potentiometric titration of hydrosulfide should be accurate if performed in a moderate excess of sodium acetate using a sulfide-coated electrode. The accuracy of this method was tested by comparing potentiometric and bromometric titrations, and coiiiplet’e agreement K R S found in 0 . 1 s hydrosulfide solution-, The accuraq- of the method in more dilute solutions was determined b y the titration of progressively diluted solutions of sodiuni sulfide prepared in the nianner described before. The titrations were performed in a beaker esposed to the atmosphere using a sulfide-coated silver electrode. The ,sample of ;odium sulfide was added to the beaker containing oxygen-free 0.LY sotliuni acetate and the volume adjusted to 100 nil. with the acetate solution. The titrations were performed with 0.1. 0.01. or 0.0015 silver nitrate. The wwlt:. of two series of such deterniinntioiis arc shor7-n in Table 111 and wveral of the titration curves are s1ion.n in Figurr 7 . Thc error of 1.0% which is shown by thc titration of the approximately lOW4-Y solution is traceable to the deviation of 0.01 nil. of the titrating solution. \yliich \vas in this case 0.01S silver nitrate. K h e n 0.0OLY silver nitrate \vas used to titrate these very dilute mlutions. the error was eliminated and :I 10 times more dilute solution, approsiinately 1O-”S, could still be aiial!-zed with the same error of 1%.
Because the maximum deviation in any of the determinations amounted to 0.02 ml. of the titrating solution, the accuracy of this method is considered to be within the limits of accuracy of the volumetric equipment used. Greater errors than the above may be caused by delay leading to oxidation of yery dilute hydrosulfide solutions. I n most cases the removal of oxygen from the solutions used (sodium acetate, silver nitrate) is sufficient; but for the titration of I O - S S or less concentrated solutions, the additional precaution of carrying out the titration in an atmosphere of nitrogen is recommended. LITERATURE CITED
( 1 ) Bottger, IT.,Ann. 223, 235 (1884). ( 2 ) Dutoit, P., von Weisse, G., J . chim. phys. 9, 608 (1911).
(3) Freyberger, 77’. I,.. de Bruyn, P. L., J . Phus. Chem. 61, 586 il9571. ( 4 ) Goite te... J. R..‘ Cole. -1.‘ G.. Grav. E. L., Faux, 3 . ’D., J . Ani. Chem. Sic: 73, i o 7 (1951). ( 5 ) Golding, R . AI.. J . Chem. SOC. 1959, 1838. ( 6 ) Kolthoff, I. AI., Furman, S . H., “Potentiometric Titrations.” 2nd ed.. n. 7 . Wilev. Sew York. 1933. ( i j Kolthoff, I. >I., Lingane, J. J., J . Am. Chem. SOC.58, 1524, 2457 (1936). (8) Kolthoff, I. M., Verzijl, E. J.. Rec. trat~.chim. 42,1055 (1923). 19) Ravite. S. F.. J . Phus. Chem. 40. 61 (1936). ’ (10) Schmid, A., Winkelmann, It7.>Vogele P., Helu. Chim. ilcta 16, 398 (1933), and previous articles. (11) Stone, H. W.,J . 9ni. Chem. SOC. 58,2591 (1936).
Table 111. Accuracy and Range of Applicability of Potentiometric Hydrosulfide-Silver Titration
Concn., Equiv./Liter Dev. In cel1,a Found by from calcd. from potentiometric Calcd. dilution method Value, % Titrating Solution, 0.01S AgXO3 0 02470 0 08 0 02472 n on989 n 00989 on 0 00494 0 00494 0 0 0 002472 0 002472 0 0 0 1 0 000989 0 000988 0 0000989 0 0000980 10 Titrating Solution, 0.001.V ilgSO3 0.02273 0.02273 0.00 0 00091 0 00091 0.0 0 000091 0 000091 0 0 0.0000091 0 0000090 1.1 Determined in 0.1S solutions by bromide-bromate method. 0
(12) Tamele, 11. IT., Ryland, L. B., IND.ENG. CHEX, ANAL. ED. 8, 16 (1936). (13) Tamele, 11. \T-., Ryland, L. B., Irvine, J-. C., Zbid., 13, 618 (1941). (14) Treadwell, IT. D., Mayr, C., 2. nnorg. Chem. 92,127 (1915). (75) Treadwell, W. D., JTeiss, L., H e l l . Chzm. Acta 2,680 (1919). (16) Van Rysselberghe, P., Gropp, A. H., J . Chem. Educ. 21, 96 (1944). (17) Killard. H. H.. Fenmick. F., J . Ani. Cheni. SOC.45, 645 (1923). for !‘eVieV October 22, 1959. Accepted April 11, 1960. RECEIIED
Simultaneous Determination of Hydrogen Sulfide and Mercaptans by Potentiometric Titration M. W. TAMELE, L. B. RYLAND, and R. N. McCOY Erneryville Research Center, Shell Development Co., Emeryville, Calif.
b Potentiometric titration procedures using silver nitrate for the determination of hydrogen sulfide or mercaptans have been extended to the simultaneous determination o f mixtures of the two. Accurate results are obtained for mixtures of hydrogen sulfide and lower molecular weight mercaptans when titrated in an aqueous electrolyte which i s 1 N in sodium hydroxide and 0.05N in ammonium hydroxide. For the analysis o f waterinsoluble samples, if higher molecular weight mercaptans are present or sodium acetate in ethyl alcohol i s used as electrolyte, coprecipitation affects the accuracy of the individual determinations. In the aqueous electrolyte hydrogen sulfide recoveries are low and mercaptan values are corrctspond-
ingly high; in the alcoholic electrolyte the effect i s reversed, but results are correct for the sum of the two components in both cases. If separation into components i s necessary, the amount of mercaptan i s determined b y titration of a second sample from which hydrogen sulfide has been removed b y treatment with cadmium sulfate solution. Elemental sulfur interferes with the direct titration, but procedures for its detection and means of averting its interference are described.
A
numerous methods are available which give approximate amounts of sulfur compounds in petroleum products, often specific, acLTHOEGH
curate, and sensitive methods are required for determination of the wide variety of sulfur compounds which occur in the many products that are derived from crude petroleum ( 2 . 11). Certain of thebe compounds, such as hydrogen sulfide and mercaptans (thiols) or their soluble salts. occur both singly and together in many petroleum products and in the aqueous extracts employed in refining (13). Karchmer ( 6 . 7 )has published an integrated scheme for determination of mercaptans and hydrogen sulfide mixtures in the presence or absence of elementary sulfur and of various sulfur compounds. Simultaneous titration of bulfide and mercaptans is part of this scheme ( 7 ) . Khile the potentiometric titrations of individual sulfur compounds ’iT’ithsilver niVOL. 32, NO. 8, JULY 1960
1007
trate and silver electrodes are capable of great accuracy (14-16), the titration of various mixtures is beset with unexpected difficulties (6, 7 ) . Hydrogen sulfide and mercaptans are differentiated by two distinct drops of potential in the titration curve which occur in widely different zones characteristic of the respective compounds. The method has been studied quantitatively and its limitations have been pointed out. THEORY
The procedure is based on the potentiometric titration of mixtures of hydrogen sulfide and mercaptans with silver nitrate solution using a silver wire coated with silver sulfide as the indicating electrode. Aqueous samples are titrated in 1 S aqueous sodium hydroxide to which ammonium hydroxide has been added to prevent precipitation of silver oxide. Samples not soluble in the aqueous electrolyte are titrated with alcoholic silver nitrate in alcoholic sodium acetate. Silver sulfide is precipitated first a t a high negative potential and a marked change in potential occurs when the precipitation is completed. Then the precipitation of silver mercaptide commences a t a lower potential and a t the completion of the mercaptide precipitation another marked change in potential occurs. The precipitation reactions are quantitative and, for mixtures of hydrogen sulfide and low molecular IT eight mercaptans in aqueous samples, directly applicable to mixtures of these materials in almost any proportion. Coprecipitation occurs in the aqueous electrolyte TT ith mixtures of hydrogen sulfide and higher molecular weight mercaptans which results in low recoveries of sulfide and correspondingly high values for mercaptan; a reversed coprecipitation effect occurs in the alcoholic electrolyte which results in high values for sulfide and correspondingly low values for mercaptan. In these cases it is necessary to titrate a separate portion of the sample from which sulfide has been removed. Elemental sulfur interferes with the procedure, as it reacts with the mercaptan and the reaction product imparts to the electrode a potential too close to that of sulfide ion. Its presence is evidenced by formation of a yellow color when the sample is added to the electrolyte, by a high potential, and by formation of black silver sulfide during titration of a sample from which hydrogen sulfide had been removed. I n this case the total titer to the mercaptan end point, if mercaptan is in excess of sulfur, accurately represents the mercaptan initially present. If elemental sulfur is in excess of the mercaptan, all the mercaptan appears as sulfide; the excess sul1008
ANALYTICAL CHEMISTRY
Figure 1. Typical titration curves for sulfide and mercaptans 0.1 N sodium acetate electrolyte Shape in aqueous sodium hydroxide-ammonium hydroxide electrolyte Glass reference electrode
__--
sufficient' for a large number. of titrabions, but if anomalous titration curves are obtained it is advisable to remove the sulfide coating completely and to repeat the above procedure. In most of this work the cr11 solutions ryere not protected from air during tiEXPERIMENTAL tration, but later experience has shovn Apparatus. Two different pot'enthat, particularly n-hen 0.01-V titrant tiometric titration systems wei'e used was used, stable electrode potentials in this work. Initially, the potentiowere reached more rapidly in the vicinmetric circuit described by Tamele ity of the end point if the caell was and Ryland ( 1 ~ 7which )~ utilized a 0.1-V blanketed with inert gas. -4 static sodium acetate reference elect'rode conblanket was preferable to flowing gas benected to the titration cell by a salt cause of t'he volatility of certain merbridge and a galranometer to indicate captans, even from alkaline solutions the null point', was used. More recently t'he direcbreading vacuum-tube (16). Presumably, this niore rapid type Titronieter described by Penther rquilibration was due to prevention of and Rolfson (10) was used. With this air oxidation of the small amounts of latter apparatus a glass referrncse elecsulfide or mercaptan present in solution trode may be used as shown by Lykken as the end point was approached. and Tucmmler (9). There lyas no Reagents. All reagent solutions significant difference in the results obtained with either system; with a glass must be free of dissolved air and peroxidic impurities t'o prevent losses reference electrode, the observed potenof the easily oxidized sulfur comtials were more positive by about 150 pounds. Percolation of solvents mv. The latter system is niore convenient and is preferred. through activated alumina, removal of dissolved air by bubbling nitrogen The silver sulfide indicating electrode for a few minutes, and storage of t h e consisted of a straight silver (99.9% Ag) t'reated solvents under inert gas rod about 2 mm. in diameter that was are convenient for maintaining a immersed a few centimeters in the supply of solvents. titrated solut'ion. It was polished to a Acid Cadmium Sulfate Solution. clean silver surface with fine emery cloth, then further cleaned by soaking Dissolve 150 grams of cadmium sulfate octahydrate in distilled water: add 10 in potassium cyanide solution, and finally coated with silver sulfide either nil. of 45% sulfuric acid, and dilute t o electrolytically or by performing a 1 liter. Alcoholic Titration Solvent. Dispreliminary titration of sodium sulfide. solve 13.6 grams of sodium acetate triAfter coating, the electrode was rinsed, wiped with a soft tissue, and burnished hydrate in 25 ml. of distilled wat'er and lightly with fine emery cloth. The add 975 ml. of anhydrous ethyl alcohol. U. S. Treasury Depart,ment denaturant electrode was soaked in alcohol containing sodium acetate and 0.5yc silver formulas 2B and 3A are satisfactory. nitrate for 5 minutes before use and Aqueous Titration Solvent. Dissolve 40 grams of sodium hydroxide in disstored in the same solution d i e n not in use. Before each titration the electilled water, add 3.3 nil. of ammonium hydroxide (specific gravit'y 0.90), and trode was burnished lightly with a soft cloth. This treatment is ~ioi~rnally dilute to 1 liter with distilled water. fur remains unreacted. d method for the direct determination of elemental sulfur by this titration procedure is a t present limited in its application and requires further development (6. 15).
Silver Nitrate Solution, standard 0.1A' aqueous. Silvei Nitrate Solution, standard O.lAYalcoholic. Dissolve 17 grams of silver nitrate in 170 ml. of distilled water and dilute to 1 liter with isopropyl alcohol. Silver Xitiate Solution, standard 0.01.Y aqueous. Prepare by exact dilution of the 0.1S aqueous silver nitiate solution with di5till~dwater. Silver Kitrate Solution, standard 0.01A' alcoholic. Prepare by exact dilution of the 0.1A- alcoholic silver nitrate solution with 927, isopropyl alcohol. Solutions of 0.01S silver nitrate should not be stored longer than 24 hours. Procedures. DETERMINATIOX OF
continue without interruption. Plot the titration curve and select end points at the bottom of the steep portions of the titration curve as shown in Figure 1. ADDITIONAL TITRATIONS.If the titration curve s h o w only the presence of mercaptan-when both hydrogen sulfide and elemental sulfur are absentno additional titrations aIe required.
Table I.
If the titration curve shows the presence of hydrogen sulfide and elemental sulfur is not known to be absent, it is necessary to perforni an additional titration aft'er t.straction of hydrogen sulfide (3). HYDROGEN SULFIDEEXTRACTION. Measure at least three times the quantity of hydrocarbon sample used in the
Titration of Mixtures
Equiv.
Ratio, Recovered, (7 Sulfide/ 1ierSulfide, Meq. Mercaptan, N e q . MerMercaptans captan Added Found Akided Found Sulfide captan Total HYDROGEN SULFIDE A K D MERCAPTAN Sodium Sulfide and Various Mercaptans with 0.1.V Silver Nitrate in Aqueous 1S Sodium Hydroxide, 0.05Il' .4mmonium Hydroxide IN AQUEOUSSOLUTION. Measure a quantity of sample into the titration 101 9 Methyl 1.020 99.0 100.3 1,010 0,785 0 800 cell t h a t will require, preferably, from 0.510 0 ,392 100.0 90 5 99,5 0.510 0 390 2 t o 10 ml. of 0.1 or 0.01S silver 0.201 0,302 0 400 0,200 98.0 102 0 100.7 98.0 102 0 101.2 0 400 0.102 0,100 0.392 nitrate solution t o titrate t h e sulfide 99.5 $10 2 99.5 1.015 1.020 0.078 0 075 and mercaptan present. Add 100 ml. 09.0 101 0 100.0 1.010 Ethyl 1.020 1.040 1 050 of aqueous titration solvent ( I S 99.0 0,520 0,510 0 520 9 8 . 0 100 0 0,500 in sodium hydroxide and 0 . 0 5 s in B9. 4 0.204 98.0 100 0 0 520 0.200 0.520 ammonium hydroxide), immediately 0 520 98.0 100 0 99.7 0.102 0.520 0,100 place the cell on the titration stand, 99.0 105 8 09 . 6 0.104 0 110 1.010 1.020 place the tip of the buiet below the 100 0 09.5 1 000 99 . 0 1.000 1.010 1.000 n-Propyl surface of the solution, and adjust the 100.0 09,O 101 0 0.505 0,500 0.500 0 505 99 . 7 90. 0 100 0 0.202 0,200 0,500 0 500 stirrer to give vigorous stirring nithout 100 0 99.8 99.0 0,100 0 500 0.101 0,500 diau-ing air into the solution. Record 99.0 100 0 99.1 0 100 1.010 1.000 0,100 the initial cell potential. 4 d d small 100.8 1 015 98.0 103 0 1.000 Isopropyl 0 . 980 0,980 portions of silver nitiate solution and, 08.0 104 1 101 .0 0.490 0 510 0.500 0.490 aftcr each addition, wait until a con104 1 101.4 0 510 05.0 0.190 0,490 0.200 stant potential has been obtained and 100.0 90.0 102 0 0.490 0 500 0,100 0,090 iecord the buret and meter readings. 96.5 0 090 97.0 c)1 8 0.098 1.000 0 . 970 101 [I At the start of the titration and in 100.0 1 070 08.0 0.990 1.050 Isobutyl 1.010 $19.0 101 0 100.0 0.525 0 5.30 0,505 0,500 I egions between inflcctions n here the 99.7 100 0 99.0 0 525 0.202 0.200 0.525 late of potential change is low, add 09 , 8 80.1 101 0 0 535 0.090 0.525 0.101 volumes of silver nitrate solution as 104 8 90.6 0 110 09.0 1,010 1,000 0.105 large as 0.5 nil. When the rate of 100.3 1 000 9 8 . 5 102 0 0,995 0,980 0.980 sec-Butyl change of potential becomes greater 100.2 98.4 102 0 0.490 0 500 0.498 0 . 490 than 5 mv. for a 0.1-ml. increment, add 100.1 0 5 . 5 10%0 0 500 0,199 0.190 0.390 0.05-ml. increments of 0.LV or 0.1-ml. 102 0 100.0 90 . 0 0,090 0.490 0 500 0.100 increments cf 0.01N titrants. Con09,1 99.5 102 0 0 100 0,995 0,990 0,098 0 930 1KI.0 101 1 100.0 1.030 1.020 0,920 tert-Butyl tinue the titration until the rate of 100 0 99 . 5 99.0 0 460 0.510 0 . 460 0.515 change of potential is less than 2 mv. 102 2 100.6 E .1 0,200 0.460 0 470 0.206 for a n 0.1-ml. increment of titrant 103.0 0 180 97.1 104 :3 0.460 0.103 0,100 and the meter reading is approximately 0 100 100.0 108 7 100,1 1.030 0.092 1.030 +200 mv. (when using the glass refer101 0 1 00,5 97.1 1 030 0.990 1.020 0,990 n-Butyl ence electrode). Plot the titration 99,5 04.1 105 1 0,495 0 520 0.510 0,480 curve obtained, and select end points 0.495 0 540 58.8 100 1 100.5 0.102 0,060 for the sulfide and mercaptan present 111 1 100.1 0 110 09.0 0,099 1.020 1.010 1 105 79.3 122 8 100.0 0 , 900 a t the bottom of the steep portions of 0.785 0.990 n-Amyl 09.5 70.7 131 1 0,495 0.450 0 590 0.350 the titration curves as shown in Figure 1. 98.8 0,100 0 510 50.5 120 0 0,198 0.450 Calculate the amounts of sulfide and 100.2 0 . 0 11"' 2 0,009 0.00 0,450 0 550 mercaptan sulfur present using a n equivalent weight of 16 for sulfide and 32 for mercaptan. Blank titrations are Hydrogen Sulfide5 and Llercaptansb in 0.1S .Ilcoholic Sodiuni .\cetate unnecessary if pure reagents are used. 103 2 W 4 99 . 3 0 668 0 707 0 873 0 901 DETERMINATION OF HYDROGEN SUL- n-Propylc 99.6 92 6 0 655 105 2 0 707 0 869 0 914 FIDE AKD ~ I E R C A P TIN A PETROLEUM KS 99.4 106 0 01 5 0 655 0 716 0 869 0 921 PRODUCTS. Measure a quantity of 105 8 (13 1 99.8 0 0774 0 0819 0 0700 0 0652 sample as directed above into a titration 07.3 61 0 150 0 o no41 o o m 0 00$2 0 0063 cell containing 100 ml. of alcoholic $12 6 99.3 0 649 106.3 0.601 0 655 0,616 n-Butylc o.. mi 99.3 92,6 _ . ~ 106.3 titration solvent. Limit the sample 0 655 0.6i9 0.616 98.6 94.3 0,0612 103.1 0 0649 0.0607 0 0626 size to a maximum of 25 grams; sam00.0 99 2 100.7 0.302 0 238 0 300 0.240 ples nhich are not soluble in alcohol 90.2 108 4 99.5 0,590 0.532 0.616 0.668 may he dissolved in u p to 40 ml. of 90.2 99.5 0.532 108 4 0.668 0.590 0.616 benzene before addition of the titration 9 6 . 6 100.2 0,0607 0.0629 0.0588 0.0568 103.6 solvent. Proceed with the titration 104,2 94.3 99.1 0.623 0.661 0.616 0.642 n-Heptylc as directed above, but continue the 99.5 95.2 0.629 104,2 0,661 0.616 0.642 titration until a cell potential of approxiAdded as aqueous solution of sodium sulfide. mately +250 mv. has been reached Added as iso-octane solution. (if 0.1N sodium acetate electrode is Ratio, 1 to 1. Average of six determinations, mercaptan dissolved in isopropyl alcohol. used). It is important to commence the titration as soon as possible and to VOL. 32, NO. 8, JULY 1960
e
1009
initial titration into a nitrogen-flushed separatory funnel and add, if necessary, enough iso-octane to make a total volume of a t least 25 ml. of hydrocarbon phase. Add an equal volume of acid cadmium sulfate solution and shake vigorously for several minutes. A l l o ~to stand until a good separation is obtained, drain, and discard the aqueous phase containing the yellow precipitated cadmium sulfide. Repeat the extraction and separation until no further cadmium sulfide is precipitated. Filter the hydrocarbon layer through a dry filter paper into a volumetric flask. Rinse the separatory funnel with isooctane as necessary. transfer an aliquot to a titration cell containing 100 ml. of alcoholic titration soh-ent, and titrate as before. Disregard any apparent hydrogen sulfide end point and calculate the mercaptan content of the sample from the total titration to the mercaptan end point. RESULTS
Because complications niay arise in the titration of more than one component-Le., coprecipitation in the potentiometric titration of mixtures of halide ions with silver nitrate (8)-mixtures of known amounts of sulfide and various mercaptans in different ratios rvere t'itrated to determine the reliability of the results. Solutions of sodium sulfide in oxygen-free distilled water and various mercaptans (ranging from methyl to n-amyl) in oxygen-free isopropyl alcohol werp prepared a t concentrations of approximately 0.l.V. The individual solutions were standardized shortly before use by the potentiometric tit'ration procedure. Xixtures of sodium sulfide and the individual mercaptans were prepared from these solut,ions such that the equivalent rat'io of sulfide to mercaptan varied from 0.2 to 10. These mixtures were titrated using the reconimended procedure for aqueous samples and the end points were selected a t the loiver end of the steep portions of the titration curve as shown in Figure 1. Selection of the end points in this manner has been shown necessary for accurate determinations (14-16). Table I s h o w the particular mixtures tested and the reroveries obtained. These data show that results are generally satisfactory for the mixtures of sulfide and met'hyl. ethyl. n- and isopropyl. and sec-, tert-! and isobut'yl mercaptans. The results obtained for nbutj-l and n-amyl niercaptans shon- a marked coprecipitation effect even when the sulfide and merraptans are present in approximately cquivalent amounts. This effect is evident even with the l o w r molecular \\-eight mercaptans, and is considered to be caused by adsorption and subsequent coprecipitation because the total titer obtained for the various mixtures is correct, although the sulfide recovery is 1011- and the mercaptan recover?- is high. I n this case sulfide
1010
ANALYTICAL CHEMISTRY
ion is not completely precipitated a t the selected end point. This was illustrated by an experiment in which two solutions, one containing 0.1 meq. of sulfide and 0.5 meq. of nbutyl mercaptan and the other 0.1 meq. of sulfide and 0.5 meq. of tert-butyl mercaptan, were prepared in the aqueous 1 5 sodium hydroxide-0.0LY ammonium hydroxide electrolyte. These solutions were titrated potentiometrically until the titration curves indicated that the precipitation of silver sulfide was completed; then the titrations were interrupted and the suspended precipitates were allowed to coagulate, after which they were separated. Atlditional silver nitrate \\-as then atldcd to the clear solutions.
-1 black precipitatr fornicd in the solution containing the n-butyl niercaptan indicated that sulfide i m s \Yere still present; a white precipitate formed in the tert-butyl mercaptan solution indicated that significant amounts of sulfide ions were no longer present. The coprecipitation effect observed n-ith tert-butyl mercaptan is less than n-ith nbutyl mercaptan (Table I). The applicability of the alcoholic titrat'ion medium to the simultaneous determination of hydrogen sulfide and mercaptans was tested in a similar nianner? except that the mercaptans were dissolved in iso-octane. As a source of sulfide ion, aqueous sodium sulfide solutions were used because of the difficulty of preparing and handling st'able standard solutions of hydrogen sulfide in hydrocarbon solvents. The quantity of aqueous solution used was such that' the concentration of alcohol in the titration solvent was always greatcr than 90%. Approximately equivalent amounts of sulfide and various mercaptans. ranging from n-propyl t'o n-heptyl, rvere prepared and titrated by the recommended procedure using the alcoholic titration medium. Table I s h o w the mixtures tested and the recoveries obtained. These data also show that a marked coprecipitation effect occurs but, unlike that observed in t h r aqueous solvent, high values are obtained for sulfide and correspondingly low recoveries for mercaptans. Thus it appears that some silver mercaptide is precipitated in the sulfide portion of the titration curve. Titrations in more dilute solution with 0.01S titrant did not markedlJ- decrease this coprecipitation. DISCUSSION
The procedure for the simultaneous titration of aqueous solutions of hydrogen sulfide and mercaptan in the same solution is directly applicable to the determination of mixtures of sulfide and the lower molecular Tveight mercaptans ovpr equivalent ratios of a t least 0.2 to 10. Mixtures containing n-butyl and higher mercaptans give results n hich
tend to be low in sulfide and correspondingly high in mercaptan. Although in the latter case the results for individual components are not' quantitative, the total recovery is correct and significant results are usually obtained for the individual components. The direct method is useful for control work where speed is necessary and large numbers of similar samples are analyzed. The procedure for the simultaneous determination of hydrogen sulfide and mercaptan in petroleum fractions using the alcoholic electrolyte is less accurate for the determination of thc individual compounds! as coprecipitation effects, observed even wit'h the low nioleculir weight mercaptans, result in high v a l u e for hydrogen sulfide and a correspondingly loiv recovery of mercaptan. The presence of elemental sulfur in samples complicates the application of the potentiometric tibration procedure and interpretation of the titration curves, particularly for the determination of hydrogen sulfide. In strongly alkaline aqueous solutions elemental sulfur is generally considered to react with mercaptides according to the following reaction (12) : S
+ 2SaSR = S a 2 S+ R &
This reaction is rapid and complete. Because sulfide is produced in an amount equivalent to the mercaptan consunied. the potentiometric titration curve will show the presence of sulfide; however. t,hr total titration to the mercaptan end point represents the amount of mercaptan initially present, provided that t'hc mercaptan is in excess. If sulfur is in excess of the mercaptan. the titration curve will not show the presence of mercaptan, but the hydrogen sulfide titer will represent the amount of mercaptan initially present. If hydrogen sulfide is also present in the sample. correct interpretation of a single titration curve is impossible. In the less alkaline alcoholic solvent a different reaction occurs which niay bc represented as follom:
S + 2SaSR
+ 2A4g' =
+ R& + 2 S a -
.4kS
The reaction apparently proceeds in two steps : S 2SaS8R
+ RSSa
+ 2Ag+
-+
-t
SaSSR
Ag.8
+ R2Sa + 2 S a -
This reaction \vas observed in this laboratory many years ago by Tamele and Ryland, whose interpretation was quoted in 1943 by Davies and hrnistrong in their report on estimation of mercaptans in presence of elementary sulfur in use a t the laboratories of the Shell Devclopment Co. (4). It has been made a basis of a determination of sulfur in presence of mercaptans. The method is limited in its application.
It has bren recently studied by Karchmer (6),who believes that the poor results observed under certain conditions can be attributed to the presence of inorganic polysulfides. h better understanding of this reaction would be desirable for development of an exact potentionietric procedure for the det'ermination of elemental sulfur in hydrocarbons alone and in the presence of other sulfur compounds. When elemental sulfur is present togetlior n.ith mrrcaptan in the alcoholic solwrit, t,he rlect'rode potentials are slow to reach equilibrium in the vicinity of the end point. This is believed to be a phcnonienon associat'ed viith the complex reaction mechanism by which silver sulfide is precipitated from a solution that doe3 not contain free sulfide ions. If elemental sulfur is known to be present, it is preferable t'o remore it by extraction with metallic mercury ( 1 , 5 ) , although a t a risk of losing some mercaptan (6). Interfering Materials. Only a limited number of materials interfere with t h r potentiomet'ric titration procedure. Previous n-ork (14-16) con-
Thermal Properties
cerned with t h e application of t h e potentiometric procedure t o t h e determination of hydrogen sulfide or mercaptan alone showed t h a t t h e majority of substances normally encountered in petroleum processing did not interfere. Certain anions such as iodide, bromide, and cyanide, which form very insoluble silver salts. precipitate in the same potential region as silver mercaptides and thus interfere. Thiosulfate and sulfite ions cause difficulty with the determination of mercaptans in the alcoholic solvent, but they do not interfere in the aqueous titration solvent. Occasionally strongly reducing qubstances are encountered which interfere by reducing the silver titrant to metallic silver; this interference I' evidenced by a rapid darkening of the solution and formation of a silver mirror on the nalls of the titration vessel. LITERATURE CITED
(1) Ball, J. S., U.S. Bur. Mines, Rept. Invest. RI 3591 (December 1911). ( 2 ) Ball, J. S., Rall, H. T., Kaddington,
G., Smith, H. IT.,"Sulfur Compounds in Petroleum," Division of Petroleum
Of
Chemistry, 119t.h Aleeting, ACP, Cleveland, Ohio, April 1951. (3) Borgstrom, P., Reid, E. E., I s u . EXG.CHEIM., ASAL.ED.1, 186 (1929). (4) Davies, E. R. H., Armstrong, J. IV., J . Inst. Petrol. 29, 323 (1943). (5) Faragher, W. F., Morrell, J. C., Monroe, G. S.,I n d . Enq. C h e m 19, 1281 (1927). (6) Karchmer, J. H., .%SAL. CHEM.29, 425 (1957). ( i )Ibid., 30, 80 (1958). (8) KolthofY, I. LI., Furma;!, ll-. I T . , "Potentiometric Titrations, 2nd ?(I., pp, 141-58, \Tile>-,S e w Tork, 1!131. (9) Lykken, I,., Tueniniler, F. 11.) ISD. EXG.CHEM.,SAL. ED. 14,BT (1942). (10) ,Penther, C. J., Rolfson, F. 13., Ibzd., 15,337 (1943). (11) "Progress in l'etrolerim Tt,chiiology," Advances in Cheni. Series, S o . 5 (1951). (12) Stagner, B. A , , Itid. En!/. Chew. 27, 275 (1935). (13) Tait, T., -4dz3ances in Cheui. Se/.ies, s o . 5 , 151 (1951). (14) Tamele, 11. IT'., Irvine, I-. C., CHEX 32, 100'2 Ryland, L. B.. .ls.i~. (1960). (15) Tamelr, hI. IT,, Ryland, L. B., ISD. Esti. C H E ~ I . hsar,. , ED. 8, 10 (1936). (16) Tamele, M. W., Rylarid, L. R. Irvine, V. C., Ibid., 13, 618 (1941 1. RECEIYED for review October 2 2 , 195!) Accepted April 11, 1960.
Q u i nolinium Phosphomolyb d a te
WESLEY W. W ENDLANDT Department o f Chemistry, Texas Technological College, Lubbock, Tex.
WILLIAM M. HOFFMAN Fertilizer Investigations Research Branch, Soil and Water Conservation Research Division, U. S. Department of Agriculture, Beltsville, Md.
b The thermal properties of quinolinium phosphomolybdate, which i s useful for the gravimetric determination of phosphorus, were studied b y use of the thermobalance and differential thermal analysis. The air-dried compound began to lose weight a t 107" C., giving a good horizontal weight level from 155" to 370" C. which corresponded to the anhydrous (CgH,N)3H3P04.1 2Mo03. From these studies it i s concluded that the precipitate possesses excellent thermal properties and can be safely dried to constant weight in the 150" to 300" C. temperature range. method for the determination of phosphorus by the alkalimetric titration of the quinolinium salt of 12-phosphomolybdic acid, (C9H;K)3H3P0,.1211003, has been developed by Wilson ( 7 ) . His attempts to establish a gravimetric method based VOLCMETRIC
upon drying the salt a t 105" C. were unsuccessful. Perrin ( S ) , however, found that the quinolinium phosphomolybdate could be ignited a t 550" C. to phosphomolybdic anhydride, P205.24hloO3, or dried to the theoretical molecular weight a t 200" to 250" C. He presented a gravimetric procedure, based upon a drying temperature of 250" C., which is nearly as fast as the volumetric method and is less tedious than the method based on the precipitation of ammonium magnesium phosphate. Fennel1 and K e b b ( 1 ) developed a gravimetric semimicrodetermination of phosphorus based on quinolinium phosphomolybdate and found that the precipitate could safely be dried to constant weight a t 160" C., and that no decomposition occurred below a temperature of 370" C. Jacob and Hoffman ( 2 ) also used the gravimetric quinolinium phosphomolybdate method in a collaborative study on the determination of phosphorus in fertilizers.
Because the thermal behavior of thc quinolinium phosphomolybdate under the conditions of its drying is important for a gravimetric determination. the thermolysis of this compound was studied on the thermobalance and by differential thermal analysis. EXPERIMENTAL
Thermobalance. T h e thermobalance and procedure used in t h e thermal decomposition studies h a w been described ( 4 , G ) . A heating rate of 4.5" C. per minute was employed with sample sizes ranging in weight from 95 to 100 mg. The isotherms were determined by therniostating the furnace a t a constant temperature using a TTrst Instrument Corp. indicating pyrometer, magnetic amplifier, and saturable-core reactor. Differential Thermal Analysis (DTA) Apparatus. T h e D T A appa-
ratus has been described (6). Sample sizes ranged from 150 to 200 mg. and calcined alumina was used as the referVOL. 32, NO. 8, JULY 1960
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