavior of unusual compounds should be inves-
1000
ANALYTICAL CHEMISTRY
1 AMIDE-/'
Figure 3. Titration of tri-nbutylamine, and n-propylamine as n-propylacetamide in acetic anhydride
~
$
t PPROPYLAMINE IN ACETiC A C ' O
400
2
4
6
8
IO
I2
14
MLGF 0 IGON HCIG,
tigated before indiscriminate application of the procedures described. Determination of Aromatic Amines. All primary aromatic amines, together with S-alkyl aromatic amines, are sufficiently basic t o be titrated with perchloric acid in acetic acid. These compounds can be determined uniquely because of t h e speed of acetylation. On t h e other hand, diaryl amines are not basic enough t o be titrated with t h e color indicator titration. They do not interfere with the acetylation of othcr amines. For all the compounds studied, the reaction was better than %yowithin 5 minutes a t room temperatures. The application to mixtu1 es of aiomatic amines and alkyl amines shoned that the decrease in basic nitrogen aftei 5 minutes was a reliable measuie of the aromatic amine content with the restriction previoudy cited in mind. Most alkyl amines do not acetylate mole than 1 to 2y0under the conditions of the test. Benzylamines acetylate more rapidly but still have a rate slow enough to make corrections unnecessary, The major interference is from ethylenediamine and from S-p-hydioxyethylcthylenediamine.These compounds are fairly reactive, although much less so than the aromatic amines. To use the reactivity in a determination, the sample is prepared in acetic acid, and acetic anhydride is then added in an amount equal to the total amine as previously determined in a separate basic nitrogen titration. The amount of amine converted to the nontitratable substituted acetamide after 5 minutes is taken as the combined primary and secondary aromatic amine content of the sample. Diary1 amines, however. are not measured because they are normally nonbasic and, therefore, do not titrate. For qualitative identification, the rate of acetylation can be studied by repeated titration of aliquots of the reaction mixture. Misleading results can be obtained in those cases where a small amount of aromatic amine is determined in a large amount of relatively fast acting amine. However, in general the determinations are good. and qualitative identification of aromatic amines through acetylation rates is usually reliable.
Although all basic aromatic amines react very rapidly, some react' much faster than others so that it is possible to analyze many mixtures of known aromabic amines. Aromatic amines require very dilute solution and very rapid analysis. Total Primary and Secondary Aliphatic Amines. Combined aliphatic primary a n d secondary amines, whether hindered or not, can be calculated b y difference from the total basic nitrogen, the aryl amine value, and tertiary amine value. Again provision must be allon-ed for those amines which d o not acetylate rapidly because of steric factors. Determination of Amines in Their Mixtures. The t'itration-acetylation scheme of analysis is rat'hrr !vel1 suited t o the det,rrmination of individual amines in known mixtures where the amines have been qualitatively identified. For example, a mixture of mono-, di-, and tri-n-but'yl'amines can be measured b y this method. T h e tri-n-butylamine is readily determined by the tert'iary amine procedure. Kinetic data indicate that n-butylamine is completely acetylated by a tenfold excess of acetic anhydride in hour, whereas di-n-butylamine requires a 100-fold excess and 1-hour reaction time. By choosing appropriately, a wide variety of quantitative determinations can be made. Determination of Nonbasic Nitrogen. The preceding discussion has been on nitrogen compounds which ordinarily titrate in glacial acetic acid. The definition of basic nitrogen is related t o this titration. Frequently, t h e basic nitrogen is only a small portion of the total nit'rogen content of petroleum. Recently, a procedure for the titration of amides has been reported by T i m e r (IO). Amides are ordinarily too n-eakly basic to be bitrated in glacial acetic acid, but they can be t'itrated in acetic anhydride. Perchloric acid in glacial acetic acid or in dioxane is again used as titrant, and a glass-calomel electrode system is used for end point detection. Color indicators were not used for this titration. 1Iethyl violet, Crystal Violet. and other indicators which perform well in glacial acetic acid do not work as well in acetic anhydride.
Figure 2 shows the titration of tivo amides, S,S'- diethylacetamide and LY,IY'- dibenzylacetamide, in glacial acetic acid and in acetic anhydride. This illustrates well the Feakly basic nature of the amides which give no titration in glacial acetic acid and shows that in acetic anhydride well-defined inflections are obtained. Figure 3 illustrabes the differentiating property of acetic anhydride. I n this figure, tri-n-butylamine and n-propylamine are titrated. Only one break is obtained in acetic acid, but the tlvo breaks in acet'ic anhydride indicate titration first of the tertiary amine, and at the second inflection point, the titration of S-propylacetamide which is formed by the reaction of n-propylamine with acetic anhydride. I n an extension of the work of IYimer, it was ohservcd that in acetic anhydride not only amides but also diphenylamincs having an alkyl substituent in each ring (hercinaftcr called dialkyldiplienylamincsj will titrate. This class of compound is ordinarily nonbasic or even acidic, but its titration in acetic anhydride is illustrated by Figure 4. Diphenylamine itself is a borderline case and doc^ not titrate very satisfactorily. Thc iiiflcction duc t,o dialkyldiphcnj-laniine cowrs about the same potential range as t,hat covered in the titration of amidcs (Figure 4). Therclfor(>, the second inflection in acetic anl-iydi,ide niay in part include amidcs, diaryl amines, and perhaps pyrroles. I n the first experiments the latter also tit'rated, b u t not well enough for quantitative dctcJrmination. Teverthclcss, many of them give an inflection point which is an interference in the titration for amides. Before all pyrroles can be titrated quantitatively, some transformation to a more basic material may be necessary. Nitrogen Distribution in Refinery Streams. A combinat'ion of selective acetylation with titration in a differentiating solvent n-as applied t o various petroleum streams. Table I1 illustrates t h e wide variety of stock t h a t was analyzed, and i t also indicates t h e type of nitrogen compounds occurring in these sbocks. Samples ranging from crude oil to finished gasoline were analyzed. The nitrogen types should be taken only as examples, becaust. considerable variation occurs, depending on tlic origin of t'he crude oil and on the nature of the processing operation. The samples described in Table I1 were not chosen to illustrate specific nitrogen distribution in petroleum stock, but bo sholv the kind of data that have been obtained. I n some cases antioxidants or other inhibitors containing nitrogen had been added and in other cases the stocks were blended to meet specific production requirements.
..
0
400
A-
'a
IO
-2
UL OF 0 IOCN HC10,
Figure 4. Typical titration of dialkyldiphenylarnines in acetic anhydride Although the solvent for the determination was usually acetic acid or acetic anhydride, some work not reported here required the addition of toluene, chloroform, or dioxane to meet solubility requirements. This did not change the results when compared t o runs in which the cosolvent was eliminated. For comparison, t,lie solubility problem \vas met' by using small samples and large volumes of acetic acid or acet'ic anhydride, but nith some loss of scnsitivity. The determinations are normally sensitive enough for measuring a few parts per million of titratable nitrogen. Some pyrroles and diaryl amines n-ill not titrate complet'elj- in the procedure, so that the amide titration may not be t'he sum of all weak bases. Figure 5 illustrates the titration of stove oil in acetic anhydride. This stove oil is knoivn to contain mainly pyrrole nitrogen. The top curve shows the titration of the stove oil directly in the usual way. The bottom curve illustrates bhe same titration with exccss t'itrant follo\wd by a back-titration with trin-butylamine after standing overnight.
Table II.
I
2
4
6
0 ~
2
ML OF 0 IOOh .IC.%
Figure 5. Titration of stove oil in acetic anhydride The inflection point is an approximate measure of the total nitrogen content of the sample. Figure 5 shows that the pyrroles may have reacted in some way, as they are not very stable in acid solution. The transformation niay have been to a compound Jvhich is more basic than the original, thus accounting for the changed titration curve. K i t h finished gasolines, some interference from tetraethyllead can be expected if the sample is heated in t h e presence of acetie anhydride. Heat'ing is not required if a sufficient standing period is allowed for t,he acetylation steps. I n early experiments, heating was used to speed up the acetylation. The result was a higher apparent basic nitrogen after a~et~ylation than before acetylation, sometimes by a factor of two or more. This was traced to the interference of tet'raethylleatl, but no interference occurred \Then thc samples w r e not heated. Further Characterizations. Most' of the characterization work in this study was a t room temperature. Rates increase with temperature, and further differentiations might be obtained if t h e rates change differently
Nitrogen Distribution in Petroleum Fractions Kitrogen Base Titration, X e q . of Amine per Liter Primary Primary and and secondsecondary ary aliTertiVerv nroTertiaryC weakc Totala matic"* ary" phatic" Totalc 11 2 10.7 3 .5 1.4 17.0 6.3 6.3 1 28 0.41 0 . 6 4 0.23 ... ... ... 1.4 11 4 20.0 2.8 1.2 4.4 15.6 15 0 1.0 14.0 . . ... 1.3 ... 0.61 6.7 7 29 6.60 0.05 1 2 , l 5.3 1 57 1.13 0.44 0.05 , . . ... ... 1 27 0.80 ... ... 5.4 ... 5.4
Sample Cycle oil, raw Hydrogenated Raw Gas oil, hydrofined Heavy Waxy Stove oil Crude distillate cuts Start-200" C'. 200"-405" C. 405O-615O C. Diesel fuel Hydrogenated naphtha Reformer feed Finished gasoline Crude oil a In acetic acid. b Excluding diarylamines. c In acetic anhydride.
,..
0 18 2 0
...
65 8
84 1 1 90 24
0.65
...
...
...
, . .
1.72
0.05 15.0 1.16
1.28
...
1.8
...
0.05
, . .
24
0.05
2.0 0.05 ...
...
...
0.05 0.1 0.75
...
1.08
...
0.7 24
... ...
I . .
...
0.05
0.05
1.08 ... 0.7 24
0.05
...
VOL. 32, NO. 8, JULY 1960
...
...
... , . .
1001
LITERATURE CITED
(1) Conant, J. B., H Chem. Sac. 49, 3047 (2) Dea1,JT. Z., Weis!, _-
Figure 6. Reaction of nbutylamine with anhydrides in acetic acid
ANAL.
(3) Hall 5115 ( 141 Kolk Chem. 144, 193-201 (1942). ( 5 ) Moore, R. T., ZIcCutchan, P., Young, D. A , , A K A L . CHEW23, 1639 (1951). (6) Pifer, C. W.,Wollish, E. C., lhzd., 24,300 (1952). \
TIME IN MINUTE
n i t h temperature changes. Some work in this laboratory has confirmed this point. Further selectirity may also be possible with different anhydrides for the amidization step. Other anhydrides may favor specific characterization. Some first experiments with phthalic anhydride and n-octylsucciiiic anhydride indicate that the amidizatioii follows roughly in the same order as with acetic anhydride; but the kinetic curve is not the same, in-
dicating a possible differencc in mcclianism involved. This is shown in Figiuc 6, where the curves for two differmt amidizations of the same amine c ~ o s s each other. Further approaches to the general pi oblem of characterizing nitrogen compounds are suggested in the reaction of amines with other reagents, such as carbon disulfide. phenyl isothiocyanate, and oxidizing agents to attain changed titration characteristics.
,
(7)-Richter, F. P., Ceasar, P. D , Meiseh, 6. L., Offenhaur, R. D., Ind. Eng. Chein. 44, 2601 (1952). (8) Sakami, W.,Toennies, G., J . Bzol. Chen2. 144, 203-217 (1942). (9) Wagner, C. D., Brown, R. H , Peters, E. D., J . A m . Chem. Soc. 69, 2609 (1947). (10) Wimer, C., ASAL. CHEW 30, i7-80 (1958).
RECEIVED for review December 22, 1958. Accepted February 15, 1960. Division of Petroleum Chemistry, 135th Meeting, ACP, Boston, Mass., April 1959.
Potentiometric Determination of Sulfide ions and the Behavior of Silver Electrodes at Extreme Dilution M. W. TAMELE, V. C. IRVINE', and L. B. RYLAND Emeryville Research Center, Shell Development Co., Emeryville, Calif.
b An accurate and sensitive analytical method for the potentiometric titration o f sulfide ions with silver nitrate has been developed. The behavior o f silver electrodes was studied over a broad concentration range of sulfide and silver ions. The optimum conditions were established, the true equivalence point was located, and the nature of the titration curve was explained. At moderate concentrations the silver electrode potential varies normally with concentration o f hydrosulfide ions (pH 8 ) or of silver ions. Below lO-'N hydrosulfide concentration, the high negative potential drops abruptly on further dilution and then approaches a low constant value, determined b y the inert electrolyte used. The electrode potential changes regularly with silver ion concentration down to lo-", and then gradually approaches the same potential determined b y the inert electrolyte. This potential determines the position of the equivalence point.
B
of the great insolubility and easy flocculation of silver sulfide, potentiometric titration of sulfide ions by silver ions is capable of great ECAUSE
1002
ANALYTICAL CHEMISTRY
accuracy. However, no exact procednre has been reported. Dutoit and von Weisse (22) were a p parently the first to attempt potentiometric precipitation of heary metal sulfides and reported errors as large as 10 to 200~0',. Treadwell and Weiss ( I S ) described only one inconclusive determination of sulfide with silver nitrate. Willard and Feiinick (17') reported accurate results in fairly concentrated solutions using ammoniacal silver nitrate soliit'ions. Kolt'hoff and Verzijl (8) studied the precipitation of a number of sulfides of heavy metals and observed that the precipitation of silver sulfide gave Patisfact,ory results when the sulfide solution was more than 0.025; in 0.0001S solutions the error iyas found to be 8 to lOYc. The titration is perfornied in a solution huffered by sodium acetate, using a sulfide-coated silver electrode to a predetermined potent,ial. The technique is basically the same as used by the authors previously for the potentiometric determination of mercaptans (thiols) ( l a , 2 3 ) . Previous workers in the titration of sulfide ions had difficulty in locating the proper end point by classical methods. The titration curve is asym-
metrical, and just prior to reaching the equivalence point, shows a n unusualljlarge and rapid drop in potential without a readily detectable inflection point. EXPERIMENTAL
Reagents. Sodium sulfide of high purity as first prepared by t h e method of I3ottger ( I ) , but later material of the same degree of purity could be obtained by selecting large crystals of Merck reagent sodium sulfide and removing the surface layer of oxidation products by washing with distilled water. Solutions of approximately the desired strength were prepared in oxygen-free distilled I\-ater immediately before use. The exact normality of the solution n-as established by the bromide-bromate method ( I C ) , and in several cases checked by the iodonietric method. The absence of sulfite and thiosulfate impurities was evident from the potentiometric titration curve n i t h standard silver nitrate. Both these salts are precipitated by silver nitrate a t a more positive potential than sulfide, and when present in a small amount, cause a detectable bend in the steepest portion of the curve. The solutions of silver nitrate were prepared from hlerck reagent grade 1 Present address, Shell Chemical Corp., San Francisco, Calif.
