809
OCTOBER 1947 the level of the top of a thimble, the water level within that thimble should drop faster than that without. If the water used for leaching contains enough mineral matter to interfere with later steps, it can be removed by cutting off the water a t the end of leaching and pouring distilled water through the thimbles as many time; as necessary. If water pours into the box from above, it is best to have it pour into a compartment a t one end, which also contains the leveling device and is separated from the rest of the box by a partition which extends n-ithin a fraction of an inch of the partition between upper and lower compartments. This is to prevent the turbulence of the entering water from dislodging glass plugs or particles of samples from thimbles. Between the acetone and benzene extractions it is necessary to
free the thimbles of acetone. At this stage the samples are very sensitive to heat and it has been found best to remove the acetone by sucking air through them a t room temperature. By cutting the water supply off, the same equipment can be used for freeing the thimbles of acetone. For this purpose the vacuum system will have to have a greater capacity than is needed for leaching, and the tail pipe and vacuum regulator are not necessary. ACKYOWLEDGMEYT
Credit is due to E. A. Meldrum of the Emergency Rubber Prop ect for perfecting the details of construction and for constructing the apparatus used. RECEIVED June 24, 1946.
Routine Method for Determining Selenium in Horticultural Materials JUSTIN S. RICNULTY, Battelle Memorial Znstitute, C o l u m b u s , Ohio ERCHLORIC acid and a vanadium catalyst are used with nitric ‘acid and sulfuric acid to destroy seleniferous organic matter. Selenium is distilled with hydrobromic acid from the digested sample into the vessel wherein it is titrated by a modified XorrisFay procedure. Greater recovery and less working time per analysis have been observed with this than with previous methods. Numerous investigators have written (1, 3, 4, 7 , 8, 11) and reviewed (6) papers on various means of converting organic to inorganic selenium, isolating it, and determining the quantity present. The first of these steps was improved in this laboratory, by devising a continuous digestion xvhich required less attention than digestions in common use. Smith (12) has described his own and Kahanes’ extensive work with perchloric, nitric, and sulfuric acids and various catalysts to destroy organic matter rapidly and retain inorganic constituents. Hoffman and Lundell ( 2 ) showed that selenium was incompletely distilled, with 15 ml. of hydrobromic acid, from mixtures in which perchloric acid.predominated. DISCUSSION
I t was found in this laboratory that, after 10 grama or less of vegetation were digeSted with 5 ml. of 60Y0 perchloric, 50 ml. of concentrated sulfuric, and 75 ml. of concentrated nitric acids a t a temperature not exceeding 210’ C., the perchloric acid remaining permitted complete recovery of added selenium on distillation with 15 ml. of 48% hydrobromic acid (Table I). The high concentration of organic matter in pot soils (one part manure, one part peat, two parts native soil, and one part sand) interfered Tvith the recovery of added selenium by the Robinson et al. (8) procedure. A vanadium-catalyzed sulfuric-nitric-perchloric acid digestion, however, enabled the recoveries shown in Table I. Using these same investigators’ (8) method for evaporating solutions, the present distillation and titration were adapted to nutrient salt solutions.
Table I.
Recovery of Selenium Added to Organic )latter
Type of Organic M a t t e r Alfalfa meal
Pot soil
Selenium Added, P.p.m.
2.0 4.0 5.0 8.0 15.0 50.0
,50.0
S e t Selenium Recovered, P.p.m. 50.8 101 250 250 248 330 1.9 3 5 5 1 7.9 15 1 49.4 50.1
The proposed distillation procedure resembles Hoffman and Lundell’s ( 2 ) . A hydrazine sulfate solution, in which the distillate is received, serves to suppress bromine and selenious acid until bromide and hydrogen attain sufficient magnitude t o keep HSe04- insignificantly small in the system (10):
Coleman and McCrosky ( 1 ) discussed the application of this relationship in preventing the conversion of selenium to the hexavalent state, preceding a Norris-Fay titration. In the present method, the Norris-Fay reaction
+
+ 4HBr +
4?\Ta2s203 H2Se03
+ iSa2SaOe + 3H20 + 4NaBr
Na2SeS406
proceeds simultaneously with the reaction 32HI
+ 8Hk3e03
--.f
321
+ Sea + 24H10
The small quantity of potassium iodide recommended in the present procedure pi favors a sharp starch-iodine end point but limits the second r e a c t i o n As a result, only a tint of suspended selenium and a trace of elemental i o d i n e a r e produced. The iodine eliminates the usual back-titration. The elimination ot transfers between the distillation and the titration, the h j drolyeis of seFigure 1. Digestion Flask and lenium t e t r a b r oDistillation Head mide before contact with the atmosphere, and the degradation of natural selenium compounds (6) incompletely oxidized by nitric acid are factors which largely account for the higher selenium values frequently obtained with the present method than with methods nom in general use. The apparatus, shown in Figure 1, combines features of Scherrer’s (9) and Pavlish and Silvrrthorn’s ( 7 ) apparatus. The
V O L U M E 19, NO. 1 0
810 flask (19 cm. from A to B ) is made by attaching a 34/45 female standard-taper joint to the bulb of a 275-m1. Johnson flask. The thermometer, M (Fisher Scientific Co., Catalog KO.15-002), has a range of 0" to 250' C. PROCEDURE
Digestion. TISSUES.Quickly moisten 10 grams of prepared Aample, contained in a digestion flask, with 30 ml. of "starting solution" and 5 ml. of water. To prepare the starting solution, dissolve 1.6 grams of ammonium metavanadate in 300 ml. of water mixed with 1500 ml. of concentrated nitric acid. After seething stops, add 75 ml. of concentrated nitric, 5 ml. of 60% perchloric, and (carefully) 50 ml. of concentrated sulfur'c acids. Place a t'hermometer in the flask and slowly heat to 140" to 150" C. \Then nitrogen dioxide no longer is in evidence, slowly increase the temperature to 210" C., then cool. Wash the thermometer with 10 ml. of water. If the solution becomes green during digestion, add 1 ml. of perchloric and 10 ml. of nitric acids, then decrease the heating rate. SOILS.Heat 50 grams of soil wit'h 30 ml. of starting solution, until the foam breaks. Then add 75 ml. of nitric, 7 ml. of perchloric, and 100 ml. of sulfuric acids. Incline the flask in a 600ml. hforoney antibumping cup and digest as for tissues. SUTRIEKT SOLUTION.Evaporate an appropriate volume of solution to 30 or 40 ml., with 0.5 gram of sodium peroxide (8) in a digestion flask. .4dd 50 ml. of concentrated sulfuric and 1 ml. of 60% perchloric acids and 5 to 10 mg. of ammonium metavanadate. Add nitric acid only if the color due to vanadium changes from yellow to green. Heat to 210' C. Distillation. TISSCES . ~ N DSOLGTIONS. Apply silicone grease to the joints and assemble the distillation apparatus on a ring stand. Start air flowing into the tube, J , a t a rate such that 2 or 3 bubbles per second rise from outlet L, which is immersed in 50 ml. of 0.1% aqueous hydrazine sulfate solution, contained in a cooled 250-ml. Beraelius beaker. Run 5 ml. of 48% hydrobromic acid into the sample through the funnel, 1. Heat the flask until most of the bromine is driven from it, then allow 10 ml. more of hydrobromic acid to drain into it, at the rate of 1 ml. per minute, while a vapor temperature of 125" to 135" C. is maintained. Heat the thermometer hole, E , as required to remove condensate. SOILS.Distill as above, but thoroughly mix 10 ml. of hydrobromic acid with the sample before heating. Add only 5 ml. of hydrobromic acid a t the rate of 0.5 ml. per minute while heating. Titration. Add about 3 grams of urea and 2.5 ml. of 90% formic acid to the receiving beaker and heat until the bromine is reduced. Seutralize to phenolphthalein v-ith 4 5 7 , sodium
hydroxide solution. Add 13 ml. of 18 N sulfuric acid and cool Dissolve 1.0 gram of potassium iodide in 100 ml. of 0.1% wheat starch paste. ildd 5 ml. of this reagent to the sample and immediately titrate with 0.005 to 0.01 A' sodium thiosulfate. When the change from purple to pink is stable for more than 7 seconds, the titration is complete. Standardize the sodium thiosulfate by carrying a pure selenite or selenium dioxide through the appropriate steps of the above titration. h reagent blank should accompany a series of samples. After this paper had been completed, attention was called to an improved van der Meulen titration developed by XlcCullough, Campbell, and Krilanovich ( 5 ) . In the present method, which employs hydrobromic acid and about 1/200 as much potassium iodide as XIcCullough et al. use, only a fraction of the selenium being titrated appears as the element. The remainder combines colorless compound in the Sorris-Fay reaction. LITERATURE CITED
Coleman, W. C., and McCrosky, C. R., IND.ENG.CHEM., ASAL. ED., 9, 431-2 (1937). Hoffman, J. I., and Lundell, G. E. F., J . Research, Satl. But. Standards, 22, 465-70 (1939). Klein, A. K., J . Assoc. Oficial A p . Chem., 24, 363-80 (1941). Lyons, R. E., and Shinn. F. I,., J . Am. Chem. SOC.,24, 1087-93 (1902).
RlcCullough, J. D., Canipbell, T. W.,Krilanovich, S . J., ISD.ESG.CHEM.,ANAL.ED..18, 638-9 (1946). Painter, E. P., Chem. Rem., 28, 179-213 (1941). Pavlish, A. E.. and Silverthorn, R. W.,J . Am. Ceram. Soc.. 23, 116-18 (1940). Robinson, IT. O., Dudley, H. C., Williams, K. T., and Byers, H. G., IKD. EKG.CHEM.,ASAL. ED.,6, 274-6 (1934). Schemer, J. .4,*J . Research, S a t l . Bur. Standards, 16, 253-8 (1936).
Sherrill, >I. S.,and hard, E. F., J . Am. Chem. SOC.,50, 1665-74 (1928).
Silverthorn, R. W., Chemist-Analyst, 30, 52-4, 62-3 (1941). Smith, G. F., "Mixed Perchloric, Sulphuric and Phosphorio Acids and Their .4pplications in Analysis," pp. 10-51, Columbus, Ohio, G. Frederick Smith Chemical Co., 1934. R E C E I V E D October 2 , 1946
Saturated Potassium Hydrogen Tartrate Solution as a pH Standard JiAIES J. LINGAKE Department of Chemistry, Harvard University, Cambridge 38, .)lass.
SBTURATED aqueous solution of potassium hydrogen tartrate is a more convenient secondary standard for the calibration of pH-measuring' instruments than any of the buffer Jolutions generally used for this purpose. The preparation of the solution is extremely simple; it is only necessary to shake an excess of the pure salt wit'h distilled water of good quality for 2 or 3 minutes a t room temperature to obtain a solution whose p H is reproducible to 10.02 unit. Potassium hydrogen tartrate is available commercially in a high state of purity, and further purification is achieved easily by simple recrystallization from water. A sample of the C.P. commercial salt was recrystallized twice, and the pH values of eaturated solutions of the original material and the two recrystallizates agreed to * 0.005 unit. Since the pH of potassium hydrogen tartrate solutions is about 0.4 unit smaller than that of water saturated with carbon dioxide a t one atmosphere, the small amount of carbon dioxide dissolved from a normal atmosphere has no significant effect on the pH. Solutions of potaqsium hydrogen tartrate appear to be more stable than the commonly used potassium hydrogen phthalate solutions; a saturated solution showed an increase of only 0.03 pH unit after standing for a year in a stoppered Pyrex bottle.
However, since the preparation of the solution is so simple, it should be prepared freshly as needed, and not stored, so that the possibility of accidental contamination is avoided. Hitchcock and Taylor ( 2 ) determined the pH of an exactly 0.03 M potassium hydrogen tartrate solution by means of the hydrogen electrode and obtained a value of 3.567 a t 25" C. on the same empirical but thermodynamically consistent scale recently recommended by NacInnes, Belcher, and Shedlovsky (3, 4) and Bates, Hamer, Manov, and Acree (1). The writer found that the pH of a saturated solution of the salt (0.034 M a t 25') does not differ significantly from the foregoing value. For all pra,ctical purposes the value 3.57 * 0.02 for the pH of the saturated solution may be used. The influence of di!ution on the pH of a solution of potassium hydrogen tartrate, originally saturated at 25 was determined with the result shown by curve 1 in Figure 1. For comparison the dilution effect observed with 0.05 M potassium hydrogen phthalate is also included (curve 2). In this figure A pH i s the apparent difference in pH between the original and diluted solutions and it includes any effect resulting from changes in the liquid-junction potential between the saturated calomel reference electrode and the glass electrode half-cell. The dilution factor, O,
