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January  2 8,   1986

Inthe late   1950s, wehad  been carrying out  workat theDepartmentofOceanography,  UniversityofLiverpool,   on the   cycle  of phosphate   in the   sea.Our  work   was  held back by the lack  of a suitable

method for   determination   of the very   low levelspresent   ( <   1-16OMgl~).Photometric methods de-

pending on the  formation ofphosphomolybdenumblue  complexes   had   been in  use   since   the   1920s.However, the intensity   of the colour   that was oh-.tamed   with   a given amount   of  phosphate   verymuch   depended   on   thereductant   used   to reduce

the  intermediate phosphomolybdic acid.1

Tin (II) chloride  had  been used   forthis purpose,

as   it gave   a very   intense blue colour and   hence   avery high sensitivity.   It had the  disadvantages thatthe   colour was  only stable for a  very   few minutesand   that  in   seawater   the formation  of   the colourwas   delayed and its intensity   decreased   by   thepresence  of the major ionsof the seawater.  We setabout trying   to  find  a   reducing   agent free fromthese   defects.

Aftertesting a numberof compounds,  we  foundthat  ascorbic   acid (vitamin C ) was   satisfactory and

allowed the analysis to be carried out with a singlereagent rather than by adding the   reducing agentafter the acid molybdate   as   had   been   donebefore.

2It  had   the  disadvantage  that   the colour

development required 12   hours,   leading   to   thepossibility   that   organic  phosphorus compoundsmight be hydrolysed and their  phosphorus includ-ed in the  analysis.  Ascorbic acid had  been used  a sa reductant in the  analysis before,

3but thecondi-

tions employed   were   not optimal,   and maximumcolour w as  not,  therefore, developed.

We decided, therefore,  to se e   if  the rate   ofcol-our development  could   be increased.   I   had   readthat   the   incorporation   of   bismuth in the reagentdid  speed  up the reaction.

4When we tried this, we

found  that this w as   the case but that  hydrolysis  ofthe bismuth led to a   turbidity that   made   thetechnique   useless   for the   determination   of   lowlevels  ofphosphate.   Consideration of the periodictable   suggested that it might be  worthwhile substi-tuting   antimony—its immediate   neighbour inGroup   Va—for   bismuth. Immediate   success   wasachieved   when this w as done, colour developmentbeing complete in two   to three   minutes.   Onceformed, the colour,  which was  due to a complexcontaining  antimony,   phosphorus,   and   osolybde-num,  was   stable for   many days.   Furthermore,   thetechnique   gave   a sensitivity   slightly   superior   tothat obtained with tin   (II)  chloride,  was free frominterference   from   most   other   elements,   and   w as

- not  affected by seawater  salts.I suppose   that the  reason this paper has been   so

extensively cited i s   that itprovides a simple, highlyreproducible   technique for the   determination   ofmicrogram amountsofphosphate.  Almost 25 yearslater, the method   is  still  basically the  recommend-

ed standard   procedure for the analysis offresh  andpotablewaters,5’6 as  well as seawater.7’8 Althoughit   was   originally   developed for the  analysis   ofphosphate in natural   waters,   it   has   been   widelyadopted in  many  other fields,   including, for  examSpIe, botany, zoology, biochemistry, geochemistry,metallurgy, and clinical  medicine. Indeed,  kits forthedetermination   are   available commercially for

use  in   physiological investigations and water anal-•   ysis.

This  W e e k ’s   Citation   C la s s ic ®    ~[ Murphy   J  &   Riley I  P.Amodified single solution  method for the determination  of phosphate   in   natural waters.   Anal.   Chim.   Acta   27:31-6,  1962.

[Department  of   Oceanography.   University of  Liverpool,   Englandi

This   paper   presents   a simple   and   reproduciblephotometric   procedure for the   determination   ofparts   per   billion   concentrations  of  phosphate innatural waters.   It is   adaptable tomany  other typesof sample   because  of   its   freedom from   interele-ment   interferences.   [The   sow  indicates   that   thispaper   ha s   been   cited in over   1.520   publications,making   it   the most-cited paper   ever   published   inthis   journal.]

James Murphy  and John   P .   RileyDepartment  of Oceanography

University   of   LiverpoolLiverpool   169  38X

England

I.   BurtonI D &   Riley  I P. The  determination of   soluble phosphaie  an d   tOtal   phosphorus   insea  waters   and   of totalphosphorus  inmarine   muds.  Mikrochinr.  Acm   9:1350-7,   1956.

2.   Murphy  I&   Riley   IP. A  single   solution   method  (or the determination of   soluble  phosphate   insea  water.

J.  Ma,.  Biol.   ,

4 

sxn.   UK   37:9-14. t958.   (Cited 60  times.(3.   Greenfleld  L I  &  Kelber F   A.   Inorganic  phosphate   measurement in  seawater. Bull.   Ma,. Sci.   Gulf  Caribb.   4 :323-35.   1954.

4.   lean  M. etude   du   d o s age   du phosphore   et de l’arsenic   dans   es  aciers   par cotorim4trie.Anal.  Chim.  Ado   14:172-82.   1956.

5.   Her Majes*y’e Stationery   Office.  Methods foe the  examination   of  waters  and   assodated materials:   phosphorus   in waters,   effluents and  sewages.   London: HMSO.   1981.   31   p.

6.   American Public Health Association.  Standard   methods for   th e  examination of water and   oastewater.Washington,  DC: American Public Health Association,   1984.

7.   KoroleffK.   Determination of nutrients.   (Grasshoff   K,  Ehrhardi  M&  Kremling   K.   eds.(   Methods  of seawater analysis.   Weinheim. FRG: Vertag  Chemie,   1983 . 419 p .

8,   Parsonu IR.   Make  Y &  Lelil C  M.A manaal of   chemical and   biological   methods for   seawater  analysis.Oxford: Pergamon   Press.   1984.   t73 p.

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