Determination of Trace Amounts of Carbonyl Sulfide in Gaseous Hydrocarbons ROBERT E. SNYDER and R A L P H 0. C L A R K Gulf Research and Development Co., Pittsburgh 30, Pa.
A simple method for determination of trace amounts of carbonyl sulfide in hydrocarbon gases is based on the reaction with piperidine in alcoholic solution to form piperidine oxythiocarbamate, which is determined spectrophotometrically. The method is applicable to gases containing hydrogen sulfide, sulfur dioxide, and aromatic hydrocarbons not exceeding the equivalent of 50 p.p.m. of benzene; higher concentrations of aromatics can be tolerated, provided they are known or can be determined. The results on synthetic blends are w-ithin the range of 2 to 75 p.p.m. of carbonyl sulfide and are reliable to within 5% of the amount present. ..iprocedure is described for preparing carbonyl sulfide having a purity approaching 99.9%. The sulfide can be stored at 0" C. for several months in a stainless steel bomb without decomposition, provided its initial assay is better than 99.5%.
T
HE presence of carbonyl sulfide in gaseous hydrocarbons
is an important problem to the petroleum industry, especially in processes where the catalysts employed are sulfursensitive and conventional metliods for sulfur removal do not effectively remove carbonyl sulfide. Carbonyl sulfide has been speculated also to contribute to the corrosion of refinery equipment and may be responsible for the reduced efficiency of amine Berubbers (5'). If the actual concentration of carbonyl sulfide could be determined in the gaseous products, then processes contributing to its formation could be studied either to remove more effectively the undesired constituent or to effect improvements in process design. Sumerous methods have been presented in the past few years for the determination of organic sulfur compounds in gaseous streams, but only a limited number are applicable to the determination of carbonyl sulfide. Avdeeva ( 1 ) described a quantitative method whereby the carbonyl sulfide was absorbed and oxidized in an ammoniacal calcium chloride solution, and then was determined by precipitating the sulfuric acid with barium chloride. Riesz and Wohlberg (6) proposed a method for the determination of cai bony1 sulfide in hydrogen in which the constituent was absorbed with carbon disulfide in a piperidinechlorobenzene reagent and the proportions were resolved colorimetrically. lIacHattie and LIcNiven ( 5 ) and Hakes ill and Rueck ( 4 ) have described methods shereby carbonyl sulfide can br determined by scrubbing gaseous samples with selective solvents and estimating the residual sulfur content of the gas by burning it, and then determining the oyides of sulfur which are formed. These methods are time-consuming and lack the
sensitivity required for determining trace amounts of carbon) 1 sulfide in refinery gas streams. The method for determination of carbonyl sulfide described by Brady ( d ) , with cwtain modifications, has been found to be the most suitable from the standpoint of ease and rapidity with which it can be performed, and is not subject to interference from other sulfur-bearing constituents. The principle of the method is based on the observation that carbonyl sulfide can be quantitatively scrubbed from a gaseous sample as piperidine oxythiocarbamate ahich has a strong absorption band a t 230 mp. The method presented is an adaptation of the Brady procedure and differs from it in several significant respects. The apparatus has been modified to permit the determination of a few parts per million of carbonyl sulfide. Synthetic standards are prepared for calibration purposes by blending gaseous hydrocarbons with measured quantities of pure carbonyl sulfide gas. The interference of hydrogen sulfide and sulfur dioxide, constituents of many refinery streams, has been eliminated. A procedure is described for the preparation and storage of pure carbonyl sulfide for long periods of time. REAGENTS
Hydrocarbon Blending Gas. Any commercial grade of Cz,CB, or Ca hydrocarbon gas which is carbonyl sulfide-free. Piperidine-Ethyl Alcohol Absorbent. The absorbent is prepared by dissolving 0.5 gram of piperidine in 1 liter of 95% ethyl alcohol and saturating the solution with the hydrocarbon blending gas. The reagent is stored in an amber-colored, glass-etop pered bottle. Manganese Dioxide. This reagent must be specially prepared and is available from the Laboratory Equipment Co., Benton Harbor, Mich., Catalog No. 501-C. Carbonyl Sulfide. The calibrating standard is prepared according to the procedure described. The purity of the standard should be ascertained just prior to use. APPAR4TUS
-1 Beckman quartz spectrophotometer fitted with two calihrated 1-cm. quartz absorption cuvettes and equipped for ultraviolet spectroscopy was employed. The gas handling apparatus, constructed of borosilicate glass, is illustrated diagrammatically in Figure 1. The gas buret, G, is a 1-ml. Mohr pipet which is sealed to a capillary, 2-mm. bore T-stopcock. A suitable length of Tygon tubing connected to a 25-ml. gas leveling bulb, L , is fastened to the lower end of the pipet, with mercury being used as the confining liquid. Any leveling device may be used for raising and lowering the leveling bulb. The reaction flask, R, consists of two 4-nim. bore stopcocks sealed diametrically onto a 5-liter capacity glass bulb. A 12/5 outer spherical glass joint is sealed to one stopinner joint to the other. The cock and a 12,s' blending flask., B., consists of a 5-liter flask with two 2-mm. bore stopcocks and a set of 12/2 TO M A N O M E T E R M~ spherical glass joints attached in the manner a8 the reaction bulb. The total volume of each bulb was determined to the nearest 5 ml. The U-shaped manganese dioxide absorber, A , is a 50-em. length of (6-mm. inside diameter) glass tubing, the ends being sealed to 12/5 outer spherical glass joints. The tube is nearly filled with manganese dioxide absorbent and the ends loosely plugged with glass wool. Diagram of gas handling system The remaining portion of the manifold is
-
Figure 1.
1167
ANALYTICAL CHEMISTRY
1168 made of 12/2 spherical glass joints and 2-mm. bore capillary glass tubing. All joints and stopcocks were lubricated with a siliconetype stopcock grease. The dispensing tube (Figure 2) is a borosilicate glass tube closed on one end and fitted with a 12/5 inner spherical glass joint on the opposite end, and having a capacity not less than 50.5 ml. or greater than 51.0 ml., not including the bore of the glass joint.
where
V
volume of carbonyl sulfide added to blending flask, in milliliters P = pressure introduced into the reaction flask from blending bulb, in millimeters of mercury T = temperature in degrees Kelvin = density of carbonyl sulfide at standard condi2.G80 X tions =
EXPERIXIElTAL
With the apparatus assembled as in Figure 1, the manifold u a s evacuated to the reaction flask and the system checked for leaks. The left-hand stopcock of the blending flask was closed, the evacuated portion of the manifold was filled with carbonyl sulfide, and approximately 0.9 ml. collected in the gas buret. The manifold was isolated from the buret, re-evacuated, and filled with about 0.85 ml. of carbonyl sulfide. The volume of gas remaining in the buret, the barometric pressure, and temperature at which the volume measurements were made were recorded The carbonyl sulfide was transferred into the bleriding bulb with the hydrol 2 Y 5 $ INNER carbon blending gas and the flask was filled to atmospheric pressure with the hydrocarbon gas. The reaction bulb and by-pass were evacuated and the manometer reading, .If2, was recorded. Approximately 100 mm. of gas pressure IT as admitted into the evacuated reaction bulb from the blending flask, a t a rate not 20MM I D eaceeding 250 ml. per minute, and the manometer reading again recorded. Blending gas was admitted by means of the bypass system until the pressure within the bulb was twice that of Figure 2. Dispensing tube atmospheric pressure. (A pressure of i atmosphere in the reaction flask may be employed if concentrations of carbonyl sulfide between 2 and 4 p.p.m. are not of interest.) Fifty milliliters of the piperidine-ethyl alcohol solution were pipetted into the dispensing tube, the spherical joint was lubricated, and the vessel attached to the reaction flask by a spring clamp. The stopcock between the tube and bulb was opened, and the liquid displaced into the latter by shaking vertically. after the liquid had been transferred, the stopcock was closed, the tube detached from the flask, and the liquid in the flask shaken vigorously for 10 minutes. The lubricant was then completely removed from the inner spherical joint, including the bore, and the solution transferred into a 50-ml. flask. The flask was loosely stoppered and the contents allowed to stand for approximately 1 hour or until effervescence due to the dissolved gas had subsided. Gas blends of increasing carbonyl sulfide concentration were prepared by introducing increments of approximately 160 and 275 mm. of gas pressure into the reaction flask. A blank solution was prepared as described, except the addition of carbonyl sulfide was omitted. .4 portion of the piperidine oxythiocarbamate solution was transferred into a quartz absorption cell and the absorbance measured a t 230 mp against the prepared blank. The cuvette containing the blank solution was emptied, cleaned, dried, and refilled with a fresh portion of solution after each measurement. (It has been found advisable to work with only two cells and to complete the measurements as quickly as possible to avoid creep of the solution from the cuvette.) The following formula was used to ralculate the concentration of carbonyl sulfide added t o the blending gasin the preparation of the calihratingstandards. ~
A calibration curve was prepared by plotting the measured absorbances against the milligrams of carbonyl sulfide in 50 ml. of solution. A typical calibration curve corrected for the blank values appears as a straight line with absorbance increasing as the milligrams of carbonyl sulfide increased. Analysis of Samples. For applying the method t o routine test samples, the portion of the manifold to the left of the manganeFe dioxide absorber was removed. The sample bomb was attached to the manifold and the system evacuated t o less than 1-mm. mercury pressure and checked for leaks. The valve on the qample bomb was cautiously opened arid the gas permitted to fill the evacuated system a t a rate not exceeding 250 ml. per minute. The addition of gas was continued until the pressure of 2 atmospheres was attained. The reaction flask was removed from the system and the barometric pressure and the room temperature recorded. The piperidine-ethyl alcohol solution, 50 ml., was pipetted into the dispensing tube and the extraction performed as described; the blank is prepared from the carbonyl sulfide-free hydrocarbon blending gas. A portion of the piperidine oxythiocarbamate solution was transferred into a quartz cuvette and the absorbance measured a t 230 mp against the prepared blank. The photometer reading was converted to the concentration of carbonyl sulfide in milligrams per 50 ml. using the prepared calibration curve. The following formula was used to calculate the concentration on a weight basis in parts per million of carbonyl sulfide in the test sample. Carbonyl sulfide, p.p.m. =
760
x
22,414 x 103 ( c ) ( T ) 273.1 ( V ) ( P ) ( - l T
where
C
= carbonyl sulfide concentration, in milligrams per
50 nil.,
corresponding to the measured absorbance T = temperature in degrees Kelvin V = volume of gas sample in milliliters P = barometric pressure in millimeters of mercury, and .If = molecular weight of the sample. Preparation of Carbonyl Sulfide. Pure carbonyl sulfide is not available commercially and must be specially prepared. The gas may be conveniently prepared, however, from the reaction of potassium thiocyanate and sulfuric acid, then purified by passage over Ascarite in combination with repeated low-temperature fractionation. The apparatus (Figure 3) consists essentially of two parts, the portion to the left of the U-tube condenser being the gas prepara-
1169
V O L U M E 2 7 , NO. 7, J U L Y 1 9 5 5 Table I. Analysis of Gas Blends for Carbonyl Sulfide Blend KO.
Found 2 ,0
!.?
3.2
23.5 23 4 22.8 33.0 3ti. 9 48 0 59 2
Carbonyl Sulfide, P.P.M. Added Difference 2.1 -0.1 4 0 -0.1 5 3 +0.2 6
+0.9
2 0 7 2 49 0 i;o 0
+0.2 +0.8
22 23 25 33 36
-0.7
+0.7 -1.0
-0.8
Standard deviation, 3 . 1 q .
tion system and that to the right the purification and fractionation system. All components of the gas manifold are fabricated from borosilicate glass. The gas generator consists of a 500-ml. flat-bottomed flask fitted with a 24/40 outer tapered glass joint. The inner portion of the glass joint is sealed to the stem of a 100-ml. separatory funnel and carries a side arm sealed to a 12/5 outer spherical glass joint for subsequent attachment to the manifold. Stirring of the reaction mixture is accomplished with a magnetic stirrer. The brine bulb is fabricated from a capillary, 2-mm. bore Tstopcock and a 4-mm. bore stopcock, sealed diametrically to a 5-liter bulb. The bulb is connected to a 2-gallon siphon bottle containing 6 to 7 liters of acidified water saturat,ed with sodium chloride. The gas scrubber is a conventional 125-inl. capacity gas nashing bottle fitted with a coarse porosity fritted-glass disk and filled with 150 ml. of 30% aqueous potassium hydroxide solution. The purification side of the system consists of a U-tube condenser conforming to the dimensions s h o m in Figure 4 and a single bulb absorption tube sealed to a 12/2 inner spherical glass joint, containing hscarite and stoppered a t both ends with glass wool. Two open-end manometers, a cold finger manifold (Figure 5 ) , and three 1-quart capacity D e m r flasks complete the unit. The pure carbonyl sulfide is collected in a 150-ml. capacity stainless steel bomb having a working pressure of 1800 pounds per square inch when fitted with a stainless steel needle valve. The bomb and valve sold by Hoke Inc., Englewood, X. J., Catalog No. HS-150 and 343, respectively, are satisfactory for this purpose. A safety shield is recommended for use in the carbonyl sulfide purification kchniques. The gas generator is disassembled and a glass- or plastic-encased stirring bar is inserted into the flask followed by 400 ml. of 55% by volume sulfuric acid. The male portion of the tapered joint is lubricated and the flask is attached and fastened to it with springs. The magnetic stirrer is turned on, the separatory funnel stopcock closed, and 40 ml. of a saturated aqueous solution of potassium thiocyanate are introduced into the funnel. Approximately 10 ml. of the thiocyanat,e solution are added to the flask and the generator is flushed with gas for 10 minutes to an exhaust hood. The generated gas is then collected i n the brine bulb by
100 M L . C A P
displacement; additional portions of the thiocyanate solution are introduced periodically as required. The brine bulb, filled with impure carbonyl sulfide, is closed off from the apparatus and the latter is evacuated to less than 1-mm. mercury pressure and checked for leaks. -1Dewar flask containing liquid nitrogen is placed around the U-tube condenser and the gas in the brine bulb displaced through the potassium hydroxide scrubber as rapidly as possible, collecting the condensable fraction in the U-tube (wndenqer. The flow rate for this operation should be 750 to 1000 ml. per minute; a slower rate results in the absorption of excessive amounts of c a r b o n y l sulfide. The U-tube is evacuated periodically, during the condensation process, to pump out noncond e n s a b l e s . .4pproximately 5 cm of the cold finger a r e i m m e r s e d iri liquid nitrogen, the Dewar flask is removed from around -2 0 M M . 0 . D the U-tube condenser, and the liquid alloned to vaporize over the Ascarite and condense in the cold finger. (The coatingof frost vihich forms on the outside of the Utube condenser tends to control the frate of vaporization Figure 5 . Cold finger manifold and should not be removed.) When all of the liquid has been transferred, a white powdery residue should remain in the Utube. The U-tube condenser is then evacuated until all of this volatile residue has been pumped out and the pressure has been reduced to less than 1-inm. mercury pressure. A small section of the U-tube is immersed in liquid nitrogen and the liquid in the cold finger is allowed to vaporize over the Ascarite and recondense in the U-tube condenser. This process is continued until only a T\ hite powdery residue remains in the cold finger. The needle valve on the carbonyl sulfide bomb is opened and the system evacuated until the volatile residue in the cold finger is completely removed. The cold finger is reimmersed in liquid nitrogen, and the liquid allowed to vaporize over the Ascarite and recondense in the cold finger. The system is then evacuated to less than 1 mm. of mercury pressure for 5 minutes. Approximately one third of the carbonyl sulfide bomb is immersed in liquid nitrogen. The purified carbonyl sulfide is vaporized from the cold finger and condensed in the evacuated sample bomb until approximately one tenth of the original liquid volume remains in the cold finger. The bomb is then evacuated to less than I-mm. mercury pressure for 5 minutes, the needle valve is closed, and the cylinder is removed from the liquid nitrogen and stored at 0 " C. With experience, carbonyl sulfide piepared in this manner should assay 99 8 to 99.9 mole %. Carbonyl sulfide may be assayed for impurities by a mass spectrometer using techniques normally employed in determining purity. Carbonyl sulfide prepared in the manner described usually does not contain acid gases and the amount of carbon disulfide present is generally too small to cause interference; hence, the carbonyl sulfide may be used directly to establish a calihration.
r
DISCUSSIOY
Figure 4.
U-tube condenser
The main objective was to develop and to evaluatr a nietliod for the determination of carbonyl sulfide, particularly as applied to the relatively low concentrations usually found in rehnery hydrocarbon gas streams. After the procedure had been demonstrated to show promise, a series of gas samples was prepared from ethylene, nitrogen, sulfur dioxide, and hydrogen sulfide containing known amounts of carbonvl sulfide. The analytical results shoun in Table I demonstrate the applicability of the
1170
ANALYTICAL CHEMISTRY
method with such a sample type and represent analyses obtained with two different lots of carbonyl sulfide gas. The method has been applied with equal success on a control basis to the analysis of ethylene produced in commercial quantities. I n general, an analysis can be completed in 2 hours, including the time required for preparation of the reference, or blank solution, and for the effervescence due to dissolved gas to subside. Effect of Carbonyl Sulfide Concentration. The relation between the absorbance of the piperidine oxythiocarbamate complev and the concentration of carbonyl sulfide is illustrated by a straight line. Within the limits, 0.0 to 0.7 mg. of carbonyl sulfide per 50 ml. of solution, the absorbance of the complex follows Beer’s law. The minimum concentration which can be determined reliably with a 1-cm. light path cell is approximately 0.05 mg. of carhonyl sulfide in 50 ml. of reagent. Effect of Time on Standing. .4 series of gas samples was prepared containing known amounts of carbonyl sulfide. These samples together with the blending gas were processed according to the procedure. Absorbance values taken a t hourly intervals indicated that the measurement may be made mithin an 8-hour period (Table 11). The time required for effervescence of the dissolved gas may vary somewhat with temperature and with the solubility of the gas in ethanol; hon-ever, 1 hour is usually sufficient.
Table 11. Stability of Piperidine Oxythiocarbamate Solution Blend NO.
1 2 3
1 hr. 0.237 0.260 0.268
Absorbance, 1-Cm. Cell 4 hr. 0.240 0,263 0.269
8 hr. 0,239 0,263 0.269
During the early part of the investigation, a shift in the calibration curve occurred on successive days. This was ultimately traced to the reference solution creeping out of the cell and over the outer surface of the cuvette windows when multiple absorbance measurements were made. For this reason, only two cuvettes are recommended to be employed, preferably of a glassstoppered type, and the measurements should be completed in the minimum length of time. If the measured values of the blanks, when compared to the original reagent, deviate by an amount greater than that equivalent to 1% transmittance, the blank determination should be repeated until this precision of measurement can be achieved. Interference. This procedure was designed to function primarily as a control on the amount of carbonyl sulfide contamination. Refinery gas streams contain other sulfur-bearing compounds that react with the reagent or interfere in some other manner. These diverse compounds are believed to be predominantly hydrogen sulfide and sulfur dioxide with smaller amounts of thiophene, carbon disulfide, and possibly methanethiol. If the interference of hydrogen sulfide and sulfur dioxide could be eliminated simply, or reduced to a nominal level through the use of a suitable absorbent, the method would have greater flexibility. Therefore, a study was made to determine x h a t effect various types of absorbents for these diverse constituents might have on carbonyl sulfide. The two absorbents selected first were asbestos fibers impregnated with lead acetate, and manganese dioxide. The experiment consisted of blending hydrocarbon gases with known amounts of carbonyl sulfide, hydrogen sulfide, and sulfur dioxide and passing the mixtures over the absorbent a t various flow rates. Tests demonstrated that lead acetate completely absorbed hydrogen sulfide and that only a minor portion of the sulfur dioxide was retained. When blends of similar composition u-ere passed over manganese dioxide, hon-
ever, the contaminating gases x-ere removed completely and the carbonyl sulfide was recovered quantitatively. A limited number of experimental results selected a t random have been tabulated in Table 111. These data substantiate the statement that hydrogen sulfide and sulfur dioxide interference is eliminated without affecting the validity of the analytical result for carbonyl sulfide by passing the sample over manganese dioxide prior to the formation of the carbamate complex.
Table 111. Removal of Hydrogen Sulfide and Sulfur Dioxide by Manganese Dioxide Added, P.P.M. Hydrogen Sulfur sulfide dioxide
Carbonyl Sulfide, P.P.SI. Added
Found
Standard deviation, 1.2%.
iiromatic hydrocarbons, such as benzene, in concentrations not exceeding 50 p.p,m. do not appreciably interfere in the method. The correlated data in Table IV indicate that a blend containing 56 p.p.m. of aromatics, such as benzene or toluene, has an absorption which is equivalent to about 1 p.p.m. of carbonyl sulfide, an error which can be disregarded except when the method isextended to its lower concentration limit. For application of the method to samples containing aromatics in excess of the concentration mentioned, the sample may first be scrubbed with 95% ethyl alcohol and the transmittance of the alcoholic solution measured a t 255 mp against an ethanol blank for correction of the absorbance owing to the aromatic compounds. Interference from thiophene and carbon disulfide can be compensated by measurement of the absorbance of the piperidine-alcohol scrubbing solution a t 230, 240, and 290 mp and application of standard principles to the analysis of a three-component system ( 2 ) .
Table IV. Aromatic Benzene Toluene
Effect of Aromatic Hydrocarbons Concn., P.P.11. 66 560
56 500
Equivalence as Carbonyl Sulfide, P.P.M. 1 0 7.5 0.5 G O
The grease should be removed from the inner spherical glass joint of the reaction bulb prior to transfer of the ethyl alcohol solution into a receiver. All silicone-type stopcock lubricants tested exhibited absorption in the vicinity of 230 mp and hence would lead to high results for carbonyl sulfide if dissolved in the ethyl alcohol-piperidine reagent. -4minimum amount of the lubricant should be employed to seal the inner joint, as well as its connecting stopcock, and particular care exercised to preclude subsequent contamination of the alcoholic solution. Contamination and Storage of Carbonyl Sulfide. Carbonyl sulfide produced from the reaction of sulfuric acid with potassium thiocyanate is contaminated with hydrogen sulfide, sulfur dioxide, carbon dioxide, carbon disulfide, oxygen, and nitrogen. To obtain carbonyl sulfide of sufficient purity for calibration purposes, a purification technique for the removal of these contaminants was developed so that a sufficient yield of carbonyl sulfide could be obtained conveniently for subsequent blending purposes. Aqueous potassium hydroxide and Ascarite are excellent absorb-
V O L U M E 27, NO. 7, J U L Y I 0 5 5 ents for acid gases but also absorbed exorbitant amounts of carbonyl sulfide. Therefore, the genefated gas is passed as rapidly a~ practical through the caustic scrubber a3 we11 as the Ascaritefilled absorption tube. Repeated scrubbing of the impure gas over Ascarite will effectively remove the remaining traces of hydrogen sulfide, sulfur dioxide, and carbon dioxide. Various methods were tested for removing the carbon disulfide, repeated lon- temperature fractionation being most successful. Evacuation of the product a t liquid nitrogen temperature eliminates the remaining undesirables, oxygen and nitrogen, and materially shortens the time for purification without introducing subsequent variable factors. With this preparation procedure, approlimately 1 gram of pure carbonyl sulfide should be obtained. Carbon?-1 sulfide can be stored for several months a t 0" C. in a stainless steel bomb fitted with a stainless steel valve. I n the early stages of the m-ork, a stainless steel bomb fitted with a brass needle valve and connector was employed. Over a short period of time the composition of the gas had changed appreciably, the decomposition apparently being catalyzed by the brass. Accuracy. The reliability of the method is illustrated by the data in Tablee I and 111. Analyses performed for the purpose of checking the performance of the method or evaluating various lots of freshly prepared carbonyl sulfide indicated a maximum abso-
1171 lute deviation of 1 p.p.m. from the thCweticd v a h e within the range of 2 t o 60 p.p.m., or 3.1% when expressed as an estimated standard deviation. ACKNOWLEDGMENT
Acknowledgment is made t o Matthew S. Norris for the ultraviolet measurements and to Nathan F. Kerr for conducting the carbonyl sulfide mass spectrometer assays.
LITERATURE CITED
(1) (2) (3) (4)
Alvdeeva, A. V., Zavodskaya Lab., 7, 279 (1938). Brady, L. J., ANAL.CHEM.,20, 512 (1948). Cornell, P. W., private communication. Hakewill, H., and Rueck, E. AI., Am. Gas Assoc., Proc., 28, 529
(1946). ( 5 ) MacHattie, I. J. W., and McXiven, K. L., Can. Chern. Process Inds., 30, 87, 92, 94 (1946). ( G ) Riess, C . H., and Wohlberg. C.. Am. Gas Assoc., Proc., 25, 259 (1943). RECEIVED for review September 24, 1954. Accepted January 8, 1955, Presented before the Symposiuni on Methods for Testing Liquefied Petroleum Gases, ASTAI Committee D-2. Satiiral Gasoline Association of America, a n d California Xatural Cramline .*ssociation, St. Louis, Ma., September 27 t o 29, 1954.
Determination of lead in Urine WILLIAM M. MCCORD and JOHN W. ZEMP Department o f Chemistry, The M e d i c a l College of South Carolina, Charleston,
A method is described for the determination of lead in urine which eliminates the necessity for time-consuniing precipitation and ashing. Lead, as lead iodide, is extracted quantitatively from acid solution w-ith meth>l isopropyl ketone. The lead is then removed from the ketone layer with an aqueous sodium hydroxide solution and is determined colorimetrically by the dithizone method of Snyder using the lead-bismuth separation of Bambach and Burkey.
T
HE detection of lead poisoning is greatly facilitated by the determination of micro quantities of lead in the urine of the individual ( 2 , 6, 10). The majority of methods currently in use (photometric, polarographic, etc.) (1,3-6,8, 11)involve precipitation of the lead as the phosphate, and usually ashing, for the removal of 7-arious organic and inorganic constit,uents which are present in relatively large amounts in urine. I n the experience of this laboratory, the initial precipit,ation of the lead as t.he phosphate in alkaline solution, with calcium and other ions, is the tedious and t ime-consuming part of the analytical procedure, especially when larger volumes of urine must be used n-hen amounts of' lead present are small. The introduction of niethyl isopropyl ketone as an organic solvent for the quantitative extraction of lead as lead iodide by West and Carlton (12) furnishes a means for considerably reducing the time required for an analysis. I n the proposed method, lead, in an acid solution and in the presence of excess potassium iodide, is extracted quantitatively by methyl isopropyl ketone. This is followed by estraction from the ketone into basic solution and deveiopment of color n+h dithizone.
S. C.
The dithizone method is that of Snyder (9) using the lead-bismuth separation of Bambach and Burkey ( 3 ) . REAGENTS AND APPARATUS
A blank must be run to determine the amount of lead in the reagents and distilled water supply before their use. If the total amount is more than 0.2 y, the reagents must be purified. All chemicals should be reagent grade. Sitric acid, concentrated. Diethyl ether. Methyl isopropyl ketone, Eastman KO.3146. Potassium iodide. Dissolve 155 granis of potassium iodide in 100 ml. of distilled n-ater. This includes a slight excess to ensure saturation. Sodium hydroxide, 0.5%. Dissolve 0.5 gram of sodium hydroxide in 100 ml. of distilled water. Thymol blue indicator, 0.1%. Dissolve 0.1 gram of thymol blue W S in 100 ml. of distilled water. Ammonium citrate, 5%. Dissolve 5 grams of ammonium citrate in 100 ml. of distilled water. Sodium cyanide, 2%. Dissolve 2 grams of sodium cj-anide in 100 ml. of distilled water. Ammonium hydroxide, 14%. Mix equal volumes of concentrated ammonium hydroside and distilled water. Dithizone solution. Diseolve 25 mg. of dithizone (Eastman S o . 3092) in 1 liter of chloroform. Store in a glass-stoppered bottle in the refrigerator. Buffer, p H 3.4. Dilute 9.1 ml. of concentrated nitric acid t o approximately 500 ml. with distilled water. Adjust the pH t o 3.4 with ammonium hydroxide. Add t o this solution a mixture of 25 ml. of 0.2M potassium acid phthalate and 4.98 ml. of 0.231 hydrochloric acid. Dilute t o 1 liter with distilled water. Ammoniacal cyanide solution. Mix 5 volumes of 14% ammonium hydroxide, 1 volume of 1 yosodium sulfite, and 1 volume of 201, sodium cyanide. [The sodium sulfite is added to reduce oxidants which attack the dithizone ( 7 ) .]
