Short communications
369
Summary-A quick, selective method for molybdenum(V1) is based on the formation of a yellow thiolactic acid complex. The complex is formed at pH 1.0-1.6, and the absorbance is measured at 365 nm. Zusanunenfassung-Ein rasches selektives Bestimmun~verfahren fur MolybdLn(V1) beruht auf der Bildung eines gelben komplexes mit Thiornilchsiiure. Der Komplex bildet sich bei pH l,O-1,6, seine Extinktion wird bei 365 nm gemessen. R&um&Une methode rapide, selective du molybdBne(VI), est bade sur la formation dun complexe jaune avec l’acide thiolactique. Le complexe se forme a pH 1,0-l ,6 et l’on mesure l’absorption a 365 nm. REFERENCES 1. F. Will and J. H. Yoe, Anul. Chem., 1953,25,1363. 2. R. PHbil and J. Adam, Tufunru, 1971,18, 349. 3. A. I. Busev, A. Nacu and G. P. Rudzit, Zh. Anah’t. Khim., 1964,19,337.
Talanta, 1972, Vol. 19, pp. 369 to 372.
Pemmon
PTCS. Printed in Nonhem
Ireland
Determination of traces of iron in the presence of iron@) bathophenanthroline method
by the
(Received 4 May 1971. Accepted 17 October 1971) THE BATHOPHENANTHROLINE method is generally used for determination of the sum of iron(I1) and (III) in boiler water. In investigation of the deposition mechanism of iron oxides and corrosion of boiler tubes, it is necessary to know the concentration of the iron(H). Titration methods,’ Clark’s bathophenanthroline methoda and the ammonium thiocyanate-thioglycol method’ cannot be applied to boiler water, because the concentration of iron(I1) is too low, of the order of ppM (parts per milliard). Lee’s bathophenanthroline method& gave a positive error for solutions with a high ratio of iron(II1) to iron(I1). The present paper describes attempts to determine iron(I1) with bathophenanthroline after masking of iron(II1). EXPERIMENTAL Reagents A standard solution of iron(I1) was prepared by dissolving reagent-grade ferrous ammonium sulphate in water acidified with sulphuric acid, and diluting with demineralized water. This solution was prepared daily, and was saturated with carbon dioxide and kept in an atmosphere of carbon dioxide. Bathophenanthroline (4,7-diphenyl-l,lO-phenanthroline) was dissolved in isopropanol. Other chemicals were chemically pure or reagent-grade. Procedure Sample solution [200 ml, 0.2-6 pg of iron( was adjusted to a pH between 4.2 and 4-7 with 2 ml of 3M hydrochloric acid and 5 ml of 2.6M ammonium acetate, and transferred to a 300-ml separatory funnel; 2 ml of 2.5% w/v sodium pyrophosphate solution were added and the solution was left standing for 5 rain for the iron(II1) pyrophosphate complex to form. Then 2 ml of 0.25M bathonhenanthroline in isonronanol were added and the solution was shaken mechanicallv for 1 min: 25 ml of n-butanol were-added and the shaking continued for 1 min. After settling (for 5 min) the aqueous phase was discarded, and 10 ml of methanol were added to the organic phase, which was then shaken to dissolve any water adherent to the funnel. The absorbance of the organic phase was measured in a IOO-mm cell at 533 nm against a reagent blank. RESULTS
AND
DISCUSSION
Choice of stabilizing agent A major problem was to establish a solution environment that would keep the iron species in their original oxidation states. Several potential stabilizing agents including citrate, tartrate, oxalate, malonate, salicylate, phosphate and pyrophosphate were tested.
370
Short communications
Only pyrophosphate iron(II1).
masked
iron(II1)
without
suppressing
the colour reaction
or reducing
Choice of solvent The ferrous bathophenanthroline complex has usually been extracted with isopentanol and any droplets of water in the extract have been dissolved with isopropanol. However, when pyrophosphate is used, the colour of the isopentanol-isopropanol extract is unstable. Chloroform, carbon tetrachloride, n-hexanol, n-butanol, ketones and esters were tested as extraction solvents, and isopropanol, ethanol and methanol as solvents for residual water. With solvents other than alcohols no coloured product was extracted. Even with alcohols as solvents, the colour faded with time, and a relationship existed between the rate of fading and the molar concentration of hydroxyl groups (sum of alcohol and water OH groups) in the organic phase. Although no explanation for this relationship was forthcoming, it appeared that the lower alcohols gave the more stable colour. Therefore n-butanol and methanol were the solvents selected. The shaking time required for the complete formation of ferrous bathophenanthroline complex was about 2 min and after shaking, its colour showed no change for 15 min and then increased. Effect of sodium pyrophosphate Figure 1 shows the effect of sodium pyrophosphate on the absorbance for solutions containing iron(U) with and without iron(II1). In the range from 2.6 to 60 mg sodium pyrophosphate masked iron(II1) and permitted iron(I1) to react with bathophenanthroline. Therefore, 25 mg (1 ml of 25 % solution) were used in further work. This amount was adequate to prevent interference of iron(II1) in up to 20-fold ratio to iron if a 10% error could be tolerated (Table I). However, the masking reaction is not instantaneous, taking ~3 min at 15” and ~2 min at 20”. At 32” the absorbance begins to fall at -6 min reaction time, so 5 min were allowed at room temperature. The time needed for
Y
I”
0
20 Weight
40
60
of sodium
60
too
pyrophospha38,
120
mg
FIG. 1.-Effect
l-Fe(I1)
of sodium pyrophosphate. 5 ppM, Fe(II1) 25 ppM: 0-Fe(I1) 5 ppM.
TABLE I.-EFFECTOPIRON(III)ON
[Fe(IIMFe(II)I in aqueous phase 0 1 5 10 20 40 60 l
ABSORRANCEOPIRON(II)COMPLEX
Absorbance 1 PPM O-026 0.028 0.029 0.028 0.028 0.030 0.037
Sodium pyrophosphate
5 PPM 0.121 0.124 0.125 0.125 0.130 0.129 0.138
was not added.
of Fe(I1) complex 5* PPM 0.129 0.134 0.176 0.233 -
30 PPM o-703 0.710 0.734 0.741 0.780
Short communications
I
0
2
3
371
I
4
5
6
PH
O-Fe(H)
FIG. L-EtIect of PI-I. 5 ppM. Fe(III) 50 ppM: O-Fe(n)
complete extraction was 2 mirr; the absorbance increased.
5 ppM.
remained constant for S min but then gradually
Eflect ofpH The absorbance was unaffected by pH in the range 3G-47 if iron(III) was originally present and 4.2-47 if it was absent (Fig. 2). The latter range was therefore used. Absorption spectrum and stability of the colour lie absorption maximum was at 533 nm and the molar absorptivity was 2.03 x ICF I.moIe-l.mm-z, slightly less than in isopentauol-isopropanol medium. Beer’s law was obeyed for iron(H) concentrations up to 30 ppM in the aqueous solution (coefficient of variation 4% for 5 ppM). The absorbance in n-butanol is constant for 20 min and for at least 3 hr in butanol-methanol. E$ct
of diverse ions
As shown in Table I, nickel, zinc, mugger and chromium~I1) had littie effect on the absorbance. Copper(H) gave a negative error, because it catalysed the oxidation of ferrous ion with oxygen. Cobalt(H) gave a positive error, because cobalt reduced ferric ion in the presence of bathophenanthroline.* Only 25 ppM of copper and 5 ppM of cobalt could be tolerated. Technical Laboratory Central Research I~titute of Electrical Power fndust~ Iwato, ILmae Tokyo, Japan
TAKAYUKI MIZUNO
Summary-Traces of iron(H) (l-30 ppM) in the presence of iron(III) were determined (error
372
Short communications RbumLOn a determin6 trace de fer(I1) (l-30 ppM) en presence de fer(II1) (erreur 10%) par la methode de bathopenanthroline. On a elimine l’interference de fer(II1) par la dissimulation avec pyrophosphate de sodium (2,560 mg). On a extrait la complexe de fer(I1) avec n-butanol a pH 4,2-4,7.
REFERENCES 1. Vereinigung der Grosskesselbesitzer. Analysenverfahren fiir den Kraftwerksbetrieb, Vulkan Verlag Dr. W. Classen, Essen, 1962. 2. L. J. Clark, Anal. Chem., 1962,34,348. JZ,S KOlOl-1966, p. 96. :: G. F. Lee and W. Stumm, J. Am. Water Works Ass., 1960,52, 1567.
p. 240.
Talanta, 1972, Vol. 19, pp. 372 to 377. Per&zmmn Press. Printed in Northern Ireland
Determination of inorganic impurities in vinyl chloride by activation analysis (Received 15 March 1971. Accepted 24 June 1971) LACK of experimental data concerning the traces of inorganic compounds found in vinyl chloride is due to the difficulties in using classical analytical methods on gases or liquids at low temperature for the detection of trace impurities. The object of this paper is to examine the estimation of trace inorganic impurities in vinyl chloride. The high sensitivity of the activation method compared with the usual chemical methods makes it suitable for this purpose. Thevinyl chloride obtained by synthesis from acetylene and hydrogen chloride (mercuric chloride as catalyst) is likely to be contaminated by such inorganic constituents as arsenic and phosphorus which are present in acetylene asAsH,and PH,. A vinyl chloride sample irradiated with a thermal neutron flux gives mainly oneradioactiveisotope, Yl, via an (n, y) reaction, in high yield. 1 For an irradiation time of a few hours all other nuclear reactions may be neglected. The presence of elements other than C, H and Cl (e.g., As) might lead to the formation of radioisotopes which could be determined. A slow neutron flux of 1.2 x 109n/mmz/sec with a fast neutron component of 6.2 x 10’ n/mm%ec was used. Under these conditions the existence of nuclear reactions such as (n, a) and (n, p) for chlorine are possible and could result in the formation of szP and *YS,the fist of which would interfere in the determination of phosphorus by p-counting. The (n, p) reaction occurs even with slow neutrons but a cross-section for reactor neutrons of 590 f 90 mb is given.* The effect of YS is easily eliminated, owing to its low /?-energy of 0.2 MeV. 82Pmay result either from the IllP(n, y) raP reaction (which is directly related to the phosphorus present as impurity) or from %l(n, a)S2P. The contribution of the latter nuclear reaction is low, owing to the subcadmium neutron spectrum having a cross-section of 0.08 mb.‘L In the presence of fast neutrons this value might be higher.
THE
EXPERIMENTAL Sample irradiation Vinyl chloride samples were introduced into quartz vials cooled with liquid nitrogen in a special aluminium vessel insulated with expanded polystyrene. In order to keep the sample cool the facility of adding further refrigerant was required and the only reactor channel suitable for this purpose had the low neutron flux of 10’ n/mniQec and could not be used for analytical determinations. This difficulty was overcome by taking advantage of the tendency of the liquid monomer to polymerize. This oolvmer is solid and stable at 2.5” and could be used as the samole I in the normal channel which has a’neutron flux of 1.2 x lo8 n/mma/sec. The industrial synthesis of vinyl chloride uses acetylene and hydrogen chloride. Acetylene is contaminated by ASH, and PH, generated from calcium arsenide and phosphide present in calcium carbide. The acetylene and hydrogen chloride are also contaminated with traces of oxygen, which can give oxygenated compounds. Other organic contaminants of the monomer may be produced during the reaction process. These organic compounds and, more efficiently, vinyl chloride react with arsine and phosphine, resulting in the retention of these elements in the polymer. The same is true for bromine (existing as HBr in HCl).