Microtitration of Selenium As Applied to Abnormal Amounts of Selenium in Urine GRANT WERNIMONT
AiVD
F. J. HOPKINSON, Eastman Kodak Company. Rochester, N. Y.
T
pieces of 4-mm. soft-glass tubing. It was found that the electrodes sometimes developed microscopic cracks around the seal which made them useless for detecting the end point. When the cracks were closed in a flame, the electrodes regained their original sensitivity. Erratic behavior was observed when the platinum was not completely immersed in the solution. The dead-stop end-point apparatus was used as follows :
HE volumetric determination of very small amounts
of selenium has been successfully carried out b y Mathews, Curl, and Osborn (8), who used an iodine-thiosulfate method and determined the end point electrometrically. Later, Curl and Osborn (a) used starch to determine the end point with somewhat less precise results. The method reported here is simpler than either of these methods, and gives results of about the same precision as the electrometric method.
Method The organic matter in a 100-ml. sample of urine was destroyed with concentrated sulfuric acid in the presence of mercury. The selenium was separated from the digestion mixture by distillation in the presence of hydrobromic acid and then precipitated (using sulfur dioxide and hydroxylamine hydrochloride), filtered, washed, and redissolved in a solution of hydrobromic acid and bromine. The bromine was destroyed and an excess of standard sodium thiosulfate was added along with a little potassium iodide. The unused sodium thiosulfate was titrated with standard potassium iodate using the dead-stop method (2, 6, 7 ) for determining the end point.
The solution of sodium thiosulfate, to be titrated, was made up to about 25 ml. in a 125-ml. beaker and 1ml. of potassium iodide solution was added. The beaker was placed in the apparatus and, with mechanical stirring, the rheostat, R, was adjusted so that a small current was flowing through the electrodes as was indicated by the displacement of the galvanometer reading from its rest point by several scale divisions. The potential between the electrodes was usually about 100millivolts. Standard 0.001 N potassium iodate was added steadily until the galvanometer began to show deflections. As the end point was approarhed, the galvanometer showed a temporary deflection after each drop of reagent was added. At the end point, one drop of reagent
Reagents SODIUMTHIOSULFATE. A standard 0.1 N solution was made by dissolving 25.0 grams of the c . P. crystals in freshly boiled and cooled distilled water to make 1 liter of solution. This solution was standardized against c. P. potassium dichromate after i t had stood for 24 hours. It was stored in a glass-stoppered bottle in a cool place and remained standard for several weeks without the addition of any preservative. Standard 0.001 N sodium thiosulfate was made up daily by diluting exactly 10 ml. of 0.1 N solution to 1 liter with freshly boiled and cooled distilled water. It was found that this solution would remain standard for a t least 36 hours. One milliliter of 0.001 N sodium thiosulfate equals 19.74 micrograms of selenium. POTASSIUM IODATE. Potassium biniodate, c. P., was recrystallized twice from water and the crystals were dried in an electric oven a t 105 C. A standard 0.1 N solution was made by dissolving 3.250 grams of the dry crystals in water to make 1 liter of solution. Standard 0,001 N solution was made by diluting exactly 10 ml. of 0.1 N solution to 1liter. The dilute solution was standardized daily against freshly prepared 0.001 N sodium thiosulfate using the dead-stop method and the same amounts of all reagents as were used in titrating the selenium. The ratio of this solution to various 0.001 N sodium thiosulfate solutions made by diluting 0.1 N sodium thiosulfate remained constant for many days, showing that dilute solutions of potassium iodate do not deteriorate. Potassium iodide (free from iodate), 5 per cent solution prepared fresh daily Sodium sulfite, 5 per cent solution Phenol, U. S. P., 5 er cent by volume in water Hydrobromic acid-gromine solution, 30 per cent hydrobromic acid containing 1 per cent of liquid bromine by volume Hydrobromic acid-bromine solution, 48 per cent hydrobromic acid containin 1 per cent of liquid bromine by volume Sulfur dioxije, liquid, in a steel tube Hydroxylamine hydrochloride, Eastman reagent No. 340 STANDARD SOLUTION OF SELENIUM.Selenium dioxide was prepared from c. p. selenium by dissolving the metal in concentrated nitric acid, evaporating the solution to dryness, and twice subliming the solid selenium dioxide. The calculated weight of dry selenium dioxide (0.1405 gram) was dissolved in water containing 5 ml. of 0.5 N sodium hydroxide and the whole diluted to make 1liter of solution. One milliliter contained 100.0 micrograms of selenium.
Dead-Stop End Point The apparatus (Figure 1) consisted of two platinum wire electrodes hooked u p as shown in Figure 2. The electrodes were made b y sealing small loops of platinum wire into short
FIGURE 1. DEAD-STOP END-POINTAPPARATUS 308
MAY 15, 1940
TABLE I.
ANALYTICAL EDITIOS TITR.4TIOS O F SOLCTIOSS O F
SELENIUM
(1 ml. of 0.000994 N NazSxOs = 19.62 micrograms of selenium) KIOs NarSrOs KIOs h'et 0.000994 0.000966 0.001008 N N N NarSpOs Se Found Se Added Difference MI. MI. M1. 1111. -Micrograms----0.1 149.9 150.0 1.34 9.00 7.64 -2.1 147.9 150.0 1:io 7.54 9.00 -0.8 125.0 124.2 1:65 6.33 8.00 -2.5 100.0 2.00 97.5 4.97 7.00 -1.1 100.0 2:02 98.9 5.04 7.00 75.0 +0.3 75.3 i:i4 3.84 5.00 +0.7 75.0 75.7 i:i7 3.86 5.00 0.0 50.0 50.0 1:43 2.55 4.00 -0.2 50.0 49.8 1:io 2.54 4.00 +1.1 25.0 26.1 1:65 1.33 3.00 4-0.5 26.0 25.5 1:i5 1.30 3.00 4-1.8 10.0 11.8 1:is 2.00 0.60 1-0.2 l ~ . ~ 10.0 1:i2 0.52 2.00 +2.3 0:61 I .J 5.0 .. 0.38 1.00
..
TABLE 11. PRECIPITATIOS AXD TITRATION OF SELENIL-I (1 ml. of 0.000994 N NarSaOa = 19.62 micrograms of selenium) KIOs Nazi08 KIO8 Net 0.000994 0.000994 0.001008 N h ' N NaiSSOs Se Found Se Added Difference . . MI. M1. M1. M1. ----.Wicroorams-1.9 148.1 150.0 7.55 1.45 9.00 -0.9 149.1 150.0 113s 7.60 9.00 100.0 -0.3 99.7 1:i8 5.08 7.06 -1.3 100.0 1:94 98.7 5.03 7.00 +0.5 75.0 75.5 i:i5 3.85 5.00 f0.4 50.0 50.4 1.43 2.57 4.00 -0.8 50.0 1:47 2.51 49.2 4.00 25.0 26.5 1:65 3.00 1.35 10.0 1:50 9.4 0.48 2.00 +2.8 12.8 10.0 1:35 0.65 .. 2.00
..
+A:; -
caused a permanent galvanometer deflection of 3 to 5 scale divisions, and further addition of potassium iodate sent the galvanometer reading completely off the scale. The end point was sharp and reproducible, so that 0.05 ml. of excess 0.001 N potassium iodate solution could be detected in 50 ml. of titration mixture. The dead-stop apparatus is simple to assemble and convenient to operate. Various authors have used i t successfully (9, IO),although one author (1) failed to get satisfactory results with it.
Experimental Procedure The first series of experiments was made by measuring various amounts of standard selenium solution from a 10-ml. buret into 150-ml. beakers containing 3 ml. of 30 per cent hydrobromic acidbromine solution and 15 ml. of water. Sodium sulfite was added, dropwise, until the color of bromine was almost gone. (If too much sodium sulfite was accidentally added, the bromine color was restored with a few drops of hydrobromic acid-bromine solution.) The last trace of bromine was removed with 2 to 3 drops of phenol solution. Standard 0.001 N sodium thiosulfate was then added to the solution, so that 2 to 3 ml. were present in excess. After the addition of 1ml. of potassium iodide solution, the excess sodium thiosulfate was immediately titrated with 0.001 N potassium iodate using the dead-stop method to determine the end point. The results of two series of experiments are given in Table I. A second series of analyses was made in which various amounts of standard selenium solution were measured from a 10-ml. buret into 150-ml. beakers containing 40 ml. of 30 per cent hydrobromic acid-bromine solution. Gaseous sulfur dioxide was passed slowly through the solutions until the bromine was just destroyed, avoiding an excess. One-half gram of solid hydroxylamine hydrochloride was added to each beaker and the mixtures were warmed for 10 minutes on the steam bath. After the mixtures had cooled for 2 hours, the red selenium was filtered off through a No. 1 porous-bottomed porcelain crucible and washed thoroughly with distilled water. Three 1-ml. portions of hydrobromic acid-bromine solution were poured through the filter, which was then washed with six to eight 2-ml. portions of distilled water. The filtrate and washings were collected in the original beaker, and treated first with sodium sulfite solution and then with phenol solution to destroy the bromine. The analyses were completed as described above. The results of two series of experiments are found in Table 11.
309
The last series of analyses was made using 100-ml. samples of urine to which various amounts of standard selenium solution were added from a 10-ml. buret,. Twenty-five milliliters of concentrated sulfuric acid were added to each sample (in 300-ml. Kjeldahl flasks) along with 0.7 gram of mercuric oxide and several glass beads. The flasks were heated over a gas flame until the water was evaporated and the organic matter completely destroyed (about 3 hours). The clear solutions were transferred to 300-ml. flat-bottomed flasks using 60 ml. of dist'illed water to wash out the Kjeldahl flasks. Twentyfive milliliters of concentrated sulfuric acid were added to each distilling flask along with 50 ml. of 48 per cent hydrobromic acidbromine solution, and the ground joint was moistened with concentrated sulfuric acid. The flasks were then attached to small Liebig condensers similar to those used by Robinson et al. (11) and the mixtures distilled until 35 to 40 ml. of distillate collected. The distillate was treated by the same procedure as v.-as used in the second series of analyses. The results are given in Table 111.
L
,i1
Discussion
It is believed that this method is capable of giving results as precise and as accurate as any method reported in the literature. As little as 3 micrograms of selenium can be positively identified by its character- --istic red color on the filter pad. The precision is about * 2 micrograms in the range 2 to 150 micrograms and the accuracy is within these FIGURE2. WIRIXGDIAGRAM limits, 07.1 the basis of a B. 1.5-voltdry cell 100-ml. sample, the method E . Platinum eleotrode G. Leeds d: Northrup galvanomis sensitive to 0.02 part per eter of enclosed lamp and scale type. Sensitivity, 0.025 million, which is sufficient microampere per scale difor the present purpose. It msion R . Potentiometer-type rheostat is believed that increased of about 1500 ohms' resistance sensitivity may be attained by starting with a large sample, say 1liter, rather than by using mort: dilute reagents. The method has been used satisfactorily for determining u p to 2000 micrograms by taking an aliquot portion of the hydrobromic acid-bromine distillate for the final titration. Two alternate methods of making the titration were tried and abandoned in favor of the one used.
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1. Potassium iodide solution was added to the selenite solution after the bromine had been destroyed with phenol, and the liberated iodine was titrated directly with standard sodium thio-
OF SELENIUM IN URINE TABLE 111. DETERMINATION (1 ml. of 0.001087 N NaiStOa = 21.46 micrograms of selenium) NatSrOa KIOs Net 0.001087 N 0.001004 N NarSnOs Se Pound Se .4dded Difference MI. MZ. M1. r-.Micrograms-9.00 8.00
7.00 5.00 4.00 3.00 2.00
2.14 2.44 2.56 1.67 1.78 1.93 2.16
7.02 5.75 4.64 3.46 2.36 1.22 0.01
130.6 123.4 99.6 74.3 50.6 26.2 0.2
150.0 125.0 100.0 i5.0 50.0 23.0 0.0
+0.6 -1.6 -0.4 -0.7 +0.6 +1.2 +0.2
310
INDUSTRIAL AKD ENGINEERING CHEMISTRY
sulfate solution. The dead-stop end point was not so sharp nor so reproducible when iodine was titrated with sodium thiosulfate as when sodium thiosulfate was titrated with potassium iodate in the presence of potassium iodide. 2. Potassium iodide solution was added to the selenite solution as above and the iodine was reduced with standard sodium thiosulfate solution. The excess sodium thiosulfate was then titrated with potassium iodate solution. I n these titrations, red selenium was precipitated and the results were not so precise as when selenium was not present. However, i t will sometimes be necessary to finish a titration by method 2. If sufficient sodium thiosulfate is not added to react with all the selenite in an unknown, iodine and selenium will be formed when potassium iodide solution is added just before the final titration. I n such a case it is necessary to continue the addition of standard sodium thiosulfate until no iodine remains and then finish the titration in the presence of a small amount of selenium. KO corrections are necessary because one mole of selenite reacts with 4 equivalents of sodium thiosulfate in one case and liberates 4 equivalents of iodine in the other. The recommended procedure gave satisfactory results when selenium was precipitated from all solutions of hydrobromic acid which contained between 20 and 35 per cent of hydrobromic acid by weight. (Dudley, 4, recommends 25 to 30 per cent of hydrobromic acid.) The complete precipitation of selenium required two hours. It mas found that filtration may be delayed as long as 24 hours, with 100 per cent recovery of the selenium added. The results given in Table 111 show that there is no loss of selenium during the Kjeldahl digestion in the presence of mercury. Sreenivasan and Sadasivan (12) have shown that selenium is oxidized to selenic acid by hot, concentrated sulfuric acid in the presence of mercury. I n this form i t is apparently not volatile. Destruction of the organic matter in urine with ammonium persulfate according to the method of Duret (5)was also tried.
VOL. 12, NO. 5
Selenium v a s oxidized to selenic acid during this digestion and 100 per cent of the selenium was recovered in every case. The method was abandoned, however, after several rather violent explosions. The explosions seemed to be spontaneous and in the gaseous phase throughout the reflux condenser. No glassvare was ever shattered. 30mention has been found in the literature of the danger of explosions Tyhile using the persulfate in the wet way to destroy organic matter. It is quite possible that in this case volatile, unstable organic peroxy compounds were formed.
Summary A new method for titrating very small amounts of selenium has been shown to give good results when applied to solutions containing known amounts of selenium, and when used to determine small amounts of selenium in urine.
Acknowledgment The authors wish to thank Dr. J. H. Sterner and hIiss Viola Lidfeldt for many suggestions during the course of this work.
Literature Cited Bratton et al., IND. EXG.CHEM.,Anal. Ed., 10, 600 (1938). Clippinger and Foulk, Ibid., 11, 216 (1939). Curl and Osborn. J . Assoc. Oficial Agr. Chem.. 20, 197 (1937). Dudley, Publ. Health Rpt., 58, No. 8, 281-92 (Feb. 8, 1938) (Reprint 1910, p. 8). Duret. Compt. Tend. 167, 129 (1918). Foulk and Bawden, J. Am. Chem. Soc., 48, 2045 (1926). Kolthoff and Furman, "Potentiometric Titrations", 2nd ed., p. 122, New York, John Wiley & Sons, 1931. Mathews, Curl, and Osborn, J . Assoc. Oficial Aor. Chem., 21, 231 (1938). Pagel and Miers, IND. ENG.CHEM.,Anal. Ed., 10, 334 (1938). Pring and Spencer, Analyst, 55, 375 (1930). Robinson, Dudley, Williams, and Byers, IXD.ENQ. CHEM., Anal. Ed., 6, 274 (1934). Sreenivasan and Sadasivan, Ibid., 11, 314 (1939).
Determination of Minute Amounts of Potassium Iodometric Evaluation of the Cobaltinitrite Precipitate, Using Ceric Sulfate IRVING ALLEN KAYE Pediatric Research Laboratory, The Jewish Hospital of Brooklyn, Brooklyn, N. Y.
S
MALL amounts of potassium are usually determined
titrimetrically by precipitation with sodium cobaltinitrite, followed by solution of the washed precipitate in a known excess of a n oxidizing reagent. Potassium permanganate was originally used and continues to be used as the oxidant for dissolving the precipitate, its excess being determined by titration with standard sodium oxalate. RIore recently (3) the excess permanganate has been determined iodometrically, a procedure capable of detecting very small quantities of potassium. Harris and others (2, 5 ) have pointed out that the use of ceric sulfate instead of potassium permanganate ia advantageous. When potassium permanganate is used, a precipitate of hydrated manganese dioxide is often formed, which necessitates the addition of a n excess of sodium oxalate to effect its solution, after which the end point is reached by a n
additional titration with permanganate. This difficulty is , of course, not encountered when ceric sulfate is used. Ceric sulfate solutions are more stable and consequently require fewer standardizations. Brown, Robinson, and Browning (2) have lately published a procedure using ceric sulfate for dissolving the precipitate of sodium potassium cobaltinitrite. The excess ceric sulfate is titrated with ferrous ammonium sulfate, using o-phenanthroline ferrous complex as indicator. Using this procedure, potassium in amounts ranging from 0.2 to 1.0 nig. could be detected with an average error of 2 per cent. -4variation of the ceric sulfate procedure applicable to very small amounts of potassium has been developed and used for over 2 years. An excess of potassium iodide is added to the excess ceric sulfate and the iodine liberated is titrated with standard sodium thiosulfate using starch as indicator. Be-
