ANALYTICA CHIblICA ACT,\ 559

SHORT COMMUNICATIONS

Freeze concentration of dilute solutions

The clctcrmination of traces of inorganic ions in de-ionized water, natural waters and various solutions suspected to contain micro amounts of contaminants becomes cvcr more exacting in the demands for precision at cxtremcl~* low levels of concentration. T!IC tlifficultics associated with the determination of ions at solution concentrations of 113s than 1 p.!>.m. arc frcclucntl~- clue to uncertaintics conccrnin(= the purity and reproducibility of rcagcnts. .A mctlroc! by wllicll tllcsc wry dilute solutions could bc conccntrntccl was sought so tlirrt they might then be brought within the range of mow accurate analysis without incurring the llilZilr<lS due to

contamination, and possible losses resulting from vol;~tility or clwnical rcnction. The mctlioc! of frcczc concentration clcvcltqwd by SHni~rlto* secmccl to provitlc

an attractive anSwcr to tlic problem. ‘I’llis tnctllot!, shown 1,~ I<OI~AYASHI ANI) LEES to bc cffcctivc for the determination of N&I ilt conccntratiww down to I p.p.m.. stimulated an investigation into its application to the dctcrmination of a variety of inorganic sulx+tanccs, prcscnt in solution at conccTltriLtiotls of less than I l>.p.m.

I<sscntia!!y. the mcthoc! consists (Jf the transfer of t!lc solution to !XJ conwn- tri1td to a SUitill>lC container, dlowiiig room few cXpilrMion on frlaezing ; it is then sllakcn mechanically in il freezing cllilnlher. After iI few hours most of tlw solution is frozen lonving iI small ov:ll c;ivity in tllc ccntrc wliich COutiliflS tllc conccntratcd solution.

‘I’lic cquipmcnt used for the !wscnt invcstigiltiotl consisted of a Microitl flask stlilkcr, with which .+ samples could be concentratctl Simult;lncously, l>lilcctl insidc a Frigidnirc frccxcr cabinet. ‘I’hc optimum 0pXlting t~IllpWiltUrl! \va found to Ix

-20°*20. and the Shking rate \vilS atljustctl to bc just insufficient to cikusc splashing of the solutions.

‘I’lw vcdumc <If solution tnkcn \vi\S itlwilys 100 ml, contained in !w~ytllcnc bottles (of cn. x20-ml capacity) wliich wcrc found to lx the best contnincrs for tllis volume. Larger bottles introduced tlw tlangcr of some loss of solute, !~robnbly IWL*ilUSc

tlw larger surfilcc area inducccl -too ra!>icl freezing. ‘L‘lw snmplcs could he left to slxtkc in tllc freezer for 3 h without attention, after which frcqucnt inspection Wi\S IlCXWSilr!,

to ensure thnt tlw samples could be rcmovccl from the frceiscr when the volume of solution remaining unfrozen lliltl rcaclicd about 5 ml. OCGNiOnidly it was nC!ccSSilr~ to

make a llolc in the ice bcforc tlw conccntratcd solution could be poured off, but more usudly a small Iiolc rcmilinctl tlircmgli tllc top of the icx. After drilining off the concentrate, tlw cavity in tlic ice was rinsed out twice hy pouring in about x ml of \Viltcr at room tcmpcrnturc, ant! rii!dly swirling tlris iWOUnd before adding it to tllc concentrated solution. ‘fhc final volume of solution WilS ncvcr mow than I0 ml, rcprcscnting a ro-fold concentration of the solute.

The results dcrivcd from tlicsc studies arc shown in Table I, togctlicr with the method of dctcrmination used for cacll solute. ‘I’ri!Ax!istillccl water was used tlirougli- out, and, as necessary, its purity was checked by blank dctcrmination.

\Ve wish to thank Mr. I-I. FOSTEIC for csperimcnta! assistance.

NilI’

NJlCI

‘TA13L.E I

CONCi?NTHATJON OF VAltlOUS SALTS

-- - ..- .~-_-_.._ --._

Itccovery Calc11IuIcc1 .WellJod 01 delermJinrJfion Solirlc CfJllcclrlruf ion f)/ sotltlc (,U/,‘:I IfJO PIIf)

--- --.....-

Niti 1~1’0.1 20.0

20.0

20.0

20.0

5.0

5.0

5.0 5.0 5.0 5.0

-

21.0

22.0

45.0

45.0

J 5.0

‘S.0

IS.0

15.0

Mg(NWa GI.0

C~l.0

(1 I .<)

I.)..)

2.1.4

12.2

12.2

(PC) ‘%,wv rrrovcry

------_ _.-- -._- _. --_. - ._ . --_--_- 19.7 38.5 Counting Of 3aI’ I~l~cllcCl NiLI-121’04 J3.2 gG.0

20.0 100.0

J ‘,.<I 93.5

4.5 00.0

4.‘) 98.0 5.0 IOO.0 .I.8 go.0 4.3 g8.0 4.7 94.0

Zl.tl ‘)‘). I Colorinwtric dctcrniinntion of I:- 2J.o 3H.2

45.2 X00..) I%bmc plmtornctry 45.0 100.0

15.0 100.0 r4.H 38.7 ‘5.3 x02.0

15.0 100.0

Or.0 100.0 Ng by iltolllic-;rbuorption qwctro- photometry

CJI .o 100.0

s0.t) 98.0

23.8 97.5 24.4 100.0 12.2 100.0

J2.2 100.0

SiO, JO.0 10.0 100.0

(;ie Na&iOa) 10.0 10.0 100.0

5.0 5.0 100.0

5.0 5.0 1OO.O

3.0 3.0 100.0

1.0 I .o 100.0

1.0 I .o f 00.0

0.5 0.55 J 10.0

0.5 0.50 100.0

Colorimctric

5GO SHORT COMMUXICATIONS

Natioual Clrcrrrical Laboratory, Teddington, Mititilcsex (E:,r&and)

I J. SIIAWHO, Scicucc. 33 (J~GI) 2063.

2 S. I<oa~u~s~rr hNi> 6. I:. l_r:n. Anrnl. Chew., 36 (19G4) 2x97.

(Rcccivccl April zgrd, x&j)

G. k-I. SMITH’

M. P. TASKER**

+ Prcscnt nddrcsu: Lhsic T’hysics Division, Xationnl I’irysical Laboratory, Tcddington, Middlcscx. l * Prcscnt addrcus: Ninistry of Dcfcncc, Fort Ihlstcad, Kent.

Awnf. ChinJ. Acla, 33 (J965) 559-560