Volumetric Determination of Sulfates Tetrahydroxyquinone as an External Indicator S. W. LEE, J. H. WALLACE, JR., W. C. HAND,
AND
N. B. HANNAY, Wallace Laboratories, New Brunswick, N. J.
S
ULFATE ion may be determined volumetrically, using tetrahydroxyquinone as a n internal indicator (3). Earlier workers point out:
centration. It can be seen from the two curves that the spectrophotometer can be used to check the visually determined end point, and that 525 m p is the best wave length for this purpose. When two drops of a n end-point solution vere added to a cuvette containing a water-alcohol solution of the sodium salt of tetrahydroxyquinone, a sharp drop in the per cent transmission occurred.
The unpracticed eye may have severe difficulty in accurately detecting the end point of the titration. False indications of an end point are often given by the color chan e to pink in the bulk of the solution and on the surface of any sofiurn chloride precipitate ( 2 ) . When using solutions low in sulfate (less than 0.1 per cent sulfur in coal) more care must be exercised, for, owing to a rather indistinct color change, there is a tendency to overrun (1).
Procedure A 10 per cent (approximately) aqueous solution of the sodium salt of tetrahydroxyquinone was used as the indicator. Only a few drops of this solution were used for each determination, thus effecting a saving over the internal method. It was most satisfactory when made up in small quantities immediately before use. However, the addition of a small amount of mineral oil to prevent contact of the indicator solution with the air stabilized the indicator for hours.
The various precipitates in the solution make i t cloudy, and there is great difficulty in Seeing the slight color change in the transmitted light, or upon the surface of the silver chloride precipitate, if silver nitrate is added, Figure 1 shows the change in color which a solution containing tetrahydroxyquinone undergoes at the end point as determined spectrophotometrically. Peabody (3) recommended the use of tetrahydroxyquinone externally in microtitrations of sulfate, but the spot-plate technique which he used does not have some of the advantages of the method described here. However, a method similar to the one which the authors have used, but employing sodium rhodizonate as the indicator, has recently been noted (4).
TABLEI. SULFATEDETERMINATIONS USING TETRAHYDROXYQUINONE EXTERNALLY
100
(All determinations in duplicate) Net Volume of Sulfate Barium Chloride Source of Sulfate Added Solution Used Gram Ml. 0,0045 NazSOd (c. P.) 1 . 0 3 (0.0946 N )
Sulfate Found Gram 0,0047
Aln(SO4)r.l8HzO (c. P.) HzSOb (Standard Solution)
0.0226 0.422
4 . 8 5 (0.0946 N ) 7.75 (1.126 N )
0.0220 0.419
NanSOb ( 0 . P.)
0.0366
8.04 (0.0946 N )
0.0365
(Solution saturated with NaC1)
80
A known weight of sulfate, in an aqueous solution, was titrated with standard barium chloride solution. A microburet calibrated to 0.01 ml. was used when less than 5 ml. of barium chloride solution were added. The volumes were chosen so that at the end point there was a total of about 30 ml. As the end point (determined approximately in a preliminary titration) was approached, a drop of the solution being titrated was added to a wet spot of indicator solution on a piece of filter paper. The color which formed was observed by transmitted light. The indicator spot was conveniently made with an eye-dropper. The first immediate (2 or 3 seconds) change from yellow to pink in the indicator s ot indicated the end point. Only the changes of color whic! occurred immediately were of significance. The colors of older spots (15 seconds or longer) were found to be misleading. The indicator blank was found to be 0.20 ml. for titrations with 0.1 N barium chloride, and 0.04 ml. for titrations with 1 N barium chloride. The value of this blank, of course, depends on the final volume of the titrated solution. If the final volume differed greatly from 30 ml., the indicator blank was multiplied by the factor obtained by dividing the volume by 30, since the concentration of the slight excess of barium ion was being measured, Table I shows the range of the amounts of sulfate determined.
60
40
It was found that there were several advantages in using the end-point reaction between barium ions and tetrahydroxyquinone as a spot test on filter paper. One unfamiliar with this method has no trouble in finding the correct end point. This type of outside indicator is fairly rapid and offers a means of eliminating interfering substances. The method gave good results in titrations employing 0.1 N barium chloride, or for sulfate-ion concentration greater than about 150 p. p. m. Spectrophotometric absorption curves of the water-alcohol solutions of the barium and sodium salts of tetrahydroxyquinone were run (Figure 1). T h e curves are similar in shape, as one would expect. The absorption maxima of the barium compound are at approximately 500 and 510 mp. The barium salt has a higher tinctorial value at the same molar con-
Elimination of Interfering Substances The substances which ordinarily interfere in the tetrahydroxyquinone titration of sulfate are hydrogen and aluminum ions and sodium chloride, when present in large amounts. Solutions from peroxide fusions always contain excessive amounts of salt after they are neutralized. The sulfuric acid solutions were first neutralized with ammonia, and then titrated as described above. The aluminum ion was precipitated as the hydroxide with ammonia, heated to boiling, filtered, and washed. The filtrate was concentrated to 30 ml., and titrated as described.
83sI
INDUSTRIAL AND ENGINEERING CHEMISTRY
840
The titration of sulfate solutions which were saturated with sodium chloride was best carried out in 50 per cent alcohol. The total volume was kept to 30 to 40 ml. The following procedure for the indicator spot was found to be more satisfactory in this case. A drop of the indicator solution was placed near a drop of 15 to 20 per cent silver nitrate, so that the spreading boundaries just met. The drop from the titration flask was put inside the silver nitrate spot, close to the indicator spot, and the color change was observed at the boundary. A second silver nitrate spot-indicator spot pair was used as a control.
Use of Spectrophotometer in Confirmation of the End Point Qualitative absorption curves of the sodium salt of tetrahydroxyquinone in 50 per cent alcohol (in such concentration that maximum absorption in 2-em. Cuvettes was approximately 50 per cent) and of the barium salt were run (Figure 1). It is clear from the curves that the optimum wave length for the detection of the end-point color change is about 526 mp. Accordingly, the end point was checked mith the aid of a Coleman Universal spectrophotometer. The spectrophotometer wave-length dial was set at 525 mp, and the per cent transmission of the indicator solution was de-
Vol. 14, No. 10
termined. The titration was then carried out in the manner described above. When a drop was taken from the titration flask just prior to the end point and added to the cuvette, no change in the per cent transmission occurred. However, a drop of the endpoint solution lowered the per cent transmission from 52 t o 45. This method could be used for confirmatory purposes only, for the addition of pre-end point solution t o the cuvette would build up a sulfate-ion concentration and lead to a false value.
Summary A method of using tetrahydroxyquinone as an outside indicator in the determination of sulfate is described, which can be conveniently and rapidly used, and with considerable accuracy. It offers a means of removing interfering substances such as ion and a large of sodium chloride. The possibility of using a spectrophotometer in in this titration is pointed out. the end
Literature Cited (1) Brunjes, H. L., and ilfannlng, IT. J., IXD. EXG.CHEM.,ANAL. ED., 12, 715 (1940). (2) hfahoney, J. F., and Michell, J. H., Ihid., 14, 97-5 (1942). (3) Peabody, JT. A., and Fisher, R. S.,Ibid., 10, 651 (1938), (4) Zararov, G. V., Zavodskaya L a b . , 8 , 933-5 (1939).
Analvtical Use of Sodium Rhodizonate' J
FRITZ FEIGL AND HANS A. SUTER Rlinisterio da Agricultura, Laboratorio Central da Produ7Ho RIineral, Rio de Janeiro, Brazil 0 O=C-
S O D I U M rhodizonate
O=
ii-C-ONa 4
-C-
I
-0Na
produces colored
i-i precipitates when added to solutions of barium or strontium salts. Feigl ( 2 ) was the first to apply this fact as a basis for spot tests for barium and strontium. Barium rhodizonate is stable toward acetic acid and dilute hydrochloric acid but is decomposed by sulfate ion, and accordingly Gutzeit ( 5 ) used i t in his spot-test method of detecting sulfate. The formation and decomposition of barium rhodizonate have also made i t possible to use sodium rhodizonate as indicator in the volumetric determination of barium or sulfate. A considerable number of papers have dealt with this method of determining the equivalence point of the sulfate-barium reaction (1, 4, 6, 8,9, 10). This laboratory has been studying the possibilities of further analytical uses of sodium rhodizonate and the behavior of this reagent toward various metal ions has been examined. It has been found that this salt exhibits a selective action. With certain univalent and divalent metal ions i t forms colored precipitates, whereas i t produces no visible reaction with trivalent or quadrivalent cations. Bismuth is an exception, presumably because i t can function in its salts as the bismuthyl radical, BiO+. Table I exhibits the findings obtained mith 1 per cent neutral and acidified salt solutions. The behavior of the respective hydroxides and oxides is also recorded.
I n the presence of fluoride ion the behavior of Fe++ salts is remarkable. The yellow of the rhodizonate solution is immediately discharged. No explanation can be given for this phenomenon, though it may be due to the increase of the reduction potential of Fe-+ salts in the presence of fluoride ion, so that the oxidation of rhodizonic acid is induced by autoxidation of Fe-- salt. From the analytical viewpoint, the formation of the slightly soluble red and blue-violet lead rhodizonates is particularly interesting. Under certain tvell-defined conditions, specified below, these precipitates can be utilized as a sure test for lead. These nen- tests are so sensitive that positive results are immediately given b y even such slightly soluble materials as lead sulfide, lead sulfate, lead chromate, alloys, and a wide variety of other lead-bearing materials. Rhodizonic acid can be used only as its sodium salt. This is a black-green solid; its water solutions are orange to yellow, depending on the concentration, and are stable for only a few days, even if stored in a refrigerator. The color changes and finally disappears, owing to a gradual decomposition arising from the ease with which rhodizonic acid is oxidized to TABLEI. REACTIONS WITH SODIUMRHODIZONATE Metal Ion
Metal Salt Neutral Black Brown-red D a r k brown Blue-violet Orange-red Ked-orange Brown-red Brown-red
.......
.......
The behavior of iron salts is to be noted. Fe++ salts in neutral solution form a red-bron-n precipitate, which quickly becomes black-blue, probably through oxidation. I n solutions of pH 2.8 there is no reaction. F e + + + salts form no precipitate but produce a blue-green color; rhodizonic acid is a phenol and therefore reacts mith Fe+'+ salts like other phenols. In the presence of fluoride no color reaction occurs, owing to the formation of the complex FeFe--- ion. 1
Translated by Ralph E. Oesper, University of Cincinnati.
Brown-violet
. . . . . .
Brown
......
Red-brown Red-brown
Solution p H = 2.8 Hydroxide Oxide Black ....... ....... Brown-red (disappears on standing) ....... ....... Dark bronn Scarlet Blue-violet Blue-violet
.....
.......
....... .......
Gray-brown Brown-red
.......
.......
Brown-red
.......
.......
.......
......
.......
....... ,
.,.,..
....... .......
....... ....... ....... .......
Brown-violet
....... .......
....... .......
....... .......
Brown-red Red-brown Red-brown Violet
Brown-red Red-brown Red-brown
.......
Ked-brown Tiolet
....... ....... .......
.......
.......
....... ....... .......
Brown-violet
.., ., ..
.,.,.,.
.......
...... ...... ......
