ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
963
Analysis of High Purity Chemicals: Examination and Improvement of the Residue after Evaporation Test for Solvents Bruce H. Campbell and Lisa G. Hallquist” Research and Development, Analytical Section, J. T. Baker Chemical Co., Phillipsburg, New Jersey 08865
A new residue after evaporation procedure was developed using a Thiers assembly. Determinate and nondeterminate errors were investigated and minimized. These errors included atmospheric contamination, dish construction and cleaning, transfer losses, dish cooling times, static electricity effects, and balance error. The recommended procedure has a detectlon limit of 0.4 ppm (pg/g).
In t h e assessment of the purity of solvents, specific tests are used to determine the content of selected components, for example, a Karl Fischer titration for water or gas chromatography for a volatile impurity. However, many proximate tests, such as “Heavy Metals (as Pb)” or “Sulfur Compounds (as SO,)” are not only useful but provide an index to the overall purity. Another such proximate test is residue after evaporation (RAE). Residue after evaporation is usually defined as the percent by weight of material remaining after evaporation of a volatile liquid and heating of the dish and residue a t 105 O C for 30 min. Both “Reagent Chemicals, American Chemical Society Specifications” ( I ) and “ASTM Standard Methods” (2) provide widely used procedures. However, these procedures have shortcomings, as was confirmed in this study. As part of our studies of low residue and special purity solvents, the sources of error in the procedure for determining residue after evaporation have been examined. T h e errors can be conventionally divided into determinate and nondeterminate ones. During the course of experimentation, the procedure presented below was evolved. In terms of elapsed time and operator time, t h e recommended procedure is equivalent to those given in the above references. The accuracy is better in the procedure as written, and allows detection of residues down to 0.4 ppm. By modification, lower limits can be attained. EXPERIMENTAL Cut a circle of “household” aluminum foil, 0.006-cm thickness, about 20 cm in diameter. Wash with acetone and air dry. Wearing clean rubber gloves, press the foil around the bottom of an 800-mL beaker. Heat the dish at 105 “C for 30 min. (For a solvent that attacks aluminum, such as halocarbons, lower alcohols, or those containing peroxides, substitute a platinum dish.) Cool the dish in a desiccator (charged with calcium chloride) placed near the balance for 30 rnin (45 rnin for platinum). Discharge static electricity with a radioactive source and weigh. Put the dish into a Thiers assembly (3) having nitrogen filtered through a 0.8-pm membrane filter, flowing into it (see Figure 1). Add a volume equivalent to 200 g of solvent. Heat the assembly using a heat lamp placed above the assembly and a low-temperature hotplate below. Adjust the heating so thar. the evaporation rate does not exceed 8 mL/min. If condensation is seen on the inside top surface of the Thiers assembly, adjust the position of the heat lamp or increase the gas flow until the condensation disappears. When the solvent has evaporated, place the dish, still in the Thiers assembly, in the oven. Heat at 105 0003-2700/76/0350-0963$01 .OO/O
“C for 30 min. Cool in the same desiccator as above. After 30 min (45 min for platinum), discharge static electricity, and weigh.
RESULTS AND DISCUSSION Dish Construction. Aluminum was chosen as t h e dish material because it is easily cleaned, available, and, most importantly, cools and heats rapidly. As indicated above, peroxides in solvents, halocarbons, and lower alcohols can attack t h e aluminum dishes; platinum dishes are therefore used for these solvents. Determinate Errors. The major, identified ones consist of (1) atmospheric contamination, (2) contamination from apparatus, (3) loss of residue if transferred, (4) cooling time of the dish before weighing, ( 5 ) moisture pick-up from the air, and (6) loss of residue through entrainment (misting) in escaping vapor. Atmospheric contamination was assessed by comparing results from the evaporation of identical 200-g samples of acetone in a 400-mL borosilicate beaker in a fume hood with the results using the procedure outlined in the Experimental section. The beaker was covered with a ribbed watchglass and heated on a hotplate. The residues using the beaker and Thiers assembly were 6 and 0.6 mg, respectively. Clearly, evaporations must be performed in a “clean air” environment for assessment of low levels of residues. Contamination from dishes or other labware is avoided by adequate rinsing. For aluminum foil dishes, high results are obtained if not rinsed. T h e dishes are dried in the oven in the Thiers assembly to minimize atmospheric contamination. During our studies, dishes were also dried in covered 2-L beakers or under an aluminum foil “tent” in the oven and all three drying methods were equivalent. Transfer losses occur when the evaporation is performed in one dish and the residue is transferred to another vessel for the final steps. Transfer losses were investigated by spiking acetone samples with vacuum pump oil (shown to be nonvolatile under the test conditions). When the residue was transferred from a large aluminum dish t o a smaller one, the losses were as much as 2570, with none less than 10%. When residues were transferred from a Rotovap assembly to a small aluminum dish, the losses were up to 40%. With no transfer step, recoveries were about 100% with no loss greater than 5%. Insufficient cooling time can also be a source of error. If the dish, after heating a t 105 “C, is not cooled to the same temperature as the air in the weighing chamber of the balance, convection currents in the chamber result, causing a negative weighing error. T o prevent this, the dish must be cooled for a t least the established length of time. The shortest cooling time for an aluminum dish in a desiccator is 30 min (for platinum, 45 rnin). Both of these times were determined experimentally using a semimicro balance. Also, the desiccator should be placed close to the balance to assure thermal equilibration. Moisture pick-up was investigated by placing cool tared aluminum and platinum dishes in a closed vessel containing 1976 American Chemical Society
964
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
PETR
Fl L T E R ED NI'ROGEN INLET DISH
Table I. Recommended Options Sample size, g Balance" Detection limit, ppm 400 SMB 0.3 200 SMB 0.4 200 AB 2 100 AB 5 50 AB 10 ' SMB = semi-microbalance;AB = analytical balance.
,-HOTPLATE
n
Y
Flgure 1. Thiers assembly for evaporation of solvents
water in the bottom. After 1 h in the vessel, the dishes were directly reweighed. Under these conditions the aluminum foil dish showed a slight increase in weight of 0.04 mg; in contrast, the dish made of platinum had an increase in weight of about 0.19 mg. Even though a dish made of aluminum foil might be cooled outside of a desiccator, this is not recommended. The desiccator not only ensures that moisture pick-up will not be a problem but also provides protection against contamination by particulate matter. If the rate of evaporation is too great, residue might be lost by physical entrainment in the escaping vapor. This possibility was investigated by spiking acetone with vacuum pump oil and evaporating the spiked samples at various rates. At or below an evaporation rate of 8 mL/min, recovery was 100%. At between 8-16 mL/min, the recovery decreased to zero. At 16-20 mL/min, the recovery was also zero. Indeterminate Errors. These errors arise from (1) weighing, (2) static electricity, and (3) room temperature changes. The magnitude of the weighing errors associated with the tare and final weighings of the dish plus residue can be calculated from a knowledge of the standard deviation of the balance. In the recommended procedure, the balance is zeroed and then the dish weighed. These steps are repeated after the evaporation. Thus, because four weighing steps are involved, the absolute standard deviation of the weighing operation is f(4)lI2 X standard deviation of the balance. For the semi-microbalance used in this study, this is f(4)"' X 0.01 mg or A0.02 mg. This f0.02 mg corresponds to a relative standard deviation of weighing of f0.1 ppm for a 200-g sample. Using a constant-load analytical balance with a standard deviation of f0.05 mg, the relative standard deviation of weighing is f0.5 ppm for a 200-g sample. T h e effect of static electricity during weighing was most noticeable during the winter months. The errors due to static electricity can be obviated by proper use of a radioactive static electricity eliminator. Statistical Treatment. In one series of experiments, 47 determinations of the RAE values of five low residue solvents were run by the recommended procedure. The pooled
standard deviation was f0.20 ppm with a mean amount of residue of 0.00 ppm (0.00 mg). The relative standard deviation of weighing, as shown above, is f0.1 ppm. Thus, by subtracting variances, the relative standard deviation due to nonbalance sources amounts to f0.17 ppm. A commonly accepted definition of the detection limit is two times the measured standard deviation of a solvent with no detectable residue. Therefore, the detection limit for the procedure as written is 0.08 mg (0.4 ppm for a 200-g sample) using a semi-microbalance. A 1600-g sample was evaporated in triplicate to see if the relative standard deviation could be proportionately decreased by a factor of eight. The result (f0.04 ppm) was a decrease by a factor of only five. This unexpected result is attributed to additional airborne contamination admitted during repeated openings of the Thiers assembly necessary to evaporate the large sample. Consequently, use of sample size larger than 400 g may not be advantageous. C0NCLUS IO NS The procedure as developed for the determination of residue after evaporation has been shown to be superior to those listed in literature and widely used in industry. Contamination errors have been significantly reduced without noticeably affecting the convenience of the method. The recommended method, use of a Thiers assembly, etc. is superior in accuracy to those using unprotected containers. Use of the semi-microbalance provides excellent detection limits with a 200-g sample. If one is determining residues larger than 50 ppm, reduced sample sizes and/or an analytical balance can be substituted. Based on this study, the recommended options are given in Table I. LITERATURE CITED (1) "Reagent Chemicals, American Chemical Society Specifications", American Chemical Society, Washington, D.C., 5th ed., 1974. (2) "1974 Annual Book of ASTM Standards: Paint-Fatty Oils and Acids, Solvents, Miscellaneous; Aromatic Hydrocarbons; Naval Stwes", American Society for Testing and Materials, Philadelphia, Pa., 1974, Method D1353-64, pp 135. (3) R. E. Thiers, "Separation, Concentration, and Contamination", in "Trace Analysls", J. H. Yoe and J. H.Koch, Ed., John Wlley arid Sons, New York, N.Y., 1957, pp 638-640.
RECEIVED for review January 26,1978. Accepted April 3,1978.
