Because approximately 5 nil. of liquid are required to wet the sieve properly and liquid volumes larger than 10 ml. give a liquid level above the sieve, the volume of sample used should be within the 5- to IO-ml. range. With a highly paraffinic stock, where a 5ml. sample may contain more than 1.0 ml. of n-paraffins, the stock should be diluted with cyclohexane. I n this case, a dilution such that a sample of 5 to 10 ml. contains about 1 ml. of n-paraffin has given optimum accuracy. Application of thc technique to several prepared sampleq is illustrated in Table I. The n-paraffins were diluted with cyclohexane as indicated and a 24-hour adsorption time was used. The standard deviation for this set of measurements is 0.016 ml. Each volumetric flask wap calihrated with the pipet and buret used in the nicthod to minimize the volumetric
errors. Kithin the accuracy of the volumetric measurements. the adsorption of the n-paraffins in the range studied appeared to be quantitative. Thus by suitable dilutions the technique may be applied to hydrocarbon mixtures representing the full range of n-paraffin content. Consideration was given to the effect of volume change on mixing unlike molecules. as there is grnerallg a small increase in volume when hydrocarbons are mixed. This could occur during dilution of a sample or adsorption of the n-paraffins from the sample. and on the addition of Cyclohexane in the final step of the procedure. Thr volume changes could result in a measurable error. but they would tend to be conpensating. I n one test of this volumc change. 10 ml. of benzene mere added to a 25-ml. volumetric flask containing
10.00 grams of the Molecular Sieve and the volume of cyclohexane required to fill the flask to the mark was compared with a blank on cyclohexane. Because the adsorbent tends to prevent mixing of the hydrocarbons, the increase in volume was much less than would occur in its absence. The volume increase of 0.02 ml. which was detected was, in fact, within the experimental error. As the benzene-cyclohexane mixture represents a rather extreme case for volume changes, for practical purposes, the effect may be neglected. Typical analyses on two paraffinic stocks were as follows: 18.8, 18.5, and 19.2 volume % n-paraffins for one stock, and 23.3, 23.3, and 25.0 volume % n-paraffins for a second stock. These samples were diluted to give 40 volume % solutions with cyclohexane and 10-ml. samples of the dilutions rvere used for analyses.
Preparation of Silver Crucibles for Determination of Iron in Silicates Edward B. Buchanan, Jr., and Harvey Diehl, Department of Chemistry, Iowa State University, Ames, Iowa
HAT erratic results are obtained for T i r o n in silicates by determinations involving fusions in platinum crucibles has been noted on several occasions (1, 9.4 ) . Shell (qj and later Collins, Diehl, and Smith ( I ) found that the difficulties were obviated by effecting the fusions (sodium carbonate, sodium carbonate-sodium tetraborate, lithium metaborate) in cruciblcs of silver. A trace of silver is dissolved during the fusion and is removed as the chloride with any precipitated silica after taking up the melt in hydrochloric acid. The crucibles recommended h a w thick walls, 2.5 to 3 mm., to minimize the danger of overheating and melting the silver. Such crucibles were made by casting molten silver into a graphite mold (4) or by electrodeposition onto a nickel crucible which was later dissolved away with hydrochloric acid as described by Collins Dichl, and Smith (1). .Ilthough the c,l(~ctrocl(.position (1) u a s made froni R cyanide bath ahich normally results i n a smooth, bright deposit of CilT-er, in forming a massive objrct s w h a': n thick-walled crucible, warts and dendritic grouths form even with vigorous stirring. We have now found that the periodic, reverse current electrodeposition proccdure of Jernsted (Sj leads to a smooth, dense deposit and 1s ideal for the preparation of crucihles for analytical work.
Apparatus and Materials. T h e dirert current for the electrodeposition n a s supplied by two H e a t h kit
12 16
ANALYTICAL CHEMISTRY
battery eliminators (Heath Co., Benton Harbor. Mich.). TTO separate sources were employed so t h a t both t h e forward and reverse currents could be varied independently. The timing cycle was controlled by a Wilson repeat cycle timer (C. G. Wilson & Co., Chatham, N. J.). This timer functions as a dipole-dithrow switch, the length of time spent' in either position being adjustable. A 5000-r.p.m. variable speed motor in which the chuck made electrical contact with the frame was used to rotate the crucible and provide stirring. Addit'ional stirring in the viciniby of the anode was provided by a magnetic stirrer. The 50-ml. nickel crucible used mas t'n-isted firmly in place on a one-holed rubber stopper mounted on the end of a brass rod. The brass rod served as a shaft and was mounted in the chuck of the stirring motor. Electrical connection between the brass shaft and t,he nickel crucible ]vas made by means of a coil of wire extending below the stopper. The plating solution was prepared by dissolring 30.5 grams of silver nitrate, 36.5 grams of sodium cyanide, and 112.5 grams of potassium nitrate in 1 liter of distilled water. The solution was filtrred to remove any reduced silver formed upon the addition of the sodium cyanide. A stationary solid silver bar iyas used as anodc. A definite disadvantage was found t,o enclosing the anode in a tightly wox-en cloth bag. for this led t o a suspension of fine silver in the electrolyte and the production of warts and dendritic growths on the crucible. Plating Procedure. The nickel
crucible was rinsed with acetone and distilled water and placed immediately in t h e plating bath. T h e assembly was adjusted t o rotate smoothly before beginning the plating, as a n y later adjustment or handling of t h e crucible usually led t o the production of flaws. T h e initial plating was done with t h e crucible negative and a current of 0.5 ampere. After a thin layer of silver had been deposited, the repeat cycle timer was started and adjusted to provide a plating current of 2.0 amperes for 7.5 seconds and a reverse or stripping current of 4.0 amperes for 1.0 seconds. The deposition was continued for about 24 hours and the weight of the silver deposited was 90 to 150 grams. The nickel crucible was then dissolved in hot dilute (1 : I) hydrochloric acid. The crucibles produced in this manner varied in thickness from 2.0 to 3.0 mm. The inner and outer surfaces were smooth but not polished. Occasionally small blisters appeared when the crucibles were fired. These were not always detrimental and were much less frequent than when steady direct current was used for the plating. LITERATURE CITED
( 1 ) Collins, P. F., Diehl, H., Smith, G. F., k A L . CHEM. 31, 1862 (1959). ( 2 ) Ellestad, R. B., Lithium Corp. of -4merica, Bessemer City, N. C., private
communication, 1958.
(3) Jernsted, G. W., Metal Finishing 4 5 , Xo. 2 , 68 (1917). (1)Shell, H. R., ANAL. CHEM.2 2 , 326
(1900).
