ANALYTICAL CHEMISTRY
908
tion of sulfur in sulfanilamide in the Grote combustion procedure even at a temperature as low as 510 a C. KO statistical treatment of these data has been attempted. Reference to Tables I and I1 shows that all the sulfur values obtained under various conditions fall within &0,3% of the theoretical sulfur content. If a deviation of &0.370 from theory is the usual acceptable tolerance in microchemical analyses, then all sulfur values obtained in the present investigation are acceptable. This does not iniply that all combustions should be run a t the lower temperature ranges, but the data indicate t'hat successful combustions can be carried out within a wide temperature range (540' to 730" (2.). -4s the average values for sulfur a t the higher temperature (730" C.) are slightly higher (by 0.17'%, Table 11), it would appear to be better practice to carry out the conibustion at the higher temperature. During this investigation the temperature of the end auxiliary furnace was varied betn-een 350 and 400" C. I t is believed that this temperature range is adequate to prevent condensation of sulfur trioxide or sulfuric acid in the ground joint connecting the absorber with the combustion tube. According to the literature, sulfur trioxide boils at 44.9" C. and H2S04.H20a t 210" to 335" C. -4 temperature in excess of 400" C. is not recommended, since a
correspondingly longer time is required to cool the ground joint to room teniperature. A recently calibrated Leeds and Xorthrup potentiometer and a Chromel-Alumel thermocouple were used in checking the combustion temperatures. All measurements were made by inserting the thermocouple into the combustion tube, so that its tip was located at the center of the furnace. This, of course, does not define the heated areas and it is possible that the Calco furnaces ( 2 , 3) produce better combustion conditions than those used by others. LITERATURE CITED (1) Ogg, C. L., Willita, C. 83 (1948).
O., and Cooper, F. G., ;INAL. CHEM.,20,
(2) Royer', G. L., Norton, A. R., and Foster, F. J., IND.ENG.CHEM.. ANAL.ED.,14,79 (1942). (3) Royer, G. L., Norton, A. R., and Sundberg, 0. E., Ibid., 12, 658 (1940). (4) Steyermark, A. L., Bass R., and Littman, B., Ax~L.CHEM..20, 587 (1948). (5) Sundberg, 0. E., and Royer, G. L., TND. ENC).C H E x . , ANAL.ED., 18, 719 (1946). (6) Walter, R. N., ANAL.C H E M . , 22, 1332 (1950). RECEIVED for review July 14, 1951.
Accepted November 19, 19.51.
Volumetric Determination of Barium THO,\I.IS J. RIINNS, M I R G 4 R E T U. HESCEIOI'SKY', AND ANTHONY J . CERT-I Tiibe Development Laboratory, Philco Corp., Lansdale, Pa. of television and other cathode ray t,uhes, a I SthinTHElayerproduction of phosphor powder is deposited on the inside face
of the tube t o convert the electrical energy of the elect,ron beam into visible light. The most satisfactory met'hod of depositing t h e powder is by settling through an aqueous mixture of a dilute electrolyte and dilute potassium silicate. One of the most widely used electrol~tesis barium acetate. \Then bariurn acetate is employed, the control of solution strength is part,icularly critical because variations in electrolyte strength produce fairly wide variations in settling conditions. It is obvioug, therefore, that an accurate control method for determination of barium acetate solution strength is necessary, and rapid analysis is desirable. A reviex of established analytical procedures indicates that few methods are accurat,e, rapid, and adaptahle for control purposes. The authors have developed a rapid and accurate volunietric method for the determination of the strength of dilute barium :icetatmesolutions, eniploying 1-eraene (the tetrasodium salt of ethglenedianiiiie tetraacetic acid) as titrant, The conventional gravimetric procedures for the det,ermination of harium as sulfate or chromate were considered too long n.ith respect to both attention time and elapsed time. The conventional volumetric procedures, in which the carbonate or chromate of baiiiini is precipitated, require comparatively long digestion periods and a filtration step, with the result that there is little advantage over the gravimetric procedures. T h e end point is verj- poor in the titration with standard potassium sulfate using tetrahydroxyquinoiie as indicator ( 7 ) . The colorimetric procedure of Fretliani and Babler ( 5 ) \vas considered good, but somewhat more involved than a simple titration. The conductometric titration (6) was considered most likely to meet requirements, but the required instruments were not immediately available. PRINCIPLE OF THE METHOD
A rapid voluhetric method was developed, using a standard solution of disodium Versenate (the disodium, dihydrogen salt of ethylenediaminetetraacetic acid) as titrant, and the dye, Erio1
Present addiess, Bell Tower Farm. Gardenrille, P a
chrome Black T, as indicator. Sufficient sodium hydroxide is included in t.he standard Versenate solution to form the tetrasodium salt,. Versene forms very slightly ionized soluble chelates with polyvalent metal ions in aqueous solution. At p H 10 the alkaline earths are stoichiometrically complexed by Versene, and chelation is preferential-Le., one alkaline earth ion will be complexed before another. The normal order of chelation at pH 10 is: calcium, magnesium, strontium, barium, with calcium the most strongly complexed ( I ) . A very important application of the preferential chelation of the alkaline earths is found in the Versenate titration for the determination of hardness of water, using Eriochrome Black T as indicator (f-4). Eriochrome Black T (also k n o ~ nas F241) forms a slightly ionized, soluble, Tine-red complex with magnesium ions in aqueous solution, It also forms less stable complexes with other alkaline earths. The dye itself is blue between pH 6.3 and pH 11.5, in the absence of reacting ions. The dye complex with magnesium, however, is ionized t o a greater extent than is the magnesium Versenate complex; with the result that magnesium ions can be extracted from the dye complex by TTersene, changing the color of the solution, in daylight, from wine-red t o blue. This color change is used as the end point in the barium titratioii as well as in the titration for hardness of water. However, the titrations are best performed under a tungsten filament light source (2, 4) which aids in recognition of the end point. Under this light source, the color change is from pink t o nearly colorless. I n the titration for hardness of water containing both calcium and magnesium, the sample is buffered to pH 10 and Eriochrome Black T indicator is added. Immediately the wine-red color appears due t o formation of the magnesium dye complex. -4s the titration proceeds, t.he calcium ions are first chelated by the added Versene, free magnesium ions are chelated next, and finally the magnesium is extracted from the magnesium-dye complex, prcducing the color change from n-ine-red to blue. I n actual practice, magnesium is added to the Versenate solution before standardization t o assure the presence of the required magnesium ions for the color change. The authors have found that barium can be titrated using essentially the same procedure as for water hardness if mag-
V O L U M E 2 4 , NO. 5, M A Y 1 9 5 2
909
nesiuni is present. On the basis of the normal order of chelation of the alkaline earths, it would not seem possible to perform this titration, as it would not be expected that barium would extract magnesium from the Versenate complex. On first consideration, it would appear that under the conditions of the titration of barium, the normal order of chelation of the alkaline earths is changed. This, however, is not the case. The most probable explanation of the apparent change in order of chelation, which was advanced by Singer (8), is based on the competition between the two complexing agents, Versene and Eriochrome Black T. Singer reported the formation constaqhs of the various complexes involved to be: magnesium Versenate (108.69), barium Versenate magnesium erio (107.0), and barium erio (102 or less) (erio designates the Eriochrome Black T radical). If, then, only the coinplexes of magnesium and barium capable of formation in the solution are considered, the competition of complexing agents may be indicated by the following formula: hlg \-ersenate (108 69) Ba erio (lo2) e Ba Yersenate ( 107J6) Mg erio ( lo7.”)
+
+
I t is apparent that the product of the constants on the right of the equation is greater than the product of the constants on the left, with result that reaction tends t o proceed to the right. In the barium titration, aa in the water hardness titration, the magnesium required for obtaining the color change a t the end point is added to the Versenate solution before standardization. This eliminates the necessity for a blank correction. The authors have found that the best end point is obtained if the magnesium present is an appreciable proportion of the barium titrated A sharper end point is obtained if calcium is present in addition to magnesium. The titrant, therefore, is approximately 0.025 S with respect t o magnesium Versenate, 0.025 N with respect to calcium Versenate, and 0.1 N with respect to free S’ersene. INTERFERENCES
Because the dilute barium solutions being analyzed are relatively pure (c.P. barium acetate in deionized water), additive errors by other reacting ions in the solutions are negligible. However, it xas found necessary to remove carbon dioxide from the sample solution before addition of the buffer, to prevent precipitation of barium carbonate. Precautions are also taken to exclude carbon dioxide from both buffer and titrant. The dye used was found to be unstable when prepared in both aqueous and alcoholic solution. This resulted in poor color change a t the end point. The indicator now employed is a dry mix of the dye with sodium chloride. Satisfactory prepared solutions of the dye are available from the Bersworth Chemical Co. and others. The indicator should be added after the pH has been adjusted. REAGENTS
0.1 N Versenate Solution. Dissolve 2.6 grams of magnesium chloride hexahydrate, 1.9 grams of calcium chloride dihydrate, and 28.0 grams of disodium Versenate, analytical reagent (Bersworth Chemical Co., Framingham, Mass.), in carbon dioxidefree distilled water containing 7.9 ml. of carbonate-free alkali oleum (5070 sodium hydroxide). Dilute to 1 liter with carbon dioxide-free distilled water and mix well. Standardize against 50 ml. of a 1%barium acetate (analytical reagent) solution previously standardized gravimetrically. Buffer Solution. Dissolve 8.25 grams of ammonium chloride in several hundred milliliters of carbon dioxide-free water in a 1liter volumetric flask. Add 113 ml. of concentrated ammonium hydroxide, and dilute to 1 liter with carbon dioxide-free water. Mix throughly and preserve in a well-stoppered bottle. Eriochrome Black T Indicator. Weigh 25 grams of sodium chloride and place about 1 gram of it in a mortar. Add 0.026 gram of Eriochrome Black T, and grind with a pestle until a homogeneous powder is obtained. h d d the remainder of the salt and grind lightly until homogeneous. PROCEDURE
In a 500-ml. Erlenmeyer flask place a weighed sample of the barium acetate solution (containing approximately 0.5 gram of
Table I.
Comparison of Results Obtained Gravimetrically and by Versene Titration % Ra(CHsC00)1
Detn. S o .
Gravimetric &s sulfate
0,996 0,979 1,002
1.026
0.577
s9
1.018 0.986 1.006 1.000
Volumetric Versenate titration
0.987 0.976 1.002 1.018
0.578 1 ,007 1.000 1.006
0.998
barium acetate) and dilute to 250 ml. with distilled water. Add 2 drops of glacial acetic acid (or enough to reduce the p H to 5.0) and boil gently for about 5 minutes. Cool rapidly t o room temperature. Add 10 ml. of buffer solution and 0.4 gram of indicator, by means of a suitable scoo Immediately titrate rapidly with the Versenate solution wit! constant swirling, but taking precautions to introduce a minimum of carbon dioxide from the air. As the end point is approached, add the titrant more slowly with good mixing. Using a tungsten filament light source, the end point is taken as the complete discharge of ink, and the addition of an extra drop produces no color change. &he calculations are straightforward, RESULTS
A series of samples was analyzed by the conventional sulfate gravimetric procedure for comparison with the Versenate titration. The results (Table I ) are within the limits of requirements and show improvement with the changes t o dry indicator, and increase in magnesium and calcium content of the titrant made recently (determinations 8 and 9). ACKNOWLEDGIIENT
The authors acknowledge the interest and encouragement of Meier Sadowskp of this laboratory. LITERATURE CITED
(1) Bersworth Chemical Co., Framingham, Mass., “The YerseneJ,” Tech. Bull. 2 (1951). (2) Bets, J. D., and Noll, C. A,, J . Am. Water Works Assoc., 42, 49 119.50). >----,
(3) Conners, J.J., I b i d . , 42 33 (1950). (4) Diehl, H., Goetz, C. A,, and Hach C. C., Ibid., 42, 40 (1950). ( 5 ) Frediani, H. A,, and Babler. B. J.. ISD. ENG.CHEM.,AXAL ED., 11, 487 (1939). ( G ) Kolthoff, I. hI., and Laitinen, H. d.,“pH and Electro Titrations,” S e w York, John Wiles Cpi Sam, 1941. (7) Schroetier. W. C., IND.EKG.C H E M .XS.LL. , E D . .5 , 403 (1933). (8) Singer, J. J., Bersworth Chemical Co., Framingham. 1Tass.. personal communicatioii. RECEIVED for review July 28, 1031. Accepted J a n u a r y 23, 1952.
Corrections The authors wish to correct a typographical error which appeared in their paper, “Infrared Spectrophotometric Determination of oil and Phenols in TT’ater’’ [*is.kI,, CHEhI., 23, 1384 (1961)]. The value 0.1 p.p.m. at the top of the f h t phenol column should read 0.01 p.p.m. WILLIA ai B AYDARCK R. G. SIMARD ICHIRO H.ISEGI~.\ C. E. HEAI)ISGTOX In the article on “Fluorometric Analysis” [ANAL.CHEV.,24, 85 (1952)] on page 86, first column, the seventh line of the second paragraph should read: Fletcher, May, and Slavin. Reference (49) on page 89 should read: Fletcher, 11.H., May, I., and Slavin, hi., U. S. Geological Survey, Trace Elements Investigations, CHARLES E. WHITE Rep!. 6 (1949).
