Figure 2 shows the results of varying the acidity and amount of molybdate reagent. The 10% molybdate solution was prepared without sulfuric acid ( 1 ) ; the 5% solution contained 1 equivalent per ml. Curves 1, 2 , and 3 of Figure 2 represent the result of varying the acidity before addition of the molybdate reagent, with the amount of molybdate held constant. At the higher p H the maximum n a s reached 0.2 to 0.4 minute after addition of molybdate; however, the results are dficult to reproduce because of the sharp and variable peak. An increase in the amount of molybdate reagent, with the acidity held constant, also increased the rate of approach to the maximum, but the sharp peak was not present (curves 1 and 4, Figure 2). As all curves were stabilized in the 1-minute region and small differences in p H had minor effect, 1 minute was selected as the best time interval. The effects of large excesses of molybdate or of higher acidities on the formation of the yellow molybdogermanic acid complex noted by Kitson and Mellon (6) also apply to the forniation of the blue complex. Color intensities were very low and variable
Table I. Determination of Germanium Dioxide in Typical Samples
Concn.,
hIg. GeOg/ 100 M1. 0.0483 0.0965 0.104 0.106 0.145 0.145 0 193
o.24i 0.290 0.338
70
Error 3.4 - 4.3 1.5 2.8 - 0.6 0.6 - 2.7 - 0.06 - 6.8 -10.7
+ + + +
if the solution mas neutral to litmus when the molybdate was added. Above p H 7, the intensities were again high, but results were not reproducible in this range. The readings did not vary significantly when samples contained equivalent germanium dioxide, from either pure germanium dioxide or carefully prepared sodium germanate. Under the conditions recommended in the general procedure, the optimum concentration of germanium diovide at the end of the analysis is between 0.1 and 0.3 mg. per 100 ml. of solution.
The results were variable below 0.1 mg. per 100 ml. and fell off rapidly above 0.3 mg. per 100 ml., with an error of -10.7% a t 0.338 mg. per 100 nil. (Table I). LITERATURE CITED
(1) Boltz, D. F., bfellon, M. G., ANAL. CHEM.19, 873 (1947). ( 2 ) Geilman, LV., Brunger, K., Biochem. Z. 275, 375 (1933). Xikro( 3 ) Hecht, F., Bartelmus, chemie ver. Mikrochtm. Acta 36/37. I , 466 (1951). i 4 ) Hoffman, J. I., Lundell, G. E. F., J . Research iVatl. Bur. Standards 22 , 465 (1939). Kenyon, 0. A., Bewick, H. A., ANAL. CHEM.25, 146 (1953).
e.,
Kitson, R. E., Mellon, M. G., IKD. EKG. CHEM., A 4 ~ED. ~ 16, ~ . 128 (1944).
Krause, H. H., Johnson, 0 . H., ANAL. CHEM.25, 134 (1953). Snell, F. D., Snell, 0. T., "Colorimetric Methods of A4nalvsis." Vol. II, 3rd ed., p. 233, 'i'an"Sostrand, Yew York, 1949. RECEIVEDfor review July 15, 1957. Accepted February 24, 1958. Supported by U. S. Air Force through the Air Force Office of Scientific Research, Air Research and Development Command, under contract No. A F lS(600) 1490.
Determination of Traces of Water in Hydrocarbons in Gasoline Boiling Range Sample Handling and Interferences J. WEST LOVELAND and THOMAS B. WEBSTER' Sun Oil Co., Marcus Hook, Pa. CHARLES P. HABLITZEL and GEORGE W. REED Sun Oil Co., Toledo, Ohio )Reliable water contents in the part per million range in hydrocarbons in the gasoline boiling range can b e obtained by titration with Karl Fischer reagent. Contamination by adsorbed and atmospheric moisture is avoided in the recommended technique by using ovendried glass sample bottles and neoprene gaskets and by making transfers of sample into the closed titration vessel with a hypodermic syringe. Mercaptan interference is corrected for by determining apparent water before and after drying the sample on a column of Drierite. For naphtha samples from the same crude source, a constant correction factor for mercaptan interference can be applied;
13 16
ANALYTICAL CHEMISTRY
this considerably reduces the time required for a water determination.
I
of reformer unih for producing aromatics or high octane gasoline, the amount of water present in the naphtha feed stocks is important because of possible deactivation of catalyst. Generally, water contents are in the range of 10 to 50 p.pm Of the various methods available for determining water in naphtha, the Karl Fischer technique (3) is the most suitable, Hanna and Johnson (5) determined water contents of hydrocarbons by first extracting the water with dry ethylene glycol and titrating the glycol N THE OPERATION
extract with Karl Fischer reagent. Most of their work was on samples containing more than 50 p.p.m. of water. Peters and Jungnickel ( I d ) extracted water-saturated hydrocarbons with glycol and titrated in the same vessel. Wiberley (15) used a Karl Fischer micromethod with color end point in a two-phase solvent system for gasoline and kerosine. In most cases a singlephase electrometric titration is preferable. The literature is meager on t8hesubject of the effect of sample handling on the !determination of water in hydrocarbons 1 Present address, Esso Research Ltd., England.
containing about 10 p.p.m. of water. The authors' experience is that sample handling is an important source of error in the determination of water in this low range. At higher concentrations (50 p.p.m.) it is not so critical. The problem of desorption of traces of moisture from the sample container became apparent when some dried naphthas showed higher water contents than corresponding undried naphthas. The first part of this paper discusses different drying techniques for sample bottles. A recommended procedure for sample handling is given. Although the Karl Fischer method is free of interference from almost all compounds found in naphtha charge stocks, any mercaptans present react with the Karl Fischer reagent (7, 8) to give apparent high water contents. Hydrogen sulfide also interferes with the water determination. Mitchell and Smith (8) state that mercaptans react with the Karl Fischer reagent as given by the following reaction: 2 RSH
+ I? --+ RSSR + 2 HI
According to these authors, it is possible to inactivate mercaptans by first reacting the mercaptan with a reactive olefin and boron trifluoride
(IO). Sneed, Altman, and Mosteller ( I S ) found no interference TI ithin experimental error when 0.005~oof either tert-butyl or tert-dodecyl mercaptan sulfur was added to base jet fuels. Howerer, a t 0.020 meight yo sulfur of either mercaptan, apparent water content increased by about 0.002 weight %. Evidently, these particular mercaptans do riot react quantitatively with the Karl Fischer titrant or else they react so slowly that the end point is reached before all the mercaptans react. Wiberley (15) reports that a correction for mercaptans in gasoline is obtained by subtracting three tenths of the mercaptan content in parts per million. However. no data are shorvn to substantiate how this factor was obtained. For mercaptans in naphthas, the authors find that more than 10 p.p.m. of mercaptan sulfur interferes with the Karl Fischer determination of water. A method is described for obtaining true water contents of naphthas in the presence of mercaptans. SAMPLE HANDLING
The following procedure was used as part of the study on sample handling of hydrocarbons in the 1- to 100-p.p.m. water range. The method is recommended as a routine procedure for determining part per million quantities of u-ater in hydrocarbons of the gasoline boiling range.
Reagents, Solvents, and ApparaREAGENT. The tus. KARLFISCHER Karl Fischer reagent was prepared according to the procedure of Mitchell and Smith (9). The strength was reduced from 3 5 mg. of water per ml. to about 2 mg. per ml. by diluting with a 3 to 1 mixture of methanol and pyridine. Some titrations were done with commercially available stabilized Karl Fischer reagent (5.6 mg. of water per ml.) which mas diluted with pyridine to a reagent strength of about 2.5 mg. of water per ml. The methanol and pyridine used for diluting purposes contained less than 0.01% water. Standardization of the reagent was performed daily using a weighed drop of water. Work on the stabilized Karl Fischer reagent indicated that standardization was needed less frequently than with unstabilized reagent.' SAMPLESOLVENT.The sample solvent consisted of 100 ml. of chloroform and 25 ml. of methanol, as suggested by Acker and Frediani ( I ) . Chloroform was dried by passing over a column of activated alumina. Commercial methanol was used only when it contained less than 0.01% water. SAMPLEBOTTLES. Eight-ounce glass sample bottles were provided with neoprene gaskets and metal screw caps having a '/,inch hole drilled in the center. The gaskets (1-inch top, 5,/8inch bottom, and inch thick) are obtainable from Firestone Rubber Co. Akron, Ohio, and may be cut to fit the inside of the screw cap. The bottles, caps, and gaskets were dried in an oven at 150' F. for 8 hours or overnight. After the bottles mere dried, they were capped tightly. TITRrlTIOh' APPARATUS. All work was done using a Serfass electrometric titration unit (A. H. Thomas Co., Philadelphia, Pa ) with Magic Eye end point detector. The five-necked, 500-ml. titration flask illustrated in Figures 1 and 2 mas a modification of one described by Aepli and JIcCarter ( 2 ) . The 10-ml. buret was graduated to 0.02 ml. and had a three-way Teflon stopcock to facilitate filling of the buret from the reservoir. The platinum indicating electrodes were sealed through glass capillary tubing. These m-we fitted a t the top with a taper joint. Connection of the Serfass titration unit to the platinum electrodes was made by dipping copper Tire leads into mercury in the glass capillaries. A glass tube extending close to the bottom of the flask permitted removal of sample and solvent bv suction. Solvent was added to the flask from the top when the drying tube was removed. The sample was admitted by means of a 50-ml. hypodermic syringe through an opening fitted with a serum bottle cap. End points for samples and for pretitration of the solvent were taken when the Magic Eye remained open for 30 seconds after 3 drops ( - 4 . 0 5 ml.) of reagent had been added. Addition of Sample to Titration Flask. Whenever a sample was taken a hypodermic needle attached to a Drierite tube was inserted through the gasket to prevent a vacuum from
tj
I LITER
DRYING/
3WAY TEFLON STOPCOCK--,
,
' o ' 3 0 $'
i-y-mcrE;kc L-.4r,
Figure 1 . Front view of Karl Fischer titration apparatus
PT WIRE\!,/
Figure 2. Side view of titration flask indicating placement of platinum electrodes forming. A 50-ml. syringe n a s then inserted through the gasket and from 20 to 25 ml. of sample used to flush the syringe. The syringe was then filled with 50 ml. of sample. The syringe needle was inserted through the rubber serum cap of the titration vessel and the sample was admitted. I n this manner it was possible to analyze samples without exposing either the sample or the pretitrated solvent t o moisture of the atmosphere. For cases where samples were to be analyzed directly after filling of the sample bottles, oven-dried rubber serum caps were used instead of gaskets and screw caps. Filling of Sample Bottles. For obtaining samples in the plant a sample line of 1/4-inch copper tubing was fitted in a tn-0-holed rubber stoppei and extended to about inch from the bottom of the sample bottle. A l/c-inch copper elbow-shaped outlet tube extended about 1 inch below the rubber stopper. A small rubber plug was inserted in the outside end when not taking samples. The unit was protected from moisture by tightly inserting the rubber stopper in a bottle until a sample was taken. When a sample was being taken, the lines were flushed for 10 or 15 seconds before the protective bottle was replaced by the dried %ounce bottle. The sample bottle was flushed with about twice its volume of sample and recapped immediately with the neoprene gasket and VOL. 30, NO. 8, AUGUST 1958
1317
screw cap top. The protective bottle and plug were replaced on the sampling line. Treatment of Sample Bottles. For several years samples of naphtha had been taken in 8-ounce bottles t h a t were previously dried with air passed through a calcium sulfate drying tube;
Table I.
Effect
of Oven Drying a t
150" F. of Neoprene Gaskets on Water Content of 50 to 50 BenzeneToluene Mixture"
Days on Shelf 0 2 3 7 Gasket Treatment Water, P.P.M. Dried 4 11 15 15 Undried 4 25 29 29 Dried 20 25 27 27 Undried 25 33 35 35 Dried 54 55 56 58 Undried 57 59 61 64 a Results are average of duplicate determinations; largest deviation was 1 p.p.m.
Table 11. Comparison of Effect of Glass and Polyethylene Bottles Dried Overnight at 150" F. on Water Content
Days on Shelf Sampleand Bottle Used Benzene Glass Polyethylene Glass Polyethylene Glass Polyethylene Xaphtha charge Glass
Polyethylene
0
1
3
Water. P.P.M.
5
5
50 50 120 120
7
7 7 8 7 7 1 0 50 50 51 59 77 113 121 121 130 128 132 137
120 120 120 120 120 126 126 126
Table 111. Comparison of Air-Dried4 and Oven-Dried* Glass Bottles Using Plant Samples
Sample and Bottle Treatment Reformer charge Oven ilir
Oven Air Oven Air
0
Days on Shelf 1 3 7 Water, P.P.M.
6 6 6 8 6 8 9 1 0 52 52 52 55 52 52 56 56 102 105 108 115 102 103 106 105
Motor reformate Oven -4ir Oven
5 6 8 9 5 9 9 1 0 62 63 65 66 $ir 62 69 70 70 Oven 109 109 110 109 Air 109 110 113 110 a Purged for 0.5 hour with air dried by passing through calcium sulfate. Dried at 150" F. overnight. 13 18 *
ANALYTICAL CHEMISTRY
the bottles were capped with neoprene gaskets. Test work indicated that such samples containing 10 p.p.m. of water would increase by 20 p.p.m. after remaining in the bottles for 3 days. Other work indicated that reliable results might be obtained by allowing the sample t o equilibrate with the walls of the sample bottle overnight before refilling with fresh sample for analysis. However, this procedure was not ideal because of the elapsed time involved. The work indicated that it was not possible to obtain reliable results up to equilibration times of 5 hours. It also indicated that an equilibrium existed between water in the sample and water on the glass surface and/or vapor phase equilibridm with water in the neoprene gasket. Investigations of possible sources of errors were limited to two aspects: moisture from the neoprene gasket and inadequate drying of bottles.
DRYINGOF YEOPREXE GASKETS. When two neoprene gaskets were placed in a desiccator, each lost 3.0 mg. in weight in 24 hours. After 48 hours an additional 0.2 and 0.4 mg. were lost. This is equivalent to about 15 p.p.m. of water for an 8-ounce sample of naphtha. Drying in the oven for 25 hours a t 150' F. caused gaskets to lose about 27 mg. Higher temperatures made the neoprene unusable. Most of this loss in weight on oven heating is probably due to volatile material other than water. A Karl Fischer titration of a gasket cut into small pieces indicated 5.0 mg. of water to be present. Data showing the effect of drying gaskets in an oven a t 150' F. overnight on water contents of a 50 to 50 mixture of benzene-toluene are given in Table I. Bottles were air dried for 15 minutes. Separate bottles were used in obtaining each result. All samples showed an increase in water content after standing for 2 days. However, in all cases the increase was much larger for the undried gaskets than for the dried gaskets. Larger increases were observed for the samples containing the lower water contents, indicating that an equilibrium system is probably involved. From these data it was established that water in the gasket was one factor which contributed to the increase in water contents of hydrocarbon samples on standing. DRYINGOF SAMPLE BOTTLES. A comparison of oven-dried (150" F.) glass and polyethylene screw cap bottles was made. Neoprene gaskets, also oven dried, were used with the glass bottles. A few seconds before taking the sample for titration, a serum bottle cap taken from a desiccator was substituted for the polvethylene top. Concentration ranges of 5, 50, and 120 p.p.m. of water in benzene and 120 p.p.m. in naphtha charge stock were studied. This wide range made it possible t o determine if water would be picked up from the bottle's surface at
low concentrations or adsorbed at the bottle's surface at higher concentrations. The data obtained after various times of standing are shown in Table 11. Separate samples were used in obtaining each result. In general, samples taken in ovendried glass bottles showed no significant increase in water content on standing for 3 days and only a slight increase after 7 days. This is not true for polyethylene bottles. This suggests that polyethylene may be very slightly permeable to water vapor and unsuitable for sampling hydrocarbons. At the 120-p.p.m. range there was no tendency for water to adsorb on the surface of the bottle for either the naphtha charge or the benzene samples. Comparison of Air and Oven Drying. Data were obtained to compare air drying t o oven drying (150' F.) of glass bottles by using reformer charge stock and product (Table 111). The neoprene gaskets were oven dried for all experiments. Any mercaptan in the charge stock, which might interfere in the Karl Fischer method, was removed by washing with aqueous silver nitrate solution. I n order to obtain the low water contents, naphtha was dried over Drierite prior to filling the 8-ounce bottles. The sample can remain in oven-dried bottles for at least 3 days without any significant increase in water content, except possibly for the more aromatic reformate in the 10-p.p.m. range. Samples in the air-dried bottles in the 10- and 50-p.p.m. range showed larger increases in water content than those in oven-dried bottles. No decrease in water content was observed in the 100-p.p.m. range, which would indicate that water was not absorbed by the glass surface. Based on data of Tables I, 11, and 111, hydrocarbon samples are taken in glass bottles which, together with neoprene gaskets, are heated overnight in an oven at 150" F. Analyses on all hydrocarbon samples are made within 3 days after receipt of samples, except aromatic samples, which are analyzed within 1 day. INTERFERENCES FROM MERCAPTANS
When determinations were made of water contents of naphthas from high sulfur crudes which had been dried, the results were much higher than expected. Although it is well known that mercaptans interfere with the Karl Fischer water determination, Sneed, Altman, and Mosteller (IS) had shown that mercaptans added t o jet fuels in concentrations of less than 50 p.p.m. of sulfur did not influence the water determination within their experimental accuracy. Results of the present work show that, within experimental error. if there is as much as 10 p.p.m. of mercaptan sulfur in naphthas, high anomalous results will be obtained for water. -4s many charge stocks con-
tamed more than 10 p.p.m. of mercaptan sulfur, it was necessary to develop a procedure to eliminate or correct for this interference. The method using boron trifluoride and an olefin (10) for eliminating mercaptan interference did not appear attractive because of the high water blanks obtained on the reagents. Several drying agents for naphthas were investigated with the thought of determining apparent water before and after drying the naphtha sample. The difference between the two results would be equal t o the true water content. One prerequisite of the drying agent would be that it did not alter the mercaptan content. The following procedure is recommended for determining part per million quantities of water in hydrocarbons containing mercaptans. Reagents and Apparatus. Karl Fischer electrometric titration a w a ratus (Figures 1 and 2). Karl Fischer titrant. eauivalent to 2.0 to 2.5 mg. of water peimi. of titrant. Drierite (18-mesh calcium sulfate). This may be used directly from a freshly opened bottle. Benzene, stored over Drierite.
Table IV. Drying of Hydrocarbons and Mercaptan-Free Naphthas with Drierite
Water, P.P.M. After Drying Sumber of Passes Before 1 2 3 4 Sample drying Benzene 178 6 3 2 .. %-Decane Xaphtha"
330 95 95 95 15 44
35 18 19 22
.. ..
5R
..
~~
55 55 55 55
..
..
10 11 10
11
..
..
2.?I 13 12
3
..
6 6
.. .. ..
5 9 3 18
5 4 ..
.. 5 4
5 Contained less than 1 p.p.m. of mercaptan sulfur.
Each time the column is charged with Drierite, clean it of adsorbed moisture in the following manner. Charge about 100 ml. of dry benzene to the separatory funnel and stopper the funnel with the drying bulb. Allow the benzene to drip onto the calcium sulfate a t a rate of 3 to 5 drops per second. Shut off the column a t the bottom when all the benzene has passed into the sample bottle. Recycle the benzene up to the head of the column by pumping on the rubber bulb. When all of the sample has been transferred, shut off the recycle flow a t the top of the column and open the column a t the bottom. Keep the stopcock of the separatory funnel open during these manipulations. Repeat the recycling operation a second time. Discard the third pass of benzene and immediately attach a dry, &ounce sample bottle. Charge about 50 to 60 ml. of sample to the separatory funnel and cycle three times in a manner analogous to that
__
Table
Before Sample drying 1
32
2
52
3
24"x'/< GLASS PACKED I&MESH
ID COLUMN WITH
CoSOq-
V.
Drying of Naphthas Containing Mercaptans
Apparent Water, P.P.M.
56
4
60
5
72
6
96
After Drying No. of PaEses 3 29 28 29 28 31 29 29 32 32 35 38 40
4 27 25 27
27 28 28 21 25 27 29 29 32
5 24 24 24 24 28 28 21 "1 27 27 29 28
ON NG
Mercaptan Sulfur, P.P.X. After Drying Before No. of Passes 3 4 5 drying .. 51 48 56
Ratio,
+-'%':
Mercaptan Sulfura 0 46
54
54
53
51
0 50
56
51
51
52
0.50
44
43
48
..
0 52
G5
65
64
62
0 43
59
57
59
59
0 32 -'v. 0 . 4 9
Std. dev. 2.4 1.7
