absorption spectrophotometry, providing an adequate sensitivity for these determinations. Calibration curves made by dilution of standard stock solutions were linear in the range of work, i.e., 0.05-10 mg I.-' for copper; 0.03-10 mg 1.-' for lead; 0.01-5 mg 1.-' for nickel; 0.02-1 mg 1.-l for zinc, and 0.015-1 mg 1.-l for cadmium. All the absorbance measurements were made at constant temperature (25.0 f 0.5 O C ) and were repeated about twenty times. This procedure enables us to give an average of the experimental values. Summary of the results are gathered in Table 11. In this treatment, Equations 6 or 9 are used with molar concentrations of species, instead of activities. This involves the assumption that the work is done in dilute solutions. It should be noted that the constants determined by the atomic absorption method at very low ionic strength should approach the thermodynamic solubility constants. When working in sea-water (average ionic strength: 0.7M) snluhility of precipitates will increase slightly. According to the various procedures published in the literature, when sea-water or natural-water are analyzed for trace elements with APDC, insoluble compounds are generally formed at the submicrogram levels (with kinetic considerations apart) but remain often invisible due to the very small quantities present in solution. These insoluble substances dissolve in organic solvents such as ketones and esters (MIBK, butyl acetate, etc.) and allow the use of solvent extraction which is probably the most useful technique for concentration of metals from dilute solut,ions aqd is a very convenient, way to improve sensitivity and remove interferences. As pyrrolidine dithiocarbamic acid (PDCH) is a weak acid with a dissociation constant around K A = 10-:3.2 ( 1 1 , (11) K. I. Aspiia, C. (1973).
L. Chakrabarti,
and V. S. Sastri, Anal. Chem., 45, 363
IZ),solubility S of the precipitates are pH-dependent. The following expressions can be deduced: S = [M2+] = f/2{[PDC-]
+
[PDCH]}
(12)
or
For pH values greater than ~ K Asolubility , will be minimum while for pH values smaller than PKA solubility will theoretically increase proportionally with the acidity of the medium. These pH variations have not been studied experimen tally.
CONCLUSIONS It is clear from the conditional solubility product constants that APDC forms very insoluble precipitates of the same degree of stability for all the divalent metal ions investigated. The stoichiometric composition of the precipitates is M(PDC)2. Zinc and copper are slightly more soluble by two orders of magnitude. If it can be assumed that the molecular solubility can be neglected compared to the ionic forms, conditional solubility products are identical to solubility constants commonly used in the literature. From the experimental stand-point, results show that it is best to add first the solvent, then the chelating APDC solution and mix right away. This procedure might prevent the formation of precipitate or a t least might facilitate a better dissolution of the water-insoluble matter in the organic phase.
RECEIVEDfor review January 14, 1974. Accepted June 18, 1974. The authors are grateful to the National Research Council of Canada for the financial support of this work by Grant A 6408. (12) R. Zahradnik and P. Zuman, Collect Czech. Chem. Commun, 24, 1132 (1959).
Semimicro Analysis for Silicon in Textiles Arthur Bradley and Domenica Altebrando Surface Activation Corporation, 1750 Shames Drive, Westbury, N. Y. 7 1590
The standard method for determination of silicon in textile materials involves oxidative decomposition in an acid (sulfuric-nitric-perch1oric)"wet ashing" environment, followed by evaporation of the acids and weighing the silica residue ( I , 2). An alternate gravimetric method utilizing 8hydroxyquinoline (oxine) is preferred by some authors (3. 4 ) . For small samples of low silicon content. this procedure requires repeated ignitions of a platinum crucible and contents to constant weight and skillful use of a microbalance. We have found it convenient to determine silicon in textiles by a colorimetric method first described by Isaacs ( 5 ) and later discussed by many authors (6-8). (1) G.M. Petty. Anal. Chem., 28, 250 (1956). (2) AATCC Monograph No. 3, "Analytical Methods for a Textile Laboratory," Research Triangle Park, No. Carolina, p 159. (3) J. A. McHard, P. C. Servais. and H. A. Clark, Anal. Chem., 20, 325 (1948). (4) J. H. Walters, and R. C. Smith, Anal. Chem.. 41, 379 (1969). (5) isaacs. Bull. SOC.Chim. Biol., 6, 157 (1924). (6) J. H. Foulger, J. Amer Chem. Soc, 49, 429 (1927).
Silicate reacts with ammonium molybdate in mildly acid medium to yield a fairly stable yellow silicomolybdic acid (7, 8). A blue color is formed when this solution is warmed with sulfite (6, 8) or benzidine (9, IO). The colorimetric method described by Horner ( 1 1 ) measures absorption at 815 mp. We found a broad absorption maximum with sufficient sensitivity to permit accurate assays anywhere in the red region of the spectrum, as long as the same conditions were employed in preparing the calibration curve. The wet ashing procedure can be shortened and simplified by charring the sample first (12).No sulfuric acid is re(7) H. W. Knudson, C. Juday. and v. W. Meloche. lnd. Eng. Chem., Anal. Ed., 12, 270 (1940). (8) H. L. Kahler, lnd. Eng. Chem., Anal. Ed., 13, 536 (1941). (9) Ref. 2, p 49. (10) F. Feigl, "Spot Tests in Inorganic Analysis," Fifth ed.. Elsevier. New York, N.Y.. 1958, p 335. (11) H. J. Horner, "Treatise on Analytical Chemistry," Part ii, Voi. 12, Interscience, New York. N.Y., 1965, p 287. (12) W. Garner, "Textile Laboratory Manual, Volume I, Quantitative Methods," Third ed.. Elsevier, New York, N.Y., 1966, p 3.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
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I 0 1
Table I. Comparison of Colorimetric and Gravimetric Silicon Analyses SLliion,
; I
1
/
/ /
Sample
Fabric
Colorimenic
Control Polyester 1 Rayon 2 Polyester 3 Cotton 4 Polyester 5 Cotton 6 Polyester 7 Polyester 8 Polyester 9 Polyester 10 Cotton 11 Cotton 12 Cotton
0.0 0.24 0.43 1.34 1.42 1.56 1.93 2.84 2.93 2.96 3.69 5.56 6.73
Gravimetric a
/
Difference, PO
/
/
"not detected" 0.27 0.39 1.20 1.50 1.37 2.30 2.99 3.39 2.31 3.62
5.56 6.60
Average of twelve
/
/
/
+ 12
I
/
-9 - 10
+6 - 12
19 +5 ~ 1 6 -22 -2 t
0 -2
110%
..'
Schwarzkopf Microanalytical Laboratory, Woodslde. New York, N Y
01
02
SILICON
03
01
0 5
'
mg/ml
Figure 1. Calibration curve
quired and often one or two careful evaporations of a little nitric acid result in a quantitative conversion to soluble inorganic salts. Twelve samples were prepared in this fashion and subjected to colorimetric analysis for comparison with the results determined gravimetrically by an independent specialist. The fabrics, which included cellulose and polyester, ranged from 0.24 to 6.73% silicon content. The agreement between our method and that employed by the professional microanalyst was excellent. The average error was &IO% with no systematic error (Table I).
EXPERIMENTAL Apparatus. Light absorption measurements were made with a Klett-Summerson Photoelectric Colorimeter, Glass Cell Model (Arthur H. Thomas Co., with test tube adaptor A45). Liquid samples were contained in standard 12- X 100-mm test tubes selected for matching optical clarity. An interference filter with maximum transmission a t 650 mp (Baird-Atomic, Inc.) was employed. The scale on this instrument is logarithmic, graduated from 0 to 1000. When adjusted to zero with solvent blank, the scale readings are directly proportional to concentration of the absorbent, for solutions obeying Beer's law. Calibration Curve. A stock solution is prepared by dissolving 1.00 f 0.01 g of reagent grade hydrated sodium silicate (NaaSiOa 9Hz0) in 250 ml of deionized water containing one pellet of sodium hydroxide. This solution, which contains 0.40 mg/ml of silicon, is stored in a polyethylene bottle. Further dilutions for temporary use are made by cutting to half strength and half again until a series of 4 solutions ranging down to 0.05 mg/ml are available for calibration of the colorimeter. A 1.00-ml portion of each standard was placed in a test tube and the following aqueous reagents were added in this order: 1.0 ml of 12%potassium chloride, 1.0 ml of 6% sodium acetate, 3.0 ml of 10% acetic acid, and 5.0 ml of 10% ammonium molybdate. The test tubes containing the standards and a reagent blank are warmed for 3-5 min by immersion in a water bath that is heated to near the boiling point. The potassium chloride is present only to simulate inert salts left by the wet ashing process, and is omitted when this procedure is applied to a fabric sample. Immediately after removal of each tube from the hot water bath, a 3.0-ml portion of saturated sodium sulfite solution is added. The blue color forms quickly. After about 15 min, when the contents of the tubes have cooled to near room temperature, each standard solution is diluted to 25.0 ml with deionized water. The instrument is adjusted to zero with the reagent blank (which should be virtually colorless) at 650 mp. About 8 ml from each standard is filtered into a Colorimeter sampling tube for measurement of absorption at this wavelength. The calibration curve prepared for the analyses reported here is shown in Figure 1. 2062
Wet Ashing. A sample of approximately 0.10 g of fabric is cut with scissors into small pieces, dried, and weighed to the nearest milligram. The fragments are placed in a small porcelain evaporating dish, covered with a crucible cover, and charred in a muffle furnace set at 500' for a period of 15 to 20 min. The dish is removed and allowed to cool, taking care not to lose any fragments of ash. Portions of concentrated nitric acid (5.0 ml) and potassium chlorate (0.3 g) are added and the dish is warmed on a hot plate to complete the digestion of the fabric. The charred residue is gradually converted to light colored salts, often to a clear solution. If all acid evaporates before digestion is complete, another portion of nitric acid is added and, if necessary, an additional increment of potassium chlorate. It is relatively easy to distinguish between charred fabric residue and inorganic salts in determining that the ashing step has been completed. To dissolve the solid silicate, evaporate the acid just to dryness (do not overheat!) and add 8-10 ml of 2% aqueous sodium hydroxide. A little warming may be necessary before the contents of the dish are washed into a 50-1111 flask with deionized water and diluted to the mark with the same. Procedure. This solution of unknown silicon content is suitable for processing via the steps outlined above for preparing the calibration curve. Thus, a 1.00-ml (or 2.00-ml) portion is transferred to a clean 15-cm test tube and followed with sodium acetate, acetic acid, and ammonium molybdate solutions in the amounts and strengths previously described. I t is customary to process several samples of duplicates of unknowns, at least one control or standard, and a reagent blank a t the same time, all immersed in the hot water bath together. Samples and standards are alike reduced with sodium sulfite solution, cooled, and diluted to 25.0 ml. If any salts precipitate at this time, the solutions are filtered before making the absorption measurements.
RESULTS AND DISCUSSION If silicon had been present in the fabric sample, the characteristic blue color of reduced silicomolybdic acid shows a t this stage. The color is stable for hours in the buffered medium. The absorption is translated directly to milligrams of silicon by reference to the calibration chart. The data plotted in Figure 1 relating silicon concentration in mg/ml (C) and absorption number ( N ) ,may be expressed as the following linear equation:
c = 5. 5,v x 10-4
(1)
As an example of how this equation is applied, an unknown specimen weighing 105 mg was digested and the recovered salts were dissolved in 50 mi at slightly alkaline
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
pH. A 2.00-ml aliquot was worked up as described above, yielding a deep blue 25-ml sample that gave an absorption number of 575, a little high for best use of the method. When the work-up was repeated with a 1.00-ml aliquot, the absorption number was 280, representing 0.154 mg of silicon. The unknown, therefore, had a total of 7.7 mg of silicon or 7.4% based on specimen weight. I t is not practical to take more than a 5.0-ml aliquot of the 50-ml digested unknown solution. If this amount yields too little absorption, it may be necessary to start again with a larger piece of fabric. A reliable assay for silicon can be obtained by this method for as little as 0.2% and a sure qualitative indication for 0.1%. Interference. This investigation did not include any study of interference, since it was applied only to fabric samples of known chemical composition, free of extraneous finishes. Phosphate has frequently been mentioned in the literature (7, I O ) as a possible complication, but Foulger concluded that phosphomolybdate does not give a blue reduction product when treated with sodium sulfite in the presence of acetic acid (6). Kahler (8)confirmed that in the p H range of 2.4 to 2.7 there is no significant interference by phosphate or iron. Nevertheless, Feigl ( I O ) recommends removing phosphoric and arsenic acids before carrying out a spot test for silicon involving molybdate and benzidine. Preparation of Silicon Impregnated Fabrics. This method was developed to monitor a laboratory program in which new routes to permanent silicone and silane attachments to various textiles were being explored. In a typical preparation, a doubleknit polyester specimen, approximately 15 X 20 cm, was extracted with toluene, dried, and weighed a t 8.72 g. I t was immersed for one minute in a solution of 150 g of vinyl triethoxysilane (Union Carbide Corp., catalog No. A-151) in 500 ml of toluene a t l l O o . The reagent solution was boiling and had been under reflux for a t least four hours prior to the fabric dip. The sample was dried in air, then washed twice with detergent in a standard home laundry cycle. When dried, it weighed 9.51 g. The silane weight add-on was 9.1%, corresponding to 1.5% silicon if the reagent remained intact. Colorimetric analysis gave 1.93% and gravimetric 2.30% (see Table I, sample 6) suggesting that some ethoxy groups in the silane were lost
Table 11. Dudicate Ashines - on Laundered Fabrics, % Silicon Cotton (1) (2) Average (3) g r a v i m e t r i c Polyester (1)
(2) Average
Unwashed
Z x LVashed
l o x Washed
3.72 4.15 3.93
3.83 3.75 3.79
3.75 3.62 3.69 3.62
2.42 2.42 2.42
2.42 2.75 2.59
2.74 2.60 2.67
en route. The different results of the two methods could be entirely due to non-uniform impregnation of the sample. I t was of interest to note that although the silane polymer could be gradually extracted with hot organic solvents (and was therefore not “grafted” in a true chemical sense), it was almost indefinitely stable to aqueous laundering. The sensitivity of the method was such that a slight increase in silicon content was detected on an unhemmed spun polyester sample that was washed ten times with detergent (below). I t is believed that non-silane coated fibers were preferentially lost in laundering. Reproducibility of Method. Sets of duplicate wet ashings and colorimetric determinations were carried out on samples of cotton and spun Dacron (100% polyester) that had been impregnated with vinyl triethoxysilane. Specimens for analysis were taken before the first wash and after the second and tenth launderings with household detergent. The largest discrepancy between any of the six pairs of results was 12% (Table 11). When averaged, they indicate a gradual decline in the silicon content of the cotton with washing and a similarly gradual increase in the polyester assay. Besides this evidence of internal consistency, there is a match with the gravimetric analysis: 3.69% us. 3.6296 for the 1OX washed cotton (also in Table I, sample 10).
RECEIVEDfor review May 7,1974. Accepted July 22,1974.
Determination of Stearic Anhydride in the Presence of lsopropenyl Stearate Michael F. Kozempel and James C. Craig, Jr. Eastern Regional Research Center, Agricultural Research Service, U S . Department of Agriculture, 600 E. Mermaid Lane, Philadelphia, Pa. 19118
Work is in progress in this laboratory to develop a commercially feasible process to make isopropenyl stearate (IPS) from stearic acid. Stearic anhydride is an undesirable reactor by-product. One constraint imposed by the process is that the stearic anhydride concentration be equal to, or less than, 2% by weight in the catalyst free product stream. The current method of determination for stearic anhydride, IR, is neither sufficiently accurate nor precise in this concentration range.
Johnson and Funk ( I ) reported a potentially suitable method for anhydride determination. They reacted morpholine with the anhydride and then back titrated the unreacted morpholine with HCl. They stated that ketene, diketene, and acid chlorides could interfere. The chemistry of isopropenyl stearate as an acylating agent is sufficiently similar t o these compounds that it could be expected to in( 1 ) J. 6.Johnson and G. L.
Funk, Anal. Chem., 27, 1464-5 (1955).
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