1251
V O L U M E 23, NO. 9, S E P T E M B E R 1 9 5 1 solution is boiled gently for 15 minutes, while the volume is kept approximately constant. Enough water is then added to bring the volume to about 800 ml. and 2.50 grams of twice-recrystallized linear A-fraction potato starch are added slowly with stirring to the gently boiling solution. After complete solution of the starch, or after approximately 5 minutes of stirring, the solution is filtered with suction through several thicknesses of dense (“barium sulfate retention” grade) filter paper. Enough water is added to bring the volume to exactly 1 liter, after the solution has cooled to room temperature. Inasmuch as the extraction of the crude linear A-fraction starch may exceed the facilities of some laboratories, the simpler method described by Krishnaswamy and Sreenivasan (9) might prove satisfactory, although this method has not been attempted. At any rate, two recrystallizations from hot aqueous solution saturated with 1-butanol should follow any extraction method used. Detailed directions for the extraction and recrystallization are to be found in publications b j Schoch (11, 18, 19). The crystalline linear starch so prepared is stable when kept dry. As the concentrations of starch and of cadmium iodide adopted in the original recipe for the reagent proved satisfactory in every respect, no changes have been attempted. The amounts of starch and iodide found by Gross, Wood, and McHargue (6) to give the optimum conditions for color development with arrowroot starch and potassium iodide, were transposed by using a weight of cadmium iodide equivalent to that of the potassium iodide in potential iodide ion concentration. The linear A-fraction potato starch used throughout this work was supplied by T. J. Schoch, Corn Products Refining Co., Argo, 111. Undoubtedly the linear starch fractions would become commercially available if there was sufficient demand for them. The reagent should be stored in brown glass bottles if it is to be kept for long periods of time, but small amounts may be kept in clear glass bottles for convenience. ACKNOW LEDGM EhT
The author wishes to acknowledge especially the contributions of Paul Arthur and Thomas E. Moore, Oklahoma iigricultural
and Mechanical College, Stillwater, Okla., with uyhom he worked on a research project supported by a grant from the National Institutes of Health, United States Public Health Service, during which the cadmium iodide-linkar starch reagent was developed. LITERATURE CITED (1) Arthur, Moore, and Lambert, J . -4m. CAem. Soc., 71, 3260 (1949). (2) Baldwin, Bear, and Rundle, Ibid., 66, 111-5 (1944). (3) Doucet, Compt. rend., 207,362-4 (1938). (4) Goading and Walton, J . Phys. Chem., 35, 3612-7 (1931). (5) Green and Schoetaow, J . Am. Pharm. Assoc., 22,412-3 (1933). (6) Gross, Wood, and McHargue, ANAL.CHEY.,20, 900-1 (1948). Haldar, J . Indian Chem. Soc., 23, 205-10 (1946). Howells, J . Chem. SOC.,1946, 203-6. Krishnaswamy and Sreenirasan, J . Bid. Chena., 176, 1253-61 (1948). Lambert, Arthur, and Moore, - 4 s ~CHEM. ~ . 23, 1101 (1951). Lansky, Kooi, and Schoch, J . Bm. Chem. SOC.,71, 4066-75 (1949).
Latimer and Hildebrand, “Reference Book of Inorganic Chemistry,” revised ed., p. 132, iYew York, Maemillan Co., 1940. Liebhafsky and Sharkey, J . Am. Chem. Soc., 62, 190-2 (1940). hIcBain, 2. EZektrochem., 11, 215 (1905). Pieters and Hanssen, Anal. Chim. Acta, 2, 712-26 (1948). Rowe and Phelps, J. Am. Chem. Soc., 46,2078-85 (1924). Rundle, Foster, and Baldwin, Ibzd., 66, 2116-20 (1944). Schoch, “Advances in Carbohydrate Chemistry,” Val. I, ed. b y Pigman and Wolfrom, pp. 247-77, Sew York, Academic Press, 1945. Schoch, J . Am. Chem. Soc., 64, 2957-61 (1942). Schoch and Jensen, A s . 4 ~ CHEY., . 12, 531-2 (1940). Scett, J . Am. Water Works Assoc., 26, 634-40 (1934). Stein and Rundle, J . Chem. Phys., 16, 195-207 (1948). U. S. Pharmacopoeia, Tenth Decennial Revision, p. 35, Philadelphia, J. B. Lippincott Co., 1926. Van Kame and Brown, Am. J . Scz., 44, 105-23 (1917); 44, 453-68 (1917). Tan Winkle and Christiansen, J . 4nz. Pharm. Assoc., 18, 124750 (1929). RECEIVED December 26, 1950. Contribution C 438 from t h e Department of Chemistry, Kansas State College.
linear Starch Reagents Linear Starch-Iodate Reagent Selective for Iodide Ion JACK L. LAMBERT, Department of Chemistry, Kansas State College, Manhattan, Kans. This study was made on a colorimetric reagent containing linear A-fraction potato starch, iodate ion, and cadmium ion which, at the proper pH, indicated a selectivity for iodide ion. The colorless reagent so prepared proved to be stable for periods up to 6 weeks and to be selective for iodide ion when used in a solution of a weak acid such as formic. The reaction of iodate ion and iodide ion to produce triiodide ion (I,-), which reacts with the linear starch to form the well-known blue complex, gives repro-
T
HE development and application of a stable and reproducible linear starch-cadmium iodide colorimetric reagent (1) suggested the possibility of a reagent containing linear “A-fraction’’ starch and iodate ion which would be selective for iodide ion. In previous work ( 2 ) involving the study of iodate ion (produced by oxidation of iodide ion) as an interference, it was observed that iodate ion is a particularly sluggish oxidizing agent toward most reducing substances except iodide ion when the pH is not extremely low. The chemistry involving the use of iodate ion to oxidize iodide
ducible and quantitative results. Yery few substances were found to interfere under such wnditions. The rate of color development and the effect of temperature were studied, and the extinction coefficient was calculated. This reagent makes possible the selective colorimetric determination of iodide ion in very dilute solution. Analytical procedures for very small amounts of iodide ion are of current interest in such fields as medicinal chemistry and water purification.
ion to form iodine and the combining of the iodine with excess iodide ion to produce the color-forming triiodide (Ia-) ion is not stoichiometrically advantageous. However, the very intense blue linear starch-triiodide ion complex permits the determination of iodide ion in concentrations of the magnitude of 10-4 P. A study of the reactions indicates that eight iodide ions are necessary to produce three triiodide ions:
513181-
++ +
108312 108-
+ 6H++ +312 + 3Hz0 31,+ 6H +318- + 3HzO +
ANALYTICAL CHEMISTRY
1252 Further oxidation of the iodide in the triiodide ion proceeds a t a very slow rate and is negligible under the conditions of the following procedure. Presumably the triiodide ion is somewhat protected from oxidation by its poiition inside the linear starch molecule helix. The present study also shows that an excess of iodide ion is not necessary for proper formation of the blue complev DEVELOPRiEhl OF THE RE4GENT
Iodate ion is inert in solutions where the pH is not lo\\ and i q an oxidizing agent only in acid solution. In order to obtain a stable reagent containing linear starch, the resulting solution should not only have the pH (5.9 to 6.3) recommended by Schoch (4)to prevent hydrolytic breakdown of the starch on long standing, but must also be protected against the growth of microorganisms. Cadmium iodide had been found to meet both rpquirements in the development of the linear starch cadmium iodide reagent, but could not be used in this reagent. The chloride and bromide salts of cadmium appeared to permit more rapid retrogradation of the linear starch in solution than did cadmium acetate. Cadmium acetate, which, in the concentration of 1.0 gram of the trihydrate per liter of reagent specified for this reagent, produced a stable linear starch solution with a pH of 6.3 to 6.5, was found to be satisfactory in retarding mold growth, Other mold inhibitors may prove superior to cadmium acetate. Potassium chloride in 15% concentration, which was suggested by Beans in a personal communication to Schoch (S), is fairly successful in preventing mold growth but introduces a concentration of chloride ion which is high enough to cauqc some precipitation of the linear starch-triiodide ion blue coniplev a t the higher concentrations of iodide ion studied.
Cadmium ion approaches inerruric ion in toxicity and has the important advantage of not precipitating an insoluble iodide salt. In an accelerated test to determine the effectiveness of cadmium ion in preventing mold gro-n-th, 75 ml. of a solution containing the concentrations of linear potato starch and potassium iodate specified for the reagent, together with 1.0 gram of CdC12.21/2H20 r r liter of solution, and 75 ml. of another solution containing t e h e a r starch alone, were exposed together in open dishes for 6 hours. The solutions were then sealed in brown glass bottles and placed together in diffuse sunlight on a shelf. The solution without cadmium chloride became highly turbid within 3 weeks, n-hereas the one containing the cadmium chloride and iodate remained practically unchanged. On longer standing, cadmium acetate proved supeiior to cadmium chloride, perhaps because of thr highrr pH of the solution.
I
M L. 1.6I. 0
1.4-
1.2-
1.0-
0.8-
>-
‘. 0.6v) z W
cl -I 0.4-
a
0 +
a
0 0.2-
I 1 C
1.0 5
0
IO
15
20
I 25
I 0 I
30
MINUTES
Figure 2.
Hate of Development of Absorption of Blue Complex at 625 mp
Temperature and total volume of sample conetant
Crude linear A-fraction potato starch wab rerrptallized t n i w from aqueous solution saturated with 1-butanol in the same manner as for the preparation of the cadmium iodide-linear starch reagent ( 1 ) . To prepare 1 liter of the reagent, 2.5 grams of the twice recrystallized linear starch ale dissolved in somewhat less than 1 liter of boiling distilled water and the solution is allowed to cool. C.P. grade potassium iodate 0.40 gram, and 1.0 gram of C.P. cadmium acetate trihydrate, (!d(C?H,O?), 3H,O, are dissolved in the starch solution and the volume 1s brought up to 1 liter. The solution is then filtered with suction through several layers of very dense (“barium sulfate retention” grade) filtei paper and stored in bromn glass bottles. This solution ?Y stablr for 6 weeks or longer arid can be refiltered if turbidity becomes marked. -4 very slight precipitate, which can 1)c filtered off or left in the bottom of the bottle where it does no hxrm, is usually noticeable n ithin the fire ncek.
WAVELENGTH, mp. Figure
1.
Absorption Curves of Linear Triiodide Ion Blue Complex
Starch-
Produced by various eoncentrations of iodide ion in sample
EXPERIMENTAL
F iodide ion wits prepared by d stock solution of 2.0 X dissolving 0.0732 gram of cadmium iodide in 2000 ml. of distilled
V O L U M E 23, NO. 9, S E P T E M B E R 1 9 5 1 1.6
1253
1 A X
3.0 F SULFURIC ACID
A
SATURATED OXALIC ACID 8 5 % LACTIC ACID
0
SAT. TARTARIC ACID SAT,
sequent forniation of nioi L' oi the blue staeh-triiodide ion coniplex, or merely an equilibrium effect, I n or?er to determine the effect of pH and the nature of the acid used for acidification, several acids were studied with the results shoan in Figure 3. The values for the optical densities nere obtained a t 625 mp at the elid of 15 minutes allowed foi rolor development a t approximately the same temperature (23" =t1" C' ). Only the results obtained using 3.0 F sulfuric acid difffried significantly from the others. This might be explained as a result of either the lower pH of the solution or the tendency of the blue starch-triiodide ion complex to precipitate in the presence of sulfuric acid, especially a t the higher concentrations of iodide ion. Formic acid was chosen as the acidifying dgent in the determinations made in this study, because acids stronger than formic-e.g., oxalic-lower the pH to a point where iodate ion begins to oxidize chloride ion a t an appreciable rate and causes rapid decolorization of the blue complex formed.
CONCENTRATION, x I O ' F
Figure 3. Effect of Various .icids on Development of Optical Density of Blue Complex at 625 mr As function of iodide ion concentration
water. Chdmium iodide had Iieen found in previous work to give much more stable solut,ions at higher concentrations than did alkali metal iodides, in whose solutions iodide ion is oxidized to iodine after exposure to atmospheric oxygen for a time. As a check during the course of the investigation, a month-old cadmium iodide stock solution was compared with a freshly prepared solution of potassium iodide made up to t'he same formality o f iodide ion. The two were found to be identical in iodide ion concentration as determined spectrophotometrically using the linear starcoh-iodate reagent. The month-old cadmium iodide stock solution gave no test for free iodine on testing with the cxadmiuni iodide-linear starch reagent (I). The solutions prepared for use in ohtaining the data presented F cadmium iodide stock soluwere made up from t.he 2.0 X tion and distilled water to give 20.0-nil. total volume of the proper iodide ion concentration. This solution was then acidified with 1.0 ml. of 98 to 100% formic arid and 1.0 ml. of the linear 3t arrh-~~iotlate reagent W R S atl(ltd, Figure 1 shows the typical alisorption curves of the lilue linear starch-triiodide ion complex produced by several concentrations of iodide ion. The maximum optical densit,y is seen to owur a t ti25 nip. All measurements in this study were obtained with a Beckman Model DU quartz spect,rophotometer using 1-cm. matched Corex cells arid :t tungsten filament light. source. The rat,e of color development is indicated in Figure 2, which : t l s ~shows the effect of adding t.wice as much reagent to the solution, keeping the total volume the same in all cases. These Curves indicatc that the optimum time for color development is 15 to 20 minutes, considering the convenience for the oprrator and the desirability of keeping side reactions a t a miriirnum. Flocculation of the blue complex does not occur under these c o l i tiition8 in that period of time. While the absorption is uniforniljhigher when twice as much linear starch-iodate reagent is added, it is also apparent that the amount of reagent added need not be measured with more accuracy than is used in the other measurements. The higher absorption may be the result of the slow reduction of iodate ion and iodine by the formic acid, xith the suh-
CONCENTRATION, x 1 0 5F Figure 4. Effect of Temperature on Absorption \t
625 mli of blue complex formed after 15 minutes at various concentrations of iodide ion
Porniic acid is also desiralde because of its strong rcducing properties; by bring present in relatively large amount, formic acid ~ v o u l dtvnd to tie preferenti:illy oxidized instead of iodide iori by traces of osidizing suhtancw. .It thc~p?I ol)t:tined using formic acid, no osidation of iodide iori by diauolved atmospheric oxygen occurb. Acetic acid, added in the amount of 1.0 ml. of the concwitr:ttetl : i d , does not, permit full color development within a reasonnhle time. T f enough cmcentrated acetic acid is added to give rapid enough color development, the color of the blue complex is altc,rfacl slightly, probably because of change in the nature of the solvent. Temperature had small effect on the color intensities developed in the study of the cadmium iodide-linear starch reagent, where
ANALYTICAL CHEMISTRY
1254
a large excess of iodide ion mas always present during color development. In the reactions of the linear starch-iodate reagent, in which all of the iodide ion present enters into the color-producing reaction, a greater temperature effect is noted, as shown in Figure 4. While the temperature effect is marked, precise temperature control would probably be necessary only where the most accurate results are desired. The intercept of the straight line, obtained for the change of optical density with concentration, with the abscissa, shown in Figures 3 and 4, was entirely unexpected and indicates that a threshold concentration of iodide ion is necessary for formation of the linear starch-triiodide ion complex. Beer's law is not obeyed, as the lines do not pass through the origin, but straight lines are obtained that are readily usable for quantitative spectrophotometric determinations. The extinction coefficient can be determined by means of the following equation, which takes into account the intercept on the concentration axis. The calculations are based on known concentrations of iodide ion.
where e is the extinction coefficient, D the optical density a t concentration C, D Othe optical density izero) a t concentration Co a t the intercept of the concentration axis, and L the length of the light path in centimeters through the solution. On this basis the value for the extinction coefficient, which is the slope of the lines in Figure 4,is found to be 15,500. In a more general equation, where L is 1 cm.
D
E =
c-
[4
+ 0.25 ( T - 23)] X 10"
where T is the temperature (" C.) a t the timeof the measurement. The extinction coefficient obtained by this equation is 15,300. Equation 2 could be applied safely between 19' and 35' C., but it would obviously be more convenient to use a graph such as Figure 4 to obtain the concentrations of iodide ion in the solutions being analyzed than to calculate it using the extinction coefficient. This value for the extinction coefficient agrees very closely with the value of 15,300 found for the extinction coefficient per unit valence change of the oxidizing agent, ee-, for the cadmium iodide-linear starch reagent ( 1 ) . A study of the reactions involved shows that there must be an equivalence between a unit valence change of the oxidizing agent (103- in the case of the linear starch-iodate reagent) and the production of the chromogenic triiodide ion:
BrOs-
108-
+ 91- + 6H+ + 313- + Br- + 3H20
+ 81- + 6H'
+313-
+ 3HzO
(e- == (e- ==
Table I.
Substances Tested for Interference
Negative Positive C1- (sodium chloride) B r - (potassium SeOs-- (selenious acid) bromide) PiOz- (sodium nitrite) see discussion and Fe + (ferrous sulfate) (ferric su!fate) Figure 5 Fe HAsOn-- (disodium hydrogen arsenate) So%--(sodium sulCu (cupric chloride) fite) see dish"4+ (dihydrogen ammonium phosphate) cussion for HzPO4- (dihydrogen ammonium phosphate) masking N O % - (sodium nitrate) S - - (sodium sulS O I - - (sodium sulfate) fite) see disFormaldehyde cussion for Ethyl alcohol eliminating SzOa-- (sodium thioPhenol Oxalic acida 4sOe- (sodium sulfate) metaTartaric acida Lactic acida arsenite) Formic acida Acetic acid" 1.0 mi. of saturated aqueous solution or most concentrated commercial grade available. + + +
+ +
termed a "threshold" color, it was assumed that the substance added was not an interference if the proper intensity of color w1t8 obtained a t the same rate as in the control. The interferences by arsenite and thiosulfate were not investigated further. Because sulfide ion could be precipitated out by formation of an insoluble sulfide salt, its interference could be easily eliminated. The interference due to small amounts of sulfite ion can be eliminated by allowing 0.2 ml. of 40% aqueous formaldehyde to react with it in neutral solution for some time before the iodide is determined. Large amounts of bromide ion (0.1 F ) produce an orange color in the solution, which changes gradually in the course of 30 minutes to purple. Concentration of 0.01 F bromide ion gives a purplish color of about the correct intensity, while 0.001 F bromide ion produces a color practically identical to the control. The absorption curves produced by these various concentrations of hromide ion with three different concentrations of iodide ion a t 23" C. are shown in Figure 5. They indicate that very little interference will be encountered where bromide ion is present in
zIa -) l/J3-)
INVESTIGATION OF INTERFERENCES
Iodate ion in solution in the presence of weak acids is not a strong oxidizing agent, but exhibits a preference for oxidizing iodide ion. Of the possible interfering substances, strong oxidizing agents would to a great extent be precluded because iodide ion would have been already oxidized to iodine or iodate by them. As a rule, then, some knowledge of the composition of the solution being analyzed is taken for granted. The interferences investigated in this study were for the most part substances that might tend to reduce iodate ion. The effect of various inert electrolytes had been found ( 2 ) to interfere only when in sufficiently high concentration to cause precipitation of the colloidal linear starch-triiodide ion blue complex. Table I summarizes the effect of the possible interferences that were studied. Solutions of 6.0 X 10-5 F iodide ion were prepared and 0.05 to 0.10 gram of the solid, or 0.10 ml. of the liquid, was added. The solutions were kept a t 23" C. and 1.0 ml. each of the 98 to 100% formic acid and linear starch-iodate reagent was added to each. As this concentration of iodide ion produced what might be
Figure 5 .
Absorption Curves of Blue Complex
Produced by three concentrations of iodide ion with varying concentrations of bromide ion
V O L U M E 23, NO. 9, S E P T E M B E R 1 9 5 1 concentrations less than one hundred times that of the iodide ion. From the apparent enhancement a t the violet end of the visible spectrum, and the shifting of the absorption peak, the presence of the iodine bromide (12Br-) ion might be inferred. DISCUSSION OF RESULTS
The reagent can be used with solutions that do not contain concentrations of interfering substances large enough to cause erroneous results and with solutions that have had the interfering substances marked or removed. The study of interferences indicates that small amounts of many common inorganic and organic substances can be tolerated. The peculiar interference introduced by bromide ion is of such a nature that some knowledge of its concentration must be known before analyzing with this reagent. Large concentrations of bromide ion, however, will make its presence known by the orange or purplish color developed.
1255 The failure of the line of optical density versus concentration to pass through the origin sets a lower limit of iodide ion concentration that can be determined, No explanation is immediately available to explain why no absorption occurs until a threshold concentration of triiodide ion is present. Further work is contemplated both on the nature of possible interhalogen ions, such as I*Br-, and on the nature of the formation of the blue linear starch-triiodide ion complex. LITERATURE CITED (1) Lambert, AKAL.CHEM.,23, 1251 (1951). (2) Lambert, Arthur, and Moore, I b i d . , 23, 1110 (1951). (3) Schoch, “Advances in Carbohydrate Chemistry,” Vol. I, p. 257, ed. by Pigman and Wolfrom, New York, Academic Press, 1945. (4) Schoch and Jensen, ISD. EKQ.CHEW,A N ~ LED., . 12,531-2 (1940).
RECEIVED April 20, 1951. Contribution C 450 from the Department of Chemistry. Kansas State College, Manhattan.
Evaluating Dynamic Fatigue of Adhesion of Tire Cords to Rubber Stocks W. JAMES LYONS Chemical and Physical Research Laboratories, Firestone Tire & Rubber Co., Akron I?, Ohio
A
VARIETY of tests have been devised in the laboratories of the tire and rubber and associated industries for measuring the adhesion of textile fabric, especially tire cord, t o natural and synthetic rubber stocks. [A few of these static tests and their variations have been briefly described by Lyons, Nelson, and Conrad (6).j Most of the tests involve measurements, generally at room temperature, on specimens which are given no special treatment after preparation. It has, however, been widely recognized for same time that the testing of unfatigued specimens at room temperature is an inadequate simulant of fabric conditions in a tire a t the time of adhesion failure. For a decade or more, tests devised by Gibbons ( 2 ) and Lessig ( 4 ) have been used to evaluate the resistance to separation of combination rubber and fabric specimens. The results of these tests, however, have been interpreted more as evaluations of the fatigue properties of rubber stocks and rubber-fabric structures than of fatigued adhesion per se. To overcome partially the deficiencies of the simple, static test, on the theory that the deterioration of adhesion in tire service is in part due to the heat generated in the flexing of the tire, there was added to the H adhesion test a t the Southern Regional Research Laboratory (6) a feature which consists of heating the specimens a t 275” F. for 1.5 hours before the actual breaking test a t that same temperature. A mechanical fatigue test, the results of which depend more nearly on the dynamic adhesion alone than do those in the foregoing tests, has been in use during recent years in the Rubber Laboratory of the I. G. Farbenindustrie in Germany ( 3 , 7). In this test, the adhesive bond is fatigued by means of a periodic shearing force, until the bond is ruptured and the cord is pulled free of the rubber strip. More recently Gardner and JTilliams ( 1 ) and Pittman and Thornley (6) have described tests in which the loss of adhesion between cord and rubber, as a result of flexing the test piece, is actually measured. In the Gardner and Williams test the flexing is done on a Goodrich Flexometer and consists of the periodic compression of a small rubber block containing a single test cord. After a period of flexing, on the order of 10 to 40 minutes, the force required to extract the cord axially, against the residual adhesion, is measured on a standard cord-testing machine. The Pittman and Thornley method subjects the rubber-encased cord sample to periodic flexure and
lateral compression for a definite number of cycles. The fatigue of adhesion is measured as in the foregoing method. Described in the present paper is a dynamic test in which the fatigue or deterioration of adhesion is likewise measured. I t h m been under development and use at the Firestone Research Laboratories since 1947. The well-known Goodyear H test ( 5 ) is employed to measure the “pull-out” adhesion of cord to rubber, in both unflexed and fatigued samples.
ROLLERS
Figure 1.
Schematic Side View of Single Unit of RollerFlex Adhesion Fatigue Machine
In the conventional H specimen, the single cord which is pulled out is entirely surrounded by rubber, whereas in the plies of a tire the cords lie side by side, very often in close contact. To simulate this condition, it was decided to use a band of cords, so that each test cord could be extracted from between two cords, all embedded in rubber. Flexing a band of parallel cords also would appear to be more realistic than the flexing of a single cord cured in rubber. APPARATUS AND METHOD
Adhesion-Fatigue Machine. The nature of the flexing t o which the specimen is subjected is shown in the schematic drawing of Figure l. The test strap, 6.5 X 1 X 0.1 inch, has a band of five cords passing longitudinally through the middle. Affixed a t one end (the right in Figure 1) to a stationary mounting, the strip passes
