Determination of Trace Amounts of lnorgank Chloride in GIycoIs and Water-Soluble Alcohols Roland P. Marquardt The Dow Chemical Company, Midland, Mich. 48640
A NEED AROSE for a rapid, accurate, and sensitive colorimetric method for trace amounts of inorganic chloride in ethylene glycol. The analytical method that was developed, generally suitable for the determination of inorganic chloride in glycols and water-soluble alcohols, is based on the method for trace amounts of chloride published by Iwasaki, Utsumi, and Ozawa ( I ) . In their procedure, slightly dissociated mercuric thiocyanate reacts with chloride ion to form mercuric chloride and thiocyanate ion; the thiocyanate ion gives a red color with ferric ion that can be measured with a spectrophotometer. The following reactions were given as the basis for their colorimetric method: 2C1-
+ Hg(CNS)z * HgClz + 2CNSCNS- + Fe Ft: FeCNS
(1 1
(2) At first it appeared that this spectrophotometric method would be directly applicable with no modifications to the determination of microgram amounts of inorganic chloride in glycols and water-soluble alcohols. However, variations were noticed in the amount of color produced by microgram amounts of chloride apparently related to the original amount of color produced by the chloride in the reagents. Microgram amounts of chloride in an alcohol produce less additional color if there is very little reagent blank than when there is considerable reagent blank to begin with. Apparently there is a threshold value for the amount of FeCNS2+ to be present in the reagent blank before maximum color is produced by additional amounts of chloride ion and Beer’s law is nearly obeyed . A ferric iron solution of low chloride concentration was prepared. The amount of reagent blank color obtained with mercuric thiocyanate in the analytical procedure was then varied by adding suitable amounts of either mercuric acetate or hydrochloric acid to the iron solution. The amount of color formed by microgram amounts of inorganic chloride in methanol at each reagent blank level was measured. In 10-cm cells, when the absorbance of the reagent blank was regulated to a threshold value of about 0.3 of an absorbance unit, the sensitivity to microgram amounts of chloride ion in alcohol had reached maximum. The principle is shown diagrammatically in Figure l . The procedure will be described for the determination of trace amounts of inorganic chloride in ethylene glycol. Since chloride-free ethylene glycol was not available, redistilled methanol having an inorganic chloride content of less than 0.01 ppm was used for the preparation of the calibration curves. Extensive laboratory experiments showed that the same amount of color was produced by inorganic chloride in methanol as in ethylene glycol. *+
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ANALYTICAL PROCEDURES
Reagents. Methanol was redistilled, chloride-free. Mercuric thiocyanate, AR grade, methanol solution. Make a saturated solution of mercuric thiocyanate in re(1) I. Iwasaki, 226 (1952).
s. Utsumi, and T. Ozawa, Bull. Chem. SOC.Jap., 25,
03 REAGENT BLANK
CHLORIDE
Figure 1. General curve for absorbance os. chloride concentration showing threshold absorbance value for reagent blank using 10-cm cells in order to obtain maximum sensitivity
distilled methanol. Dilute 100 ml of filtered saturated solution to 1000 ml with redistilled methanol; this is the solution used in the procedures. Mercuric acetate, 0.1 % in methanol. Dissolve 1.0 gram of AR grade mercuric acetate and 1.O ml of glacial acetic acid in redistilled methanol and dilute the solution to 1000 ml with redistilled methanol. Ferric iron solution. Place 10.0 grams of AR grade iron wire in a one-liter volumetric flask. Add 750 ml of water. Warm the water to lukewarm temperature by use of a steam bath and then pipet 20.0 ml of 90% nitric acid (ACS Reagent Grade, sp gr 1.49-1.50) into the flask. After most of the acid has reacted with the iron wire, pipet in another 20.0 ml of 90% nitric acid. After all of the iron has dissolved, heat the solution by placing the flask in a steam bath and shake the flask and contents frequently to release the dissolved gases (take care that foaming is not so great as to cause some of the solution to leave the flask). After the solution has been heated to maximum temperature in the steam bath and the dissolved gases released, cool the solution to room temperature. Then add concentrated sulfuric acid to the solution in suitable increments (cooling the contents of the flask each time with cold tap water) until the solution has a volume of 1000 ml at room temperature. Mercuric nitrate, approximately 0.1M solution. Dissolve 3.50grams of mercuric nitrate, Hg(NO&.HzO, in 100 ml of water containing 1.0 ml of concentrated nitric acid. Sodium chloride was AR grade. Apparatus. Spectrophotometer, Beckman Model D U or equivalent. Cells, absorbance, 10-cm. Cells, absorbance, I-cm. Adjustment of Reagent Blank. Using 50-ml portions of redistilled chloride-free methanol instead of ethylene glycol in the first procedure described below using IO-cm cells, adjust the absorbance of the reagent blank within the range
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Table I. Absorbance Data-0.05 to 1.00 ppm Chloride in Ethylene Glycol c1-, Abs Av Net abs (485 m d abs (abs-blank) ppm Blank 0.318 0.319 ... 0.05 0.10 0.20 0.30
0.50 0.70 1.00
0.320 0.361 0.364 0.416 0.415 0.513 0.501 0.590 0.594 0.771 0.774 0.946 0.951 1.22 1.22
0.363
0.044
0.416
0.097
0.507
0.188
0.592
0.273
0.773
0.454
0.949
0.636
1.22
0.901
Table 11. Absorbance Data-0.5 to 10.0 ppm Chloride in Ethylene Glycol Cl-, Abs Av Net abs ppm (485 m d abs (abs-blank) Blank 0.031 0.032 ... 0.5 1.0 2.0 3.0 5.0 7.0 10.0
0.033 0.071 0.072 0.118 0.122 0.211 0.210 0.290 0.290 0.460 0.462 0.656 0.656 0.966 0.968
ABSORBANCE DATA
Absorbance data for 0.05 to 1.00 ppm and for 0.5 to 10.0 ppm inorganic chloride in ethylene glycol are given in Table I and Table 11, respectively.
0.072
0.040
DISCUSSION
0.120
0.088
0.211
0.179
0.290
0.258
0.461
0.429
0.656
0.624
0.967
0.935
Utsumi (2) states that the analytical procedure is not specific for chloride. Bromide, iodide, cyanide, sulfide, thiosulfate, bromate, iodate, and nitrite react in the same way with mercuric thiocyanate as chloride and will interfere. Ferrocyanide and ferricyanide can interfere. The sensitivity and accuracy of the colorimetric method for chloride developed by Iwasaki, Utsumi, and Ozawa ( I ) has been greatly improved by the use of mercuric acetate and/or hydrochloric acid to regulate the amount of reagent blank in order to obtain maximum sensitivity for inorganic chloride. Furthermore, the sample of alcohol or glycol is used for the reference solution, thus balancing any variations in the absorbance at 485 mp that the sample may have. The principle here illustrated for determining inorganic chloride in water soluble alcohols and glycols should with modifications be applicable to other analytical problems concerning trace amounts of inorganic chloride. For chloride in aqueous solutions, sulfuric acid should not be used to prepare the ferric iron reagent as the development of the color is retarded by the presence of large amounts of sulfate ion.
of 0.30-0.32 absorbance unit by the dropwise addition of 0.1M mercuric acetate or 0.1N hydrochloric acid to the ferric iron reagent solution and then determine the value to the third decimal place. Preparation of Calibration Curves. Prepare a series of standard solutions of inorganic chloride in redistilled, chloride-free methanol (assumed to be in ethylene glycol with a density of 1.115) as follows: STOCKSOLUTION.Dissolve 1.838 grams of sodium chloride in 10 ml of water and dilute the solution to 1000 ml with redistilled methanol. This solution is equivalent to 1000 ppm chloride in ethylene glycol. STANDARD SOLUTION 1. Dilute 10.0 ml of the stock solution to 1000 ml with redistilled methanol. Concentration: Equivalent to 10 ppm chloride in ethylene glycol. STANDARD SOLUTION 2. Dilute 100.0 ml of standard solution 1 to 1000 ml with redistilled methanol. Concentration: Equivalent to 1.O ppm chloride in ethylene glycol. Properly dilute aliquots of standard solutions 1 and 2 with redistilled methanol to obtain other standard solutions of desired chloride concentrations. Obtain net absorbance data (absorbance minus reagent blank) in the desired ranges according to the procedures. Prepare calibration curves by plotting net absorbance us. ppm chloride in ethylene glycol. Procedure (0.05 to 1.0 ppm Inorganic Chloride). Place two 50-ml portions of a sample of ethylene glycol in respective 100-ml beakers. Add 10.0 ml of 0.1% mercuric acetate 278
solution to the portion to be used as a reference solution. Add 10.0 ml of mercuric thiocyanate solution to the other portion. Add 2.0 ml of ferric iron solution to the contents of each of the beakers. Stir each solution well and let the color develop for 15 minutes. Fill a IO-cm absorbance cell with the reference solution. Fill another 10-cm cell with the other solution and read the absorbance at 485 mp. Subtract the reagent blank from the absorbance to obtain the net absorbance and read the number of ppm inorganic chloride in the sample from the suitable calibration curve. Procedure (0.5-10.0 ppm Inorganic Chloride). Proceed as described in Procedure (0.05-1 .O ppm inorganic chloride) and let the color develop for 5 minutes. Fill a 1.0-cm absorbance cell with the reference solution. Fill another 1.0-cm cell with the other solution and read the absorbance at 485 mp. Subtract the reagent blank (one tenth of the figure used in the other procedure) from the absorbance to obtain the net absorbance and read the number of ppm inorganic chloride in the sample from the suitable calibration curve.
RECEIVED for review July 28, 1970. Accepted October 26, 1970. (2) S. Utsumi, Bull. Chem. Soc. Jap., Pure Chem. Sect., 73,835-8 (1952).
Correct ion Rapid Method of Determining First-Order Rate Constants from Experimental Data In this article by R. 0. Wale [ANAL.CHEM.,42, 1843 (1970)l there is an error in the caption for Figure 2. It should read as follows: The theoretical r f ( r ) function whenf(r) = 0.1 exp (-0.lt) exp ( - t ) demonstrating one maximum.. .
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
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