The net colorimetric readings plotted against the concentration of sesquimustard per milliliter on coordinate paper resulted in a straight line showing that Beer's law was readily satisfied, The same procedure was followed using standard solution of mustard in ethyl alcohol t o determine the extent t o which mustard entered into this reaction. These data are plotted in Figure 1. #?(P-NITROBENZYL)PYRIDINE
Standard solutions of sesquimustard and mustard in ethyl alcohol were prepared and analyzed by this procedure. The data obtained are plotted in Figure 1. From the slopes of the four curves, equations were established and expressions for sesquimustard and mustard were derived as follows:
W S = 0.714 RA - 0.00539 RE W M = 0.253 RB - 0.671 RA where = y
of sesquimustard per milliliter of solution
Net Colorimeter Quantity Reading Recovered, Y Units Y RE S 11 RA 21 32 28 212 19 35 48 32 44 314 30 50 80 54 76 524 51 82 71 640 114 2800" 66 642 a Dilution, 1 to 10. RA = colorimeter reading by coppero-dianisidine. RB = colorimeter reading by 4-(p-nitrobenzy1)pyridine. Quantity Sought,
TEST.
An alkylation method of analysis ( 2 ) was modified slightly so that ethyl alcohol could be used as the solvent in place of diethyl phthalate. Instead of heating the mustard and sesquimustard with 4(pnitrobenzyl)pyridine reagent for 5 minutes at 100" C., the bath temperature was lowered t o 70' C., and the heating time increased to 15 minutes. These conditions gave reproducible results.
Ws
Table 1. Recovery of Sesquimustard (S) and Mustard (M) from Mixtures
W M= y of mustard per milliliter of solution RA = net colorimeter scale units obtained by copper-o-dianisidine method RB = net colorimeter scale units obtained by 4-(p-nitrobenzy1)pyridine method Several mivtures of mustard and sesquimustard were prepared and analyzed by the methods described above. Recoveries greater than 90% with an accuracy to &3% in the 75-7 range were obtained. The data are shown in Table I. I n handling field samples, the filter mats having sesquimustard and mustard on the collection surface, were added t o a given volume of alcohol.
Aliquots were taken from this extract and analyzed by the two methods, and the quantity of each compound was calculated by substituting the colorimeter readings in the equations. LITERATURE CITED
(1) Copley hi. J., Foster, L. S., Bailer, J. d., Jr., Chem. Revs. 30, 227
(1942). (2) Geckel,' P. T., TCR 80, Chemical Corps, Chemical and Radiological Laboratories, Army Chemical Center, Md., "Colorimetric Method for Estimation of Mustard and Sesqui in Mixtures of Mustard and Sesqui," February 1951. (3) Hanker, J. S., Master, Irwin, Mattison, L. E., Witten, Benjamin, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February 1956. (4) National Defense Research Council, Division 9, OSRD 4288, Sept. 30, 1944. ( 5 ) Randles, J. E. B., J. Chem. SOC. 1941,802. (6) Tschugaeff, L., Ber. 41, 2222 (1908). (7) Tschugaeff, L., Compt. rend. 154, 33 (1912). (8) Tschugaeff, L., Kobejanski, A., 2. anoru. Chem. 83. 8 (1913). (9) Tschugaeff, L., Subbotin, W., Ber. 43, 1200 (1910). '
RECEIVEDfor review January 5, 1957. Accepted Xovember 15, 1957. Division of Analytical Chemistry, 130th Meeting, ACS, Atlantic City, N . J., September 1956.
Photometric Determination of Zinc Oxide in Rubber Products Absorptiometric and Turbidimetric Methods Using Sodium Diethyl Dithiocarbamate K. E.
KRESS
The Firestone Tire & Rubber Co., Akron,
b Zinc is precipitated from a weakly alkaline medium as zinc diethyl dithiocarbamate, which is then extracted with ether. The absorbance of the complex in ether solution may b e measured a t 262, 280, or 295 mp. Alternatively, the absorbance of a colloidal aqueous suspension of the precipitate may b e measured a t 448 mp. Interference from other cations is uncommon but is detectable when it occurs, and can b e corrected for b y an absorbance difference method. W e t ashing with perchloric acid on a micro scale eliminates errors observed 432
ANALYTICAL CHEMISTRY
Ohio with conventional dry ashing of some rubber products.
Z
INC OXIDE added to activate vulcani-
zation is the ,most important isorganic material in nearly every type of rubber product. The ASTM standard volumetric procedure for zinc oxide determination in rubber products calls for ferrocyanide titration with uranyl acetate as a n external indicator (1). This method has proved less satisfactory than titrations with an internal indicator such
as diphenylbenzidine ($0). However, the slow rate of titration required with diphenylbenzidine and the difficulty in determining the exact end point have at times resulted in considerable error. The need for removal of interfering iron and limitation of the method to samples preferably containing in excess of 100 mg. of zinc oxide for accurate results complicates the analysis. Other volumetric methods such as iodometric titration and precipitation as the oxalate have been proposed, but as far as is known, they have not been applied to analysis of rubber products.
The Spectranal, a visual spectroscopic instrument, offers a rapid method of determining semiquantitatively the zinc oxide content of rubber ashes ( I d ) , but a more accurate method was desired Dithizone reagent and its analogse.g., dinaphthylthiocarbazone-were the only reagents known up to 1950 which permitted satisfactory direct colorimetric determination of traces of zinc (18). Determination of zinc by dithizone is subject t o interference from several other metals which also form colored compounds with the reagent. The critical control of p H and the volume of dithizone reagent (in mixed color methods particularly), as well as the photosensitivity of the colored complex, are additional drawbacks of the method. Use of sodium diethyl dithiocarbamate as a complexing reagent to remove interfering metals has been reported (6), but empirical calibration methods were needed to correct for partial reaction of the zinc with this reagent. iln ultraviolet absorptiometric procedure has been proposed ( I C ) more recently for determination of zinc as the 1,lO-phenanthroline complex. However, actual application to zinc was not reported and elimination of anticipated interference from iron and other elements was not investigated. Recently a colorimetric reagent for zinc, called Zincon (2-carbosy-2'-hydroxy-5'-sulfoformazylbenzene) was described (17). With this reagent copper and many other elements give colored reaction products which cannot be distinguished from zinc, and an ion exchange procedure was required to separate zinc prior to its determination. In rubber chemistry zinc is always the major component, but it is a definite advantage to be able to detect and identify certain interfering elements during the course of the zinc determination. To make this possible, the interfering elements should yield products whose absorbance is distinctly different from that of zinc, which apparently is not the case with Zincon. Zinc diethyl dithiocarbamate is a stable colorless compound with strong absorbance in the middle ultraviolet region. This has been used as a basis for quantitative control of the commercial chemical (11). Sodium diethyl dithiocarbamate is the reagent commonly used to determine traces of copper in rubber (1). Chilton (4) has used the reagent to determine copper, nickel, and cobalt simultaneously by measuring absorbance in the visible and near-ultraviolet spectral region above 300 mp. Investigation proved that an ultraviolet absorptiometric method for zinc using sodium diethyl dithiocarbamate reagent offers several important advantages over titration methods now in use. Use of this reagent for turbidimetric estimation of traces of zinc in water has
been reported (3), but the nature and amount of possible interferences were not thoroughly investigated, and accuracy of quantitative analysis was limited. Turbidimetry is not usually considered as accurate or reliable as absorptiometry. However, the simplicity of a turbidimetric method, as well as the facts that water is the only solvent needed and that measurements may be made in the visible region, are definite advantages.
EQUIPMENT
Cylinders, 25 and 50 ml., graduated to 1 ml., with close fitting, ground-glass stoppers. Borosilicate glass beakers, 5 ml. Stirring rod with short platinum wire bent a t an angle a t the end. Section of 2- t o 4-cm. diameter glass tubing restricted a t one end to serve as an adsorption column for ether purification. Beckman Model DU spectrophotometer with conventional ultraviolet accessories. including - 1.000 0.005-cm. quartz cells. Beckman Model B spectrophotometer with 1.00-em. Corex cells.
*
REAGENTS
Chromatographic grade adsorption alumina of 80 to 200 mesh (Fisher Scientific Co.). Anhydrous ethyl ether, ACS reagent grade, purified further by filtering through a short column of alumina (about a 4-cm. depth in a column 4 cm. in diameter) to remove iron. This purification step also removes peroxides ('7). Hydrochloric acid, 10% by volume. Dilute 10 ml. of 37% acid to 100 ml. with distilled water. Sodium carbonate, approximately 10%. Dissolve 10 grams of reagent grade anhydrous sodium carbonate in distilled water, dilute to 100 ml., and purify (see below) for absorptiometric analysis. Ether extraction should never be used to purify this reagent for turbidimetric analysis. For the turbidimetric reagent no more than a trace of yellow coloration or turbidity should develop when a few drops of 1% carbamate reagent is added t o an aliquot of this sodium carbonate solution. If absorbance a t 448 mp is above about 0.050, a new and purer source of sodium carbonate should be sought. Sodium diethyl dithiocarbamate, approximately 1%. Dissolve 1.0 gram of reagent grade material (Eastman No. 2596) in distilled water and dilute t o 100 ml. Purify with ether as described below. Store in an actinic brown bottle. This is referred t o hereafter as 1% carbamate reagent or indicator. Ammonium hydroxide, 10% by volume. Dilute 10 ml. of the concentrated reagent to 100 ml. with distilled water. Ammonium hydroxide, concentrated. Purify as described below,
Gum arabic, about 5%. Make a paste with distilled water and 5 grams of reagent grade gum arabic, then dilute to 100 ml. There should be no detectable increase in yellow coloration or turbidity when an aliquot of the filtered solution is tested by adding a few drops of 1% carbamate reagent. Never heat this reagent. Standard zinc solution, 500 p.p.m. Keigh 0.1000 gram of zinc metal and place in a 200-ml. volumetric flask. Add 2 mi. of concentrated hydrochloric acid and allow to stand until all the zinc is in solution, warming if necessary. Dilute to the mark with distilled wster. All aqueous alkaline reagents for the absorptiometric method must be purified before use. Add about 2 drops of 1% carbamate reagent per 100 ml. of reagent to be purified, then extract with purified ethyl ether to remove any trace metal impurities (such as copper in ammonia) which form a carbamate complex. Extract three or more times until the ether is colorless, discarding the ether each time. The sodium carbonate may be warmed to drive off the ether, but do not warm the ammonium hydroxide. These purified reagents are usable for 1 month or more.
METHODS
Absorptiometric. CALIBRATION. Determine the absorptivity, a, of the zinc diethyl dithiocarbamate solution in ether a t each wave length where data are to be recorded. The absorptivity is reproducible enough so that in many cases the values obtained on one instrument may be used for all instruments of the same make in the laboratory (if they are in good operating condition). For greatest accuracy, however, it is preferable to calibrate individual instruments. A fixed slit width may be assigned, but as the absorbance maximum (262 mp) is relatively broad, the slit width is not normally critical. Dilute the standard zinc solution 1 to 10 and use different volumes to establish the calibration curve. Calibrations in this laboratory gave absorptivity values of 550, 282, and 85 a t 262, 280, and 295 mp, respectively. PROCEDURE.Weigh 2 t o 4 mg. of a well-milled, homogeneous sample to the nearest 0.01 mg. if absorbance is to be measured a t 262 mp. If absorbance is measured a t 280 mp, use a 5- to 10-mg. sample. Place in a 5-ml. borosilicate glass beaker and ash a t 550" C. for 30 niinut8es or until completely ashed. Alternatively, drive off volatile matter a t 550" C. for 5 to 10 minutes and complete ashing in a muffle a t 800" to 1000° C. by placing the beaker inside the door just long enough to burn off the carbon black (1to 2minutes). Remove from the muffle carefully t o avoid loss of ash and allow to cool somewhat. TF'hile still warm to the touch, add about 1 ml. of 10% hydrochloric acid from a dropping pipet to wash down the side of the beaker. If necessary, break up any caked residue VOL. 30, NO. 3, MARCH 1958
433
with the qtirring rod tipped with platinum wire and stir the sample a little. Add 2 to 4 ml. of distilled water, using a small jet from a n-aqh bottle to aid mixing the sample. then pour the solution into a clean, glass-stoppered, 50nil. graduated * cylinder. Kash the beaker twice m-ith 1 t o 2 ml. of water and add to the cylinder. Keep the total volume below 8 ml. Add litmus paper and then 10% ammonium hydroxide drop by drop, while shaking vigorously, until the aqueous solution is just alkaline. Then carefully add 1 drop more. From a 50-ml. buret, add 0.5 i 0.1 ml. of sodium carbonate reagent. Mix, then add 0.5 ml of carbamate reagent. Dilute carefully to 10.0 ml. with distilled lvater, then dilute to the 50.0-ml. mark with purified ethyl ether. Stopper the cylinder and shake vigorously for about 30 seconds. Clean the 1.00-cm. quartz cells with ether and check for transparency correction if necessary. (Disregard the transparency correction if the absorbance with ether alone is 50.005 or less.) Place purified ether in the first cell as an instrument blank. Decant the separated ether layer of the reagent blank and the sample solution into the remaining cells. Take readings a t the 262-mp absorbance maximum of zinc diethyl dithiocarbamate in ether. If the absorbance is over 1.8, repeat the measurement a t 280 or 295 mp to provide three levels of sensitivity for the determination. This eliminates the need for dilution or concentration of the zinc solution. If the ether solution is colored or if lead is present, add 10 ml. of purified concentrated ammonium hydroxide directly to the sample in the 50-ml. graduate. Stopper and mix vigorously to re-extract the zinc into the 50YGaqueous ammonium hydroxide layer. dgain measure the absorbance of the ether layer a t the same wave length as used for the initial measurements, use this reading as the blank absorbance. The loss in absorbance is that due to zinc, while residual absorbance is due to interfering elements. There should be no visible trace of yellow coloration in the ether solution when observed through its 12-em. depth against a white background. Avoid mistaking the reflected color of precipitated ferric hydroxide as coloration in the ether layer. \Then the ether layer is uncolored, any interference from iron, copper, cobalt, nickel, or bismuth may then be considered negligible, particularly if the absorbance of the zinc complex itself a t 262 nip is relatively strong (above 0.5). CALCULATION.
where
A, is absorbance measured at a given wave length ilb is absorbance of the blank a t the same wave length 434
0
ANALYTICAL CHEMISTRY
azn is absorptivity a t the same wave length as determined with reagent grade zinc metal, with concentration (c) in mg. per nil. or grams per liter (azn =
A/c)
Cell thickness is a constant and need not be included in the calculation. BLANK.Always run a reagent blank.
It should be measured against ether as the instrument blank rather than used to zero the spectrophotometer. This ensures that the reagent blank is low enough to provide satisfactory accuracy. A blank above 0.050 a t 280 mp indicates that some reagent needs further purification. A blank absorbance as high as 0.10 may be tolerated if the sample absorbance is relatively high, preferably more than 10 times that of the blank. Any dilution of the sample (ether extract) requires a comparable dilution of the blank. SPECIALSAMPLES. White Sidewall Stock. Black Stocks of High Zinc Oxide Content. Reduce sample size to 1 to 2 mg. and warm the ash with the acid. Normally add 2.0 ml. of carbamate reagent; this is adequate for complete zinc precipitation. Measure the absorbance a t 280 mp; less accurate data a t 295 mp may be recorded if needed. Neoprene Stock. In the case of all stocks containing neoprene (black or white), much of the zinc present is in the form of zinc chloride in a cured stock. Always use either wet-dry ashing with sulfuric acid or wet ashing with 1 ml. of concentrated nitric acid and 3 drops of perchloric acid to avoid zinc loss by volatilization. Foam Rubber. Reduce the sample size to 1 to 2 mg. Absorbance measurements a t 280 mp may be required. If there is reason to believe all zinc present is not recovered by dry ashing, wet ash the sample. Organic and Inorganic Zinc Salts. Follow the procedure for white sidewall stock. Weigh powdered pigments in small capillary tubes if not easily pelletized. Use wet-dry ashing with sulfuric acid to retain all zinc, or dissolve directly in acid where possible. Dry ashing should be tried first; wet-dry or wet ashing should be used when dry ashing is inadequate. Turbidimetric. CALIBRATION. With the standard zinc solution determine the “apparent” absorptivity due to turbidity for each instrument. The apparent absorptivity, a , is the slope of a plot of absorbance for the suspension us. zinc concentration in milligrams per milliliter. It should be used in calculations only over the absorbance range that is linear. The absorptivity determined in this laboratory was 100 for a 15-ml. volume. PROCEDURE. For zinc oxide content in the range of 0.5 to 4% weigh 10 to 15 mg. of sample to the nearest 0.01 mg. Ash as for the absorptiometric method. Dissolve, then pour the ash into a 25-ml., graduated, glass-stoppered cylinder. Wash the beaker twice with 1 to 2 ml. of water. The sample volume should be between 5 and 7 ml.
Add litmus paper t o the solution and neutralize with sodium carbonate reagent added 2 drops a t a time with vigorous shaking in between (this is important) to drive off carbon dioxide. Add exactly a 3.0-ml. excess of sodium carbonate from a 50-ml. buret. Then dilute t o exactly 14 ml. with distilled water and mix by stoppering and shaking. -4dd 3 to 5 drops of gum arabic and mix again. Finally dilute to 15.0 nil. with 1% carbamate indicator and mix immediately. Allow the sample to stand for 5 minutes or longer. Pour the sample into 1.00-cm. Corex cells to within about 0.5 inch from the top. Record the sample absorbance, A,, a t 448 mp with the Beckman Model B spectrophotometer, using water in the first cell as blank. Then remove the cell holder and add 4 drops of concentrated ammonium hydroxide to each sample cell. Stir by moving a small glass rod up and down in the cell until all the turbidity due t o zinc has dissolved. Measure the absorbance of the sample again and record this as the blank absorbance, A b . Calculate zinc oxide using the equation given for the absorptiometric method.
ASHING
Dry. Much work has been done with controlled ashing a t or near 550’ C. in determining the copper content of natural rubber (2). The micro-sized sample (normally 2 to 4 mg. and short ashing time a t 550’ C. reduce, but do not eliminate, the possibility of loss of zinc oxide through volatilization. Presence of considerable carbon black after the ashing does not interfere with the absorptiometric method if other organic matter has been burned off, thus enabling the acid t o come in contact with the zinc oxide. Although borosilicate glassware is preferred for the ashing, it may contain about 10 p.p.m. of zinc (18). Other chemical glasswares may contain up to about 10% zinc oxide (9). However, determinations on new beakers and those which had been used repeatedly shoxed no detectable zinc before or after heating in the muffle a t 550’ for 30 minutes. As a precautionary measure though, all glassware for the determination should be thoroughly washed with a strong solution of hot hydrochloric acid before the first use, then reserved for this determination only. Wet-Dry. Part of certain organic compounds of zinc added t o the stock or formed during the vulcanization process may be lost through volatilization on dry ashing (Tables I and 11). Zinc in the form of zinc chloride, as in the case of neoprene stocks, may also be lost on dry ashing. This is particularly true with a macro-sized sample (1 gram)
Table I.
Effect of Method of Ashing on Zinc Recovery from Rubber Compounding Materials
yo Zinc Foundb Wet-dry Present in Stock Theoretical ash a t Direct Formed on % Zinc in Dry ash 550" C.; solution Added with ZnOa vulcanization Pure Material at 550" C.c H2SOa usedd in HCl Zinc oxide ... 80.2 ... ... 81.211 5 Zinc sulfide 67.1 62 61 Stearic acid Zinc s'tkarate 11.5 13.2 f 0 . 6 1 3 , i G0 . 3 ... Neoprene (tire white- Zinc chloride 47.9 14 47.9 1 0 . 4 50 wall) MBT (mercaptobenzo- Zinc RlBT 16.4 14.2f0.2 15.4i0.4 ... thiazole) TETD (tetraethyl Zinc TETD 18.1 10 to 14 17.2 zk 0 . 4 ... thiuramidisulfide) a Commercial grade except zinc oxide and zinc chloride, which were reagent grade. Plus-minus values represent mean deviation from average of 2 or 3 determinations. c Micro-sized sample inserted directly into muffle with adequate air supply. Evaporated to dryness with a few drops of sulfuric acid before placing in 550' C. muffle.
in a closed muffle a t constant high temperature (550' C.), where oxygen may be limited. However, for normal tire and tube compounds the loss of zinc oxide in this manner appears to be negligible. A combination of wet and dry ashing ensures total recovery of zinc from neoprene rubber products which may contain zinc chloride (15). The sample is first taken to dryness on a hot plate with a few drops of concentrated sulfuric acid, then ashed in the 550" C. muffle as usual. Wet. Certain samples, notably the high silicate samples referred to by Tyler (.20), may need to be wet ashed. The use of a nitric acid-perchloric acid mixture for rubber oxidation has often been demonstrated (119, 20). However, despite its recognized convenience the use of perchloric acid for wet ashing in rubber chemistry is restricted in routine
Table II.
1.6
-
1.4 12
y
IO
.
i080.6. 0.4
-
02
-
200
20
40 WAVE
Figure 1.
--_-
60
80
LENGTH
WO
20
(mA)
40
Absorbance of carbamates Zinc in ether Sodium in 2% hydroxide
aqueous ammonium
cl, Recovery of Theory
Dry ash
Wet-dry ash
92 114 29
117 100
86
94
66
95
Direct soln. 101 91
104
analysis because of the potential explosive hazard when used on a macro scale. These hazards are increased by the introduction of synthetic rubbers, such as Butyl and Vistanex polymers, which are harder to oxidize than natural rubber. Oxidation of samples of Butyl rubber of about 100 mg. with either concentrated or fuming nitric acid mixed with a few drops of perchloric acid has resulted in explosive oxidation. The sensitivity of the absorptiometric procedure makes it possible to work in a micro sample range, where perchloric acid oxidation is considered safe. After most of the work reported here had been completed using a dry ashing technique, the following modification of the wet ashing procedure was developed. Digest a 5- to 15-mg. sample in a borosilicate glass test tube (16 X 100
Comparison of Volumetric and Absorptiometric Methods for Determination of Zinc Oxide in Rubber Products
Product Tire stocks Natural tread Synthetic GR-S tread
% ZnO Found Absorptiometricb Dry ash Wet ash a t 550" C. xvith HC10,
Av.
yo Recovery of ZnO ilbsorptiometric
% ZnO Added
1-olumetrico
1.85
...
1.83i0.02 L 7 6 f.O.05
1.82 3Z0.02 1.82 i0.02
1.49
1.3gC
1.51 5 0.06 1.45 f 0 . 0 3
1 . 4 8 i 0.04 1 . 4 0 3Z 0.00
36.0 5 0 . 2 3 8 . 4 3Z 0 . 8
35.0 1 0 . 4 37.6 f 0 . 4
6.00 1 0 . 2
...
92.6
...
...
4.21'&0.10
16.4 (wet, 103) 9.2
86.5
lOOd
101
lOOd
Volumetric
93.2
Dry ash
Wet ash
97.2
98.5
101 97.2
99.3 94.0
101 98.1
98.1 96.1
White side walls WSW-89 91 4.98%asZnS
wsw
Mechanical goods Bushing Foam rubber F
35.75 34.93 4.16 39.09
6.02
5.58
s-1
... ...
s-2
...
5 37 0.69 (Wet, 4.31) 0.46
a
b
5,52fO.l 3 , 5 8 f 0.12 3.71 1 0 . 0 1 3.54 f0.07
3.50&0.16
99.7
Titration with potassium ferrocyanide using diphenylbenzidineindicator; dry ashing. Plus-minus values are mean deviation. Average of best data by six different analysts. Recovery by wet ashing arbitrarily set at 100%.
VOL. 30, NO. 3, MARCH 1958
435
V
' 02
I
0.4
VOLUME
Figure 3.
0.6 OF
OB 1%
2
4
6
8
VOLUME % CONCD. NH40H
Figure 2.
IO 12 14 16 IN APUEOUS PHASE
Factors affecting extraction 0
A X
1 -minute shaking 2-minute shaking 4-minute shaking
mm.) with about 0.5 ml. of fuming nitric acid and 5 drops of 70% perchloric acid. After complete oxidation add 1 ml. of water and 2 drops of purified saturated ammonium chloride solution, then transfer to a 50-ml. graduate with water. Neutralize the sample to litmus with sodium carbonate (no ammonia) and proceed with the absorptiometric method,
More critical application to the determination of zinc in brass-plated, wire tire cords indicated that excess ammonium hydroxide should be controlled closely for best results. Following the recommendation of Chilton (6),who also found excess ammonium hydroxide a critical factor, the use of sodium carbonate t o replace ammonium hydroxide was investigated. Because sodium carbonate did not precipitate the iron as EXPERIMENTAL W O R K completely as did ammonium hydroxide, it was added to adjust the alkalinity afAbsorptiometric Method. PRECIPI- ter the iron had been precipitated with ammonium hydroxide. The amount of TATION. The ultraviolet absorbance sodium carbonate was not otherwise curves of the zinc and sodium salts of critical, and zinc was quantitatively exdiethyl dithiocarbamic acid are illustracted even in the presence of an excess trated in Figure 1. The sodium salt is relatively insoluble in ether and reas high as 25% in the aqueous layer. The amount of excess carbamate remains in the aqueous alkaline layer. Thus, there is negligible direct interagent should be kept low to reduce the blank absorbance. Figure 3 shows that ference from the reagent, even though 1 ml. of 1% carbamate reagent just it absorbs strongly and selectively in the gives quantitative recovery of 100 p.p.m. ultraviolet. of zinc in the aqueous phase. For conThe data of Figure 2 show that zinc venience, about 0.5 ml. of 1% carbammay be recovered quantitatively if the ate reagent is used with a sample up to excess concentrated ammonium hy4 mg. containing up to 10% zinc oxide. droxide is between 1 and 3% by volume Actually, 0.2 ml. is sufficient for normal in the aqueous phase. This is not a pH tire and tube stocks. The analyst may sensitivity, as the alkalinity for 1 to 3% calculate the minimum requirements for ammonium hydroxide is beyond the higher zinc oxide concentrations from range of accurately measured pH. Figure 3. Below 1% ammonium hydroxide] the REMOVAL OF IRON.In colorimetric absorbance is high because of partial analysis with carbamate reagent it is extraction of the reagent itself, as shown common practice to add a complexing in Figure 2 by the absorbance of the chemical such as citric acid (4, 18) to blank. prevent precipitation of ferric hydroxide. The shaking time is not critical for The collector action of ferric hydroxide ammonium hydroxide concentrations for zinc and the inhibiting action of amfrom 1up to about 4%. Slightly higher monium chloride are illustrated in Figabsorbance at 2 or 4 minutes above that ure 4. At iron concentrations above for 1minute in this 1 t o 4% range is duabout 200 p.p.m. (Fe/Zn ratio = 20), a plicated by the blank and would be subtrace of iron not precipitated bytheslight tracted in a normal determination. 436
ANALYTICAL CHEMISTRY
1.4
1.6
REAGENT
1.8
2.0
ADDED
22
(MLa
Optimum concentration of 1 % carbamate
Ir I
1.0 1.2 CARBAMATE
excess of ammonium hydroxide then reacts with the carbamate reagent to form the extractable ferric salt of diethyl dithiocarbamic acid. This causes high absorbance and creates a positive error. As long as the iron content of the sample is lon-er than the zinc content, the interference of iron through this collector action is insignificant for rubber stock analysis, particularly if a small amount of ammonium chloride is present. EXTRACTION SOLVEKT.Carbon tetrachloride is almost universally used for the extraction of metallic thiocarbamate complex salts (4, 18). Several other solvents, all heavier than water have been used. In the normal extraction procedure with an extraction funnel solvents lighter than water have been considered inconvenient (18). Ethyl ether gives a more rapid and cleaner separation from aqueous ammonia than does carbon tetrachloride. It may be used as a solvent down to about 220 mp in the ultraviolet, whereas carbon tetrachloride is unsuitable below 265 mp; chloroform cuts off a t 246 mp. Reagent grade anhydrous ethyl appears to be easier to purify than carbon tetrachloride (18) as percolation through an alumina column is all that is required. Purity of the extract is ensured by absence of a stopcock lubricant on the ground-glass stopper of the cylinder. This is of more consequence in the middle ultraviolet region below 300 mp, where a colorless stopcock lubricant may absorb appreciably. The absorbance of an ether solution of zinc diethyl dithiocarbamate is essentially unchanged after 24 hours' standing in a window exposed to normal daylight, but not to direct sunlight. The oxidizing nature of ether (due to a trace of free peroxide) is believed to form more stable dithiocarbamate solutions than are formed with chlorinated solvents. Ethyl ether is considerably less toxic than carbon tetrachloride (10). The main disadvantage is its flammability. The volatility is objectionable, but as the ether solution is kept in a tightly stoppered cylinder until just prior to
AT
0$0
350
450
400
550
500
650
600
750
700
(m,N
WAVE L E N m
Figure 5. Absorbance of zinc carbamate colloidal sol 0
IO
20
30 40 50. 60 70
IRON/ZINC
RATIO
IN
80 90 100
AQUEOUS
PHASE
Gum arabic a d d e d immediately after carbamate Gum arabic a d d e d before carbamate No gum arabic Reogent blank
Figure 4. Interference of iron when present a t higher concentration than zinc 10 p.p.m. zinc in all samples 0 No ammonium chloride ( 1 drop of ammonium hydroxide) A Ammonium chloride ( 3 drops of hydrochloric acid 4 drops of ammonium hydroxide)
+
iI/ 1.6
-6.61
e \
\ \ \
measurement, this is no great problem. Evaporation during measurement is easily minimized t o insignificance by the required rapid measurement of a single absorbance a t the same wave length for each sample. Turbidimetric Method. The absorption of a turbid colloidal suspension of zinc diethyl dithiocarbamate in the visible region is illustrated in Figure 5 . The sloping curve exhibits no wave length of maximum absorbance. The turbid suspension cuts off or scatters more light in the blue region a t 400 to 450 mp than in the red around 600 mp. The reason for this strong absorbance in the blue region is made apparent by visual examination of the suspension. In a cylindrical test tube or graduate, the appearance is that, of a white turbidity due to diffraction or reflection of light. Hon-erer, by direct transmitted light between the flat parallel planes of the cell windows, the colloidal suspension a t moderate concentrations has the appearance of a clear yellow solution. Much below 400 nip, the sodium diethyl dithiocarbamate reagent absorbs strongly in the aqueous medium and the reagent blank absorbance would be excessive here. Therefore, it is advisable to make absorbance measurements a t some wave length above 400 mp. Measurements mere made to determine the range of zinc concentration over which the absorbance of the turbid colloidal suspension of zinc carbamate would be linear. Measurements a t 400, 448, 500, and 600 mp indicated that the
closest approach to lincarity was near 448 mp. COLLOIDALDISPERSANT. An attempt was made to use a 1% aqueous methyl cellulose solution as dispersant. HoIl-ever, it foamed excessively when the sample was shaken and offered no advantages which would outweigh the use of gum arabic. The order of addition of the gum arabic affected the intensity of absorbance greatly a t low alkalinity, but was not so significant in the presence of a 2% excess of sodium carbonate. Khen 1y0 carbamate indicator was added before the gum arabic in the presence of a slight excess of sodium carbonate, the absorptivity was almost twice that obtained when the gum arabic mas added first. Khen gum arabic was present, the absorbance was stable for more than 24 hours and was not affected by shaking. When the carbamate reagent was added in the absence of gum arabic, the absorbance dropped rapidly with a few seconds' shaking, which caused flocculation of the zinc carbamate precipitate. A mixed reagent containing both gum arabic and carbamate reagent invariably became turbid in a few hours, and a fresh mixture had to be made up daily. Consequently, the technique applied in copper determination was adoptedi.e., the gum arabic was added before the carbamate-though this caused a considerable loss in sensitivity. ALKALINITY AKD SALT COKCENTRATION. Work with the ahsorptiometric method showed that the solubility of zinc carbamate in aqueous ammonium
\ W 0
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o 0.4
1
2
3
4
5
ML. OF EXCESS IO% N(I2CO3 IN 15
6
7
ML.
Figure 6. Effect of excess alkalinity as sodium carbonate x
No shaking Vigorous shaking when neutralized
hydroxide wab appreciable. Although this fact made a turbidimetric method in alkaline solution appear impractical, investigation showed that, once precipitated, the zinc carbamate was essentially insoluble in excess sodium carbonate. Therefore, a 10% solution of this reagent was used to adjust the alkalinity of the sample. The need for excess alkalinity to obtain complete precipitation of the zinc carbamate is illustrated in Figure 6. The absorbance reaches a maximum in the range of a 2- to 3-ml. excess of 10% sodium carbonate per 15-mI. sample, or a concentration of 1.3 t o 2.0%. A commercial buffer mixture sold for pH meter calibration was not satisfactory for controlling the pH within the necessary narrow limits, The buffer VOL. 30, NO. 3, MARCH 1958
437
to be due to abnormally low bicarbonsalt concentration altered the sensitive ate or carbon dioxide content. Heating conditions of precipitation of the colthe sample after neutralization precipiloidal zinc carbaniate sol. The intensity tated most of the zinc as the relatively of absorbance and the range of linearity insoluble carbonate, which does not remere both affected by presence of such act with carbamate reagent. buffer salts. A saturated solution of At the other extreme is the step of Borax was also tried as a buffer; it is cooling the acid sample solution in ice easily prepared and offers a suitable water prior to neutralization with soalkaline pH (9.2). The results indicate dium carbonate. In this case there was that Borax would be suitable, but not almost no effervescence on neutralizasuperior to sodium carbonate. tion and the zinc recovery was low. Use of added ammonium chloride, in Apparently this was due t o the presence an attempt to extend the range of linear of an excessive amount of bicarbonate absorbance of the zinc dithiocarbamate ion, but the mechanism of this reaction system with the Model DU instrument, is not clear. proved of no value. Absorbance a t low This critical factor is easily controlled zinc concentrations was not linear in the by carrying out the acid solution and prescnce of excess ammonium chloride. BICARBONATE CONCENTRATION.carbonate neutralization a t room temperature. If the sample is vigorously When sodium carbonate is used to neushaken as the carbonate is added, the tralize hydrochloric acid in the sample, amount of residual bicarbonate ion will considerable carbon dioxide is released. be sufficient to keep zinc in solution, The rate of evolution depends on how but not enough to prevent precipitation much the sample is agitated during the of the zinc carbamate. dropwise addition of sodium carbonate. Comparable recoveries of zinc are posIf there is no shaking and the sodium sible a t zinc concentrations above 10 carbonate is added rapidly to approxip.p.m. with or without shaking of the mately 2y0 excess, there is little effersample (Figure 6). However, the sovescence. This is due to solution of the dium carbonate concentration is more carbon dioxide to form carbonic acid, critical without shaking. Other data which reacts with sodium carbonate to have conclusively proved that a t 5 yield sodium bicarbonate. The chemp.p.m. and lower zinc concentrations, ical equilibrium may be such that a the absorbance may be reduced by as small amount of dissolved carbon dimuch as 3070 if the sample is not shaken oxide remains in the solution. during the neutralization step. Investigation has proved that the RANGE O F LINE.4R ABSORBANCE. presence of some bicarbonate ion (or disTypical calibration curves for the solved carbon dioxide) in the sample is Model B glass prism and the Mcdel DU essential to prevent precipitation of the quartz prism instruments are shown in einc cation as zinc carbonate prior to Figure 7. Although these two spectroaddition of the carbamate reagent. photometers give essentially identical Frequent erratic results obtained when absorbance readings for a true colloidal the acid sample solution was warm a t suspension and for a solution, their rethe time of neutralization were believed sponse is different for the zinc carbamate suspension, particularly a t concentrations above 10 p.p.m. The range of linear response of the Model B instrument (0.2 to 1.7 A.) is more than twice the linear range of the Model DU (0.15 to 0.75 A,) for the same samples in the same 1.00-cm. cells. This phenomenon has been reproduced repeatedly and is attributed to a difference in construction of the cell compartments of the two instruments. The Model B is preferred over the Model DU for the turbidimetric determination of zinc. As the apparent absorptivity represents the slope over the linear range in Figure 7, it is constant for the Model B from about 2 to 16 p.p.m. of zinc. For the Model DU it is constant from 2 to 11 p.p.m. However, if samples with an absorbance above 0.75 are diluted, this ZINC CONCENTRATION (ERN.) range may be extended to 16 p.p.m. of zinc, as for the Model B. When the Figure 7. Beer's law plot for colloidal absorbance is above 0.75 but below 1.0, zinc carbamate a 1 to 1 dilution with 2% sodium carx Beckman Model B, 1 -cm. cells bonate is satisfactory for bringing it 0 Beckman Model DU, 1-cm. cells 438
a
ANALYTICAL CHEMISTRY
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40
VOLUME PER CENT EXCESS
60 60 NH4OH
'10
Figure 8. Solubility of carbamates in aqueous ammonium hydroxide 0
Reagent blank
x
5 . 0 p.p.m. of zinc 1 . 5 p.p.m. of elements as noted
A
within the linear range of the Model DU instrument. INTERFERENCES
Absorptiometric Method. The sensitivity of this method requires scrupulous care in using only clean equipment. Also, the ether and aqueous alkaline reagents must be purified as described above. The only common metallic ions which form colored complexes with sodium diethyl dithiocarbamate and which are normally found in rubber and rubber products are copper, cobalt, nickel, tellurium, and selenium. Besides zinc the only colorless thiocarbamates of interest are lead and antimony, but compounds of the latter two are used only in special applications. Such metals as bismuth, silver, tin, cadmium, and mercury are too rare to merit consideration as interferences. The only elements known to be added as a component of a rubber stock formula as metals are tellurium and selenium, which are added in small amounts (0.25%) as vulcanizing agents. The thiocarbamate salts of copper and lead, as well as tellurium and selenium, are used as accelerators to promote vulcanization (16). Nickel dibutyl dithiocarbamate is used to improve aging qualitiesof GR-S and neoprene synthetic rubber compounds (8). Use of any of these accelerators is relatively minor compared to use of nonmetallic accelerators (IS). The fact that sodium diethyl dithiocarbamate is not specific for zinc is considered a definite advantage of the method. Presence of added compounds containing the elements causing most serious interference as dithiocarbamates -e.g., copper, cobalt, nickel-in more than trace amounts is of interest to the rubber chemist. Carbamate reagent is commonly used
Figure
9.
interference with zinc precipitation
Concentrations of lead, cobalt, manganese, iron, barium, and calcium below 10% of the zinc present do not interfere appreciably with zinc carbamate formation, Lead is unique in that its interference represents an increase in apparent zinc carbamate precipitated, while the other elements interfere by preventing complete zinc precipitation. When none of the interfering compounds have been added t o a rubber product, the ratio of zinc to trace metal is about 500 to 1 up t o 1000 t o 1. Therefore, for all practical purposes these interferences may be disregarded. Khen elements reach a concentration a t which they interfere with zinc precipitation, their presence may be readily detected (Table 111). The coloration of the sample because of the presence of copper, nickel, or cobalt carbamates first becomes visible a t or near interfering concentrations. Likewise, the turbidity of lead, barium, and calcium is apparent after adding ammonium hydroxide to dissolve the zinc carbamate. Any increase in blank absorbance above the normal level then indicates that interfering cations are present. INTERFERENCE AS TURBIDITY. The most common interference is due to the presence of iron. The data of Table I1 indicate that as long as the iron concentration is below 2 p.p.m. in the sample, no turbidity due to iron is evident. However, a t higher iron concentrations, the presence of iron is evident as a precipitate of brown ferric hydroxide. Consequently, if no ferric hydroxide precipitate is apparent in the blank after adding ammonia to dissolve the carbamate, the iron probably has not lowered zinc recovery. Turbidity due t o traces of acid-insolu-
to determine traces of copper in raw The fact that the carbamate reagent natural rubber (1). The trace amount is not specific for zinc may be considered of copper (normally below 0.002%) or an advantage, in that a routine test for other cations is so small that interferzinc oxide can detect the presence of an ence may be neglected when zinc oxide abnormal amount of any of the interis present in excess of O . l % , as is nearly fering elements. These can then be always the case. identified and determined if desired. Tellurium, selenium, and antimony An unexpected interference with predo not form dithiocarbamate complex cipitation of zinc as the carbamate was salts under the conditions of precipitaobserved for copper, nickel, and cobalt. tion used here. Copper, cobalt, and The data of Table 111and Figure 9 show nickel diethyl dithiocarbamates impart that copper and nickel are more effective a yellow coloration to the ether. The than cobalt in preventing precipitation approximate amount of these colored of zinc carbamate. Trace amounts of complexes which will just impart a deficopper a t the level of 0.01 p.p.m. do not nite trace of yellow visible to the eye interfere with precipitation of 10 p.p.m. through the 12-em. depth of ether was of zinc (ratio, 1000 to 1). At 0.05 determined to be 0.5 p.p.m. of nickel, p.p.m. of copper (200 to 1) interference 0.25 p.p.m. of cobalt, and 0.1 p.p.m. of is slight, but a t 0.1 p.p.m. of copper copper. (Incidentally, this threshold of (100 to 1) about 30% of the zinc does definitely visible coloration a t a 12-cm. not precipitate. Interference of nickel depth of ether corresponds t o a transis not excessive a t 0.1 p.p.m., but zinc mittance of about 95% for a 1-em. cell precipitation is retarded a t concentraa t the 434 mp absorbance maximum of tions much above this. copper .) Interference from any significant coloration in the ether layer may be Table 111. Interference of Various Cations in Turbidimetric Determination eliminated by extracting the zinc carbamate complex with 50% (volume) (Concentration at which cations interfere with recovery of 10 p.p.m. of zinc) ammonium hydroxide, as described in Visible Coloration or Turbidity, Maximum the procedure. The efficiency of this P.P.M. Allowable extraction of zinc is shown in Figure 8. Cation, Just visible Interference, (trace) Definite P.P.M.a Remarks Turbidimetric Method. INTERFER-P.P.M. Absent ENCE AS CARBAMATE.Interference cu 0.1 O.I,
