Combustion-Amperometric Titration of Traces of Halogen in Petroleum Products LAWRENCE J. CALI, J. WEST LOVELAND, and DOROTHY G. PARTlKlAN
Sun Oil Co., Marcus Hook, Pa. F A sensitive analytical method for determining parts per million of halogen in petrochemical hydrocarbons was needed because of increasingly stringent specifications and process requirements. A method applicable to benzene, toluene, and naphthas uses a Beckman atomizer burner for sample combustion. The sample is aspirated into the oxyhydrogen flame and the combustion products are collected in a sodium carbonate absorber. The sodium chloride formed is titrated amperometrically with silver nitrate. The determination for a single sample requires about 1 hour of elapsed time. The range of halogen studied was 1 to 10 p.p.m. of halogen as chloride. The standard deviation of results from known values is 0.6 p.p.m. and the repeatability standard deviation is 0.3 p.p.m.
A
s
THE specifications and process requirements for petroleum chemicals become more rigid, more sensitive analytical methods are needed. The determination of parts per million quantities of chloride in benzene, toluene, and naphthas is a specific example. Direct methods for determining chloride in naphtha include neutron activation and x-ray fluorescence ( 2 ) ,but these require expensive and elaborate equipment. Other direct methods are generally not sensitive to small quantities of chloride and in many cases are applicable only to the determination of a apecific chlorine compound. The simplest approach to the problem i s offered by methods that convert organic chloride to the ionic state. The ASTM method for sulfur (1) has been modified and used to determine volatile chloride in naphthas and heavier materials (15), but it is limited b y the amount of chloride obtained from the combustion. Procedures for determining small amounts of chloride ion in aqueous solution are not sensitive enough t o measure accurately less than 100 y (the amount obtained from burning 10 grams of a sample containing 10 p.p.m. of chloride in naphtha). To obtain a suitable amount of chloride on a 1-p.p.m. sample would require burning a 100-gram sample for about 20 hours, The advent of a spectrophoto-
74
0
ANALYTICAL CHEMISTRY
metric technique (8) made it possible for Bergmann and Sanik (2) to modify the lamp procedure and tremendously increase the sensitivity of the lamp method. The spectrophotometric technique makes use of the displacement, of thiocyanate ion from mercuric thiocyanate by chloride ion; in the presence of ferric ion, a highly colored ferric thiocyanate complex is formed, the color of which is proportional to the original chloride ion concentration. However, the procedure still requires about 2 to 3 hours of burning time. Other general procedures for converting organic chlorine to chloride ion by treatment with sodium in various solvents (3, I S , 16), or treatment of the organic chloride with disodium biphenyl (12, I d ) , require removal of excess reagent, which can be time-consuming. Procedures for determining inorganic chloride in aqueous solutions include nephelometry (IO), amperometry (9, 11) and titrimetry (6). A colorimetric procedure for chloride was reported by Iwasaki, Utsumi, and Ozawa (8) and used by Bergmann and Sanik. Chapman and Sherwood (4) report a spectrophotometric method, in which the ab-
Figure 1.
sorbance of a palladium-halide complex is measured in the ultraviolet. This procedure is, however, subject to many interferences. Granetelli (6) reported a novel combustion method for determining (0.006 to 0.11%) sulfur in drip oils, by using the oxyhydrogen burner of the Beckman flame photometer. Hinsvark and O'Hara (7) used the Beckman burner and modified and extended the method to the determination of sulfur in naphtha and Diesel fuels. The sulfate formed during the combustion was determined by EDTA [(ethylenedinitrilo)tetraacetic acid J titration. The proposed procedure makes use of the Beckman burner for the determination of chloride in benzene, toluene, and naphtha. The combustion is carried out in an enclosed system and a 50-gram sample can be burned in about 0.5 hour. The inorganic chloride formed during combustion is absorbed in aqueous sodium carbonate solution and titrated amperometrically (9, 11). APPARATUS AND REAGENTS
Combustion Apparatus.
The com-
Combustion-absorption system
A . Oxygen pressure regulator B. Hydrogen pressure regulator C. Oxygen gage, 0-30 pounds D. Hydrogen gage, 0-16 pounds E. Coil resistance mire, 22 gage, 1.06 ohms/ft. F . Quartz t.ube, 30-mm. tubing, with 24/40 T joints on each end G . dtomizer burner H . Sample holder, borosilicate glass, 4-cm. diameter by 6 em. long, containing 10/18 '$ ground-glass joint with 3-mm. opening at tip of male joint I . Variac. 115 volts. 50/60 cvcles, " . oneahase. outrut 0-135 volts. 7.5 amperes J. 4-liter 'beaker ' K . Absorber (Harshaw Scientih, Code H-54260-4) L. Condenser. 300-mm. (Harshaw Scientific. Code H-16210) M , N . Spray iraps 0. Rotameter, 0 to 800 liters per hour P . Screw clamp &. Pump R. Absorber 8. Enclosed combustion chamber I
-
bustion apparatus was assembled as shown in Figure 1. The apparatus, excluding the oxygenhydrogen regulator, Variac, and rotameter, should be completely enclosed in a metal shield, for safety reasons.
A Beckman oxygen-hydrogen regulator was used in place of the oxygenhydrogen gages (C and D, Figure 1). The quartz tube is constructed from 30-mm. tubing and wound for approximately 4 inches with 22-gage, resistance wire, 1.06 ohms per foot. The wire on the tube is then wrapped with asbestos tape. Figure 2 shows in greater detail the construction of the enclosed box (8, Figure 1). The atomizer burner was an oxygen-hydrogen burner (Beckman Instruments, Inc., Code B-4020). A stainless steel tube, 1 mm. in inside diameter and 60 mm. long, is silversoldered on the aspirator tip, to increase the length of the aspirator.
Figure 2. chamber
Enclosed combustion
Titration Apparatus. A Sargent Model XXI recording polarograph was used for all amperometric titrations. The titration cell shown in Figure 3 used a mercury pool as the reference electrode and the mercury drop as the indicator electrode. A 5-ml. buret, graduated in 0.02-ml. divisions, is needed. Special Reagents and Solutions. All materials should be reagent grade unless otherwise specified. They include triple distilled mercury, and 99.99'% oxygen-free tank nitrogen (Matheson prepurified is recommended). PROCEDURE
To prevent dangerous accumulation of hydrogen in the apparatus due to undetected leaks, the apparatus should be swept out each day by turning on the vacuum pump before starting the determination. Combustion of Sample. The input t o the coil of resistance wire E is regulated through Variac I . About
NITROGEN &BEL E 8 DISK TYPE
DROPPING MERCURY
RUBBER STOPPER -OPENING FOR BURET
I
~-
Figure 3.
2"
.
Amperometric cell
7 minutes is allowed for the coil and the quartz tube t o reach temperature. This is noted by the red glow of the tube under the asbestos-coated resistance wire. The absorber, K , is charged n-ith 50 ml. of 0.1N sodium carbonate. With valve C turned completely off, the hydrogen cylinder is opened and the pressure a t gage B is adjusted to 10 pounds per square inch gage by means of regulator B. With D turned completely off, the oxygen cylinder is opened and the pressure a t gage A is adjusted to 25 pounds per square inch gage by means of regulator A . The cover is removed from the enclosed box, S, and the sample holder, H , filled to capacity with sample, is weighed. It is placed under the Beckman burner with the aspirator tip immersed as much as possible in the liquid. The cover is replaced and the retaining springs are put in place. Reservoir J is filled with distilled water, so that it encompasses as much of the absorber as is practical. Cold water is allowed to flow through condenser L in a steady stream. Pump D is turned on and screw clamp P adjusted so that about 15-liters of air per minute is pulled through absorber R, which contains soda-lime. Hydrogen is supplied t o burner G by opening valve D to about 3 pounds per square inch gage. The. burner should ignite immediately. The hydrogen pressure is then adjusted to 1.5 pounds per square inch gage and kept a t this pressure. Oxygen is supplied to the burner by slowly opening valve C until the sample begins to be aspirated into the flame. This should occur a t about 5 pounds per square inch gage of oxygen. The pressure is then increased slowly until the oxygen pressure reaches 15 pounds per square inch gage. The sample should burn smoothly. Once the sample begins to burn a t a steady rate, the input voltage to the coil of resistance mire is brought practically to zero by means of the Variac. After sufficient sample has been burned, first oxygen valve C is turned off, and then hydrogen valve D. The vacuum pump is turned off and the apparatus allowed to cool for a few minutes. The absorber containing the chloride is removed from the assembly and spray traps and N are rinsed with about 5-ml. of chloride-free distilled
water into the absorber. The absorber solution is transferred directly to the amperometric cell and the absorber rinsed with small portions of chloridefree water. Sample holder H is then removed from the combustion box and reweighed. A combustion blank is obtained by burning only oxygen and hydrogen for the time required to burn the average sample (about 30 minutes for 50 grams)-Le., all manipulations are performed with the exception of actually burning a sample. In this work the blank was negligible. Amperometric Titration. The procedure is carried out directly on the absorber solution. One milliliter of 1 to 1 nitric acid and 1 ml. of 0.1% gelatin are put in the cell and the cell is fitted with a rubber or cork stopper containing the dropping mercury electrode (DhIE). The solution is degassed and the anode and cathode are connected to the polarograph. A potential of -0.2 volt us. pool is applied to the dropping mercury electrode. When the solution is degassed, as noted by little or no current deflection on the recorder, the bubbler is raised out of the solution and nitrogen allowed t o pass over the surface of the solution. The height of the mercury leveling bulb is adjusted, so that one drop of mercury falls every 3 or 4 seconds. The current a t 0 ml. added is recorded. Then 0.04-ml. increments of 0.01N silver nitrate are added and the solution is degassed for about 30 seconds. After each increment of titrant has been degassed, the current is recorded. Titration is continued until about four points have been obtained beyond the end point (recognized by the large increases in current). Alilliliters of titrant are plotted against current and straight lines are drawn through the points before and after the end point, taken a t the point of intersection of these two lines. The cell is washed out with running water, then with chloride-free water. Any excess water left on the surface of the mercury pool is siphoned off. The cell is then ready for the next titration. Calculations Halogen,p.p.m. as C1- = (ml. of titrant sample - ml. of titrant blank X N X 0.0355 X 106
sample wt. burned where N = normality of silver nitrate titrant. EXPERIMENTAL
RESULTS AND
DISCUSSION
Discussion of Apparatus. Hinsvark and O'Hara (7) showed that drawing unpurified air through the combustion tube did not affect the sulfur results. A series of determinations for chloride made on the hydrogen-oxygen flame using unpurified air showed t h a t the air must be purified before the combustion step. I n the apparatus shown in Figure 1, the air is drawn into two absorbers containing sodalime. The purified air then travels VOL. 30, NO. 1, JANUARY 1958
e
75
into the enclosed box and up through the quartz chimney. The air performs two functions. It helps cool the Beckman burner, Tyhich ceases to operate when overheated, and it helps in evaporating from the absorber the large quantities of mater formed during combustion. Extreme precaution is necessary in niaking the chloride determination, as small amounts of contamination cause difficulty in obtaining reliable results. The determinations are best carried out in a location where no contamination from fumes of hydrogen chloride can occur. Copper tubing is the best material for the oxygen and hydrogen lines. The oxyhydrogen burner must be cleaned occasionally. The copper tubing with swagelock connections makes this operation very simple. A series of runs was made using purified air, burning only the oxygen and hydrogen for about the time required to burn a 50-gram sample. The titration for the absorber solution from the combustion was the same as for the reagent sodium carbonate solution. This carbonate solution required less than 0.04 ml. of 0.01N silver nitrate, which is equivalent to less than 0.3 p.p.m. of chlorine based on a 50-gram sample. Because the blanks were very low, no blank correction was used. DlSCUSSlON OF RESULTS
Known blends of chloride as ethylene chloride and carbon tetrachloride in halogen-free benzene were analyzed by the proposed procedure (Table I). During earlier work it was found that evaporation of the absorber solution on a hot plate to a low volume tended to give high results, which may have been due to contamination from the atmosphere. Accordingly, the solution was transferred directly to the amperometric cell and titrated. The proposed method gives quantitative and repeatable results in the 1 to 10 p.p.m. range studied. Halogen-free benzene was prepared by refluxing benzene with sodium and n-butyl alcohol. The inorganic halide was extracted with chloride-free water and the halogen-free benzene stored over sodium. I n Table I results are shown for blends of ethylene chloride in benzene which had been previously analyzed by the proposed procedure. The standard deviation of the averaged duplicate results from their known values is 0.6 p.p.m. The repeatability standard deviation of the method based on the duplicate results i s 0.3 p.p.m. over the range of 1 to 13 p.p.m. To test the applicability of the proposed procedure to the determination of inorganic chloride, blends of
76
ANALYTICAL CHEMISTRY
Table 1.
Accuracy of Determination of Organic Halogen
Chloride Added, P.P.11. Found Chloride-Free Benzene
Source of Chloride Ethylene chloride
1.3 6.3 12.6 5.4 10,s
Carbon tetrachloride
Chloride, P.P.11. .4v.
1.3,1.2 6.6,G.S 12.2,12.1 5.5,4.7 10.1,10.0
Dev. 0
1.3 6.7 12.2 5.1 10.1
+0.4
AV.
-0.4 -0.3 -0.7 0 ,4
Av.
+0.3 +1.4 $0.4 0.7
Previously Analyzed Benzene" 1 .o 5.0 10.0
Ethylene chloride
1.2, 1.3 6.8,6.0 10.3,10.5
1.3 6.4 10.4
Benzene blank contained 3.7, 3.5, av. 3.6 p.p.m. chloride. Repeatability standard deviation = 0.3 p.p.m. Standard deviation of results from known values = 0.6 p.p.m.
a
Table 11. Comparison of Ionic Chloride Method with Burning Procedure
(HCI recovery) c11-
bl
Expected, P.P.M. 1.0
5.0
c1- Found, P.P.RI. Ionic
Burning
0.5 1.7
