Heterometric micro-determination of traces of iron in thiocyanate solutions by titration with nitron

VOL.

21 (1959)

HETEROMETRIC THIOCYANATE

ANALYTICA

ACTA

4x1

MICRO-DETERMINATION OF TRACES SOLUTIONS BY TITRATION WITH

MORDECHAI De$artment

CHIMICA

of Inorganic

(MAX)

BOBTELSICY

and Altalytical

AND

Chemistry,

OF IRON NITRON

EN

A. H. I. BEN-BASSAT

The Ifcbrew University,

Jevusalem

(Israel)

INTRODUCTION

In the present paper a heterometric method is described for a very rapid determination of traces of ferric iron (0.05-0.08 mg in 20 ml of solution) in solutions containing ca. gg.g”/o of the following cations : Ca +a, I3a+z, Mg+s, Cr+3, Al+“, Mn+z, Zn+n, Cd+a, Th+4 or Ce+n.‘The same quantity of iron could be determined in the presence of the following ions : Ni** (= gg.S%), Pb+z (gg.S%), Sb+3 (gg*~S%), MoO4-* (gg.I%), VOa- (98.4%), WO4-2 (96% ) and Cue2 (98.7%). A titration usually took ca. 5 minutes. In most cases the error was I o/oor less, but exceptionally it amounted to ca.3%. The method is based on the quantitative precipitation of traces of ferric iron in concentrated solutions of thiocyanate by means of nitron. EXPERIMENTAL The same working conditions were observed as in previous heteromctric investigations. A deep red Corning fiItcr (No, 2408) was always used. reagent; this was a. A M ferric chloride stock solution was prcparod from Baker “Analyzed” then gradually diluted with hydrochloric acid solution, 50 that the latter always had a molarity IO times higher than that of the iron. The dilute iron solutions were freshly prcparcd at frequent intervals. b. An aqueous nitron stock solution (0.01 M) was prepared EIS follows: 0.7goz g of nitron (M.W. 312.16, Eastman Organic Chemicals, for chemical purposes) and 7 ml of N acetic acid wcrc dissolved in water to giV8 250 ml of aqueous solution. The dilute solutions wcrc prepared from this solution every few clays. The solutions of nitron were kept in dark brown bottles. The stock solution rcmaincd unchanged for months. All other chemicsls used wcrc either AnalaR or chomitally pure reagents. RESULTS

AND

DISCUSSION

In Table I a selection of the experiments is given. A blank experiment was carried out by titrating the concentrated thiocyanate solution with nitron in the absence of iron; a slight white turbidity formed but this never interfered with the titrations. Table I indicates the highest concentration of the foreign metal which may be present without seriously affecting the titration of iron. In a,lltitrations a horizontal maximum density line was obtained (Fig. I) after the fist maximum density point (= end-point). The flocculent precipitate obtained had a brick red colour. The molar ratio of [Fe+aJ: EN] (N = nitron) at the end-point was always I :G.6.The.precipitation of small amounts of iron depended on the concentration of thiocyanate in the solution. Thus, practically no precipitation occurred in 0.x N thiocyanate solution Relevences p. 4x5

GENERAL

:

3 3

;

;

;: 9 10

f 3

13 14 15 16 17 I8 19

0.0033 0.0033 o-oo33 0.0033 0.0033 0.005 o-*33

3

3 4

11 I2

COMPOSITION: u

TABLE I ml m&f Fe& f b ml N HCl + t ml 2 M KCNS + d ml supplements f ad Content: o.05-0.084 mg iron. T = 20'. Red filter: Coming No. 2408

4 4

3 3 3 3 3 3 3

20 21 22

3 3 3 3

23 24 25 26 27 28 29 30 31 32 33 34

3 3 3 3 3 3 3 3 3 3 3 3

$ 37

; 3

0.0025 0.2 ml M &PO4 0.5 ml M H3PO4 0.5 ml M H3FQ4 I~~N~i~te~5~2~~4Cl I ml M Na&trate + 5 ml 2 I ml M Nas_citrate

M

NHYOs

2 ml .M CaCls

3ml3MMCl3 5mlMCrCIs 5 ml 2 M MxClg 5 ml M CaCI2+ I ml M Na&xate 5 ml M BaCI2+ I ml M Na3_citrate 5mlz&~MgCl~fImlMNa&rate I ml illZn(NO& + I ml M Na&trate 5mlMCrQ+xmlMNascilxate 3 ml _MAl{NO& + I ml M Nwitrate 5ml2MbM&+rmlMNa&trate I ml M Ni(NO& + I ml M Na&trate I ml 0.01 M Co(N03)a f I ml M Nwitmte I ml 0.x M NaaJIoO4 + I ml Af Na3&rate I ml 0.1 M NaVOs f I ml M Nwitrate I ml 0.01 hf NaaWO4 f I mI M Na;tcitrate I mi M CWJo3)2 I ml 0.1 M Pb(NO& + I ml hi Napcitrate I ml 0.08 M SbCt f I ml M Napcitrate 0.5 ml 0.2 M Cu(NO& + I ml M Nas-citrate 0.5 ml 0.2 M Cu(NO?)wJ- 0.5 ml M H3PO4 5 ml 0.1 M Tb(NO& + I ml M Na&rate 5 ml 0.1 M Ce(NO& + I ml M Nazcitrate

0.1

0.04 0.04 0.02 0.05 0.02

0.04 0.15

0.04 0.12 0.02 0.17 12.3 0.86 1.6 4.3 0.07 o-5 0.85

I-3 I.3 0.07 0.12

0.0033 0.005 0.0033 0.0033 0.0033 0.0033 0.0033 0.0033 0.0033 o-0033 0.0033 o-33 0.0033 0.0033 0.0033 oJJo33 0.0033 0.0033 o.oo33 0.0033 0.0033 0.0033 o-33 o-*33 o-*33 0.0033 0.0033 o.oo33 o-0033

20

ml Ha0 + x ml nM nitron

3-s I.2

0.6 0.6 0.6 0.0 0.6 1.0 o-4 0.6 0.8

0.6 1.0 0.6 0.6 o-4 o-4 0.6 0.6 o-4 o-4 0.2

o-4 0.4 0.2 o-4 o-4 0.8

o-4 o-4 0.6 o-4 0.4 0.4 o-4 0.6 0.6

i/c 2.90 h i 2.94 h i/c 3.00 h i 3.00 h i 4_00> i 2.03 h i 4.00 h i/c 3.00 h i 2.70 h i 2.03 h i 2.95 h i 3.00 h i 3.oI h i 3.00 h i 3.00 h i 2.95 h i 3.02 h i 2.95 h i 2.95 h iqoh i 3.10 h i 3.00 h i 3.00 h i 3.10 h i 2.98 h i 2.98 h i 3.08 h i 2.95 h i 3so> i3.ooh i 3.00 h i2.goh i 3.00 h ig.ooh c 2.95 h i 2.95 h

0.46 OS3

0.72 0.77 >o-95 0.44 0.71 0.75 0.81 0.42

2.8 I.5

0.5 0.5

: 5 IO

2.0 0.5 0‘5

6 6 5

2.0

:

0.65

I.2

0.57 0.62 0.74 o-93 0.57 0.7s 0.77 0.76 0.77 o-53

0.5 0.5 0.5 0.5 0.2 0.7 0.2

: : 5 : 5

0.2 2.8

3.8

0.52

0.5

0.82

0.5 3.8

o-75 0.69

0.2

1.00

0.2

ohg

3.2

0.87 0.48 0.75 0.78 0.78 0.88 0.88 0.69

0.5 0.5 0.5 2,s 0.5 0.5

0.68

1.2

i = inbssection point; h = horizontal maximum density line: > = density increase after the end-point; c = contact poi$.

IO

5 7 i: 6 ; 5

I.2

I.2

: I5 : I7 ss 5

VOL. 21 (1959)

HETEROMETRY

OF f;C

413

(Expt. I) ; the minimum concentration necessary for quantitative reaction was 0.5 M (Expt. r-5). The reaction with nitron always occurred instantaneously. It must therefore be assumed that this reaction is of an ionic character and that the nitron acted somehow as cation. However, ,it seems unlikely that G molecules of nitron per I iron atom were necessary to give the insoluble ferric-nitron-thiocyanate salt and the insoluble compound obtained may be of a more complex nature.

0.7 -

I t

Pig. I. Titrations

I

I

I

ml --, 2 3 of iron chloride with nitron.

Some titrations of nitron with iron chloride solutions were also carried out. The results were not so clear as in the reverse titrations, but again in most cases the above molar ratio of iron to nitron was found at the end of the titration. When iron thiocyanate was titrated with nitron in hydrochloric acid solution, the titrant had to be added almost continuously, otherwise the density values measured soon decreased again. The same thing occurred with the maximum density values measured, which could not be stabilized. This phenomenon was apparently caused by the character of the “primary reaction” measured, and the tendency of the insoluble compound to be rearranged and redissolved. Many experiments were carried out in order to stabilize the readings of the galvanometer. When the influenceof different supplements on the reaction,was studied, it was food that citric acid acted very favourably in st’abilizing the measurements.: Therefore ail’ further experiments Refevences

pa 4x5

M. BOBTELSKY, A.H.I.BEN-BASSAT

4x4

VOL.

21 (1g5g)

were carried out with the addition of trisodium citrate to the hydrochloric acid solution. Some titrations were repeated with and without citric acid. In some cases, merely the presence of a foreign metal salt in concentrated solution (e.g. alurninium chloride, Expt. 16) was sufficient to stabilize the readings. The citric acid complexed the iron before it reacted with the nitron and so possibly the uncomplexed ferric iron interfered with the insoluble compo~d first formed. This is probably not the only reason, as the end-point density values decreased rapidly in the absence of citric acid. The favourable influence of the concentrated foreign salt (Expt. 16) may be caused partly by an increase in viscosity which weakened the activity of the primary compound. The titration time .and the maximum density values obtained were usually similar whether the citric acid was present or not. The concentration of iron in the solution had to be O.OOOI-o.oooz iI!+?.Within these concentrations the maximum density values were proportional to the amount of iron present @xpt. 4 and 7). As experiments s)-XI indicate, the &rong hydrochloric acid could be replaced by phosphoric acid without affecting the maximum density value or the end-point ; after the end-point, a horizontal maximum density line was always obtained. The presence of a large excess of ammonium chloride or nitrate (Expt. 12 and 13) had hardly any influence on the results although the maximum density value was slightly depressed. The results of some titrations in the presence of large amounts of foreign salts but in the absence of citric acid, are presented in Expt. 15-18. From these and the following experiments (Expt. x9-25)+ it can be seen that less than 0.1 mg of ferric iron (ca. 0.~ O/Qin 20 ml of solution can be determined in the presence of gg.9o/o of Ca +a, Ba+a, Mg+z, Al+a, Cr+“, Mn+a, Zn+2, Cd+z, Th+b and Ce+a. Thus, it is possible to determine traces of iron directly in concentrated solutions of salts of the calcium-barium and the ammonium sulphide groups. This reaction could probably also be used in qualitative analysis for the detection of iron in the presence of the above cations. METHOD

Less than 0.1 mg of ferric iron (cont. I-Z x 10-4 M) in 20 ml of an aqueous solution is titrated with a 0.0025-0.0033 M solution of nitron. The test solution must contain 5 ml of 2 M KCNS, I ml of M citric acid and 0.5 ml of N HCI.

SUMMARY

Traces of ferric iron wcrc cletcrmined hetorometricatly in concentrated solutions of potassium

thiocyanste by titration with nitron. The solutions could contain 99.9% of the following cations: C&e, B&e, M&s, Zn+s, Cr+s, Al+s, Mnj-2, Cd+s, Th+4, C&s or ~9% of Ni+s, Pb+s, Cu+*, Mo+s, V+s and W. A titration usually iasted 5 min and in most cases the error was cu. I%. The molar ratio of [Fe] : [nitron] was always equal to I : 6.6. RESUME Une methods rapide, par h&t&om&rie, est proposde ponr la dosage de traces de fer(lf1). Le titrage s’effectue au moyen de nitron, dans dcs solutions conccntr&s de thiocyanate de potassium. ZUSAMMENFASSUNG Es wird eine ,,heterometrische” Methoda bcschrieben zur raschen Bestimmung von Spuren von Eisen(IlI) durch Titration mit Nitron in ksliumthiocyana~haltiger L&sung. References

p. 415

VOL.

21 (1959)

HETEROMETRY

OF

Fe

415

REFERENCES 1 M. BOBTELSKY, Anal. Chim. Ada, 13 (1955) 172. (See literature on heterometry x953-'56 in Anal. Ckim. Acta, 16 (1957) 75) : M. BOBTP.LSKY AND R. RAFAILOFF, Anal. Chim. Acta, 16 (x957) 321.488;17(1957) 267,308; M. BOBTELSKY AND J. EISENSTADTER, Anal. Chim.Acin,16(xg57) 479; 17 (1957) 503, 579; 18 (1958) 534;zo (1959) ZI~,~~~;M.BOBTEL~KY AND A.H.I. BENBASSAT, Bull. sot. dim. France, (x958) 180, 233, 1138; M. BOBTELSKY AND J. M. E. GOLDSCHMIDT, Bull. Research Council Israel, 7A (1958) IZI ; M. BOBTELSKY AND J. SHAMIR, Bull. sot. chim. France, (1958) 1569.

GRAVIMETRIC

DETERMINATION

OF THORIUM

Received

April

28th, 1939

AND

CERIUM

WITH

N-BENZOYLPHENYLHYDROXYLAMINE S. I<. SINHA

AND

S. C. SHOME

Presidency College, Calcrclla (Imiiu)

N-Benzoylphenylhydroxylaminc was recommended for the precipitation and determination of metalsl.2 and has been used3 for the gravimctric estimation of molybdenum. SHOME’ observed that the reagent forms insoluble complexes with thorium, cerium and other metal ions in aqueous solutions. In the present investigation thorium and cerium have been cletcrmined with bcnzoylphenylhydroxylamine ; thorium has been separated from cerium by making use of the fact that thorium is precipitated at pH 5.0 whilst cerous ion is precipitated at pn 7.0. EXPERIMENTAL Solutions of metals Standard solutions of thorium nitrate and ccric ammonium nitrate were prcparecl respectively a.84 mg of thorium dioxide and 2.408 mg of cerium dioxide per ml. All the chemicals employed were of A.R. quality.

I)ETERMINATION

to contain

OF THORIUM

A known amount of the thorium nitrate solution (25 to 30 ml) was diluted with distilled water to 250 ml, and 10% ammonium acetate solution was added to adjust the pH to *about 5.0; the PH value was ascertained with indicator paper. Benzoylpl~enyll~ydroxylamine (0.3 to 0.4 g) in alcohol (IO to 15 ml) was then added with constant stirring. The white precipitate was occasionally stirred for 15 min, filtered, washed with water, ignited to dioxide in a porcelain crucible and weighed. The results (Table I) show that thorium is precipitated quantitatively with benzoylphenylhydroxylamine between PH 4.5 and 5.5 and the metal can be determined by igniting the precipitate to thorium dioxide and weighing, References 9. 417