Tetraphenylarsonium Chloride as an Analytical Reagent I
J
Determination of Rhenium HOBART H. WILLARD AND GEORGE AI. SMITH' University of Michigan, Ann Arbor, Mich.
T
final solution about 0.5 molar, a measured excess of tetraphenylarsonium chloride is added. The total volume should be 25 to 60 ml. The mixture is stirred and allowed to stand several hours, preferably overnight. The precipitate is filtered through a Gooch crucible, washed several times with ice water, and dried at 110' C. It is weighed as tetraphenylarsonium perrhenate, (C6H5)4AsRe04. Multiplying by the factor 0.3952 converts this weight t o perrhenate, Reo4-.
HE perrhenate ion unites with the tetraphenylarsonium ion, in the reaction
+
(C6H5)~As+ Reo4- = (CsH5)4AsRe04
to form a white, crystalline precipitate which is insoluble in cold water. This permits the quantitative determination of perrhenate both potentiometrically, by the titration of the excess reagent with iodine ( I ) , and gravimetrically. The latter is usually more convenient and is the method described in this paper. The potentiometric titration is, however, equally satisfactory. The determination is carried out by adding an excess of tetraphenylarsonium chloride to the perrhenate, keeping the volume as small as possible. The precipitate, which is allowed to stand several hours, preferably overnight, is filtered through a Gooch crucible, washed several times with ice water, dried, and weighed as (CeH@sRe04. The precipitation is carried out in a hot solution, in the presence of a neutral salt, such as sodium chloride or sodium sulfate, to make the precipitate more granular and more easily transferred. The most satisfactory precipitation medium is 0.5 molar sodium chloride. Nitrates, except in very low concentration, should be avoided because of the limited solubility of tetraphenylarsonium nitrate.
TABLE11. EFFECTOF ACIDITYAND ANIONS ON PERRHEKATE DETERMINATIONS (Re'&-, 22.21 mg.; total volume, 25 t o 30 ml.) Perrhenate Substance Molar Present Concentration Error, Mg. HCI 0.5 f0.04 HC1 4.8 f0.08 "08 0.7 +l.54 0.8 H&On $0.09 His04 3.6 $1.30 HaPOa 0.8 $0.07 HCiHiOi. +o. 19 6.9 Citric acid 0.2 0.00 Tartaric acid 0.6 $0.08 Oxalic acid 0.3 +0.10 NaiSOa 0.5 -0.01 NaSWOt 0.1 $ 0 . 04 NazHPO4 0.1 -0.11 NHiOH 6.0 +0.03 NaOH -0.26 0.5
I n this way quantities of perrhenate varying from 0.40 to 133 mg. have been determined with satisfactory accuracy TABLEI. GRAVIMETRIC DETERMINATION OF PERRHENATE in the presence of various other ions. Typical data are shown (Volume, 25 t o 60 ml.; NaC1, 0.5 molar) in Table I. Many of these gravimetric results were dupliPPerrhenatecated by the potentiometric titration of the excess of reagent Present Found Error with iodine. Mo. MV. MQ. 0.44 0.89 1.33 1.78 2.22 4.44 13.32 17.76 22.21 22.21 44.41 88.82 133.24
.
0.40 0.91 1.26 1.82 2.21 4.43 13.33 17.82 22.21 22.17 44.39 89.00 133.18
-0.04 $0.02 -0.07 i-0.04 -0.01 -0.01 $0.01 $0.06
TABLE111. EFFECTO F C-4TIONS ON DETERMINATION OF PERRHENATE (INCLUDING METAVANADATE, VOs-) (Volume, 25 to 35 ml.; NaCI, about 0.5 molar) Quantity Perrhenate of Ion Present Found Error Mg. Mg. Mo. MQ. AIt++ 112 22.21 22.13 -0.08 Bat+ 565 44.41 44.34 -0.07 Ca++ 360 44.41 44.34 -0.07 440 Cd++ 22.21 22.17 -0.04 Co++ 210 1.33 1.26 -0.07 Cr+++ 215 44.41 44.50 cu++ 255 44.41 44.34 200 Fe++ 2.22 2.29 $0.07 206 Fe+++ 2.22 2.13 -0.09 Fe+++ 206 44.41 44.34 -0.07 Mg+C 100 22.21 22.21 0.00 Mn++ 22.21 22.21 275 0.00 Ni++ 250 22.21 22.13 -0.08 366 Sb+++ 22.21 22.17 -0.04 UO,++ 635 22.21 22.30 $0.09 44.41 45.01 200 VO++ $0.60 22.21 23.67 800 vo++ +1.46 VO" 1000 4.44 6.97 4-2.53 vo844.41 44.57 35 +0.16 22.21 22.31 VOa650 $0.10 vos520 4.44 4.55 +O.ll Zn++ 22.21 22.17 260 -0.04
Ion Present
=to.oo
-0. a4 -0.02 +0.18 -0.06
t::;
Determinations were attempted in the presence of most of the common anions and cations. Anions, such as permanganate, periodate, perchlorate, thiocyanate, iodide, bromide, and fluoride, which unite directly with the tetraphenylarsonium ion to form insoluble compounds, should be absent. Those cations the halide complexes of which form insoluble tetraphenylarsonium compounds in the presence of 0.5 molar chloride ions interfere except in very low concentration. High concentrations, approaching saturation, of any substance should be avoided to prevent the precipitation of excess reagent. I n general, the presence of other substances in solution causes the formation of a precipitate which is more easily transferred. Procedure In all determinations a standard solution, made by dissolving pure potassium perrhenate in water containing 8.8824 m . of perrhenate ion per ml. was used. To a definite volume o f the hot solution, containing sufficient sodium chloride t o make the
Similar determinations were satisfactorily made under conditions of acidity varying from weakly alkaline to fairly strongly acidic. Sodium hydroxide has a solvent action on the precipitate, but a relatively high concentration of ammonium hydroxide is not harmful. Nitric acid or nitrates, except in very low concentration, will cause coprecipitation of tetraphenylarsonium nitrate with the perrhenate. High results are obtained with high concentrations of hydrochloric
'Present address, Vanderbilt University, Nashville, Tenn.
305
INDUSTRIAL AND ENGINEERING CHEMISTRY
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acid or other acids, probably because of the decreased solubility of the reagent under such conditions. Bromide, iodide, and fluoride, in more than traces, should be absent. Tungstate does not interfere. Results are shown in Table 11. TABLEIV. EFFECTOF NITRATEAND MOLYBDATE ON PERRHENATE DETERMINATION (Volume, 20 to 35 ml.: NaCl, about 0.5 molar)
Substance Present NHiNOs NaNOa NaNOs NaNOa NaNOa
++ 43 g.ml.tartaria NH40H acid + 4 ml. NHiOH MOOS+ 4 ml. NHaOH Moos + 3 g. tartaric acid MOOS+ 4 ml. NHiOH MOOSf 4 ml. NHiOH Moos + 2 ml. NH40H MOOS+ 2 ml. “aOH MoOa MOOS
MoOa MOOS
Concentration
0.5 M 0.3 M 0.016 M 0.1 M 0.05 M Mg. MoOa 315 315 210 210 210 100 100 100 20 100
Perrhenate Present Found
Error
Mg.
Mg.
Mg.
44.41 22.21 22.21 0.89 0.44
50.98 25.77 22.40 0.91 0.40
$6.57 $3.56 $ 0 . I9 10.02 -0.04
22.21 22.13 22.21 22.21 2.22 2.25 44.41 44.34 4 4 . 4 1 44.26 0.44 0.40 2.22 2.21 22.21 22.29 22.21 22.17 22.21 120.26
-0.08 0.00 $0.03 -0.07 -0.15 -0.04 -0.01 + O . 08 -0.04
+98.05
I n t e r f e r i n g Substances The effect of the presence of various cations is shown in Table 111. Only those ions interfere which form insoluble chlorides or whose complex halides form insoluble salts with tetraphenylarsonium ion. These include mercuric, stannic, bismuth, tellurium, lead, vanadyl, and silver. Cadmium and zinc do not interfere if the chloride-ion concentration is Iow. Metavanadate ion in fairly high concentration does not interfere. All these ions serve to make the precipitate more granular.
VOL. 11, NO. 6
The presence of nitrate even in small quantities may cause serious interference if the quantity of perrhenate is fairly large. However, the interference is not so pronounced for very small amounts of perrhenate. It is practically impossible to wash out all traces of tetraphenylarsonium nitrate from the heavier perrhenate precipitate. Typical data are shown in Table IV. Molybdate ion forms a fairly insoluble precipitate with tetraphenylarsonium ion, but this precipitation is hindered or prevented altogether in the presence of ammonium hydroxide, tartrates, citrates, and their acids. Typical data are shown in Table IV.
Summary From 0.40 to 133.0mg. of perrhenate ion can be determined gravimetrically with tetraphenylarsonium ion, in moderate excess, in volumes from 25 to 60 ml. The presence of a small amount of sodium chloride, about 0.5 molar, or of other salts, and heating before precipitation are very effective in producing a crystalline precipitate which is easily transferred and washed. Accurate determinations may be made in solutions varying from strongly ammoniacal to fairly strongly acidic. Permanganate, perchlorate, periodate, iodide, bromide, fluoride, thiocyanate, mercury, tin, vanadyl, and bismuth ions interfere. Nitrate must be absent in all but very low concentrations. Interference by molybdate may be avoided by the use of ammonium hydroxide or tartaric acid.
Literature Cited (1) Willard and Smith, IND. ENQ.C H ~ MAnal. . , Ed.,
11, 186 (1939).
Determination of Riboflavin in Milk By Photoelectric Fluorescence Measurements DAVID 33. HAND, Cornel1 University, Ithaca, N. Y. Riboflavin in milk was determined by adding 50 ml. of acetone to 25 ml. of milk, filtering, and comparing the fluorescence of the filtrate with that of a cube of uranium glass which had previously been calibrated against solutions containing known amounts of riboflavin. The comparison of fluorescence was made with a photoelectric cell and microammeter, using suitable glass filters.
R
IBOFLAVIN or lactoflavin, the principal water-soluble pigment of milk and whey, is of interest because of its nutritional value as vitamin G (8), its behavior as a co-enzyme (15,l 7 ) , as a hydrogen acceptor (16),and as a photosensitizer for the oxidation of vitamin C in milk by light (6, 6,12). The riboflavin content of milk can be determined rapidly and accurately by using suitable light filters and standards and measuring with a photoelectric cell and microammeter the fluorescence produced in a filtered acetone extract of milk or whey. Methods for the determination of riboflavin so far described have involved visual estimation of color or fluorescence
intensity by comparison with standards, br photoelectric measurement of either light absorption or fluorescence intensity. Charite and Khaustov (1) compared riboflavin extracts with a standard solution of potassium chromate in a colorimeter. Kuhn, Wagner-Jauregg, and Kaltschmitt (11) determined the chloroform-soluble, photochemical decomposition product with a stage photometer. Koschara (7’) purified the riboflavin solutions with chromatographic adsorption and oxidation by permanganate, and determined concentrations directly in a stage photometer. Sullivan (13) measured the light absorption by the use of filters and a photoelectric cell. Euler and Adler (3) and later Supplee, Ansbacher, Flanigan, and Hanford (14) and Whitnah, Kunerth, and Kramer ($0) compared visually the fluorescence of unknown and standard riboflavin solutions. Weisberg and Levin (19) described a similar method but used fluorescein solutions as standards. Cohen (2) measured the intensity of fluorescence directly with a photoelectric cell and amplifier, using fluorescein as a standard. The methods based on light absorption have the advantage that the absorption coefficients reported are in absolute units and can be determined very accurately. However, many of the colored materials accompanying riboflavin interfere with the determination. Fluorescence measurements are more
