thus quickly determined by simple titration using standard potassium iodate. About 0.8 to 1.6 mmole of di-n-butyltin bis(isoocty1thioglycolate) was dissolved in 80 ml. of glacial acetic acid and a couple of platinum (attached to input side) and silver (attached to reference side) electrodes (Coleman or Corning) were placed into the liquid until the metallic elements were submerged. Electromotive force (EMF) readings were recorded as 0.1N KIO3 was added to the sample solution. The end point is characterized potentiometrically by a sharp increase of the EMF value, as shown in Figure 1, and visually by a color change from colorless to pale yellow. The visual and potentiometric end points coincide accurately. If the end point is over-run, back-titration with 0.1N sodium thiosulfate is necessary. The calculations with potassium iodate are identical to those with iodine and in the wet method where back-titration is necessary:
where R = CH2-COOCsH1,.
The absence of precipitate during the titration does not rule out the formation of dibutyltin hydroxide (Reaction 1) or dibutyltin oxide (Reaction 2), for it is known (5) that dibutyltin oxide reacts readily with acetic acid at room temperature to yield dibutyltin diacetate (Reaction 3). Thus, Reaction 4 appears representative of the titration of the dibutyltin mercaptide with potassium iodate. Any free mercaptan present will be converted to the corresponding disulfide. After all the Bu2Sn(SR)2and free mercaptan, if any, are consumed, only one drop of 0.1N KIOI is needed to give a colorimetric end point upon liberation of free iodine. Although KBr03 is very often used interchangeably with KIO3 for oxidation-reduction titrations, it was inapplicable in this work. These methods allow the determination of total sulfur quickly, with accuracy and reproducibility. The greatest difference in percentage sulfur of any duplicate titration using K I 0 3 in acetic acid was 0.08%; the least was 0.0001% in Titer = (volume KIO3 consumed X NKIOJcomparison with 0.1 and 0.15% for titrations with the iodine(volume Na2S203consumed X NNalbnO3) hydrocarbon system. These values are well within experiCompared to a theoretical sulfur content of 10.03%, the mental error; the precision in sulfur analysis by the Parr potentiometric titration indicated a value of lO.OO%, the Bomb method is usually 0.1 to 0.2% for duplicate determinavisual method a value of 9.96z of sulfur. tions. Here again, an interesting feature of these titrations is the The simplicity of these titrations presents several advantages self-indicating end point. The following equations show over the more time-consuming procedures currently used and the chemical reactions taking place during the oxidation of a should make them a useful tool in the laboratory. Such dialkyltin mercaptide: titrations can generally be carried out in a minimum of time, 3 Bu2Sn(SR)2 2 KIO3 6Hf whereas standard gravimetric methods often require several hours. + 2 KI 3 RSSR 3 BuzSn(0H)p (1) + 2 KI 3 RSSR 3 BuzSnO 3Hz0 (2) RECEIVED for review July 20, 1967. Resubmitted June 6,
+
+
+ + +
+
+ +
+ Bu2Sn (OCOCH3)2+ H 2 0 (3)
Bu2Sn0 2 HOCOCH3 3 Bu2Sn(SR)2 KIOa 6 HOCOCH3 .-, KI 3 RSSR 3 B U ~ S ~ ( O C O C H ~3)HzO ~ (4)
+ +
+
+
1968. Accepted July 12, 1968. (5) N. I. Sheverdina, K. A. Kocheshkov, L. V. Abramova, andI. E. Paleeva, Khim.Prom., 1962, pp 707-8; C A 59, 8776.
Gravimetric Determination of Silica in Fluosilicates D. N. Tewari’ and A . C. Chatterjee Chemistry Department, Lucknow University, India
SILICAcannot be determined by usual acid dehydration when appreciable amounts of fluoride are present. The fact was noticed by Berzelius ( I ) who devised a method for the separation of silica from fluoride. Hoffman and Lundell (2) simplified the procedure slightly. Shell and Craig shortened the procedure (3, 4 ) . They incorporated ZnO in the sodium carbonate fusion with further precipitation of silica in the unfiltered solution by ammonical zinc oxide (3, 4). Residual silica was determined colorimetrically. 1 Present address, Sri Krishna Bhawan, Purana Killa, Lucknow, India.
(1) J. J. Berzelius, SchweiggJ., 16,426(1816). (21 . , J. I. Hoffman and G. E. F. Lundell, J. Research Natl. Bur. Std., 3, 581 (1929). (3) H. R. Shell and R. L. Craig. ANAL.CHEM.. 26.996 , (1954). . (4j H. R. Shell and R. L. Craig, “Synthetic Mica Inv., VI1 Chem.
Anal. and calculation to Unit Formula of Fluorsilicate” U. S. Bur. Mines Rept. Invest. No. 5158 (1956).
1892
e
ANALYTICAL CHEMISTRY
In a second approach, boric acid and borates have been used to prevent the loss of silica as fluorides. Fluoride is volatilized at the time of fusion, or complexed as fluoborates. Jannasch and Weber (5) fused silicate containing fluoride with boric oxide over an oxygen blast. No silica was lost, but fluoride was volatilized as BF3. Meulen (6) complexed fluoride by adding boric acid and Schrenk and Ode (7) used 2 0 x HCIOa saturated with boric acid to determine silica in fluorspars. This procedure is, however, subject to indeterminate plus and minus errors. The former is caused by the partial separation of boron with silica and the minus error occurs because under acidic conditions silicofluoride is more stable than fluoborates and silica is liable to loss during dehydration. However, in samples containing silica as quartz the loss is not serious because quartz is very slowly attacked by HF. (5) P. Jannasch and H. Weber, Ber., 32,1670 (1899). (6) H. ter Meulen, Chem. Weekblad, 20, 59 (1923). ANAL. ED., 1, (7) W. I. Schrenk and W. H. Ode, IND.ENG.CHEM. 201 (1929).
Shell (8) has given a new method for the determination of silica in fluosilicates. He showed that silica could also be determined in such samples by the usual acid dehydration provided F was complexed by A13+ions added as A1CIa6H20. According to Shell, the method gives satisfactory results for silica in a variety of samples containing fluoride, such as micas, amphiboles, tourmaline, topaz, or any rock containing less than 10% fluoride. We have investigated the efficiency of certain other ions in preventing loss of silica during acid dehydration when appreciable amounts of F are present. The following ions did not prevent loss of S O 2 ; U6+, Mgz+,Tla+,Ga3+; Ce4+,Mn2+, Ti4+. But Be2+, Zr4+,and Th4+ were effective in preventing such losses. Varying amounts of silica were determined satisfactorily in presence of 200 mg of NaF. Determinations with Be2+ were entirely satisfactory. A small amount of Be2+is required as compared to large amounts of A13+ (500 mg) in Shell's procedure. With Zr4+and Th4+a basic salt is often precipitated with silica. This however does not cause appreciable error because it is weighed as ZrOz or Tho2 before and after hydrofluorization. The loss on hydrofluorization is entirely due to SiOz provided care is taken with ignition of the hydrofluorized residue. Fluorides have to be eliminated and metals are to be converted to sulfates before ignition is attempted. Shell (8) has however reported ineffectiveness of Zr4+ in preventing the loss of SO2. He confirmed the appearance of a white precipitate (9). EXPERIMENTAL A solution of sodium silicate obtained by fusing a pure quartz sample (99.90z Si02)in an excess of sodium carbonate or a known weight of pure sodium silicate nonahydrate was used as a source of silica. A solution of known concentration or known weight of pure sodium fluoride was used as a source of fluoride. Tests were also made on a variety of synthetic fluosilicates prepared by mixing weighed amount of sodium fluoride to acid or basic rocks, clays and quartz samples. Procedure. The solution of sodium silicate was used as such. Insoluble silicates were fused with sodium carbonate. A 1.00- to 0.50-gram, finely gound (-200 mesh) sample was fused in a platinum crucible with 5-10 grams of sodium carbonate (the larger amount of NazCOs was used for basic rock). Fluoride in known amounts was incorporated with the sample before fusion. Fusion was conducted for 30 minutes at temperatures not exceeding 1050 "C, the melt was cooled, the crucible placed in a platinum dish, and the melt dissolved in 150 ml of 1:3 HC1 containing the appropriate amount of the metal salt used to complex fluoride. The solution was evaporated on a boiling water bath to complete dryness. Any crusts or lumps were broken by a rod from time to time, the dish was removed, and after 15 minutes of cooling the residue was moistened with 10 ml of concentrated HCl. About 100 ml of hot water was added, the dish was set on a boiling water bath, and the solution stirred to dissolve the salts. The residue was filtered through Whatman No. 40 paper, washed thoroughly with 5 % HC1 and placed in a clean platinum crucible. The filtrate was transferred to the same dish and evaporated to dryness; the silica was baked for an hour in an air oven maintained at 105 "C. The dish was taken out after an hour and dissolution of residue in HCl and filtration was done in a similar manner except that filter paper pulp was added before filtration and filtration was done through Whatman No. 42 paper. The residue was washed (8) H. R. Shell, ANAL.CHEM., 27, 2006 (1955). (9) H. R. Shell, U. S. Bureau of Mines, Norris, Tenn., personal communication, June 24, 1965.
Table I. Silica Recovery with Constant Silica, Variable Fluoride and Metal Ion (250 mg SiOz taken as sodium silicate) SiOz Sios found found Fluoride in two colorimetadded, mg Compounda added, dehydrations, rically, (as NaF) g mg mg 248.5 2.0 None None ZrOCIz 8 H 2 0 247.5 ... 75 0.5
Zr OClz 8H20
75
251 . O
...
248.8
...
235.0
2.5
248.1
...
248.5
...
1.05
Zr OC1, 8 H z 0
150
1.5
Th(NO& H2O
15
0.5
Th(NOd4 HzO
75
0.75
Th(NOd4 Hz0
150
1.5
Th(NOd4 Hz0
75
1.5b
...
0.75
Be(NO& 4HzO
75
230.0
3.5
247.5
3.5
246, Oc
4.5
1.5
Be(NO& 4 H 2 0
75
3.0
Be(NO& 4 H 2 0
150
6.0 a Metal ion used as nitrate was first precipitated as hydroxide to separate it from nitrate. The hydroxide was dissolved in hydrochloric acid and then added. b Si02 not added. c Borosilicate glass beaker used for evaporation and dehydration of silica.
Table 11. Results for Silica by Combined Double Hydrochloric Acid Dehydration and Colorimetric Determination in Presence of 100 mg Fluoride Present as NaF (0.25 gm. Be2+ion added) Sample SiOl, Z size, Gravi- ColoriTotal Sample gram metric metric found Present Granite DN, 0.5 71.94 0.42 72.36 72.53 GraniteDN2 1.00 72.86 0.35 73.21 73.45 Basic rock 20511
Impurequartz Plasticclay Flint clay
0.5 0.25 0.50 0.50
48.20 97.30 58.30 44.0
0.25 0.21 0.34 0.25
48.45 97.51 58.64 44.25
48.35 98.01 59.01 44.10
z
with hot 1 HCI and finally with hot water. Silica adhering to the dish and rod was wiped with a piece of wet filter paper and transferred to filter. This silica was also added to the platinum crucible containing silica from first dehydration and heated at a low temp. (ca. 650 "C) to burn off carbon and then ignited at 1100 to 1200 "C for 30 minutes. The crucible was removed from the furnace, cooled, and weighed. The SiOz impurities were moistened with 5 drops of 1 :1 HS04 and hydrofluorized, and then heated at low temperature until fumes of SOa were given off. The crucible was then ignited for 2-3 minutes, cooled, and weighed. The difference between the two weights is equal to silica obtained in two dehydrations. The silica escaping in the filtrate was found to vary from 2-4.5 mg (Tables I and 11). This was estimated by colorimetric method and added to the total found by hydrofluorization. Estimation of Residual Silica by Colorimetric Method. The filtrate, after evaporation to adjust the volume to 150 ml, was adjusted to a pH of 1.2-1.5 using a glass-electrode pH meter by addition of HC1 or NaOH (in the presence of Zr4+
+
VOL. 40, NO. 12, OCTOBER 1968
1893
Table III. Effect of Fluoride on Colorimetric Determination of Silica (Fluoride complexed with Be2+ion) SiO, found, mg (5 mg fluoride added) SiOppresent, mg 0.50 1.21 1.45 2.5 3.0
0.48 1.39 1.5 2.8 3.1
and large amounts of Th4+ a precipitate appeared on neutralization of acid solution). Ten milliliters of 10% ammonium molybdate was added. The solution was stirred, allowed to stand for 15 minutes and made to 250 ml in a volumetric flask, the absorbance was measured at 410 mp. Fluoride does not interfere if sufficient amounts of Bez+ or Th4+ ions are present to complex the fluoride (Table 111). Addition of borate is not necessary. Values of S i 0 2 by the colorimetric method are included in Table I. RESULTS AND DISCUSSION
Values of silica obtained by conventional method and by the described method (in presence of known amounts of fluoride) are tabulated in Tables I and 11. Recoveries of 100 to 400 mg of Si02in the presence of 50 to 200 mg of NaF are satisfactory. Small amounts of Si02in presence of large amounts of fluoride have not been determined. For determination of such amounts Schrenk and Ode’s procedure (7) might be more suitable. Large amounts of Be2+ions hinder complete dehydration of silica. The method is expected to give satisfactory values for SiOz for most fluosilicates occurring in na-
ture such as amphiboles, topaz, micas, and phosphate rocks. Most of these are expected to contain less than 10% fluoride. Fluorine must be determined in a separate portion of sample. The portion used for Si02 determination cannot be used for further analyses. The following points have been noted. The ignited Si02 obtained by dehydration from a fused silicate or fused quartz was white powdery in appearance. While that obtained by dehydration of sodium silicate was found to be transparentglassy solid especially when precipitated from solution containing Be2+ions. The recoveries of Si02 from a sulfuric acid dehydration were short of being satisfactory. Casseroles and beakers may also be used for dehydration of Si02in presence of fluorides provided fluoride is effectively complexed by Be2+,Th 4+, or Z r 4+ ions. Inhalation of fumes, dusts, powders, mists, or solutions containing BeZ+must be avoided, since it is a hazardous material which may cause death or permanent injury after very short exposure to small quantities. Also, soluble compounds of beryllium may result in dermatitis. Frequent washing of hands etc., is necessary while working with beryllium-containing materials. Thorium and its compounds are radioactive. Storage of large amounts of thorium salts near the working shelf must be avoided. The Ala+ method (8) may still be regarded as superior for the following reasons: nonpoisonous nature of Ala+, and recovery of Si02 is apparently greater. RECEIVED for review December 12, 1967. Accepted June 3, 1968. One of us (DNT) is thankful to State Council of Scientific and Industrial Research for contingent grant sanctioned for the research work.
Precipitation of Berkelium(1V) Iodate from Homogeneous Solution Boyd Weaver Oak Ridge National Laboratory, P.O. Box X, Oak Ridge, Tenn. 37830
MANY PROCEDURES for separation and determination of metals have been improved by application of the technique of precipitation from homogeneous solution (PFHS), following methods initiated by H. H. Willard ( I ) . An outstanding example is the Willard-Yu ( 2 ) method of precipitation of ceric iodate by formation of cerium(1V) ions from cerium(II1) in a solution containing an excess of iodate ions. Berkelium, still a very scarce element, has oxidationreduction properties very similar to those of cerium. Therefore, berkelium(II1) in an iodate solution should be oxidizable to berkelium(1V) and precipitated similarly to cerium (IV), if the solubility of berkelium(1V) iodate is exceeded. When present in only tracer amounts, it should be coprecipitated with cerium. Thompson et a/. (3) used cerium(1V) iodate to carry berkelium(1V) but apparently not from homogeneous solution. The study reported here established a suitable procedure for coprecipitation of tracer-level berkelium with cerium, (1) Louis Gordon, M. L. Salutsky, and H. H. Willard, “Precipitation from Homogeneous Solution,” Wiley, New York, 1959. (2) H. H. Willard and S. T’Sai Yu, ANAL.CHEM., 25, 1754 (1953). (3) S. G. Thompson, B. B. Cunningham, and G. T. Seaborg, J . Amer. Chem. Sac., 12,2798 (1950).
1894
ANALYTICAL CHEMISTRY
compared the completeness of precipitation of these two elements, and measured the degree of contamination by several other elements, especially trivalent lanthanides and actinides. Information gained from this study indicates the future applicability of iodate PFHS to the recovery and purification of larger quantities of berkelium without the use of cerium carrier. EXPERIMENTAL
Reagents. Cerous nitrate, nitric acid, iodic acid, sodium bromate, and small quantities of several other metallic salts were of analytical quality. Tracer and lj2Eu were obtained from the ORNL Isotopes Division and 249Bkand 244Cm from the transplutonium element production program. Apparatus. The gamma activity of I4Ceand ls2Eusamples was measured by a Nuclear-Chicago gamma spectrometer with a well-type NaI(T1) detector. The beta activity of 249Bk and the alpha activity of 244Cm were measured by standard techniques in a Packard Tri-Carb Liquid Scintillation Spectrometer. Procedure. A standard procedure was derived from results of preliminary tests and data of Willard and Yu. Bromate was chosen over persulfate as oxidizing agent because it gives more rapid precipitation. Chromate, useful as an
