the aromatic ring and weakened the interaction between copper(I1) ion and the *-electrons, as it did for Cu(Phen)C l r t o an even greater extent than for C u ( P y ) d N 0 ~ )and ~ Cu(NH3MNOd2. At the present time, the effects of sulfate cannot be explained. However, to rationalize the relatively greater retention of hydrocarbons, particularly saturated aliphatic compounds, one might invoke the greater ability of the double charge on sulfate to participate in London interactions and to induce a dipole in the hydrocarbon. However, another effect may be due to the influence of sulfate on the charge distribution within the coordination sphere of the cation, and the geometry of the molecule and/or crystal. For example, the sulfate effects are missing for coordinatively saturated Cu(Py)4SOa, but are very pronounced for Cu(Py)SO4 and Cu(Phen)S04. H2O. In agreement with earlier data, interaction between the copper(I1) ion and non-bonded electrons on oxygen appears
to be contributing to the retention of oxygen-containing compounds (1, 4). In addition, both 2-pentanone and 2heptanone had larger retention times than their more sterically hindered isomers (3-pentanone and 3-heptanone) even though the respective boiling points are virtually the same. Similarly, tetrahydrofuran (B.P. 65' C.) had a larger retention time than isopropyl ether (B.P. 68' C.) on these adsorbents (1, 4). The values given for theoretical plate heights were undoubtedly not the optimal ones because the particle size studied was 50 to 60 mesh. A decrease in the particle size to 80 to 100 mesh would probably have decreased the theoretical plate height by a factor of about two as it did for a column of C U ( P ~ ) ~ ( N (OI )~. ) Although ~ the effects of flow rate of carrier gas on the efficiency were not studied, the optimal flow rates probably occurred around 5 cm. per second because the adsorbents and conditions were similar to those used for earlier studies ( I ) .
ACKNOWLEDGMENT
The authors thank E. I. du Pont de Nemours & Co. for the use of the differential thermal apparatus, and C. s. Yeh for the carbon, hydrogen, and nitrogen analyses. LITERATURE CITED
(1) Altenau, A. G., Rogers, L. B., ANAL. CHEM.36, 1726 (1964). (2) Duffield, J. J., Rogers, L. B., Zbid., 34, 1193 (1962). (3) Moffat, A. J., Solomon, P. W., U. S. At. Energy Comm. Res. and Development Dept., IDO-16732,1961. (4) Rogers, L. B., Altenau, A. G., ANAL. CHEM.35, 915 (1963). A. G. ALTENAU~ L. B. ROQERS Department of Chemistry Purdue University Lafayette, Ind.
WORKsupported by Wilkens Instrument and Research, Inc., Walnut Creek, Calif. Present address, U. S. Army Natick Laboratories, Natick, Mass.
Determination of Zinc in Rubber Products via EDTA Titration SIR: Zinc oxide is commonly added to rubber stock to aid in the curing process. A rapid method for the determination of zinc as a control procedure is therefore important. Several methods have been proposed for this analysis. The American Society for Testing and Materials standard method (1) involves the titration of zinc with potassium ferrocyanide using uranyl acetate as external indicator. C. M. Roberts (6) proposed an (ethylenedinitri1o)tetraacetic acid (EDTA) titration of Ca, Mg, and Zn a t pH = 10 using Eriochrome Black T as indicator and the subsequent titration of Ca+2 and Mg+2 after masking the zinc with cyanide. Both these methods require a hydroxide precipitation of iron, aluminum, and titanium prior to the zinc determination. In tests made in this laboratory using the above procedures the ferrocyanide method gave high results and some zinc was lost in both procedures during the hydroxide precipitation of R2Oaeven on reprecipitation. G. Milner (5) proposed a polarographic method for this analysis which does not require separation prior to the zinc determination. This method is useful but involved and requires extensive equipment. Blenkin (2) presented a scheme for separation and determination of all the major fillers in compounded rubber. The zinc is determined as the oxide and as the sulfide by EDTA titration. Miksch and Hinteneder (4) developed 1436
ANALYTICAL CHEMISTRY
a procedure for the direct EDTA titration of zinc in the ash from rubber in the presence of Fe, Ca, and Mg. They used the effect of the ferri-ferrocyanide couple on 3,3-dimethylnaphthidine to indicate the end point. The end point of this titration is very slow-requiring approximately one minute after the addition of the last drop of titrating solution for completion. The procedure presented in this paper allows the rapid determination of zinc in common rubber products without prior separation from other ions commonly found in the ash. The ions commonly found in rubber products are ~ l f 3 Fef3, , Ti+4,Sb+3, SiOz, Ca+2, and Mgf2. Lead which is sometimes, although rarely, found in rubber products interferes and must be separated before titration. In the titration 0.01M EDTA is used which allows very small amounts of zinc to be determined. Because of this the rapid oxygen flask method is applicable for ashing the sample. A review of this method has been published by A. M. G. Macdonald (3). EXPERIMENTAL
If an oxygen flask is used, burn a sample weighing between 130 and 150 mg. in a 1000-ml. flask using 10 ml. of 6 N hydrochloric acid as absorbing solution. Evaporate this solution to 5 ml. or less on a hot plate. Most rubber samples can be dry ashed in a furnace a t 550' C. If large amounts of chloride are present, a sul-
fated ash must be used. In the latter case the sample is reduced to a slurry by boiling with 1.0 ml. of concentrated sulfuric acid and 1.0 ml. of nitric acid. The acids are distilled off and the residue is ashed a t 550' C. in a furnace. Transfer the ash to a 250-ml. beaker and add 10 ml. of concentrated hydrochloric acid using portions of the hydrochloric acid to wash the crucible. Evaporate to 5 ml. or less. After evaporation add to the hydrochloric acid solution 10 ml. of 3M ammonium fluoride solution, 2 ml. of 0.1M aluminum chloride solution, 5 ml. of 10% (v./v.) 2,Ppentane dione in ethanol solution and one drop of methyl orange indicator solution. Neutralize with concentrated ammonium hydroxide. Add 10 ml. of buffer solution (1M ammonium acetate-1M acetic acid), 40 ml. of ethanol, 6 dro s of dithizone indicator solution (0.1k (w./v.) in acetone) and titrate with 0.01M EDTA from an orange to a green color. In absence of Ca+Z and Mg+2 the aluminum chloride can be omitted. If the iron content is not greater than that of zinc the 2,P pentanedione can be omitted. Acetone or 2-propanol may be substituted for ethanol. DISCUSSION AND RESULTS
SiOz adsorbs Zn+2 ions. The ZnfZ ions are freed by boiling with strong hydrochloric acid. Most of the silica is dehydrated in this process and causes no further interference. Ammonium fluoride completely prevents reaction of Alfa and Ti+' with
_ _ _ _ _ _ _ _ _ ~
EDTA under the conditions of the procedure. Trivalent iron is also masked with this reagent in amounts up to the molar content of zinc present. I n amounts greater than this, high results are obtained. To prevent the interference of iron, 2,4-pentanedione is added. The 2,Cpentanedione will mask iron in the absence of fluoride but the dark red color of this chelate prevents observation of the end point. The complex formed between iron, 2,4pentanedione, and fluoride is yellow and thus allows observation of the end point color change from orange to green. I n the presence of 2,4-pentanedione and fluoride, the iron content can be 20 times that of zinc without interference. Higher amounts have not been tested. The zinc cannot be back-titrated with 0.01hl Zn+2 using ;an excess of EDTA if Fe+3, A l + 3 , or Ti+' are present using the above procedure. When Ca+2,Mg+:!,and F- are present, Zn+Zcannot be titrated directly in the absence of Al+3. A back-titration of excess EDTA with Zn+2 yields good results. This is attributed to the precipitation of CaZnR and MgZnF4 because low results and unstable end points were obtained in the direct titration. The back-titration (cannot be used as a general method because Fe+3,Al+3, and Ti+4 which are almost always found in rubber products interfere. By adding Al+3 before the titration all interferences from Ca+2 and Mg+2 are prevented, because of the greater solubility of the zinc salts than Ca3(AlF& and M a (A1F6)2. Lead ions interfere by reacting slowly with EDTA during the titration which causes poor and unfitable end point and
Table I.
~
Determination of Zn+* in Rubber Using NH4F Only as Masking Agent
No. of runs 3 6 6
6 6 3
Table II.
CaO
~~
FetOI
Contents, % TiOs
1.24 1.13
1.13 1.12
1.12
ZnO found, %
Std. dev.
1.01 1.06 10.2 10.2 10.1 10.0
1.01 1.07 10.2 10.0 10.0 10.0
0.00 0.01 0.06 0.06 6.00
Determination of Zn+2 in Rubber Using the Procedure Presented
MgO
Present, 90 TiOt Fe20s 1.12
8.48
ZnO
1.24 1.12
5.92
high results. Lead can be removed by precipitation as PbS with hydrogen sulfide (ASTM, D297-61T). Sulfide ion interferes with the dithizone indicator but can be removed by oxidation with bromine to elemental sulfur and reduction of excess bromine with a small amount of sodium sulfite or hydroxylamine hydrochloride. Rubber samples to which known amounts of zinc oxide were added were analyzed for zinc using ammonium fluoride alone as masking agent. The results are shown in Table I. Additional samples were analyzed using the procedure as written. The results are shown in Table 11. The relative standard deviation was 1.0%. Each sample was analyzed in duplicate.
ZnO
ZnO found, %
Recovery, %
1.06 10.0 2.29 1.77
1.10 9.98 2.29 1.76
104 99.8 100 99.5
LITERATURE CITED
(1) Am. SOC.Testing Materials, D29761T, sec. 40f (1964). (2) . , Blenkin. J.. Trans. IRZ 40. T123 (1964). (3) Macdonald, A. M. G., Analyst 86, 3 (1961). (4) Miksch, R., Hinteneder, L., Kautschuk. Gummi 15, WT358 ,( 1962). (5) Milner, G., "The Principles and Application of Polarography, ' p. 440, Longsmans, Green & Co., New York, 1957. (6) Roberts, C. M., Trans. Inst. Rubber Znd. 33, 97 (1957). ,
I
T. L. HUNTER Research Division The Goodvear Tire & Rubber Co. Akron, OGo 44316 PRESENTED at the Division of Rubber Chemistr ACS, meeting in Miami Beach, ay 4-7, 1965.
2
Detection of Gases Based on Entropy Considerations SIR: The ( W ) vdue for a gas is the number of electron volts of energy required to create an ion pair in the gas. Recently Lovelock pointed out that the ( W ) value for gases could be exploited for the detection 'of gases in air (9). He indicated that this property has not previously been exlploited for detection purposes. This is not precisely true because (W) values are one variable which determines response of an ionization chamber or a proportional counter. Saturation currents for pure gases are a linear function of the pressure of many gases (6) and the slope of the saturation current us. pressure plot is related to the (W) value for the particular gas. The alphameter is based on the difference in the slopes of saturation current us. pressure plots and
thus on the ( W ) value (b), although i t was not called a ( W ) value detector. Thus, the alphameter has been used to measure the composition of binary mixtures of gases at known pressures. It has been used for the analysis of mixtures of hydrogen and oxygen, carbon dioxide and oxygen, and nitrous oxide and nitrogen. The alphameter utilizes a source of 0.5 mc. of radium. Lovelock utilizes a source of 200 mc. of tritium in his ( W ) value detector, but suggests that an cu source would also be suitable. Lovelock indicates, however, that his ( W ) value detector is independent of temperature and pressure changes. A number of gas mixtures have been studied in a parallel plate ionization chamber with a Pu2*9 (Y source. A linear function of the
reciprocal of ( W ) value vs. an empirical function of pressure has been derived (11).
In a previous paper (12) pulse attenuation data were reported for a number of gases and gas mixtures mixed with 1.3% butane-98.7% helium. An attempt has been made to correlate the pulse attenuations with the known properties of the gases, ( W ) being one of these. The correlation between ( W ) values has not proved to be as fruitful as that between pulse attenuations and the absolute entropy in the hydrocarbon series. EXPERIMENTAL
The apparatus has been described previously (12). A new cylinder of the nucleonics gas 1.3% butane-98.7% VOL 37, NO. 1 1 , OCTOBER 1965
1437
