Rapid Method for Determination of Microgram Amounts of Silicon by Colorimetric Procedures Charles Hozdic, Division of Color and Cosmetic Chemistry, Bureau of Science, Food and Drug Administration, Washington, D. C.
ANEW, silica detennination has been devised in which SiF4 is RAPID
transferred from a sample solution to an absorbing solution by a current of dry air. The procedure is more rapid than that of Alon et al. (1, 2) in which the transfer is effected by diffusion. Ammonium molybdate is used as the absorbent; and p-methylaminophenol sulfate, recommended by Mullin and Riley (S), as the reductant. The silico molybdenum blue produced is measured photometrically. Using a single flowtrain assembly (Figure l ) , one determination can be completed in less than 4 hours, or 15 in 8 hours. Satisfactory results have been obtained with a modification of the prccedure of Alon et al. (2); however, the time required to carry out the diffusions is considerable (at least overnight). To expedite the determinations, therefore, a new procedure was devised. EXPERIMENTAL
Reagents. All reagents should be stored in polyethylene bottles. Distilled water was used throughout. ,4 fresh solution of acid ammonium molybdate was prepared weekly by dissolving 8 grams in 280 ml. of water, adding 24 ml. of concentrated HCl, diluting to 400 ml., and filtering. p-Methylaminophenol sulfate solution was prepared weekly by dissolving 10 grams in 480 ml. of water containing 6 grams of anhydrous NazSOI, then diluting to 500 ml. and filtering. The reducing agent was prepared weekly by combining 100 ml. of the p-methylaminophenol sulfate solution with 60 ml. of 10% oxalic acid solution and 120 ml. of 25% (v./v.) HzSO4 Table I.
I
I
\
Figure 1.
04
Diagram of the absorption train
solution, then diluting to 300 ml. with water. The standard sodium silicate solution, 100 gg./ml., was prepared by fusing 0.1 gram of Si02 (Matthey Specpure, Jarrell-Ash Co., Newtonville, Mass.) with 5 grams of Na2C03 in a 30-ml. platinum crucible, then dissolving the fused mass in 30 ml. of water (using no acid) in a platinum dish on a water bath, cooling, transferring the solution to a 1-liter volumetric flask containing approximately 900 ml. of water, and filling to the mark with water. The hydrofluoric acid reagent, a proximately 300 kg. of fluorine per $, was prepared by weighing 0.658 gram of 48% H F in a plastic container and diluting to 1 liter in a previously calibrated polyethylene bottle. Special sulfuric acid was prepared by heating concentrated H2S04 in a platinum dish a t fuming for 15 minutes and cooling in an open desiccator to minimize the reabsorption of water. A blank solution was prepared by fusing 5 grams of Na2CO3in a platinum crucible, dissolving in water, and diluting to 1 liter with water. Apparatus. A Cary Model 14 recording spectrophotometer with 5-cm. path length cells was used.
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Comparison of the Results from Three Techniques Used in the Analysis of Silicate Samples
Material Pyrophyllite: 15.6% Alios, 0.13% MgO Kaolin: 39.0% Al2Oa Talc:
/i
WN
1.570 Alzos 1 . 4 % Fez08 32.3% MgO 1.370 CaO
Gravimetric ( 4 )
Diffusion
Absorption
77.6 79.0 78.8 45.6 45.1 44.6 57.8 56.9 57.0
78.7 75.6 75.6 40.7 48.6 46.4 56.5 60.0 58.0
78.0 78.3 77.7 42.7 43.7 44.3 59.0 59.7 59.0
These samples represent, in each case separate portions of single lots of pyrophyllite, kaolin, and talc carried through the entire procedure. 1626
ANALYTICAL CHEMISTRY
Plastic pipets (1 in. l/lm ml.), centrifuge tubes (50 ml., polypropylene and Lusterloid, A. H. .Thomas Co., Philadelphia, Pa.), and clear plastic vials (50 ml. with caps and shoulder, Spex Industries, Inc., P.O. Box 98, Scotch Plains, N. J.) were used. Borosilicateglass flasks were used only to obtain final measurements. The flow meter was operated at 0.725 liter per minute. The absorption train is described in Figure 1. The direction of flow is from inlet 0 to exit 1. Polyethylene tubing ( M ) , I/,s inch i d . , is used with rubber connectors ( L ) . Tubes A , B, and D contain anhydrous CaClz; tube C contains soda lime. Generator tube E contains 2 ml. of special H2S04 plus 0.5 ml. of sample, and 0.5 ml. of HF reagent added just before analysis. Absorber tube F contains 15 ml. of specially concentrated HtsO4; absorber tube G contains 3 ml. of acid ammonium molybdate and 12 ml. of water. Tubes E , F , and G are heavy-walled polyethylene; the others are clear Lusterloid plastic. H is the flow meter; I , a vacuum; J , a perforated disc (controls excess foaming); K , a water bath; and N , the H F reagent intake (the tube has a l/Anch opening a t bottom and is extended 0.5 inch above the solution). All other inlet tubes have six '/Isinch perforations a t the bottom, except inlet to tube F which has one 3/l&xh opening. Dry air must be bubbled through the sample solution and all absorbing solutions. Procedure. To obtain d a t a for t h e standard curve add, respectively, to seven 50-ml. volumetric flasks, 0, 5, 10, 20, 30, 40, and 50 ml. of the standard sodium silicate solution (100 pg./ml,). Dilute each to 50 ml. with water and store in plastic vials. Prepare absorber tubes F and G in advance. Partially prepare generator tubes E by adding all but the H F reagent. (;illow 3 minutes for excess Na2C03 decomposition in tube E.) Position absorber and generator tubes
in the train and adjust the air flow to 0.725 liter per minute. Raise the water bath to contain tubes E and F. After 1 minute remove the HF inlet stopper at N (with air flow on) and add 0.5 ml. of HF reagent to generator tube E. Replace the stopper quickly. After 15 minutes (with air flow on and to llrevcnt any backwash) remove the H F inlet stopper, disconnect the vacuum pump, loosen the stopper in tube F , and remove absorbing tube G. Rinse the inlet bubbler into tube G with 10 ml. of water. Pour the contents of tube G into a 5O-ml. plastic vial, add 15 ml. of reducing agent, and mix for 15 minutes. Allow this to stand for 2 to 3 hours (99% of the color develops in 2 hours). Remove generator tube E and rinse the inlet bubbler with acetone. Absorber tube G with a clean and dry inlet bu1)l)ler can be replaced and readied for the next analysis. Change the special H2S01 acid in tube F after eight determinations. After 3 hours of development, transfer the samlde from the plastic vial to a 50-in1. volumetric flask and fill to the mark with water. Measure the color intensity on the spectrophotometer between 750 and 850 inp. Plot the a1)sorl)ance at the peak (approximately 812 mp) against the concentration of Si02 in micrograms for the standard curve. The procedure for samples is the same as any one determination above. High concentrations of silica are controlled by diluting the solutions to the 5- to 50-pg. level.
DISCUSSION AND RESULTS
Various solutions containing KOH, NH40H, saturated CaO, Ca(OH)*, Ba(OH)*, BaCL, and distilled water were tried as absorbents for SiF4. Because water proved t o be best, the procedure was simplified and time was saved by using aqueous acid ammonium molybdate as the absorbent. Thus, the SiF4 was simultaneously absorbed and converted to silico molybdic acid during the 15 minute absorption. Experiments proved that absorption periods less than 15 minutes gave low results. Also, a t least 10 minutes was necessary for the conversion of the SiFl to silico molybdic acid ( 3 ) . It was determined from a yield curve that 0.5 ml. of the HF reagent was enough to drive the reaction to completion under the conditions of the experiment. Various temperatures were tried in the water bath. Above 90' C., the polyethylene tubes were too pliable and too hot to handle. A 70' C. constant temperature water bath was therefore adopted. Dry air was the only medium used as a carrier in the experiments. Two separate fusion samples were used to obtain data for the standard curve. One determination at each level of concentration was made on each fusion on 3 separate days (an average of six determinations for each point on the curve). The result was a linear curve.
Table I shows good agreement in the results of the three techniques used in the analysis of the silicate samples despite the fact that they contained other elements in high concentration, that they had to be fused, and that the diffusion and absorption samples mere determined in the microgram rangc. The results obtained by absorption were in the range of 25 to 40 pg. The above technique permits rapid determinations of micro amounts of silicon in aqueous solution, or in samples in which the silicon is made soluble I)y alkali fusion. The success of the absorption technique for the determination of SiOv depends ul)on three important factors: Dry air must be bubbled through the solution in generat'or tube E and in absorption tubes F and G. No condeiisation should be present in the exit tubing from the generator to the absorber lest the SiF, be premat,iircly absorbed. Specially prepared concentrated H2SOl must be used in the generator and in the tube just behind the generator. LITERATURE CITED
( 1 ) Alon, A., Bemas, B., Frankel, lI., Anal. Chzm. n c / a 31, 279 (1964). ( 2 ) Hozdic, C., Ibzd , 33, 567 (196.5). (3) Miillin, J. B., ILiley, J. P., Zbzd., 12, 162 (19%). (4) "Standard Methods of Chemical Analysis," 6th ed., Vol. 1, p. 956, N. 11.
Furman, ed , Van Nostrand, New York, 1062.
Apparatus for Rapid Degassing of liquids Rubin Battino' and F. David Evans, Chemistry Department, Illinois Institute of Technology, Chicago, 111.
arc iiiimerous occasions when Teslic~iincnt~al studies require the )arat>ion of gas-free liquids. HERE
1iiq
alq)aratus for the rapid and efficient degassing of a variety of liquids in quant8iticsup to 1 liter is described in t,his I q r r . The apparatus was develolid s~mifically for measureinents of the solubilities of gases in liquids, but it should lirove useful for other pur1)oscs also.
G a m lrave l~ecnrcinoved from liquids :I niinibcr of incthods which can be cat ygoi'izcd as: (a) simple boiling; (b) ev:icuation above thc liquid with or without heatring; and (e) evacuation above the frozen solvent. I3unsen ( 3 ) , follo\retl by many other workers boiled thc solvciit and t.hrn allowed it to cool untlc,r vacuu~n. Leduc ( 7 ) found that e v m aTtclr 1)rolongedl)oiling, water gave 111) g : 1)iil ~ )l)lw on fiwzing. Succwsivc fi,ccsziijg ant1 iridting untlrr vacuuni is an cxffrctive i?ietlicitl of degassing, but it is rat1ic.r inconvrnirnt for largr samliles 1)y
and frequently requires more than six phase-change cycles even with small sain1)les. Hihben (6) employed the method of sublimation in vacuo, and Paunov (8) made use of the observation that ultrasonic frequency radiation affects the solubility of gases in liquids. I3aldwin and Daniel ( I ) degassed the oils they were studying by permitting the oil to drip into an evacuated chamber. They reported a n efficiency of 97-98Cr, which is insufficient for Irrcision work. (:Lever et d.(4) ptirii~)ed on the boiling solvent until 10-2UY0 of the liquid was evaporated. The liquid was t.hen transferred to another vessel and sprayed through a fine nozzle into a n evacuated flask. Although this procedure was effective, the transfer st,rl) and tlic single stage s1)raying could be iin])rovcdupon. Cook ( 5 ) degassed his solvents by evacuating above the boiling solvent and removing about 20y0 of the solvent. He checked for completeness of (le-
6061 6
gassing by two techniques: cushioning the liquid between mercury and looking for undissolved g w buhl,les, and following the degassing by observing a thwmocouple gauge placed with a cold trap between the boiling solvent and the gauge. The solvent was considered to be degassed when the pressure on tlic thermocouple gauge reached the base pressure of the vacuuni system. (Tllc boiling was continucd :rntl some ad(litional qolvciit,reinovcd T:iylor (9) cnil )loyctl a circiiht )I y system coupled with a coiidciisiirg coluinn leading to vacuuin. The circulation was achieved by heating the liquid in a side arm of the main vessel, and a mixture of bubbles and liquid moved from the side arm into the inain vcwel. The disadvaiitnqv of tlii-: l i i ' o ccvl I irc ivrre that heat-scti1-i 1 ivc 1i (11 I i t 1s Present address, Chemistry Department, Wright Stnte College, Dayton, Oliiri
Vr'l
4XIl '! '
11.
0\+'3BER
1966
1627
