A gran titration to determine the chlorinity of sea water

SHORT COMMUNICATIONS

A Gran

titration

407

to determine

the

chlorinity

of sea water

Determination of the chlorinity of sea water by potentiometric titration seems to have been first suggested by WEST AND ROBINSON’ in rg4r. The procedure was later examined by HINDXIAN, ANDERSON AND MOHERG~, HERR~IANN~, BATHER END RILEY~ and PROCTORS in order to improve the accuracy of the titration procedure and . -to evaluate the errors. None of these authors have, however, used the Gran II methoda for evaluating the equivalence point. As we believe that this method increases the precision of the titration irrespective of whether volume or weighing burets arc used, we wish to report an experiment at BornB station, which was carried out by 2 of our students, SIV GRIA~VALL and LENNAHT JAKSON, during a course in analytical chemistry. Water sampled in Gullmarsfjord (57”N, xr”E) at Born6 from a depth of 20 m (S=2g.S2O/oo, temp. x0.29”) on the 19th of November 1965 was used for the titration. In a roo-ml bcakcr 5.01 6 of sea water was diluted to 50 ml with distilled water. The emf was measured with a Radiometer PH Meter 25 with a scale expansion, which allowed the potential to be read to I mV and to be estimated to tenths of a mV, Polished silver rods were used as clcctrodes and the cell was as follows: Ag/diluted sea water/o.1 M 1
([Cl-] + [Br-1)

=m~+m~--v

*

0.1000

At the equivalence point: v. 0.1: = mcl +mnr. Introducing ve, [Ag+] [Cl-] =10-s-8 and [Ag+] [Br-] = l

[Ag+J-1(1o-Q.e+

10-12.4

one obtains

10-12-4)= (v-vvo)* o.r/(go+v)

to [Ag+]-1 is obtained Thus when [Ag+]-l=o then v =zJ,,. A quantity proportional from E as [As+]-1 = xoexp(E -&,)/58. In Fig. I we have plotted the Gran II function Fl = (50 + v)xoexp(E - 300)/58 against v together with the logarithmic diagram for the precipitation of silver chloride and silver bromide from sea water diluted I :5. At FI=o, v=v c=23,33 ml from which the chlorinity was calculated as follows. vO/lOOO 0.1000 l

Independent 16.5x+

- 107.868 - 328.5233/5.01=

measurements

of the salinity

16.50 fo.030/00

with a gold-chain

hydrometer

gave

Cl=

0.020/00.

Anal. Chim. Acta, 35 (x966)

407-409

SHORT COMIIUNICATXONS

408 After the equivalence l’lrc

point (V >71c

ml) the following

relation is valid

(u -7&J - 0, IO00 = [:&p-j (50 f v) Gran II function for this range is 1;2= (50 +v) xo cxp(zoo - E)/58

The two functions intersect at PO in the logarithmic diagram where [Cl-}=xo-4.0 (= i/n,log J&O), i.e. O.OI% of chloride is stili untitratcd. ‘The extrapolation of 171 to [Cl-] + f&--J = o is therefore preferable although the difference is barely discernible. f*

12

Fig. I. Lognrithn~ic SC& \W.tCr by A@.

t0

6

clingrmn

6

4

(al~ovc) and

n Gran

II plot

(below)

for the titration

of Cl-

-1. J3r- in

N&s. Sea water lins a molality ratio of Cl-/Br-5Y, IO~+~that corresponds closely to the ratio of the solubility products of the silver hrrlicles ~o-~-~/xo-i~.‘~= 102+e.Thus it can be seen from the logarithmic diagram in Fig. x that silver bromide starts to precipitate very soon after silver chloride when [.A@] is increased, It has recently been shown? that Cran II plots also can be used for the evaluation of equivalence points in the potcntiomctric titration of the alkalinity and total carbonate of sea water. The work on ocean chemistry Natural Science Council (NFR).

at our department

is supported

by The Swedish

DAVII) DANIEL

dmal.

Clbim.

Acta.

35 (rgGf)

407-409

DYRSSEN JACNER

SHORT L. J. I;. J.

-.

E. WEST AND C. HINDMAN,

I<. J. R~uINs~N, J-A[mirre i&s., .+ (x94x) x. J;-Aforim Rrs.. I&. J. ANDICHSON AND 12.'-C. %~OuEHG, f, CotrsciC, Conscil IIJh?Y)I.E.vpCovaIio)t nfer, 17 (1951) AND J. P. RILEY, J. Con&f, cottscil z?atrr?t. lhfdornliorr

HERRMANN,

M.

C. M.

BATHER

Trans. Am. Gcopitys.

PROCTOR,

0 G. GRAN, 7

409

COMAIUNICATIQNS

~lwZyst.

1). DYRSSEN,

77

(1952)

Uttiora, 37 (1956)

3 I.

8 (1949) 30.

223. Mcv, 18 (1953) 277,

GGr.

Acfu Chcm. Scmzd., 19 (1965) 1265.

‘,.‘:‘.(Iieceived December

8th. 1965) .*Ittctl. Citim . .-I cla, 35 ( rgGO) 407-409

Silver(ll) oxide oxidation and spectrophotometric determination of chromium( III) Chromium(II1) in micro amounts can be quantitatively oxidized to chromium(V1) by oxidizing agents, such as potassium permanganatc, potassium persulfate obtained is dctermincd by in acidic media, or perchloric acid 1; the chromium(V1) rending the absorbance of its complex with s-diphenylcarbazidc at a wavelength of 540 rn~i~~. The present paper describes a simple, rapid and accurate method for this determination, using silver(I.1) oxide as oxidizing agent. Silver(I1) oxide has been studied as an oxidizing agent in the titrimetric determination of cl~romium(III)~ but, so far as is known, it has not been used in the calorimetric determination.

A “Hitachi-Perkin-Elmer” the absorbance. Reagents Silver(Il)

of potassium

oxide

w;s

Model

prepared

139 spectrophotometer

by adding

silver nitrate

was used to read

to an alkaline

solution

persulphateJ.

Standard chronaiatns(Il~) solvtiotz (~1.97 nig/l) was prepared by dissolving a we&bed amount of spectroscopically pure metallic chromium (Johnson, Matthey and Co., Ltd.) in 5 ml of I :I sulfuric acid and then diluting it to I 1 with water. Mevck reaged-grade diphevylcarbazide was used. The water used was distilled from alkaline permanganate. Proceduve

Transfer an aliquot containing IO--100 ,ug of chromium to a 5o-ml beaker. the proper amounts of acids to give I N nitric acid and 0.5 N sulfuric acid in a volume of about 20 ml. Add about 20 mg of silver(I1) oxide in small portions, while stirring solution until a brown colour appears 3. Continue stirring for a further 15 min and heat the solution to ca. S5” in order to destroy the excess of silver(I1) oxide. Amtl. C/rim. Ada,

Add final the then The

35 (1966) 409-410