Determination of Tin by a Modified lodometric Method T H O M A S B. M c D O W , K E N N E T H D. FURBEE, AND FREDERICK B. C L A R D Y Chemical Laboratory, Norfolk N a r y Yard, Portsmouth, Va.
An accurate method for the determination of tin is propored which avoids the uswl sources of enor encountered in precipitating tin as metastannic acid and those encountered In the reduction-oxidation procedure. The metal i$ dinolvrd in acid and, when necessary, collected with tho use of emmonia end aluminum hydroxide, reduced with nickel, and titrated with standard iodine solution under a blanket of carbon dioxide.
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T IS generally accepted that the best method for determining tin is based on the reduction of the tin to its bivalent state with a metal, snd subsequent oxidation with a standard iodine solution. Details of procedure vary widely and many difficulties are encountered that seriously affect the usefulness of the method. Sources of error encountered are: 1. Incomplete precipitation of metastamio acid in making separations from copper 2. Loss of precipitated m e h t a m i o scid in filtering 3. Failure to obtain a satisfactory titration end point 4. Incomplete reduction of the matei%l 5. Failure to prevent oxidation of stannous chloride by contact with sir of the reduced solution
It is necessary to ensure the complete reduction of the tin to the bivalent state and to prevent its reoaidatian hy sir. The former is not diffioult and can be awomplished by the use of several metals. "Ehccess in the latter depends on the maintenance of a nonoxidizing atmosphere during the entire operation, and is possible through the use of such expedients a8 the Bunsen valve or the use of a few grams of sodium oarhonste" (a). The precipitation of tin by the usual nitric acid procedure is known to be incomplete at times (4) because m e t a s h n i c acid often fails to coagulate sufficiently t o be completely retained upon filtration. By the proposed method, precipitation is complete without liltration loss. A series of determinations was made in order to select a metal to be employed &B the reducing agent. The metals used R.ere'
iron ( S ) , lead (G),
antimony (6).and nickel ( I ) . All thew metnip air cnpsble of cRecting the reduction of tin to the bivalent state. Nickel, alone, gave B very ahsrp end point. I t is unneressnry to rrmnvo the undiaolved nickel, thur eliminating fram the m d y . R atep which i3 B troublmome sonrcc of error. No elnim to originality is made as regards the individual details of the pmredure linnaily adoptd. Iloaever, because the hi.*
prdecdure outlined differs in some detail from any of those puhiishpd and the results obtained arc precise and accurate. it is b c liwed that the following method is t o bo preferred. REAGENTS REOUIRED
STAVDAEDTN SOLUTION. Dissolve 5 grams of purc tin in 100 ml. of concentrated hydrochloric wid and dilute to 1 liter i n s volumetric flssk. STLNDAEDIODINESOLUTION.Dissolve aooroximatelv 11 (rrams of iodine in about 100 ml. of distilled w&r contsiuihg 20 grams of potasrim iodide. Dilute t o 1 liter in B volumetric fl.tsk and >tnndardize s p i n i t rhc standsrd tin solution nftrr rrduction of 25 mi. o i the tin solution by the m d m d given b t h w STARCH Jo~mrox. Add a thin pmte made of 5 prnms of solublr starch. 10 zrams oi sodium bicarbonate. and wnter to about 300 ml.bf b o h g water. Boil for 1 minutk with stirring. Cool rapidly and dilute to 1 liter. PURENICKEL (shot or strip). PREPARATION OF SAMPLES FOR REDUCTION
BRASS,B R ~ N ZAND E COPPER BEAEINQMAmm.ue,(over 2% copper). Weigh i to 10 gmms of material t o give a tin content between 0.1 and 0.2 gram into 3Wml. Erlenmeyer flasks, sdd 10 t o 30 ml. of nitric acid (1to l),and heat gently until the m a t s rial is completely disintegmted. Boil until oxides of nitrogen are expelled, dilute .to ahout 100 ml. with water, and add 3 t o 6 ml. of 10% alummum nitrate solution. Add ammonium hydroxide to the blue copper complex color, then 10 ml. in excess. Heat to boiling and lilter through hard paper (Whatman No. 42 or comparable). Wash twice 4 t h 5% ammonium nitrate solution. Place the filter paper oontauung the recipitate in the original flask. Add 10 ml. of concentrated sulkrie acid, 5 ml. of concentrated erchlorio acid, and a few drops of concentrated n i t r i c mid. k a t gently, adding nitric mid dropdse a8 required to y v e n t darkening of the mlution. Evaporate to sulfur trioxide umes. Cool bat in air. then in water. and oroceed as directed SOLDER BEARINO METAZ, eto. (less than 2% copper). Weigh thesam~leto~ontsjnbetweenO.la~dO~mrunoftininto300-ml. Erlenmeyer Bask. Add 10 ml. of conEentmted sulfuric arid and about 5 grama of potawium sulfate and heat until the m a t e rial is rornolrtelv dissolved or until lead sulfate. if oresent. turns white. Gaol fir& ii9 sir, then in water, and p&& !d as d h t e d below. METHOD
.
Figure 1. AppareIus
. ..-
.
Carefully dilute Wlth warn M about 1W ml. add 75 ml. of eonoentrsted hydrochloric mid and 10 grams of kckel shot (10mesh), and connect flask with a tube, one end of which extends below the surface of the beaker of water. Boil gently for 30 minutes transferring the end of the outlet tube to a beaker of sodium bicarbmate solution (10%) several minutes before the end of the penod. Keeping the end of the tube below the surfsce of the sodium bicarbonate solution (Figure l), place the Bask in a suitable container of cold water. Allow to stand until cold. Remove the rubber stopper and, as rapidly as possible, introduce a small piece of dry ice (solid carbon dioxide). Then add more dry ice in sufficient quantity t o keep the solution very cold (below 10' C.) and blanketed with carbon dioxide gas. (If solid e8,rborbon dioxide is not avsilshle, pe!lets of sodium hiearbonate may be s u b stituted and will 've satisfaetory results if the solution is cooled by the use of ice7 Add 5 ml. of starch solution and titrate a t once agaiinst the atandardiaed iodine solution to a permanent blue end point. Calculs,te the percentage of tin in the ssmple.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
556
Table I. Determination of Tina Bureau of Standards No.
Material
Tin Present
Tin Found
%
%
Manganese bronze 0.82 Sheet brsas 1.013 37 53 Lead-basemetal 10.91 127 Solder 34.88 52 Caet bronze 7.90 63 Phosphor bropzs 9.91 540 Tin-base metal 88.61 Figures represent average of six determinations 82
a
0.82 1.012
10.91
34.87
7.90 9.91 88.01
Deviation
% 0.00 0.001
0.00
0.01 0.00
0.00
0.00
of each #ample.
DEXRlPTlON OF APPARATUS
The ap aratus consists of Erlenmeyer flasks (300-m1.), s t o p ered w i t i one-hole rubber stoppers. These are rovided with gent glaw and rubber tubing that extends to the gottom of the beakers containing, as they are used, water and bicarbonate of soda. The flask rests upon an electric heater that has a regulator e0 permit high, medium, or low heat adjustment. A ring stand,
Purification
OF
Vol. 16, No. 9
the base of which fits snugly under the heater, is provided with cross supports for the bent glass tubing. At the sides of the heater are two 80-ml. beakers partly filled with ice and water to facilitate rapid cooling. Table I shows results obtained on National Bureau of Standard samples of varying tin content. LITERATURE CITED
(1) Hallet, R.L., J . SOC.Chem.Id., 35, 1087 (1916).
(2) Hildebrand and Lundell, “Applied Inorganic Analyses”, p. 238, New York, John Wiley & Sons, 1936. (3) Lowe, A. H., “Technical Methods of Ore Analyses”, 9th ed., p. 221, New York, John Wiley & Sons, 1922. (4) Lundell and Hoffman, “Outline of Methods of Chemical Analyses”, p. 210, New York, John Wiley & Sons, 1938. ( 5 ) Lundell, G. E. F., and Schemer, J. A., J. IND. ENQ.CHEM.,14,426 (1922). (6) Stelling, E., Ibid., 16,346 (1924). TEEviews presented in this article are those of the writers and are not t o be construed as the official views of the Navy Department.
Solvents For Absorption Spectroscopy An Adsorption Method
MORRIS M. GRAFF, ROBERT T. O’CONNOR, AND EVALD L. SKAU Southern Regional Research Laboratory, N e w Orleans, La.
A simple, rapid method for removing ultraviolet-absorbing impurities from hydrocarbon solvents by selective adsorption on silka gel columns is described. Solvents suitable for use in absorption spectrum measurements have been prepared b y this method from commercial samples of cyclohexane, n-heptane, iso-octane, SkellysolveB, and Skellysolve-F. In general, hydrocarbon solvents, both synthetic and commercial, which have been subjected to exhaustive chemical and physical purification have been noticeably improved b y this adsorptive treatment. The advantages of the adsorption method over the usual methods are speed, simplicity of technique, and high yield of purified solvent.
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N RECENT years the advantages of liquid hydrocarbons over polar liquids a’s ultraviolet absorption solvents have gained recognition (I). The need for the use of pure hydrocarbons such as synthetic n-heptane, cyclohexane, and isooctane instead of the commercially available petroleum fractions, which are hydrocarbon mixtures, has been demonstrated in specific examples (2, 5, If). I n this connection the purification of liquid hydrocarbons has been investigated. The present methods for the preparation of hydrocarbons for use as solvents in absorption spectroscopy (5,8,9,11) are as a rule long and cumbersome and result in poor yields. Usually the final products still contain significant amounts of impurities, probably aromatic and unsaturated compounds, which absorb radiations in the ultraviolet region. The present paper describes a simple method for direct purification of commercial hydrocarbon solvents based on selective adsorption of the impurities by means of a suitable adsorbent. The adsorption procedure waa suggested by the work of Mair, White, and others (4, 7, IO) who, in connection with an investigation of the composition of petroleum distillates a t the National Bureau of Standards, showed that aromatic hydrocarbons can be separated from naphthenic and paraffin hydrocarbons by adsorption on silica gel.
The results which the authors obtained demonstrated the effectiveness of silica gel for purifying not only synthetic nheptane, cyclohexane, and iso-octane for absorption spectroscopy, but also petroleum ether fractions (Skellysolves), if desired. I n general, both synthetic and commercial hydrocarbons, even after they have been subjected to exhaustive chemical and physical purification, have been noticeably improved in ultraviolet transparency by adsorptive treatment. APPARATUS AND PROCEDURE
The adsorption apparatus used consists of a glass tube, 120 cm. in length and 38 to 40 mm. in diameter, constricted at the lower end. A small plug of glass wool is laced on a perforated porce lain disk a t the constricted end o f t h e column and about 400 grams of silica gel, Davco 659528-2000 (manufactured by the Davison Chemical Cor oration, Baltimore, Md.), are introduced with the aid of a pow& funnel in batches of about 100 grams. The tube is tapped occasionally to ensure ood settling of the adsorbent. Another plug of glass wool is pkaced on top of the column to prevent agitation of the adsorbent by the pouring of the solvent to be purified. The solvent is added to the column from a 2-liter separatory funnel, care bein taken not to allow the top of the column to run dry before a b the solvent has been added. The solvent is allowed to percolate through the column and the percolate is collected in the same manner as the successive fractions of a distillation. The first fraction is in all cases the purest sample; successive fractions are acce table until the adsorbent has become saturated with respect to t l e impurities. A test spectrogram is made to ascertain the extent of purification in,the successive percolates. Usually a single passa e of the liquid through the column suffices to produce a satiskctory ultraviolet-transmitting solvent, and a yield of about 90 to 9574 of the original liquid is obtained. A simple distillation may be used to remove any adsorbent . Various experiments were performed to determine the maximum amount of liquid which could be purified with a given quantity of silica gel and other adsorbents. However, this value was found to vary according to the activity of the particular adsorbent, and even more widely with the amount of impurities to be removed from different solvents or the same type of solvent
