TRACE ANALYSIS
I
Group Session on Analytical Research, Division of Refining, American Petroleum Institute, Montreal, Canada, May 14, 1956
Spectrophotometric Determination of ChIoride, Bromide, and Iodide F. W. CHAPMAN, Jr., and R. M. SHERWOOD The Atlantic Refining Co., Philadelphia, Pa.
,The determination of trace amounts of halogens in petroleum naphthas and catalysts i s becoming increasingly important because of their harmful effect in catalytic processes. A method i s given for converting the halogens in the sample to water-soluble inorganic halides, making the aqueous solution react with palladous sulfate, and measuring the absorbance of the reaction products at the prescribed wave lengths. A technique for separating chloride, bromide, and iodide by preferential oxidation provides a means for determining microgram quantities of these halides with a standard deviation of 5% relative, for both precision and accuracy. A procedure for the determination of these trace halogens in petroleum naphtha uses the disodium biphenyl reagent for their removal. Catalysts can be prepared for analysis by any standard method of fusion with sodium carbonate.
S
AMOUNTS O F HALIDES have been determined by many methods, but direct measurement of the absorbance of a complex of the halides has not been widely used. The reaction products formed between chloride, bromide, or iodide and palladous sulfate in aqueous solution exhibit characteristic absorption spectra in the ultraviolet region. Khen these halides are present together, they can be separated by selective oxidation and determined individually. This method requires the conversion of the halogens being determined into inorganic halides For the analysis of light petroleum products, where small amounts of halogens may be harmful in certain catalytic processes, the organic halogen is converted to MALL
172
ANALYTICAL CHEMISTRY
halide using the disodium biphenyl re-
1
.
@,4 )' I.6
PRELIMINARY EXPERIMENTS
Investigation of Complex Formation. Metals in the platinum family form stable complexes with halides ( 2 ) and many of them exhibit strong absorption in the ultraviolet region. -4s palladium can be converted readily to its sulfate salt in a halide-free aqueous solution, it was selected as the complexing metal. A solution of palladous sulfate was prepared by dissolving palladium metal in nitric acid and then evaporating the solution repeatedly in the presence of sulfuric acid. The absorption spectrum of the diluted palladous sulfate solution us. water is shown by curve A in Figure 1, A small amount of chloride as hydrochloric acid was added to another aliquot and curve B was obtained. The latter solution was compared to the original palladous sulfate solution and the differential absorption spectrum shown in Figure 2 was obtained. This indicated that a potential method was available for the determination of microgram quantities of chloride in aqueous solution. The effect on the differential absorption spectrum when the chloride concentration is varied is shon n in Figure 3. I n order to select the wave length a t which absorbance changed most nearly linearly with concentration, a series of curves was constructed as shown in Figure 4, using the data from Figure 3. At 230 mp the closest approach to a straight line was obtained. The effect of varying the concentration of palladous sulfate was also studied; the optimum sensitivity and reproducibility were obtained with the concentration
1"\
1.41
\
8
Pd SO4 SOLN.2 1.4mg. Pd+'/ml. A. 0.2ml. PdS04/10ml.
t \
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1
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vs'
250
300
WAVE LENGTH, mp
Figure 1. Absorption spectra of palladium solutions
Table I. Reproducibility of Chloride Determinations on Known Aqueous Solutions Chloride, y/ml. Known Found 9.5, 9 .7, 9 . 3 . 9 3 9 3 18.6
18.5, 19.0, 1 8 . 2
specified in the preparation of reagents. Using the 230 mp line of Figure 4 as a calibration curve, the concentrations of known chloride solutions were determined (Table I).
The reaction between bromide and palladous sulfate was then investigated. The palladous bromide complex was found to have an absorption spectrum sinlilar to that of the chloride complex but of greater absorbance (Figure 5). Through experiments similar to those made with chloride, a linear response \vas obtained a t 230 mp. Iodide gave a different reaction with palladous sulfate. The reaction probably forms a reduction product of palladous sulfate such as colloidal palladium and has an absorption spectrum as shown in Figure 6. The calibration
1.2
1
0.2 ml. Pd SO4 PLUS 105YCI-IN 10ml.
1.01
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$ m U LZ
0.2ml. Pd SO41N IOml.
.8-
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-
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.2
250
200
300
curve for iodide was constructed from absorbances obtained a t 390 mp (Figure 7 ) . ,4s expected, fluoride gave no reaction with palladous sulfate under the conditions of the experiment. Separation of Halides. The next step was to investigate the possibility of separating chloride, bromide, and iodide by selective oxidation. From values in the oxidation tables ( 2 ) two materials were chosen: solid manganese dioxide and lead peroxide. It appeared that conditions of pH, heating, etc., could be selected so that iodide could be oxidized by manganese dioxide with no loss of bromide or chloride, and bromide and iodide could be oxidized by lead peroxide with no loss of chloride. A series of experiments using these separate halides and mixtures of halides showed that all iodide could be removed with no loss of chloride or bromide by heating a slightly acid aqueous solution in the presence of solid manganese dioxide. The bromide and iodide could then be removed by heating a slightly acid aqueous solution in the presence of solid lead peroxide, with no loss of chloride. This provides a method for the determination of all three halides in the presence of each other. The individual halide concentrations are calculated from the absorbances obtained on the reaction products of palladous sulfate with the original aqueous halide solution, the manganese dioxide-treated
solution, and the lead peroxide-treated solution. Table I1 gives data for such a determination on a known aqueous solution.
Table It. Determination of Chloride, Bromide, and Iodide on Known Aqueous Mixture
c1Br-
I-
Known, 57.5 11.5 19.0
y
Found, y 60.0 11.5 20.0
Investigation and Elimination of Interferences. hfany materials, particularly anions, exhibit absorption in the ultraviolet region, as shown by Buck, Singhadeja, and Rogers (1). A sodium carbonate fusion step in the sample preparation effectiyely removed the common interfering anions with no loss of halide. Some anions removed by sodium carbonate fusion are: acetate, tartrate, citrate, nitrite, nitrate, sulfide, sulfite, thiosulfate, hydrosulfite, cyanide thiocyanate, hypophosphite, and phosphite, The presence of large amounts of anions such as chromate, manganate, and molybdate in the final solution would prevent the use of the present method because of high total absorbance. However, in petroleum fractions con-
WAVE LENGTH, mp Figure 2. Differential absorption spectrum of palladium solution plus chloride
I .6 1.4-
1.2-
1
0.4 ml. Pd S04/I Om1 .
2 3
IN SAMPLE AND REFERENCE CELLS
4
I. 35.0Y CI-/ml. 2. 28.0 3. 17.5 1 8 4. 14.0 1)
w 1.0- 6 0 2
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18
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7.0
11
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200
o
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t
250
300
WAVE LENGTH,mp
Figure 3. Differential absorption spectra of palladium solutions with varying chloride
Figure 4.
Absorbance vs. chloride concentrations a t various wave lengths VOL. 29, NO. 2, FEBRUARY 1957
173
centrations of these metals are
60
1.0,
low as
1
I
I
t o cause no trouble. Sodium ions decreased the relative absorbance difference between the reference solution and the sample (Figure 8). To prove that the difference was due t o the cation, various sodium salts and their free acids were used to make up solutions. I n each case the free acid had no effect, but the sodium salts did. Because other cations would have the same effect, they are removed by passing the solutions, after carbonate fusion, through a carefully prepared acid form of a cation exchange resin. This procedure has been completely successful and has been incorporated in the analytical method. The only limitation is that the total acidity of the final solution should be between 0.1 and 0 . 5 s . Other possible sources of interference are the presence of these halides in the reagents, and also small amounts of materials not removed by sodium carbonate fusion, xhich might absorb in the ultraviolet region. The method of correcting absorbances for these materials (Table V) has been found valid in a series of experimental tests. All final halide values are corrected for the halogen content of a blank containing all reagents and analyzed by the same technique.
0.4ml.PdS04/10ml. IN SAMPLE AND REFERENCE CELL
-
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Z
a m
-
cn m
-
2. 3.82 3. 2.30 4. 1.53
11
-
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250 300 WAVE LENGTH,my
200
350
Figure 5. Differential absorption spectra of palladium solutions with varying bromide
.31
I
I
I
REAGENTS AND APPARATUS
Disodium biphenyl (3). Hydrogen peroxide, 15%, Superoxol diluted 1 to 1 with water. PALLADOUS SULFATE SOLUTION.Dissolve 0.500 gram of palladium black (Fisher Scientific Co., purified) in 15 ml. of 3 to 1 nitric acid and 15 ml. of concentrated sulfuric acid in a 50-ml. beaker. Evaporate the solution to fumes of sulfur trioxide, cool, and carefully add 10 ml. of water. Repeat this procedure twice more to remove all nitrate, transfer the solution to a 500ml. volumetric flask, and dilute to about 300 ml. with mater. Dilute 1 ml. of this stock solution to exactly 10 ml. and measure its absorbance against distilled water a t 390 mp. .4dd water to the stock solution to bring the absorbance of a 1 to 10 dilution in the range of 0.15 to 0.17 a t 390 mp. A slight precipitation of palladium will occur in the stock solution after several days, but this does not affect the final results, if the absorbance of a 1 to 10 dilution is in the range of 0.15 to 0.17. The stock solution is about 1.5N in sulfuric acid; the exact normality is not critical. SODIUhl CaRBONATE SOLUTION. Dissolve 20 grams of anhydrous sodium carbonate (llallinckrodt, analytical reagent) in water and dilute to 100 ml. SULFURIC ACID, 5N. Add 35 ml. of roncentrated sulfuric acid to 200 ml. of water. cool the solution, and dilute to 250 ml. STAXDARD CHLORIDE SOLUTION. TTeight about 0.5 gram of sodium 174
ANALYTICAL CHEMISTRY
0.4ml.PdS04/10ml. IN 7.6 7I-/ml.
m U
Icm,cell
200 Figure 6.
,
400 WAVE LENGTH,mp
500
500
Differential absorption spectrum
of palladium solution plus iodide
chloride (Merck, reagent grade), dissolve in water, transfer to a 250-ml. volumetric flask, and dilute to the mark with water. Determine the exact concentration by titration with standard silver nitrate. Prepare a working s t m d ard by pipetting 10 ml. into a 250-ml. volumetric flask and diluting to the mark with water. This solution contains about 50 y of chloride per ml. STANDARD BROMIDESOLUTION. Prepare from potassium bromide (Merck, reagent grade) in the same manner as the standard chloride solution. The working solution contains about 15 y of bromide per ml. STANDARD IODIDE SOLUTION.Prepare from potassium iodide (Mallinckrodt, analytical reagent) in the same manner a s the standard chloride solution. The working solution contains about 15 y of iodide per ml. Lead peroxide (Fisher Scientific Co., Special Micro).
Table 111. Practical Range of Concentration for Various Halides in Final Solution
TlPvll. Iodide Bromide Chloride
1.0- 6 . 0 0 5- 7 . 0 2.0-25.0
Manganese dioxide (Baker's Analyzed, c.P.). CATIONEXCHAKGERESIN. Condition a cation resin such as Amberlite IR-120 (Rohm & Haas) with 5N sulfuric acid and then wash until the eluate is about pH 3, and is transparent a t 230 mp. Spectrophotometer, Beckman Model DU, with photomultiplier attachment and matched 1-cm. quartz cells.
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5
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Typical calibration curve for iodide
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ERENCE CELL) Icm.cells
230 mp
Preparation of Naphtha Samples. The determination of trace halides in petroleum naphtha samples is of particular interest. I n order to convert organic halides t o ionic halides, the disodium biphenyl method has been used (3, 4). The reagent was prepared by the method of Liggett ( S ) , the only change being prior purification of the toluene and benzene by treatment with some disodium biphenyl reagent, followed by a water wash and distillation. The halides are then determined on aliquots of the final aqueous solution obtained by the procedure given below. This method allows the use of samples in the order of 100 ml. or larger, thus providing for the determination of parts per million of halide. Place 50 ml. of benzene in a 250-ml. glass-stoppered separatory funnel and add 25 ml. of the disodium biphenyl reagent. Pipet 100 ml. of the naphtha sample, stopper, and shake for 30 seconds. Allow the mixture to stand for 15 minutes. Add 10 ml. of water to decompose the reagent. Stopper, invert the funnel, and vent the gas formed. Swirl the contents several times until the reagent is decomposed, venting the gas each time. Allow the layers to separate and drain the water layer into a 2.50-ml. beaker. Add 10 ml. of distilled water to the funnel, shake, allow the layers to separate, and drain the water layer into the beaker. Add 10 ml. of water and 1 ml. of 1 to 2 acetic acid to the funnel, shake, and drain the aqueous layer into the beaker. Wash once more with 10 ml. of water and add the water layer. Add 10 ml. of 15% hydrogen peroxide to the combined extracts in the beaker, cover with a watch glass, and bring to a boil on a hot plate. Continue boiling until the volume is reduced to about 15 ml. Prepare a blank solution, using all reagents without naphtha. Make aqueous sample and blank solutions just acid with dilute sulfuric, and add 1.0 ml. of the sodium carbonate solution to each. Evaporate to about 10 ml. and transfer to 30-ml. platinum crucibles. Continue evaporation to dryness under a heat lamp, and fuse each for about 30 seconds, using a hieker burner. Dissolve each melt with about 10 ml. of water, make just acid with dilute sulfuric acid, and swirl to expel all carbon dioxide. Then pass the solutions through a column containing about 10 grams of the acid form of the cation resin. Check the eluate occasionally, by means of a platinum wire and a flame, to assure no passage of sodium. Wash the column with water and collect about 20 ml. of the eluate in a 25-ml. volumetric flask. Then dilute the solutions to the mark with water.
I 5
IO
15
20
25
30
35
40
CI- CONC.,Y/ml.
Figure 8.
Effect of sodium ions on chloride determination
PROCEDURE
Preparation of Calibration Curves.
A plot of absorbance against concentration is made for each halide, using standard solutions. The practical range of concentrations of each halide is given in Table 111. T h e bromide and chloride solutions are measured a t 230 mp, while iodide is measured at 390 mp. The procedure for each is the same.
Appropriate aliquots of each halide solution are transferred to 10-ml. volumetric flasks, 0.4 ml. of 5N sulfuric acid is added, the solutions are diluted to 5 to 8 ml. with water, and exactly 0.4 ml. of the palladous sulfate solution is added to each. After being diluted to the mark with water and mixed, their absorbances are read a t the proper wave length in 1-cm. cells against a reference solution containing the same concentration of sulfuric acid and palladous sulfate solution.
Preparation of Catalyst Samples. Catalyst-type samples can be prepared for analysis by any standard method of fusion with sodium carVOL. 2 9 , NO. 2, FEBRUARY 1957
* 175
bonate. For silica-alumina cracking catalysts a ratio of sodium carbonate to catalyst of 10 to 1 and fusion for 1 hour over a Meker burner are satisfactory. The carbonate fusion is dissolved in dilute sulfuric acid and the solution passed through the ion exchange and diluted to volume in the same manner as the naphtha sample solution. A blank is prepared in a similar manner, using all reagents. Absorbance Measurements. TOTAL ABBORBASCE.Pipet 5 nil. each of the sample and blank solutions into separate volumetric flasks, add 0.4 ml. of 5.Y sulfuric acid each, and dilute t o about 8 ml. with water. Pipet exactly 0.4 ml. of the palladous sulfate solution into each flask with swirling, and adjust the volumes t o the mark with water. Measure the absorbance of each solution a t 390 mp against a reference solution containing 0.4 ml. of 5-I7 sulfuric acid and 0.4 ml. of the palladous sulfate solution, and dilute to 10 ml. This number is used to calculate the iodide content of the sample ( A z ) . [The absorbance of the sample is also measured a t 230 mp (A1); if it is greater than 0.7, a smaller aliquot must be taken for the subsequent bromide determination.] ABSORBANCE AFTER I O D I D E REMOVAL. Pipet 5 each of the sample and blank solutions into separate 10-ml. beakers and add 0.2 ml. of 5N sulfuric acid to each. Place the beakers on a hot plate and add about 40 mg. of manganese dioxide to each to oxidize iodide. Heat solutions at 75" to 80" C. for 25 minutes with occasional agitation. Then remove the beakers and allow them to cool to room temperature. Decant the solutions from manganese dioxide into separate 10-ml. volumetric flasks, !\-ash the beakers with water, decant the mashes into the flasks, add 0.2 ml. of 5A7 sulfuric acid to each, and dilute to about 8 ml. with Lvater. (Contamination by a small amount of manganese dioxide in the flasks mill not affect the results.) Add exactly 0.4 ml. of the palladous sulfate solution to each and adjust the volumes to the mark with water. Measure the absorbances of the solutions a t 230 and 390 mp against a reference solution containing 0.4 ml. of L 1 7 sulfuric acid and 0.4 ml. of the palladous sulfate solution (Aa and A). A B S O R B A N CAEF T E R IODIDE AND BROMIDE REMOVAL. Pipet 5 ml. each of the sample and blank solutions into separate 10-ml. beakers. Carry out oxidation in exactly the same way as for bromide, but use 40 mg. of lead peroxide to oxidize the iodide and bromide. After cooling, decant the solutions, wash into separate 10-ml. volumetric flasks, add 0.2 ml. of 5N sulfuric acid and 0.4 ml. of the palladous sulfate solution to each, and dilute to volume. Measure the absorbances of the solutions a t 230 mp against the reference solution containing 0.4 ml. of sulfuric acid and 0.4 ml. of the palladous sulfate solution (As). 176
0
ANALYTICAL CHEMISTRY
ABSORBAXCES OF NONHALOGES MAPipet 5 ml. each of the blank and sample solutions into separate 10ml. beakers, and add 0.2 ml. of 5.V sulfuric acid followed by about 40 mg. of lead peroxide. Heat solutions 75" to 80" C. for 25 minutes with occasional agitation. Cool solutions, transfer to separate 10-ml. volumetric flasks, and dilute to the mark with water. Measure the absorbances of the sample and blank solutions against distilled water a t 230 mp in 1-cm. cells (-4%).
TERIALS.
Table IV. Method for Obtaining Absorbance Due to Each Halide
.lbsorbance 230 mp
Absorbance total After iodide removal -4fter iodide and bromide removal Of nonhalogen material
*1'i
Solution NO. 1 2
'4 I
'43
A,
A,
Absorbance due to iodide Absorbance due to bromide Absorbance due to chloride
Table V.
390 mp d2
= = =
A , - d, A? - A 5 A; - A,
cracking catalysts. It could be used with other types of samples, unless a large quantity of some unremovable material absorbing in the ultraviolet region was present. The sensitivity of the iodide method could be increased by using longer cells, as the reagents do not absorb too strongly a t 390 mp. However, with the bromide or chloride determination longer cells are not feasible, because of the strong absorbance of the reagent a t 230 nip. At 230 nip it is preferable to keep a constant and narrow d i t width, oning to the shape of the absorption curve; longer cells xould require too large a slit width. The absorbance of the material formed nith palladous sulfate and bromide or chloride !vas constant for several days when compared against a reference solution of the same age. K i t h iodide the absorbance is constant for only a fern hours. Vhen it is necessary to use the oxidizing agents for separation of halides, the stability is soniediat less, although the absorbance is still constant for several hours. The time required for a complete analysiq of a petroleum naphtha sample
Determination of Known Chloride, Bromide, and Iodide in Benzene Foinid Concn., P.P.M. Iinown Concn., Element P.1'. 11. 1 2 3 1 rlv.
Chloride Chloride Bromide Iodide
4 1 10 2 2 95 12.6
Calculation of Concentrations. The corrected absorbances for the halides in the sample and blank solutions are obtained according t o Table IT'. The concentration of each halide is calculated from the proper calibration curve. The concentration of each halide in the blank solution is then subtracted from the corresponding halide concentration of the sample solution. Because of slight nonlinearity of the chloride calibration curve a t low concentrations, the absorbances of the blank solution must not be subtracted from absorbances of the sample solution. The concentration of halide in the original sample is obtained from these corrected concentrations by the use of the proper aliquot and weight factors.
4 1 10 0 2 8 11.9
38 10 8 3 2 12.2
4 4
4 3
10 5 3 0 12.5
3 0 12.5
-12 10 4 3 0 12.3
is about 12 hours. Table V shows results of replicate analyses of two benzene solutions, one of which contains only chloride and the other contains a mixture of chloride, bromide, and iodide. The halides were present as chlorodecane, ethyl bromide, and ethyl iodide. The standard deviation is in the order of 570 of the amount of halide present, for both precision and accuracy. LITERATURE CITED
(1) Buck, R. P., Singhadeja, S , Rogers, L. B., ANAL.CHEM.26, 1240 (1954). (2) Latimer, 117. M., "Osidation Potentials," 2nd ed., Prentice-Hall, New
DISCUSSION
(1954). (4) Pecherer, B., Cambrill, C. AI., Wilcox, G. IT.,Ibid., 22, 311 (1950).
This method has been applied successfully to petroleum naphthas and
RECEIVEDfor review June 11, 1956. Accepted December 13, 1956.
