44 Z. Anal. Chem., :Band 272, Heft 1 (1974)

Speetrophotometrie Determination of Silver(I) with 4-(2-Pyridylazo)-resorcinol

Spektralphotometrische Bestimmung yon Silber(I) mit 4- (2-Pyridylazo) -resorcin

Best. yon Silber mit 4-(2-Pyridylazo)-resorcin; Spektral- photometric

M. C. Eshwar and B. Subrahmanyam

Department of Chemistry, Indian Institute of Technology Bombay-400076, India

Received December 17, 1973

Silver(I) reacts with 4-(2-pyridylazo)-resorcinol (PAR) in neutral media giving a sparingly soluble orange- red chelate, which dissolves in aqueous sodium hy- droxide solution. Maximum absorbance a t 510 nm is recorded at p H 10.5. The complex is stable for a t least 3 h, and is not affected by light. Measurements were taken after 1 rain on type C ~ 4 quartz spectro- photometer with matched 10 m m quartz cells. Com- position of the complex was found to be 1 : 1 by Job ' s and mole ratio methods, l~ormation constants (logK) calculated by above methods are 5.4674 ~ 0.4 and 5.7533 =k 0.1, respectively. Ionic strength is main- tained at 0.1 with aq. KN0z. Beer 's law is obeyed in the range of 0.16 to 1.6 ~tg of Ag(I) per ml. Molar absorpt ivi ty is 2.025 • 104 and sensitivity (Sandell's)

0.0053 ~zg of silver per cm ~. Relative standard devia- tion (from 10 repeat determinations) is --q- 1.55~

The interference of about 40 ions has been studied with 40 ~g of silver per 25 ml in presence of 0.1 ~ of tartaric acid, and the following tolerance limits (in [zg) were found: F - 5000; malonate, citrate, oxalate, PO~ s- 2 500; Ru(I I I ) , Se(IV), Te(IV), Th(IV) 2000; TI(I), Ca(II), Sr(II), Ti(IV), Zr(IV), Mo(VI), W(VI), CI tzCO0- 1000; Be(II), Cr(III), Re(VII), As038- 500; AI(III) , Sb(III) , Rh(I I I ) , Ir(IV), Pt(IV) 250; Ba(II) , Pb(II) , Au([II ) 100; Bi(III), V(V), SCN-, EDTA ~- 50; Co(II), Ni(II), Cu(II), Zn(II) , Cd(II), Fe(II , I I I ) , CN- none. Bi, A1, Zr, Th and V are masked by fluoride and Pb by phosphate. The method compares favourably with rhodanine method for determination of silver, and has the added ad- vantage of not being photosensitive.

Recommended Procedure. An aliquot of silver nitrate solution containing about 40 ~g of silver is treated with 5 ml of 0.1~ tartaric acid, and the pH adjusted to 10.5 with 0.05 N aq. IqaOH. To this solution is added 1 ml of 0.1~ PAR solution, made upto 25ml, contents shaken thoroughly, and the absorbanee recorded at 510 nm.

Acknowledgements. Authors express their sincere thanks to the Council of Scientific and Industrial Research (India) for the award of J.R.F. to one of them (BS).

Dr. M. C. Eshwar Indian Institute of Technology Powai Bombay 400076, India

Indirect Determination of Phosphate and Arsenate Using Silver Ion-Selective Electrode Indirekte Bestimmung yon Phosphat und Arsenat mit einer Silber-ionenselektiven Elektrode

Best. yon Phosphat, Arsenat; Elektroden, ionenselektive; Ag-selektive Elektrode, indirekt

Takashi Tanaka, Kazuo Hiiro, and Akinori Kawahara Government Industrial Research Institute, Osaka, Japan

Received January 14, 1974

The application of phosphate and arsenate ion- selective electrodes to various fields of pollution con- trol is strongly required, but those electrodes are not available commercially. Inorganic phosphate salts impregnated in silicon rubber [1] and n-pentanol solution of heteropoly compounds [2] were proposed as usful ion-exchange materials of phosphate ion- selective electrode.

On the other hand, phosphate, and arsenate ions react with silver ion in neutral aqueous solution and form insoluble silver phosphate and silver arsenate quantitatively. In the proposed method, an excess amount of silver nitrate is added to the sample solution containing phosphate or arsenate ion, and the concentration of the silver ion remaining in the solution is determined by the silver ion-selective electrode.

Experimental Apparatus. Orion digital pH-meter Model 801 (USA). Orion reference electrode Model 90-02 with KNO 3 salt bridge. Toa Dempa silver ion-selective electrode Model AG-125 (Japan).

Reagents. 10 -1 M silver nitrate solution was standardized by potentiometric titration with sodium chloride. 10 -1 M potassium dihydrogen phosphate solution. 10 -1 M arsenate solution was prepared by oxidation of arsenic trioxide with nitric acid and by evaporation of the excess nitric acid on water bath. These solutions were diluted to proper con- centrations with water. All pH-buffer solutions were prepared

Kurze Mitteflungen 45

Table 1. Decrease of potential of silver ion-selective electrode by addition of phosphate and arsenate ions

Phosphate or arsenate Decrease of potential (En-1 -- En), mV solution added (n) (ml) Concentration of phosphate added Concentration of arsenate added

(10 -1 M) a (2.5 • 10 -~ M) b (10 -1 M) a (10 -e M) e (2.5 • 10 -a M) a

1 3.7 4.1 3.9 4.0 0.9 2 4.3 4.3 4.6 4.6 4.5 3 5.4 5.6 5.8 5.4 3.8 4 7.1 6.8 7.3 7.4 5.3 5 10.9 8.2 11.4 10.8 4.7 6 18.4 7.5 22.5 14.8 3.5

(E0 -- E 6) 49.8 mV 36.5 mV 55.5 mV 47.0 mV 22.7 mV

Acetate buffer concentration 10 -1 M; total velume a Silver nitrate solution added: 20 ml of 10 -1 M. b Silver nitrate solution added: 5 ml of 10 -I 1~.

50 ml. c Silver nitrate solution added: 20 ml of 10 -~ M. d Silver nitrate solution added: 5 ml of 10 -2 M.

by mixing proper amounts of 1 M sodium acetate and acetic acid. All reagents used were of analytical grade.

Procedure. l~easuring solutions were prepared by adding 5 ml of t M sodium acetate-acetic acid solution (pH 5.5) and certain amounts of silver nitrate solution to various amounts of phosphate or arsenate solution, and by dilution of the mixture with water to 50 ml. The concentrations of silver ion in these measuring solutions obtained were determined by the silver ion-selective electrode.

Results and Discussion

One mole of phospha t e and a r sena te reac ts wi th 3 moles of s i lver to form yel low and b rown pre- c ip i t a tes of the fol lowing fo rmulas ; AgaPO a a n d AgaAs0 a. Therefore, a m o u n t s of phospha t e and a r sena te up to 1/3 mole of s i lver a m o u n t a d d e d are measu red b y the p roposed method . The fo rma t ion of these p rec ip i t a t e s is r a p i d l y a n d comple te ly a t p H ranges of above 5 for p h o s p h a t e and above 4 for arsenate . Cons tan t e lec t rode po ten t i a l of s i lver ion- select ive e lec t rode is ob t a ined af te r 30 sec of a d d i t i o n o f s i lver n i t r a t e solut ion. Measurements of e lec t rode po ten t i a l are pe r fo rmed in the presence of the pre- c ip i ta te .

When 20 ml of i0 -I IVi silver nitrate solution are

a d d e d to the sample so lu t ion and the m i x t u r e is d i lu t ed to 50 ml wi th water , 2 x 10 -a ~ 1.2 • 10 -3 of p h o s p h a t e (190 ~ i l 4 0 p p m ) or a r sena te (278

~ - 1 6 6 8 ppm) in the final so lu t ion is de te rmined . The resul ts o b t a i n e d using var ious concen t ra t ions of s i lver n i t r a t e , p h o s p h a t e and a r sena te are shown in Table 1. The lower l imi t of the sens i t iv i ty is g iven b y the so lub i l i ty of the prec ip i ta tes . I n t he case of phospha te , the lower range of the d e t e r m i n a t i o n is 5 • 10 -4 ~ 3 X 10 -3 M (48 ~ 285 ppm) b y add i t i on of 5 ml of 10 -1 M silver n i t r a t e so lu t ion per 50 ml. I n t he case of a r sena te which shows lower so lubi l i ty t h a n phospha t e wi th silver, 5 • 10 -5 ~ 3 • 10 -4 M (7 ~ 42 ppm) is de t e rmine d b y a d d i t i o n of 5 ml of 10 -2 M silver n i t r a t e solut ion per 50 ml.

A m m o n i u m su lpha te and po t a s s ium n i t r a t e of 10 -~ M level do n o t affect these de te rmina t ions . However , ions forming p rec ip i t a t e s wi th silver, as chloride, bromide , iodide, su lphide a n d others, in ter - fere and mus t be absent .

References

1. Guilbault, G. G., Brignac, P. J., Jr . : Anal. Chem. 41, 1136 (1969)

2. Guilbault, G. G., Brignae, P. J., Jr. : Anal. Chim. Acta 56, 139 (1971)

T. Tanaka Government Industrial Research Institute, Osaka Midorigaoka 1, Ikeda City Osaka Pref., Japan