unknown solution to obtain intensity readings for each of the impurity alkalies. I n cases where the reading for a given element exceeds about 30% transmission, better results will be obtained if a more dilute sample solution (obtained b y aliquoting) is used for the determination. Estimate concentrations b y comparing the readings with calibration curves drawn from readings of pure alkali standards. Divide the estimated concentration b y the enhancement factors shown in Table I. Make a trial standard containing the theoretical concentration of the major component and the estimated amounts of the alkali impurities. Compare the intensity readings of the trial standard with the unknown test sample. Intensities at each appropriate wavelength should coincide within two scale divisions. If not, make more accurate estimations, and new trial standards until required precision is attained. Such a standard is called a simulated standard (2). Make three simulated standards which will bracket the highest of the minor alliali components, using the final trial standard for the middle of the three. ?\lake other simulated standards for each of the minor alkalies in the same way. The standards should differ sufficiently to give a 10 scale-division spread in intensity a t the appropriate wavelength. Read emission intensities on the three standards and the unknown test sample for each impurity. Use one of the following procedures: read solutions five times each and average the readings, discarding variations of more than one scale division for any given solution and take more readings so that five are available for the average; or read
standards and sample until two subsequent readings for the entire series read within 0.2 scale division of the previous reading for each solution. The order in which the solutions are read depends on the preference of the operator; however, follow the same order for successive readings. Aspirate distilled mater after each solution is burned, and leave the shutter switch in the on position for the entire series of readings. Use the emission intensity readings to draw a calibration curve on 7 x 10 linear graph paper. Plot concentration (expressed in grams per liter) against emission intensity readings. Determine the exact concentration of the minor alkali impurity from this curve. Follow this procedure for the unknown minor alkali impurity in the nest highest apparent coricentration until all have been determined. Assay Method for the Major Alkali Component. Determine the major alkali component (cesium or rubidium) by converting to the sulfate and correcting for t h e alkali impurities, as t h e sulfates, determined by flame photometry. Accurately neigh approximately 1 gram of the sample of salt to the nearest 0.1 mg. (pipet the metal sample solution) into a tared, 150-ml. platinum evaporating dish. Add 5 ml. of concentrated sulfuric acid slowly, with swirling, and evaporate to apparent dryness on the hot plate, exercising care to avoid spattering. Remove to a cold mufflefurnace and heat overnight (minimum of 16 hours) a t the following temperatures: all cesium samples, GOO” C.; all rubidium samples, 700” C. Following ignition a t these respective tem-
peratures, remove to a desiccator, cool, and weigh. The net w i g h t represents the total weight of alkali sulfates. This weight, corrected for minor alkali sulfates, is the major component sulfate and may be calculated for any compound or metal by appropriate factors. RESULTS
Table IV lists analyses of synthetic samples, using the methods outlined above. All values resulted from single determinations, and no results were discarded. The data of Table I V indicate that major components can be determined to nithin 3=0.16% of the amount present, and that minor components can be determined to nithin =t2.8% of the amount present, both a t the 95% confidence level. These confidence limits are calculated from rstiniates of standard deviations of 0.074% and 1.38% based on 10 and 39 degrees of freedom, respectively. The large error in the determination of lithium in sample 5 of CsCl was not included in the estimate of accuracy for determinations of minor components. LITERATURE CITED
(1) +chibald,
E. H., Hooley, J. G., Phillips, AI., J . Am. C h e w SOC.58, 70
(\1- 926) _-_,.
( 2 ) Baxter, G. P., Harrington, C. D., Zbid., 62, 1834 (1940). (3) Dean, J. A,, “Flame Photometry,” p. 111, XcGraw-Hill, New York, 1960. (4) Zbid., p. 11‘7.
RECEIVEDfor review July 17, 1961. Accepted November 27, 1961.
New Indicator for Volumetric Determination of Copper VIRGINIA WILLS, OSCAR F. STAMBAUGH, and
ZOE G.
PROCTOR
Nizubefhtown College, Elizabethtown, Pa.
b A method for the determination of copper using a new indicator [N,N’-bis(2-hydroxyethyl)dithio-oxamide] is described. The results are accurate and precise for samples containing from 2 to 70 mg. of copper and the indicator i s stable.
C
has been determined volumetrically in many ways including the use of thiosulfate with starchiodine indicator and E D T A [(ethylenedinitri1o)tetraacetic acid] titrations using murexide (1) or PAN [l-(a-pyridylazo)-2-naphthol] (1) as the indicator. The following method is similiar t o the other E D T A determinations but is accurate for much larger quantities 224
OPPER
ANALYTICAL CHEMISTRY
of copper. The copper is dissolved in nitric acid, placed in a 7570 acetone solution and titrated with E D T A to the end point. The indicator is S,S’bis(2-hydrosyethyl)dithio-oxamide, one of a series of substituted dithio-oxaniides which form colored complexes with many ions ( 2 ) . REAGENTS
Indicator. Prepare a solution of N ,N’ - bis (2-hydroxyethyl) dithio-osamide containing 70 mg. of indicator per milliliter of acetone. EDTA, Depending on the copper concentrations present, prepare a 0.0500iV or a 0.0100N solution of EDTA. For the 0.0500N solution dissolve 9.3064 grams of disodium (ethylenedinitril0)-
tetraacetate dihydrate in 1 liter of aqueous solution. PROCEDURE
Samples may be dissolved in nitric acid. If the copper is present as the ammine comple\-, neutralization with dilute nitric acid is sufficient. If the copper is present as the cyanide complex, oxidation with concentrated nitric acid is necessary. The copper nitrate solution is then diluted with sufficient acetone t o make the solution a t least 75% acetone by volume. The p H is adjusted t o less than 2 with nitric acid. The total volume should be in the range of 100 to 200 ml. One drop of the indicator solution is then added and the green complex is titrated with E D T A to a blue end point.
Table 1. Sumber of Samples 2 9 2 2 4
3 3 3
Present 58.6 mg.“ 29.3 mg.“ 14.7 mg.a 2 . 9 mg:
Copper
1.5m g . O
10.40yo*
9.857hb 5.
Copper Titrations Mean
Found 58.6 mg. 29.3 mg. 14.7 nig. 2 . 9 mg. 1 . 5 mg. 10,35Yo 9.89%
Error, yo 0.00 0.00 0.00 0.00 0.00
5.8370
+0.51
-0.48 + O . 40
Range 0.00 mg. 0 . 0 7 mg. 0.06 mg. 0.01 mg.
0.01 mg. 0.10y0 0.207, 0.05yo
Copper dissolved in nitric acid and analyzed by electrodeposition, thiosulfate, and EDTA (murexide indicator). * Commercial samples (’l’h~rn-Smith). RESULTS A N D DISCUSSION
The titration proceeds equally n ell n i t h other water miscible organic solvents (isopropyl alcohol, n-propyl alcohol, ethyl alcohol, 2-butyl alcohol) replacing the acetone. In water the end point is indistinct. I n solvents nonmiscible with water the reaction is slow. S o r m a l deviations in room temperature have no apparent effect on the titration, which is relatively rapid. Weights of copper between 2 and 70 mg. lvere titrated rvithout effect on accuracy or precision (Table I). hfost EDTA-copper titrations are accurate only for small amounts of copper (murexide: 6 to 12 mg., P A S : 5 to 25 mg. (3). For less than 2 mg. of copper the end point is colorless rather than blue. The color change from green t o blue is due t o the disappearance of the copper-indicator complex leaving the blue copper-EDTA. A series of titrations of commercially analyzed ores yielded a n average error of 0.46% (Table I). The effect of cations and anions was studied (Table 11). Alkaline and alkaline earth metals except
barium do not interfere in the determination. Other metal ions tend to give a late end point. High concentrations of other ions tend to give a n early end point. I n most cases the copper may be separated as either the ammine or the cyanide complex. Titrating the cyanide complex after oxidation introduces a 0.66% error. Titrating the ammine after neutralization introduces a 0.337, error. The indicator detects 3 p.p.m. of copper in water and is a more sensitive qualitative test for copper than either ammonia or ferrocyanide. KOother ions tested reacted with the indicator at such low concentrations. The method is equivalent to murexide and superior to PAIS in EDTA-copper titrations. It is preferable to thiosulfate in ore analysis since the E D T A is a primary standard and the indicator is stable for a t least 6 months. ACKNOWLEDGMENT
The authors thank the Mallinckrodt Chemical Works for the sample of N,A~’-bis(2-hydrosyethyl)dithio-oxamide.
Table II. Effect of Ions All samples contained 30.3 mg. of copper. g. ion Ratio g. copper is lowest ratio which shows interference Yith titration. G. Copper Found G. Ion Ion G. Copper Added Al(II1) 0.50 1 8G1 Ba(I1) < O . 01 0 9‘370 Ca(11) 2 00 1 002b Fe(II1)
