ANALYTICAL CHEMISTRY
1618 content determined as above. The apparent average mo1ec:ulur weight of the corn amylose in duplicate experiments was found t o be 32,000 and 39,000. SUMMARY AND CONCLUSION
Although the use of the hypochlorite-sodium phenat,e reiictioii is difficult to control for the quantitative determination of ammonia ( 6 ) , a method using hypochlorous acid gives reliable results for the det,erminatiori of minute amounts of ammonia. The procedure gives good results with simple nitrogen-containiiig compounds such as acetamide and n-gluconamide, which libemte ammonia directly upon treatment with alkalies. Thtx method could be employed in conjunction with micro or si111micro Kjeldahl determinations. I n conjunction with the Kiliani cyanohydrin reitctiori thct method has been used for the determination of the appiirc'nt average molecular weight of the polyglucosan and laminarin, which was found t o correspond t,o about 25 anhydroglucoac~ residues. As the Kiliani condensation requires considerable time to reach completion, hydrolysis of the cyanide ion must be taken into account. The effect is too large to be corrected for wcurately by a blank determination; and it seems necessary to remove ammonium ions produced by hydrolysis of cyanide ions I>y some chemical method, or by a physical method w c h ar , However, the polyqaccharide cyanohydrin (-:in be
purified by precipitation with alcohol as in the case of amylose , cyanohydrin. Water-insoluble polysaccharides such as celluloac cayanohydrin can be filtered off and washed t o remove ('ont:tniiiiating ammonium ions. ACKYOWLE:DG\I EYT
Thc authors wish t o thank t h r l 1'. I. du Pont de Kernour5 & C h . for a grant, which defi:t\ed the expenses of this work and provided a fellowship for P. G. S c h t w r r . LITER i T U R E CITED
(I) Barry, V. C.. AIcCormick, Joan E., and Mitchell, P. W . I).. .I. Chem. SOC.,1954, p . 3692. (2) Borsook, H., J . B i d . Chsm., 110, 48 (1935). ( 3 ) Connell, J. J., Hirst, E. L.. and Percival, E. G. V., J . ( ' h e m . SOC.,1950, p. 3494. (4) Frainpton, V. L., Foley. Lucia P., Smith, L. L., and Jlalone. J. G., ANAL.CHEM.,23, 1244 (1951). ( 5 ) Isbell, H. S.,Science, 113, 532 (1981). (6) Russell. J. A.. ,J. B i d . Chein.. 156. 457 (1944). (7) Thomas, P., bull. SOC. chii7z.. '11, 796 (1i12). ' (8) [bid., 13, 398 (1913). (9) Van Slyke, D. D.. and Hiller, .ilnia. .J. Bioi. Chem., 102, 499 (1933).
(LO) Zalta. J. P., Bd1. SOC. chini bid.. 36, 444 (1954). RECEIVEDfor review January 18, 1953. -\rcepted June 10, 1955.
Paper No. 3310 Scientific Journal Series, I l i n r i e w t a .igricultural Exppriinent Station.
Absorptiometric Microdetermination of Total Sulfur in Rubber Products K. E. KRESS Firestone Tire and Rubber Co., Akron 17,
Ohio
The 1.0 to 2.5'30 of sulfur normallj present in ruhber products is oxidized with concentrated nitric acidbromine reagent, followed bj perchloric acid in the presence of excess lead nitrate. Sulfur as lead sulfate is precipitated and washed with acetone. The lead sulfate is dissolved in 50% hjdrochloric acid and absorbance of the lead chloride complex is recorded at 27'0 mp. Sulfur is calculated on the basis of the nieasured lead content of the precipitate. The high sensitivity puts the method in the micro range. 411 experienced analyst can analyze 40 to 50 samples a cia!. Precision and accuracy are comparable to those of the conventional barium sulfate gravimetric method at the low sulfur concetitrations normally found in ruhher products.
T
HE literature contains numerous references to deterniinatiori of total sulfur in rubber products as barium sulfate by gravimetric, volumetric, turbidimetric, or nephelometric methods. The latter methods were developed in the course of a search for a more rapid procedure than that afforded by standard gravimetric methods. Recently an amperometric titration has been applied for the indirect seminiicrodetermination of sulfur in organic conipounds aft,er precipitation as lead sulfate (8). However, digestion of the sample in a sealed Carius tube has serious disadvantages for routine cont.ro1 of sulfur in rubber products, and the range of sulfur concentration (about 1 to 6 mg. of sulfur) recommended rcquires rubber samples of above 50 mg. h new combustion apparatus for automatic determination of sulfur in steel has been applied successfully to rubber ( 2 ) ) but the cost of this single-purpose instrument precludes its use in most rubber laboratories.
I'ei~~liloric acid has been used to speed up oxidation of ruI>ljer 1)roducts during determination of sulfur (,9). Published methods have all been for work on a niitcro scale, where the possible explosion hazard has retarded generid acceptance of this strong osidant in rout,ine analysis of rubber. Even with the more rapid osid:ition provided by perchloric acid, the conventional gravimetric barium sulfate procedure ip used to precipitate the sulfur. This limits the speed of analysk. .1 very sensitive absorptiometric method for determining lead (/i) depends on the strong selwtive ultraviolet light absorbance near 270 mp of the lead chloride mrnplex formed in strong hydrochloric acid solutions. A rapid osidation ivith nitric arid perchloric acids on a safe micro scale has been developed and is reported here. Sulfur is insoluhilized and precipitated a s lead sulfate with acetone. Absorptiometric measurenient of lead as the lead chloride coinples in 50% hydrochloric acid provides data for calculating sulfur concentration. EQUIPNIENT, REAGEYTS, AYD PROCEDURE
Equipment. Sample test tubes, such as 15 X 100 mm. lipless w l t u r e tubes, of 12- to 15-ml. volume. Capillary-tipped pipet connected to a water pump vacwum source. It may be made by drawing out the lower end of a standard pipet over a hot flame until i t is of capillary size, with thin walls and an internal bore of 1 t o 2 mm. Standard high speed semimicrocentrifuge capable of holding sample tubes. Finger stall of pure gum rubber, or a finger from a rubber glove, extracted with a solution of 5% hydrochloric acid in acetone. Beckman DU quartz ultraviolet spectrophotometer with ultraviolet accessories, or a similar instrument. Roller-Smith microbalance, torsion type, of 25-mg. capacity with V-shaped pan, or similar balance weighing to 0.01 mg. Reagents. CONCESTR.4TED NITRIC -4CID-BROMINE. Add reagent grade bromine to concentrated reagent grade nitric acid. in
V O L U M E 27, NO. 1 0 , O C T O B E R 1 9 5 5 an all-glass dropper bottle with clean rubber or Tygon bulb. IIave a layer of excess bromine on the bottom a t all times. PERCHLORIC ACID,70 to i 2 % reagent grade used as received. I t is conveniently stored in an all-glass dropper bottle with either rubber or Tygon bulb. Store away from heat and handlr only a small volume a t a time, keeping the main part of the retgent in the original bottle in the storeroom. A 1-pound bottle is sufficient for over 2000 analyses. ACETOSE,reagent grade. This must be redistilled if a hronii residue is present aft,er evaporation of the acrtone following tlit. precipitation and washing step. HYDROCHLORIC ACID,50% by volume. ; i convenient method of preparation is to mix equal volumes of concentrated hydrochloric acid (35 to 38% reagent, specific gravity 1.1778 to 1.192:j range) and distilled xater, using the same volumetric flask to m a s u r e the w i d and x a t r r . 1,c.i~hTITR.iT?;, 107G. IXssolve 10.0 grams of lead nitratc) in watci. :ind dilutt, to 100 ml. Calibration. The average specaific absorbance or h7 valuc. (also termed :ilworptivity) of lead at 270 mp is 54.0. The exact figure should he determined for earh instrument for greatest accuracy-. Prepare a stock solution containing 1000 p.p.m. of lead by dis~olving0.160 gram of reagent grade lead nitrate in distilled water in a 100-ml. volumetric flask and diluting to volume. Pipet 10.0 nil. into a seeorid 100-ml. volumetric flask and dilute to volume for a 100 p.p.m. lead standard. Use a 50-ml. buret to deliver different volumes hetween 0.5 and 8.0 nil. of the 100 p.p.m. standard into a clean glass Erlenmeyer flask. K i t h a second 50-ml. huret, add enough distilled water to make exactly 10.0-nil. t,otal volume of lead solution. Pipet, 10.0 nil. of concentrated hydrochloric acid reagent into the flask. Shake well and measure absorliance of the 5070 hydrochloric acid soiut,ion at 270 inp in I .OO-cm. quartz samplr cclls. Calculate K F Ias , follows:
- 4 270111p h-,,,, = bC
where 1: is instrument ahsorbanw at, 270 nip b is internal cell thickness arid is neglected as long a,P it, is 1.00 f 0.005 em. c is concentration of lead in grams per liter a t the dilutiori for which ahorbane? is nieamred-r.g., 100 p.p.m. X 4 ml./20 ml. = 20 p.p.m, or 0.020 gram per liter T h e K value to he used is the average of the data over t h r range of linear instrument absorbance (usually considered 0.1 to 1.8 A ) . Procedure. Keigh 1 t o 3 mg. of well milled repre.entative sample. Sheet as thin as possible :tnd weigh to 0.01 mg. on a suitahle microbalance. Check the zero point of the balance carefully before nnd after any serie5 of weighings. Brush the sample directly into the labeled sample tuhe, add 0.25 ml. of 10% lead nitrate solut,ion from a 10-nil. huret, then add about 1 ml. of prepared concrnt,rated nitric acid-bromine reagent and 3 drops (1 ml.) of 70% perchloric acid reagent,. Do not add bumping stones of any kind. Place a 3-inch square of sheet asbest,os, ahout, inch thick, on a hot plate a t 300" t o 320" C. under a well ventilated h Immediately place a 150-ml. beaker containing as many ap sample tubes on the ashestos, and heat until completely osidi (about 30 to 60 minutes). Mlien the perchloric acid solution is essentially colorless and no particles of unosidized sample are present, on the walls of t.he tube (a trace of carbon black may he ignored), remove the beaker from the asbestos and place it, directly on a bare hot plate a t 300" t o 320" C. for ahout 10 minutes. The tube should fume copiously and the liquid condensation level should rise a t least t o nithin 0.5 inch of the top of t h r ad nitrate is converted to lead perchlorate. if this occurs, the sample may be recovered h 2 drops of perchloric acid. As long as the residue is wet' with liquid acid on cooling, no heat decomposition of lead perchlorate present should result. Lead perchlorate will decompose to form some acetone-insoluble lead chloride, if heated excessively in the absence of free perchloric acid. Remove the beaker and tubes from the hot plate and place on a cool surface. If desired, accelerate cooling with a blast of compressed air. When tubes are a t room temperature and cold to the touch (acetone poured into hot perchloric acid may result in a dangerous explosion), rapidly pour about 10 ml. of acetone reagent into all tubes in the beaker. Cover the index finger with a finger stall and "stopper" each tuhe. Shake just enough to mix acetone and acid residue. Lead sulfate will be
1619 lirwent as a t,urbid suspension, but if only a trace of sulfur is present-e.g., 0.1 %-this turbidity may not always be apparent. Always wipe the finger stall on edge of sample tube t o minimize possible loss of a trace of sulfur. Fill a 150-ml. beaker about two thirds full with hot water a t 50" to 60" C., but not be so hot that the acetone boils. Immediately immerse as many as six sample tubes in this water hath and allow the beaker to stand a t room temperature for 10 to 15 minut,es. Remove tubes from bath, and centrifuge in a wmimicrocentrifuge for 1 t o 2 minutes. Remove from centrifuge. With a capillary-tipped pipet connected by rubber tubing to a v-ater pump vacuum draining down a sink, withdraw all hut about 0.3 t o 0.5 ml. of acetone from each tube. iilways hold the tuhe in a vertical position, and lower the pipet tip in the center of the tube. Be careful not to jar the tube when removing it from the centrifuge. K a s h the precipitated lead sulfate twice more with about 10 ml. of acetone, centrifuging 1 to 2 minutes each time. Three of these small tubes may conveniently be handled a t the same time during the washing operation whrn acetone is drawn off and added. After removal of the third addition of acetone add 2 dropq of water to each tube and place the beaker n i t h tubes on the 300" C. hot plate to evaporate t o dryness. Reduce the temperature if bumping occurs, but complete drying at high temperature for 5 to 10 minutes. Bcetone absorbs in the ultraviolet and must he removed completely. K h e n dry, remove from the hot plat,?, and cool in an air blast. T h e residue should be white a t this point,. If any trace of brown coloration is apparent, heat the tuhes wit,h the beaker a t 550" C. for 15 minutes until the residue is white. Then proceed as usual. When tubes are cool, carefully pipet 10.0 ml. of exactly 50% h3-drochloric acid reagent into the sample tubes. With :t finger stall over the finger to act as a stopper, shake the tuhes a few seconds until all the lead sulfate is in solution. Inspect the bott,om of the tube for undissolved material. If there is difficult,y in dissolving the precipitate (or if white side-wall stock n.ith acid-insoluble titanium dioxide is being anal, add a few small silicon carhide bumping stones and $hake ously. I n the case of white side-wall stock, or other material where acid-insoluble matter is present, centrifuge thr samplc to throw down the acid-insoluble material. Sometime during this solution operat'ion, place the Beckman DL spect,rophotometer in operation 1vit.h the hydrogen tube. An instrument slit width of about 1.1 mm. n i t h sensitivity set near the middle of its range has proved satisfactory. Rinse a 1.00 =t0.005-em. quartz sample cell t'wice with about 2 ml., and finally pour in 3 to 4 ml. of the sample solution. Itccord absorhhydrochloric acid blank. If ance a t 270.0 mp against a ahsorbance is above 1.80, pour the contents of the crll into a 10-ml. glass-st'oppered graduated cylinder and dilute to twiw it? volume accurately with 5OYGacid ( a 1 to 1 dilution). Calculate the per cent sulfur in the sample as follows, with a spectrophotometer cell thickness of 1.00 em. A reagent blank normally amounts to ahout 0,010 absorbance or less, which is about 0.01 to 0.0270 sulfur. Therefore, the experienced analyst may neglect the reagent blank, except for the most exacting work.
For 10.0 mi. of acid this reduces to
70sulfur
=
(2.87) A270 I I I ~ mg. of sample
If 1 to 1 volume dilution is necessary (20-ml. volume), the factor is doubled and is 5.74. E X P E R I M E Y T i L WORK
Evaluation of the procedure for sample preparation used for semimicro amperometric titration ( 1 ) proved that the amount of residual nitric acid was a critical point in working on a micro scale with lowsulfur compounds. A significant amount of lead sulfate is dissolved by the residual nitric acid. Use of potassium chlorate on a micro scale. following the conoxidized the sample comventional ASTM macromethod (j),
ANALYTICAL CHEMISTRY
1620 pletely in 5 to 10 minutes. However, it proved too unreliable for routine analysis, as excess lead nitrate, excess acid, degree of dryness of residue, and washing technique were critical. Use of perchloric acid was investigated as a final resort. Even this gave erratic results until the precipitation technique with an organic solvent was developed. This proved the answer to the recovery problem. High and erratic rrsults may be obtained, if fuming sulfutic acid is near when the sulfur is being determined. Oxidation. The oxidation is carried out in the presence of excess lead nitrate. Kormally about 10 mg. is sufficient for up to 5 mg. of rubber stocks, but about 25 mg. is added to ensure complete precipitation of sulfur as lead sulfate for diverse materials. It is convenient to add the lead nitrate from solution; the consistent amount added in this manner tends to result in more accurate data and to give a more reproducible blank. Oxidation of natural rubber with fuming nitric acid is almost instantaneous and violent, and may lead to loss of sulfur. Consequently, the more slowly acting concentrated acid is used for all elastomers and other materials. Though concentrated nitric acid will not decompose butyl stocks, this elastomer and similar hard-to-oxidize materials are oxidized completely when the high boiling (200' C.) perchloric acid content increases as the nitric acid evaporates. As long as analyses are carried out on a micro scale, this is a safe practice, but larger samples of butyl 100 mg.-may oxidize with explosive violence, polymer-e.g., once perchloric acid becomes concentrated by evaporation, unless fuming nitric acid is first used to decompose the polymer. Perchloric acid should never be used indiscriminately for oxidation purposes. The heavy bromine gas retains and oxidizes volatile sulfur compounds formed in the initial stages of oxidation. Though it has been said that iodic acid is superior t o bromine for this purpose (Y), the amount of sulfur lost when bromine is used in the presence of excess lead nitrate has proved negligible for routine analysis of rubber products and compounding materials. Unless perchloric acid volume is above 2 drops, incomplete oxidation of sulfur may result in some cases. A4range of 3 to 5 drops of added perchloric acid gave essentially the same sulfur recovery with natural rubber stocks but in some cases use of 5 drops gave somen hat lower recoveries-e.g., 95 to 98% of 3-drop recovery. Safety precautions, as well as lead sulfate solubility, require use of the minimum volume that Kill produce complete oxidation, which is about 3 drops (0.1 ml.). The only really critical point about oxidation with the specified amount of perchloric acid on a micro scale is possible evaporation to dryness. In such a case the heat-sensitivr lead perchlorate present drcomposes to form lead chloride, that is insoluble in acetone and ail1 lead to false high figures for sulfur recovery. The sample may be recovered by reheating to fumes n i t h 2 drops of perchloric acid. Precipitation. Perchlorates of most rations are soluble in alcohol and acetone. Potassium perchlorate is the only salt essentially insoluble in these solvents (4). T h e precipitation of lead sulfate through its insolubility in organic solvents offered a novel means of separation from interfering perchlorate salts. Several other solvents were investigated, including mter-miscible alcohols (ethanol and methanol), higher ketones (methyl ethyl ketone and methyl isobutvl ketone) and ether (dioxane) alone, as well as water-immiscible hydrocarbons (%-he\ ane) mixed with acetone. Khen 3 drops of perchloric acid were used, the alcohols all gave lox recoveries, on the order of 60 to 907, of that of reagent grade acetone. The acid in dioxane dissolved practicallp all the lead sulfate. Methy ethyl ketone gave recoveries only a little lower than acetone, but methyl isobutyl ketone precipitated lead salts other than lead sulfate in the second wash, once acid concentration was reduced by the first wash. Mixtures of hydrocarbons and acetone offered no advantage over acetone alone.
Early work with acetone was carried out with specially prepared essentially anhydrous acetone. However, it was later proved that, though the water content) of the acetone must be controlled, reagent grade acetone can be used. I n fact, 1 to 2% of water added to reagent grade acetone may a t times be beneficial. The presence of a trace of water inhibits the oxidation of anhydrous and reagent grade acetone, which normally results in development of a yellow coloration as the acetone stands in the presence of perchloric acid. However, this solvent oxidation does not normally appear to affect the quantitative recovery.
1.6
t
1.4
PO0
Figure 1.
220
940 260 280 WAVE LENGTH, M r
300
Effect of acid concentration on absorbance of lead 20 p.p.m. lead Range 0 to 80% HCI
- - - 5 0 % HC1
When acetone is first poured into the residual cold perchloric acid after sample digestion, an exothermic reaction results due to heat of solution. This has not proved hazardous even with such a large excess as 20 drops of perchloric acid, which would mean a final concentration of about 10% perchloric acid in acetone. On the micro scale involved, only 2 drops (0.07 ml.) of excess perchloric acid are diluted to about 8 ml., which is a concentration of less than 1% perchloric in acetone. Consequently it is felt that no hazard is involved in adding acetone to cold perchloric acid, as done here on a micro scale. Paradoxically, heating the acetone in hot water a t below the boiling point tends to force complete precipitation more efficiently than cooling in an ice bath. K i t h high-sulfur materials, the additional recover>- due to heating the acetone may not always be apparent, and immediatr centrifuging may be justified in many cases. However, a definite increase in sulfur recovery was noted on several occasions following a period of heating the acetone, particularly with butyl stocks, so this step is incorporated in the procedure. Heating a reagent blank in the same manner for extended periods did not alter the blank. Therefore, such heating does not decompose the acetone-soluble perchlorate salts. Interferences. Lead and iron salts and even barium perchlorate are very soluble in acetone and are probably all dissolved
V O L U M E 27, NO. 10, O C T O B E R 1 9 5 5
1621
when the first acetone is added to precipitate the insoluble lead eulfate (the sulfur could be precipitated as barium sulfate in the same manner with excess barium chloride). However, a residual volume of about 0.5 ml. remains in the tube each time after the clear acetone is drawn off folloTving centrifuging. This makes necessary two more dilutions of the residue to reduce the blank to negligible proportions. A correction for background absorbance was necessary when potassium chlorate or nitric acid alone was used as the oxidant, b u t the acetone solubility of interfering perchlorates eliminates all possible interference. Even lead present in a stock as the hard-to-dissolve oxide, and mid-insoluble barium sulfate will be converted to perchlorates and bc removed by the acetone with quantit,ative recovery of sulfur as lead sulfate. This procedure offers a sensitive and rapid method for determining lead in rubbcr product,s and compounding materials. The sample is n-et or dry ashed, then taken up in 5ooj, hydrochloric acid. The absorbance a t 270 mp is measured and per cent lead is calculated using a specific absorbance, K , of 54.0. A simple mathematical correction may be made for any background interference due t o iron. .Acet,one absorbs strongly in the ultraviolet spectral region of interest and must be rcnioved completely by evaporation before the 5070 acid is added. Some lots of reagent grade acetone had to be redistilled, as they left a brown residue when the acetone w:is evaporated to dryness after the final wash. This residue absorbs and gives high results for sulfur. The brown material was apparently a high boiling oxidized.impurity formed by osidation n.ith perchloric acid. I t could be removed by heating the saniple tubes a t 550" C. for 15 minutes. However, it is best eliminated by redistilling the reagent grade acetone to ensure purity. Addibion of 2 drops of water t o each sample before evaporat,ion to drynens vcill reduce such interferencs.
0
10
PO
30
40
50
60
70
80
V O L U M E PER CENT HCI
Figure 2.
Shift of absorbance maximuni with acid concentration
Standard micro chemical practice would call for use of a filter stick or some method of filtration for each sample, but the vacuum pipet is much faster and easier to handle Recoveries are some h i e s a little lo\v-c.g., 9SY0 of theory-for inorganic salts where no sulfur is likely to be lost by ouidation. This may be attributed to the slight soluhility of lend sulfate in the acidic acetone. Solution. I t has been shown ( 7 ) that the intensity and sclectivity of ultraviolet absorbance of lead in aqueous hydrochloric acid change drastically with change in acid concentration. This was investigated and the data are illustrated in Figure 1. The
maximum is sharp and synimetric,:d at an acid concentration above about 40%. The shift of wave length of niaximum absorbance toward longer viave lengths is observed in Figure 2 to be an asymptotic furi.ct,ion of the acid coilcentration. Above about -i070 acid the shift in wave length is slo~v,and a t 5OY0 acid concentration the wave length of the maximum (270 nip) will be essentially constant. Its location' will not be affected hy small changes in acid concentration, which might occur, owing to normal variation in normality of the concentrated reagent grade hydrochloric- acid used.
10
XI 01 vl
1
Figure 3 sholvs that the intensity of absorbance, or specific absorbance, K at the maximum is a linear function of acid concentration in the range of about 20 to 60 volume % hydrochloric acid. The K value of 50% acid is 54.0 a t 270 mp, but in 25% acid it drops to 34.0 a t 264 mp. The high ( 5 0 % ) acid concentration is preferred because the great,er sensitivity makes possible a smaller sample, which is rapidly oxidized with a minimum amount of perchloric acid, and any interfering absorbance is reduced in relative intensity t o negligible proportions by the strong lead absorbance. Use of a weaker acid-e.g., 25%-niay prove advantagroils for purified materials of high sulfur content,. However, dilut,ion to larger volumes of 50% acid than the 10 ml. used hpre is preferred because of the greater accuracy possible a t high sensitivity. The concentrated (12'V) hydrochloric acid (considered 3 i Y 0 hydrochloric acid with a range of 35 to 38%) as used to prepare a 50% acid mixture by volume would result in a range of 17.5 to 19% hydrochloric acid content in a 1 to 1 mixture. During development of thc procedure, no abnormal results were obtained which could be attributed to variations in hydrochloric acid concentration. For determination of sulfur in rubber products, no correction was made for variation in hydrochloric acid content of the concentrated acid. For the most accurate work, t,he hydrochloric acid content should be checked carefully and kept constant. Any chemically inert material can be used as a finger stall for stoppering the sample tubes containing a c i d Rubber provides a better seal for acetone washing, but polyethylene film is satisfactory for 50% aqueous acid. Pure gum rubber is satisfactory for both acetone and acid, if extracted with both solvents before
ANALYTICAL CHEMISTRY
1622 Table I.
Determination of Organic and Inorganic Sulfur in Rubber Compounding AIaterials
Conipounding Material Sodium sulfate Zinc oxide Barium sulfate Sulfur (commercial rubber grade) Accelerators M B T S (purified)'
Theoretical
Sulfur, 70 B y analysis
Mean Deviation. AV.
55
Recovery, % of Theory
Inorganic Sulfur Compounds 21.8, 22.0 0.30, 0 . 3 4 13.7 13.4, 1 3 . 1
21.9 0.32 13.2
50.1
97
5zo.02 10.2
86.3
Organic Sulfur Compounds 99.5 9 6 . 2 , W .4,98,100
98.1
11.3
99 (97-101) 96.7 (94-100)
22.6
38.8
36.2,36 4.37.9,38 5
37.3
+0.9
.53.4e
34.0, 5 2 . 6 5 1 , 8 , 5 3 ..4
53.4
*0.8
Analyses in Rubber Chemistry. The data of Table I illustrate the wide application of the perchloric acid method to both inorganic and organic rubber compounding materials. The follobying modifications of the regular perchloric acid procedure for analysis of these compounding materials are recommended
CARBOSBLACK. \Veigh 5 to 10 mg. of pelletized black 52.0 and digest Tvith a minimum Softeners amount of perchloric acid (3 Petroleum oils (rubber grade) to 5 drops). Add 3 drops, and 1.39% by 02 bomb 1.10, 1 . 2 8 1 :34 jz0.06 0.20% byO?homb 0 . 1 1 . 0 . 1 3 , o . 13 0.13 rt0.01 if oxidation is incomplete, add Coal tar origin (Bardol) 0.57,0.64 0.60 '*0.04 1 or 2 inore as rcquired for a Asphalt origin (Paraflux) 0.86,0.80 0.83 10.03 colorless n e t residiie. Carbon Blacks I S O R G A NSI A CL T S .Foi. S R F (furnace) 0.044.0.043,0.064 0.030 10.009 h i g h - s u l f u r s a l t s , such as HMF (furnace) 0.12,O. 1 4 0.13 10.01 barium sulfate, weigh 1 to 2 F E F (furnace) 0.67.0.64 0 66 10.02 WAF (furnace) 0.29,0.35 0.32 jzo.03 mg. and digest with 3 drops SAF (furnace) 0.47,0.45 0.46 7to.01 of perchloric acid. Dilute to E P C (channel) 0 . 1 5 , o . 17 0.16 10.01 2%5 to 100 ml. with 50% hya Benzothiazyl disulfide. d r o c h l o r i c acid as needed. 6 Tetramethyl thiuram disulfide. For low-sulfur inorgauic salts. e Chemical analysis by ASTN procedure gave 51.8, 51.5, and .52.9y0 sulfur in T M T D , average recovery of 97.7c.;. such as zinc oxide, make the _~___ final 50% acid volume u p t,o 10 nil. as usnal. Table 11. Total Sulfur in Conipounded Rubber Stocks by Absorptiometric Method PETROLEUM OILS ( ~ ~ h k ~ e r Absorptiometric Mean grade, essentially nonvolat,ile Polymer sulfur^", Sulfur, % Deviation. Recrlvwy". Stock a t 100" (3.). Weigh 5 to IO Type Type Range AT-. Range AI-. c;;c '-5 mg. ( 1 sinal1 drop) and digPst Standard GR-S 128to1.34 1.31 128t01.38 1.32 10.04 101 with a minimum amount of Standard Natural 2 05to2.12 2.08 2.12to2.19 2.16 +0.04 in4 perchloric acid ( 3 to 5 drops), Tire tread Natural 1.72 1 . 8 5 to 1 . 0 3 1.89 +0.03 110 Foam rubber Natural 2.62 2.52 t o 2 . 0 1 2.56 -0.03 98 Dilute to 10 ml. with 50% White side wall Natural 2.02 1 91 t o 1 . 9 8 1 95 jz0.02 q7 acid, with further dilution as 2.18 2.11tCJ2.1.5 2.15 10.01 RP Tire tread Mixed GR-S and natural nceded for hi,gh-sulfur oils. Tire tread GR-8 (LTP) 1 53 1.50t u I 3 i 1 J4 1 0 04 100 ITeighing of softenel, oils on a Hose (10% C a C 0 d Neoprene 0.60 0.59 t o 0 . 0 8 0.68 jzO.01 103 microbalnrice is facilitated by Cement Chlorinated 0.66 0 . 7 2 t o 0 80 0.7fj &0.03 115 rubber taking up .the sample on a wrighed cott,ori cord, cut,tirig % recovery by absorptiometric method relative t o recovery by ASThf gravimetric method, absorptioinetrioally X 100/70 gravimetrically. o f f the part of t,he cord with the sample, and dropping the cord and all into a sample tube. The cord must be essentin:lv sulfur-free. For sulfur below 3%, proceed as for ORGASICCOMPOUKDS. use. Iioir.ever, any possibility that acid will extract iron from comrubber analysis. For high sulfur, such as tetramethyl thiuram pounded rubber must be avoided, as ferric chloride absorbs in the disulfide accelerator, dissolve 1 to 5 mg. in 100 ml. of chloroform ultraviolet. The uncovered finger should never be used to or other suitable solvent. Pipet an aliquot of 1 to 5 ml. into a stopper the sample tubes whether acetone or acid is the solvent. sample tube and evaporate just to dryness a t low tempprnture (70" C. oven or' water bath). Proceed as for rubber analysis, Safety preparations for handling lead recommended that the and include the dilutions of the chloroform solution in the final analyst mmh his hands thoroughly after determining total sulfur calculations. .Uternatively, the final acid volume can b e made by this method. up i o 100 ml. instead of 10 nil. T M T D (purified) b
4PPLICATIO.V.5
The results obtained bj- the perchloric acid method represent true total sulfur and are comparable to those by the sodium carbonate fusion method ( 1 ) . Only in critical work, or in application to materials other than rubber products, need possible loss of volatile sulfur be taken into consideration. The results are not strictly comparable to those of the standard ASTM gravimetric method, as perchloric acid will recover sulfur from barium sulfate, which is filtered off as acid-insoluble in the regular method. Both methods determine sulfur in acid-soluble inorganic sulfates, such as sodium and calcium sulfates. T h e combustion method does not determine inorganic sulfate, as does this wet oxidation procedure. If there is need to distinguish between inorganic and organic sulfur, the material can be ashed prior to determination of sulfur by wet Oxidation on the ash. However, for the normal tire and tube stock, results are also in close agreement with the standard gravimetric barium sulfate procedure following nitric acid oxidation.
100 (97-103)
The dat,a of Table I1 illustrate the application of the method :t wide varietv of complex compounded rubber products. St) difficulty was experienced in analysis of natural, GR-S, but,yl, neoprene, or chlorinat,ed rubber-base polymer stocks. Insolubility of titanium dioxide in white side-wall t,ire stock requires an extra centrifugation, to remove turbidity, but no ot,her modification was necessary for any stock analyzed. Other Methods of Measurement. The amount of lead sulfate prwipitate can be measured in different ways-polarographic, volumetric, gravimetric, turbidimetric, or nephelometric techniques. The volumetric technique might involve the well known, but difficult, visual end point titration with tetrahydroquinone indicator, or amperometric titration (8). However, the ultraviolet absorptiometric procedure is considered equal or superior in accuracy and reliability to any of these methods. I n addition, its high sensitivity makes possible analyses on a micro scale. I t was desired to demonstrate that the procedure for sample preparation with perchloric acid and acetone could be used to advantage with other methods of measurement. Results of a to
V O L U M E 27, N O . 10, O C T O B E R 1 9 5 5 gravimetric seiiiiiiiicroprocedure are given in Table 111. Sonic modifications were necessary for the gravimetric method. Samplr size ranged from 5 to 10 mg. for high-sulfur compounds and From about i 5 to 100 mg. for lowsulfur compounded rubber prodw t s . Sulfur content was kept below 5 mg., which means a wighahle lead sulfate recovery below 50 mg. Amount of reagrrit, per mriiple was increased t o 100 mg. of lead nitrate and l to 2 nil. of concentrated nitric acid, followed by 3 or 4 nil., of fuming nitric acid added in two or three treatments. Perchloric :wid was added up to 10 drops for organic compounds contniniug no carbon I~lack,and up to 20 drops for compounded r u l h r r itociks;. Volume of acetone wash was maintained a t about, 10 nil. 1)rr w:ish, hut four washes viere used. Close attcntion to complete disintegration of rubber products \\.it11 funiing nitric acid prior to addition of perchloric acid is rswntial. No trouhle was encountered n-ith perchloric :wid oxidation of natural and GR-S rubber stocks. However, the rtlliitivcly large (100 mg.) sample of hard-to-oxidize but'yl rubber may oxidize viith explosive violence unless completely disintcgrated hefore the nitric acid h:rs evaporat,ed. h prelimiiiary svelling in chloroform and evaporation of the chloroform prior to arid treat,ment were used to speed oxidation of hutyl rul)Iicr in iiitric acid. If a significant amount oi :icitl-insoluble mat,erial from t,hc sample is present, it is necessary to dissolve the lead sulfate in 50% hydrochloric acid, centrifuge. and weigh the sample tubc agtiin to r o m c t fox the acid-insolu1)le. S o correction was niatie for any trace of arid-in .Itmiice of any signifi sulfate will shovi where Petroleum Chemistry. The general application of this method to fields other than ruhher chemistry is suggested by successful malyeis of petroleum oil softeners. The somewhat lower results obtained may indirate the presence of volatile sulfur compounds, hut agreement of the perchloric acid and oxygen bomb methods
Table 111.
Stoch
1623 is close enough to make the wet oxidation satisfactory for routine analyses. Regular analysis by oxygen bomb methods could be speeded b y using lead nitrate and measuring the lead sulfate formed by the absorptiometric method outlined here. For critis are concerned, the sample cal analyses where volatile ma tube method in the presence niay he oxidized by the Carius s of lead nitrate. Perchloric acid is then added when the t,uhe is opened and the regular procedure outlined here is followed thereafter. Absolute Structural Analyses. The method appears to tie of value for absolute determination of sulfur content in securing (lata for chemical formula reconst,ruction. Analysis of purified tetramethyl thiuram disulfide and high purity sulfur shoivs an average of 99 to 100% recovery. The perchloric acid procedure outlined here has been simplified for rapid routine determination of sulfur and some precision and accuracy have been sacrificed t.0 speed up the analysis. Modifications, such as a somewhat larger sample with 100 ml. or more of exactly 50% acid, should make the accuracy comparable to that of other methods now in USP. A preliminary slow oxidation with nitric acid and escess bromine a t room temperature prior to heating, or use of conventional Carius or bomb techniques, should eliminate loss of trace amounts of volatile sulfur compounds. Accuracy. The accuracy is d i f f i d t t o prove in analysis of i,ut)her products because sulfur is present in several compounding materials (see Table I). T o prepare a standard stock of known true total sulfur content would require a prohibitive amount of work, involving determination of sulfur in every material added t o the stock. However, a p r a r t i d :tlternative is to consider only added sulfur as such and that pi n t in accelerators added, in calculating theoretical pcr cent d f u r . .Inalysis of d:tt:L of Table I V indicated the ahsorpt8ionietric method recovcrs 97 to 108% of theoretical. This is good, considering the many f:trtors affecting the theoretical per cent sulfur.
Gravimetric Sulfur as Lead Sulfate by Perchloric Acid Oxidation Semimicrornethod Sulfur Theoretical, %
5 by BaSOr (ASTW. %
Gravimetric Lead Sulfate Sample, PbSOa, tug. mg. Sulfur
Natural tire tread
2.14
Compounded Elastomer Stocks 2.08 75.0 lL3 ( 2 . 0 5 t o 2 12) 100.8 19.1
GR-S tire tread
1.26
1.31 il 2 8 t o 1 . 3 4 )
104.7 92.6
12.6 11.6
Butyl tube
I 38
1.35
112.4 92.8 102.0 97.2
13.4 10.9 11.2 11.5
38 6
TIIT11 (purified)
53.4
of Theoretiral
07.2
hv.
2.08 103
AI..
1.27 1.33 1.30 1 27" 1 .24"
89
Elastomer Compounding Materials 2.82 10.1 5.24 17.9 10.19 34.6 Av.
Sulfur (commerrial of high purity)
Recovery, 7c
c%
2.18 2 01
AV.
N B T S (purified)
-
1.17 1.25 1.23 3 7 . .. 2 . 03
hv.
1.18
1.88 1.44
1.39
1.38
1.38 1.35
1 91
1 38
1.36
Summary of Data
Av. for sample. % Max. deviation Mean deviation
2.09 -0.18 =0.06
1 3(3 -0.08
*n.oz
-0
1,34 16
c o . 06
Comparison of .IST.\I Gravimetric and Absorptiometric Methods Av. absorptiometric S,5% 2.00 1.36 1.34 Theoretical S as conipounded, ‘70 2.14 1 20 1.38 Av. gravimetric S. “c 2 04 1.31 ... absorptiometric 9, % 0 08 1.08 0.97 Ratio theoretical S,c70 absorptiometric 9, 7 ‘ 1.02 1 04 ... Ratio - graximetric 7 ~ b, ‘70
ACKVOWLEDGJIE\T
The author acknowledges the help of Jack Z. Falcon with development work. His interest in perchloric acid m an oxidant aided materiallv in developing the final procedure. Many analvsts of the Firestone analyticd laboratories helped to determiqe the precision of the method and obtained data on its application t o diverse ruhher compounding materials. The greater part of the gravimetric barium sulfate data in tables was secured by R. 0 . Lankenau. The permission of the Firestone Tire and Rubber Co. to publish this paper is gratefully acknowledged. LITERATURE CITED
(1) Am. Soc. Teqting Rlaterials, Philadelphia, Pa., ‘‘ASTM Stand-
ards. Chemical Analysis of Rubber Products,” Method D 297-437,1952. (2) Bennet, E. L.. “Quartz-Enclosed Carbon Crucible for Induction (3) (4)
(5) (ti) (7)
Furnace,” Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. 1054. Brock, 11.J . , and Loutli, G. D., ASIL. CHEM.,27, 1575 (1955). Hillebrand, W. F., and Lundell. G. E. F., ”Applied Inorganic r\nalysii.” Ki!ey, Kew York, 19%. Kress, K. E.. -1x.i~. CHEM., 23, 313 (1981). Kress. I(.E.. and Neew, F.G. S., I b i d . , 27, 525 (1955). Nerritt, C., Hershenson, H. AI., and Rogers, L. B., Ibid., 25, 572 (19,53).
(8) Rulfs, C. L.. and Maekela, .\. d.,Ibid., 25, 660 (1953). (9) Smith. G . F., “Mixed Perchloric, Sulfuric, and Phosphoric .kcids
and Their Application in Analysis,” G. Frederick Smith Chemical Co., Columbus, Ohio, 1935.
RECEIVED for review
April 20, 1955.
Accepted July 14, 1955.
