ANALYTICAL CHEMISTRY
520 Table I.
Salts Assayed by Titration with 0.1 N Perchloric Acid
Table 111. .4ccuracy and Precision Perchloric Acid Titration, Yo
(Commercial reagent grade) Purity Found,
Salt (Cation Na) NaCzHs01 NaNs NaHCOa NaHSOr NazBzOi NaBrO3 NaBr NazCOI NaClOa NaCl NaCX NaF Na0H NsH1POZ.HaO NaIOs NaI NazMo04.2H10 NaNOz NaNOs Na202 NaH2POi NazHPOi NasPOi NalSiOi NazSOd NaaS.9HnO NatSOa NaSCX NazW04.2K10
Aluminum iimmonium Antimony Barium Bismuth
Anion of Snlt Acetate Azide Bicarbonate Bisulfite Borate Bromate Bromide Carbonate Chlorate Chloride Cvanide Fiuoride Hydroxide Hypophosphite Iodate Iodide Molybdate Nitrite Nitrate Peroxide Phosphate, primary Phosphate, secondary Phosphate, tertiary Silicate Sulfate Sulfide Sulfite Thiocyanate Tungstate
% 100.0 100.0 100.1 99.4 99.8 100.0 100.0 99.9 99.9 99.9 95.7 96.6 98.2 99.8 100.0 100.0 99.9 100.0 100.0 97.6 99.8 99.9 99.9 99.8 100.0 99.9 98.8 99.4 99.9
Table 11. Other Cations Cadmium Calcium Cobaltous Iron Lead hlagneaium
Labeled Purity,
7c
..
.. .. , .
..
..
..
..
.. 95.9 96.2 98.2
Sodium chloride A
Chloride Determination,
B
99.9 99.9 100.0
99.8 100.0 99.9 d c i d Titration
B
99.9 99.9 100.0
99.9 99.7 99.9
C Sodium carbonate 4 C
Sodium sulfate A B C
Precipitation as Bas04 100,o 99.9 100.0
99.6 99.4 99.7
..
.. ,
I
100.0 97.6
.. ..
..
Salts composed of the cations listed in Table I1 with any of the anions in Table I may also be successfully titrated in the manner described, provided they can be solubilized and react basic in glacial acetic acid.
, .
ACCURACY AND PRECISION 98.8
.. ..
Manganous Potassium Silver Strontium Zinc
performed potentiometrically. It was thus possible to dissolve all salts listed in Table I. If the anion is a chloride, bromide, or iodide, 10 ml. of mercuric acetate reagent are added and the solution is titrated with 0.1 N perchloric acid in p-dioxane, either potentiometrically or using crystal violet as indicator. Several compounds, such as those that react xith the indicator, must be titrated potentiometrically.
Accuracy and precision of titrations in nonaqueous solutions, using perchloric acid, have been evaluated ( 6 , 7 )and a reproducibility of ~k0.275or better has been reported for a majority of compounds. This is also borne out by results obtained in triplicate determinations of three different sodium salts, both by titration with perchloric acid and by conventional methods (Table 111). LITERATURE CITED
(1) Higuchi, Takeru, and Concha, Jesusa, J . Am. Phurm. Assoc., 40, 172 (1951). (2) Higuchi, Takeru, and Concha, Jesusa, Science, 113, 210 (1951). (3) Kolthoff, I. M., and Willman, A., J. Am. Chem. SOC.,56, 1007 (1934). (4) Ibid., p. 1014. (5) Markunas, P. C., and Riddick, J. A., ANAL. CHEW,23, 337 (1951). (6) Palit, S. R., IXD. ENG.CHEM.,ANAL.ED.,18, 246 (1946). (7) Pifer, C. W., and Wollish, E. G., ANAL.CHEM.,24, 300 (19323. RLCEIYED for reriew August 28, 1951.
Accepted December 6, 1951.
Determination of Small Amounts of Molecular Oxygen in Gases and liquids FRANCIS R. BROOKS, MARTIN DIMBAT, RICHARD S. TRESEDER, AND LOUIS LYKKEN Shell Development Co., Emeryville, Calif.
I
N T H E petroleum and related industries the need frequently arises for a reliable method for determining dissolved oxygen in both aqueous and nonaqueous liquids. A knowledge of the dissolved oxygen content of liquids such as gasoline, kerosene, lubricating oil, solvents, extracting solutions, and treating solutions is often of value in the solution of both plant and laboratory problems involving corrosion, emulsions, and storage stability of refined products, A review of the literature reveals that little consideration has been given to this analytical problem. The method proposed by Schulze, Lyon, and Morris (6) for application to certain liquid hydrocarbons is a modification of the fundamental Winkler method (8) used in water analysis. It consists of bringing the hydrocarbon sample in contact with an aqueous reagent containing manganous hydroxide and, subsequently, measuring the degree of
oxidation of the reagent by iodometric titration. This method is subject t o serious error if the material analyzed contains reducing agents or oxidizing agents other than dissolved oxygen. The materials to be analyzed frequently contain appreciable quantities of substances, such as organic peroxides and mercaptans (thiols), which would interfere with the method. The same limitation applies t o other methods of this general type, which involve measuring the oxidizing power of the sample by mixing it with a standard solution of a reducing agent. MacHattie and Maconachie (5) devised a method for the determination of small amounts of oxygen in gases, whereby the gas sample was passed through a tube containing copper wire wetted with ammoniacal ammonium chloride solution and the resulting copper oxide was removed from the reaction tube by washing the copper further with ammoniacal ammonium chloride solution.
V O L U M E 2 4 , NO. 3, M A R C H 1 9 5 2
521
To gain a better understanding of the effect of molecular oxygen on various processes in the petroleum industry, the MacHattie-Maconachie method for the determination of traces of oxygen in gases by reaction with copper has been improved and extended to the determination of dissolved oxygen in aqueous and nonaqueous liquids. Dissolved oxygen is determined by stripping the oxygen from solution by a stream of nitrogen, which is then passed through the reaction cell to remove the oxygen. Data have been collected which indicate that the molecular oxygen content of both liquid and gas samples can be determined within 5% of the truth over a wide concentration range.
The dissolved copper was determined colorimetrically as the copper-ammonia complex and the oxygen content of the gas was then calculated from the quantity of dissolved copper. Although AIacHattie and Maconachie obtained good results with this method, they found that 10.45 mg. of copper, instead of the theoretical 11.36 mg. for cuprous oxide, were equivalent to 1 ml. of oxygen and adopted the former figure for their calculations. Uhrig, Roberts, and Levin (’7) studied this method and demonstrated that the low ratio of copper t o oxygen observed by MacHattie and Maconachie was due to the formation of some cupric as well as cuprous ion in the reaction cell. They modified the procedure to prolong the contact of the reaction cell solution with the excess copper and were able to obtain the theoretical copperto-oxygen ratio of cuprous oxide. In addition, they found it convenient t o determine the dissolved copper volumetrically by an iodometric titration. Alorerecently, Deinum and Dam (3) investigated the method as modified by Uhrig, Roberts, and Levin and were able to simplify the apparatus somewhat and to reduce the size of sample required for analysis. Although these investigators did not apply the method to the determination of dissolved oxygen in liquids, they
NI l r o g t n SUPPlY
(Copper Shot)
“en,
2
3
5
4
6
7
did recognize the possibility that dissolved oxygen could be displaced by a current of oxygen-free nitrogen and determined in this manner. A modification of the method developed by Uhrig, Levin, and Roberts is described in this paper for the determination of oxygen dissolved in liquids as well as for the determination of traces of molecular oxygen in gases. Dissolved oxygen is stripped from liquid samples by a stream of oxygen-free nitrogen and the resulting gas mixture is analyzed in a manner similar to that used for gas samples, The copper removed from the reaction cell is determined colorimetrically by the sodium diethyldithiocarbamate method (1) as modified by Hoar (3). The high sensitivity of the method permits accurate analyses t o be made with a minimum amount of sample. The accuracy and sensitivity of the method are illustrated by analytical data that were obtained for a variety of liquid and gas samples of known oxygen content (4). REAGENTS
Acetone, C.P. Ammoniacal Ammonium Chloride Solution. Dissolve 300 grams of C.P. ammonium chloride in 1 liter of distilled water. Add 1 liter of 14 N ammonium hydroxide solution and filter. Gum Arabic Solution. Dissolve 5 grams of powdered gum arabicin 100ml. of distilled water a t room temperature and filter. Prepare fresh as needed. Vent Nitrogen, supplied a t a pressure of 2 to 4 pounds per square inch from cylinders equipped with a two-stage regulating valve. Where available, high purity nitrogen containing less than 0.01% ’ oyygen is recommended; if ordinary nitrogen containing 0.2 to 0.5y0oxygen is used, the purifier solution must be changed more frequently and blank determinations are higher and less reproducible. Sodium Diethyldithiocarbamate Solution. Dissolve 1 gram of the salt in 1 liter of distilled water. Standard Copper Solution. Dissolve 0.2000 gram of pure copper in 20 ml. of 16 N nitric acid and dilute t o 1 liter with distilled water. Dilute this solution tenfold with distilled water to obtain a solution which contains 0.02 mg. of copper per ml. Stopcock Lubricants. Both water-insoluble and hydrocarbon insoluble lubricants are required. Rotameter Assembly
u
\ / I
Reagoni
Sintered Qloec Bubbler
Recrrvoir
Figure 1. Diagram of -4pparatus
APPARATUS
The apparatus is shown in Figure 1. Fill the upper chamber of the nitrogen purifier with copper shot, cleaned by washing with acetone; support the copper shot bv a perforated glass disk. Secure the stopper firmly with wat&-insoluble lubricant and spring retainers, or with sealing wax, to prevent entrance of air. Place a sufficient number of glass beads on the perforated glass disk in the lower chamber of the nitrogen purifier to cover the delivery tube from the upper chamber t o a depth of a t least 10 mm. (The glass beads serve to trap spray from the gas leaving the nitrogen purifier.) Locate the main reagent reservoir a t such a height that the reagent scrubber can be filled by gravity flow. Fill the pressure relief device with mercury to a depth of about 10 cm. Construct the reaction chamber from a section of borosilicate glass tubing, approximately 14 mm. in inside diameter and 180 mm. long. Fill the reaction cell with copper rods, prepared by
ANALYTICAL CHEMISTRY
522 cutting KO.10 copper wire into 0.25-inch lengths; wash the rods with acetone t o remove any grease film. Leave a small void space a t the top t o permit sealing operations with a minimum oxidation of the copper packing. A photoelectric colorimeter is required to determine the copper content of the reactor solution. For the analyses described below, a Fisher Electrophotometer was used with a blue filter having a maximum transmittance a t 420 mp. SAMPLES AND SAMPLING
Collect both gaseous and liquid samples in calibrated sample tubes similar to that shown connected t o the apparatus in Figure 1. Select the sample size for analysis such that the oxygen content is from 0.005 to 2.0 ml. It is convenient t o have available several sizes of sample tubes, ranging from 25 t o 500 ml. in capacity. Gollect gaseous samples by displacement of nitrogen followed by thorough flushing with sample, in evacuated sample tubes, or bv displacement of mercury. If the sample is collected In an evacuated sample tube, fill the tube with nitrogen before evacuating. When the sampling is done by mercury displacement, fill the tube with nitrogen before filling with mercury in order to prevent trapping of air by the mercury. If it is necessary t o store the samples before they are analyzed, fill the sample tubes under slight pressure t o avoid any contamination from “breathing.” T o collect liquid samples, fill the sample tube with oxygen-free nitrogen by attaching i t t o the apparatus and purging it thoroughly. Connect the sample container to the sample tube and purge the connecting lines with sample. Turn the sample tube stopcocks to allow sample to enter the tube and fill the tube t o a predetermined calibration mark. Protect the exit of the sample tube with a water trap to prevent entrance of air. Avoid esposing liquids to air while sampling unless the liquid had previously been at equilibrium with air. Do not subject a liquid sample t o reduced pressure lvhile sampling. PROCEDURE
Preparation of Apparatus. Lubricate all stopcocks and spherical joints with water-insoluble lubricant. Connect the nitrogen source to the apparatus, adjust the reducing valve t o deliver nitrogen a t 2 t o 4 pounds per square inch, and purge the entire apparatus with nitrogen. Connect the ammoniacal ammonium chloride reagent reservoir, containing a t least 2 liters of reagent, t o the apparatus and flush the connecting lines with the reagent. Close stopcock 1 (Figure 1) and open the nitrogen purifier t o the atmosphere through stopcocks 2 to 7 . Turn stopcocks 11 and 12 to connect the reagent reservoir to the purifier and fill the lower chamber of the nitrogen purifier with reagent until the liquid level is almost in contact with the perforated plate supporting the glass beads. Turn stopcock 11 t o a completely closed position, vent the reagent scrubber through stopcock 3, and admit reagent through stopcock 12 until the liquid level in the reagent scrubber is within 20 t o 30 mm. from the top. Admit nitrogen t o the purifier a t the rate of about 100 ml. per minute, allow the gas to flow through the purifier and bubbler in the bottom of the reagent scrubber, and vent the reagent scrubber t o the atmosphere. hdjust the nitrogen rate t o obtain smooth pumping of reagent in the purifier. Draw 200 t o 300 ml. of acetone through the reaction cell t o remove any grease film from the copper packing, dry with a stream of air, and then flush with purified nitrogen. Turn stopcocks 9 and 10 t o permit reagent t o flow from the scrubber to the reaction cell which is vented to the atmosphere. When the reaction cell is about two thirds full of reagent, turn stopcock 9 to connect stopcocks 5 and 10, so that a stream of nitrogen from the scrubber flows upward through the reaction cell t o the atmosphere. Adjust the nitrogen flow t o about 100 ml. per minute. Adjust the liquid level in the cell until the copper is barely awash while the liquid is being agitated with nitrogen. After the nitrogen has been flowing through the reaction cell for .about 10 minutes, withdraw the reagent by displacing it downward with purified nitrogen through the drain connected to stopcock 10. T o do this, first close stopcock 10, turn stopcocks 6 and 5 t o connect the top of the reaction cell t o the purified nitrogen supply, then turn stopcock 10 t o drain. This order must be followed to prevent air from being drawn back into the reaction cell. Repeat thiv operation until the reagent withdrawn (cell washings) contains only a trace of copper, which can be determined as follows: Shake the cell washings vigorously t o assure aeration and convert the dissolved copper t o the cupric state, and examine for blue color. If the cell washings remain colorless, add 1 t o 2 ml. of 0.1% sodium diethyldithiocarbamate solution. A light yellow color indicates minute amounts of copper present, and with
greater copper content the color is more intense. A lightly colored blank indicates that the nitrogen purifier is operating efficiently, the copper in the reaction cell is free of oxide, and the reagent in the scrubber is free of oxygen. If the apparatus is t o be shut down for a period greater than a few minutes, introduce into the reaction cell sufficient reagent t o cover the copper completely and align the stopcocks to prevent the entrance of air.
Blank Determination. \Then it has been established that the nitrogen purifier is working properly and that the reaction cell is free of oxidized copper, make a blank determination a i follows: Turn stopcock 4 to by-pass the sample tube and displace the reagent from the reaction cell and flush the short delivery tube leading from stopcock 10 Rith a small amount of scrubbed reagent. Introduce 2 t o 3 ml. of fresh reagent into the reaction cell and pass purified nitrogen upward through the reaction cell t o the atmosphere at a rate of 40 to 60 ml. per minute for 45 minutes (blank t o be run the same length of time as the sample). At the end of that time, add more reagent until the cell is about two thirds full and pass nitrogen as before for an additional 5 minutes; adjust the liquid level in the cell until the copper is just barely awash. The agitation of the reagent in the cell ensures the conversion of the dissolved copper t o the cuprous state, which is a necessary condition of the method. Displace the reagent from the cell into a clean 100-ml. volumetric flask, allowing as complete drainage as practicable, then introduce fresh reagent into the cell and agitate for 30 seconds by passage of nitrogen. Again drain the cell as before and add t o the cell washings previously withdrawn. Repeat this operation until 10 washings have been made. Bubble oxygen or air through the cell washings while shaking vigorously to aerate the cell washings completely, and add sufficient distilled water t o bring the volume t o exactly 100 ml. Stopper the flask and reserve for colorimetric determination of copper. Analysis of Liquid Samples. Clean and lubricate the spherical joints on the sample tube, using a lubricant that is insoluble in the sample t o be tested, and attach t o the apparatus. Open stopcock B and drain any of the sample from the side tube, and flush side tube and connecting lines with purified nitrogen, including the bores of both stopcocks A and B. Introduce 2 t o 3 ml. of fresh reagent into the reaction cell, turn stopcock 4 to connect the sample tube t o the apparatus manifold, and pass nitrogen down through the side arm of the sample tube, up through the sample tube and the reaction cell a t a rate of 40 t o 60 ml. per minute for 45 minutes. Add more reagent t o the reaction cell, agitate, rinse, and collect the cell wmhings as in the blank determination. Make up t o volume with distilled water, stopper, mix thoroughly, and reserve for colorimetric determination of copper. (In cases where the oxygen content is very low, and large samples are taken, i t may be necessary t o employ longer flushing periods. The required flushing time for any type of sampling may be determined by repeatedly pmsing nitrogen through the sample until the amount of copper found in the cell washings is the same as that found in the blank.) Analysis of Gas Samples. Clean and lubricate the spherical joints on the sample tube with the proper lubricant, attach the sample tube t o the apparatus, and flush the connecting linea through stopcocks A and B with purified nitrogen. Barely immerse the tip of the vent tube from stopcock B in a beaker containing water, and pass purified nitrogen through t o drive out any air in the line; cautiously vent the sample tube through the immersed vent tube to bring the sample t o atmospheric pressure. The water vent trap is a safeguard against dramlng air into the sample in case the sample pressure is below atmospheric pressure. Record the room temperature and atmospheric pressure. Introduce 2 to 3 ml. of fresh reagent into the reaction cell, turn stopcock 4 t o connect the sample assembly t o the apparatus, and pass nitrogen through the side arm, sample tube, reaction cell, and t o t,he atmosphere at a rate of 40 t o 60 ml. per minute. Flush for 20 minutes or longer, depending on the size of the sample tube. As a general rule, a volume of nitrogen equal t o 6 or 7 times the sample volume should be used t o assure proper flushing of the sample. Introduce additional reagent into the reaction cell, agitate, rinse, and collect the cell washings as in the blank determination. Stopper and reserve cell washings for colorimetric determination of copper. Colorimetric Determination of Copper. To calibrate the photoelectric colorimeter, introduce 0-, 1-, 2-, 3-, 5-, 7 - , and 9-ml. portions of standard copper solution (I ml. = 0.020 mg. of copper) into separate 100-ml. volumetric flasks. Dilute with distilled water t o about 60 ml. and add in order 1 ml. of gum arabic solution, 10 m]. of 14 N ammonium hydroxide solution, and 1 ml. of 0.1% sodium diethyldithiocarbamate solution. Dilute to exact,ly 100 ml. with distilled water, stopper, and mix thoroughly.
523
V O L U M E 2 4 , NO. 3, M A R C H 1 9 5 2 After 10 minutes, but within 1 hour, determine the optical densitv of each solution, referred to distilled water, using a 25-mm. colorimeter cell. Subtract the optical density of the solution to which no copper was added (reagent blank) from the optical density values found for the other standards. Plot these corrected readings against the number of milligrams of copper added to the corresponding standard. T o determine the copper content of the reaction cell liquid which is obtained from the analysis of a sample, pipet an aliquot of the solution containing less than 0.18 mg. of copper into a 100ml. volumetric flask and proceed as above. Again, prepare a reagent blank and correct the optical density observed for the reaction cell solution by the value observed for the reagent blank. From the corrected optical density value, determine the copper content of the solution aliquot taken by means of the calibration curve prepared above. Calculations. Subtract the optical density of the colorimeter reagent blank from that of the cell washing sample, and convert the corrected optical density to milligrams of copper by means of the previously prepared calibration curve. Calculate the milligrams of copper in the cell washings solution by means of the following equation: R x 100 llilligrams of copper S = ___ A where R = milligrams of copper coiresponding to the corrected optiral density, and A = volume of aliquot in milliliters, taken for the copper determination. Calculate the osygen content of liquid samples by means of the followine eauation: 1000 ( S - B ) Oxygen, ml./litrr (STI-': = 11.36 X V I
.
wheie S = milligrams of coppei i n sample cell washings, B = milligrams of copper in blank cell waqhings, and V = volume in milliliters of sample analyzed. Calculate the oxygen content of the gaseous sample by means of the following equation: Osygen, % mole =
( S - B ) X T X 760 X 100 11.36 X 2 T f l X P X V
where S = milligrams of copper in sample c ~ l washings, l B = milligrams of' copper in blank cell washings, ?' = observed room temperature, degrees Kelvin, P = observed barometric pressure, niillinieters of mercury, T' = volunie, milliliters, of sample anaIyz(Jtl.
Table I.
Determination of Low Concentrations of Oxygen in Nitrogen"
Oxygen Present, %
0.r~ gen Iound ' 1 ,
Recovery,
%
96 96 98 97 97 97
n
0x62 6 0888 0.0855 0 0143 0.0144 0 0147 0.0143 0 00272 0.00501 0 00241 0.00301 0 00290 0.00301 0 00303 0.00301 I n all cases oxygen was rontainrd in 152 n i l . of nitrogen.
R.. fi 98
oxygen-free liquid. Oxygen-saturated liquids were prepared by bubbling oxygen through the liquids for several hours, then shaking the liquid in equilibrium with oxygen a t atmospheric pressure and room temperature. Liquids free of oxygen were prepared by prolonged purging with nitrogen. In this manner, samples of water, acetone, methyl acetate, and benzene containing various amounts of oxygen were prepared. These samples were then analyzed by the procedure described above for, liquid samples. The resulting data are shown in Table 11, together with values that were calculated from the solubility data of Horiuti (4). DISCUSSION
The data in Table I show that reliable determinations of oxygen in gases were obtaincd in the concentration range of 0.003 to 2%, using only 150 ml. of gas sample. The high sensitivity and accuracy of the method are attributed to three factom-(l) the sensitive diethyldithiocarbamate colorimetric method for the determination of copper, (2) the copper-rod packing in the reaction cell, vhich pcrmits the oxidized copper to be readily and completely washed out, and (3) the use of cylinder nitrogen containing less than 0.01 % oxygen. The data in Table I1 clearly show that this method is effective for the determination of dissolved oxygen in either aqueous or nonaqueous liquids. The results obtained for samples containing more than 1 ml. of oxygen per liter of liquid consistently agree within 5y0 nith the results calculated from solubility data. The results obtaincd for mater samples containing less than this amount of dibsolved oxygen indicate that recovery of the oxygen is incomplete. Apparently the method is not reliably applicable to such dilute solutions because of the slow rate a t which the last tracrs of dissolved oxygen are purged from solution. Experience has shown that it is occasionally necessary to wash the copper packing of the reaction cell with acetone to recondition the surface of the copper. Contamination of the copper is recognized by its loss of luster. T h i P tieatment is rarely necessary mhen the method is used for analysis of water samples or sweet hydrocarbon samples of Ion- vapor pressure. When analyzing extremely sour samples that contaminate the copper excessively, it may be necessary to clean the copper with 6 A* nitric acid solution followed immediately by rinsing vith distilled water. This cleaning procedure should be avoided unless it is absolutely necessary, as it expends the copper rapidly. KO attempts were made to study the inteiferences caused hy the presence of hydrogen sulfide mercaptans, sulfur dioxide, or ~~
~
~.
~
~
~-~
Table 11. Determination of Dissolved Oxygen in Various Liquids
104
99
Test
90 97
Liquid
103
~
Blended Sample Volume, RII.
Calculated
Oxygen Content,
MI./Liter
Oxygen Found, MI./Liter
Recovery,
17.5
99 99 96 99 105 101
70
96
101
Aretone
APPLICATIOR O F METHOD
In order to demonstrate the accuracy and sensitivity of the above method, it was applied to both gas and liquid sampies of known oxygen content. A series of gas samples was prepared containing from 0.004 t o 2 ml of oxvgen in 152 ml. of nitrogen. These samples were then analyzed by the procedure described ahove for gas samples. Frequent blank determinations made in the course of these analpes consistently corresponded to about 0.003 ml. of oxygen. The iesults, shown in Table I, indicate that the method may be expected to yield data that are within 5 % of the truth over a wide concentration range. Liquid samples of known dissolved oxygen content were prew i c d by mixing measured volumes of oxvgen-saturated liquid and
:; JJ
17 6 17 8
52
9 4'
52 ;I
8 8.5
Dl
10 10 15
40
110 110
b?
GI 51 51
.i4 i
3 40
17.6
9.04 8.73 3.63 3 43
28.5 31.1
29.8 31 0 25.7
95
104 95
25.1
98 96 92 78 84
0.37
.66 65
0.90 0.51 0.39
524
ANALYTICAL CHEMISTRY
acetylenes in samples. This subject has been discussed by others (8, 7) in connection with the analysis of gases, and techniques have been developed for circumventing the interferences. However, the authors have observed that chloroform and carbon tetrachloride vapors cannot be allowed to come in contact with the reaction cell packing because they react with the copper. It is not known how general the interference by chlorinated hydrocarbons is. However, the method has been used for the determination of traces of oxygen in vinyl chloride gas with apparent success, because the values obtained were very low. ACKNOWLEDGMENT
The authors wish t o acknowledge their appreciation of the assistance of R. E. Murdock, L. D. TeSelle, and W. W. Kerlin.
LITERATURE CITED
(1) Callen, T.,and Henderson, J. A. R., A n a l y s t , 54, 656 (1929). (2) Deinum, H.W.,and Dam, J. W., Anal. Chim. Acta,3,353 (1949). (3) Hoar, T.P.,A n a l y s t , 62,657 (1937). (4) Horiuti, J., Sci.Papers Inst. Phys. Chem. Research (Tokyo),17, 125 (1931). (5) MaoHattie, I. J. W., and Maconachie, J. E., IND. ENCI.CRCM., ANAL.ED.,9,364 (1937). (6) Schulze, W.A.,Lyon, J. P., and Morris, L. C., Oil Gas J., 38,So. 46,149,152,155 (1940). (7) Uhrig, K.,Roberts, F. M., and Levin, H., IND.ENQ. CHEM., ANAL.ED., 17,31 (1945). ( 8 ) Winkler, L. W., Z . NahT. Genussm., 47,257 (1924). RECEIVED for review August 13, 1951. Accepted October 31, 1951. Presented before Section 7, Fuel, Gas, and Petroleum Chemistry, at the X I I t h International Congreas of Pure and Applied Chemistry, New York, September 10 to 13,1951.
Determination of Copper in Sugar Sirups and Beverages Modified Dithizone Method PHILIP D E C . KRATZ, JAY I. LEWIS, AND ARTHUR FELDMAN' Hoffman Beverage Co., Newark, N . J. Work was undertaken to find a simpler, more rapid, and more sensitive method for determining copper in sugar sirups and carbonated beverages than methods previously used. The developed method, requiring only standard equipment and readily available reagents, allows determination of copper by a single spectrophotometric reading, in amounts at least as low as 0.001 mg. per sample taken, in sugar sirups
T
H E determination of traces of copper, utilizing the color developed with diphenylthiocarbazone (dithizone), has been reported extensively in the literature. Sandell ( 9 ) gives a thorough discussion of dithizone as a heavy metal reagent and gives a specific method ( 4 ) for copper in which the mixed dithizone-dithizonate color is read spectrophotometrically against a blank of carbon tetrachloride a t 510mp (500 to 550 mp) or a t 625 mp (600 to 650 mp). Changes in concentration or decomposition of the dithizone solution must be avoided. Snell and Snell (6) give a mixed color method where the mixed colors are read a t 510 or 625 mp presumably against a carbon tetrachloride blank. They also give a nomograph method where the mixed colors are read a t both 525 and 650 mg. Both Sandell and Snell and Snell give numerous references to original papers. Stammer (6) summarizes the results obtained by Association of Official Agricultural Chemists collaborators, using the Bendix (monocolor) and Greenleaf (mixed color-dual reading) methods. The method described here was developed to provide a sensitive, rapid, simple procedure for copper in sugar sirups and in carbonated beverages and to avoid manipulative difficulties encountered in the dithiocarbamate method (7). APPARATUS
Coleman Universal spectrophotometer Model 14 with round 19 X 105 mm. cuvettes. End Shak sieve test machine (Newark Wire Cloth Co.), 180 cycles per minute, with a metal container in which to hold the samples being shaken. Four-ounce French square sample bottles with screw caps with aluminum foil or other inert liners. All glassware is made copper-free by shaking with dithizone re1 Present address, Central Laboratories, General Foods Corp., Hoboken, N. J.
and beverages from which carbon tetrachloride does not extract color, without preliminary ashing. Other samples may be analyzed after a preliminary ashing procedure. Interference by other metals in normally occurring amounts is insignificant. The use of a reagent-dithizone blank permits a single, precise, mixed color, spectrophotometric reading. Ashing by double sulfation yields low results.
agent, rinsing with redistilled methanol, and finally rinsing with double-distilled water. REAGENTS
Carbon tetrachloride, C.P. (low sulfur). Concentrated sulfuric acid, C.P. Hydrochloric acid, 0.06 N , C.P. (in double-distilled water). Methanol, redistilled from borosilicate glasa. Diphenylthiocarbazone (dithizone) Eastman Kodak. A 0.0012% solution in carbon tetrachloride, filtered, and stored in brown bottle under refrigeration. Standard copper solution, 0.1964 gram of C.P. cupric sulfate pentahydrate made to 500 ml. with 0.06 hydrochloric acid (100 p.p.m. of copper). Diluted as required with 0.06 N hydrochloric acid. GENERAL METHOD
With Preliminary Ashing. Weigh a sample of sirup or beverage (containing 0.005 mg. of copper or more) in a silica dish. Evaporate if necessary to a sirupy consistency. Add 2 ml. of concentrated sulfuric acid and char carefully over a Bunsen burner until no more fumes are evolved. Ash in a furnace a t 500" to 550" C. to constant weight, dissolve, and transfer to a 4-ounce screw-cap bottle, with five 5-ml. portions of 0.06 N hydrochloric acid. Add exactly 10 ml. of dithiaone reagent and shake 30 minutes on a mechanical shaker. Allow carbon tetrachloride and water layers to separate and pipet the carbon tetrachloride layer to a cuvette carefully excluding the water layer. It is not necessary to t a i e more of the carbon tetrachloride layer than enough to fill the cuvette above the level of the light beam. Treat a blank containing 25 ml. of 0.06 N hydrochloric acid and the dithizone solution in the same manner. Compare the blank and the sample in the spectrophotometer with the blank set at 100% T a t 525 mp. Read the %T of the sample and obtain the co per content by reference to the standard curve for the amount o r dithizone solution used. The developed color should be between that of the pure dithizone and that of copper dithizonate. If the sample contains more
