532 ANALYTICA CHIMICA ACTA

For over 4 years the mcthocl appliecl in our laboratory for the clctcrmination of iodine in crop samples was that rccommenclcd by the Central Veterinary Laboratory (Minis- try of Agkulture, Fisheries ad Food, Great Britain)‘. In this method 200 rng of clriccl plant sample are digcstetl with 80 ml of 18 N sulplluric acid and 4 g of potassium pcrm~iiiganatc at 195”. After tlic digestion oxalic acid is added ant1 the iodine formed by reduction is clistillecl. Joclinc is tlicn clctcrminecl colorimctrically by means of its catalytic action on tlic rate of reduction of cerium(IV) by arscnic(II1) in an acidic medium. Tliougli this mc”dioc1 generally gave satisfactory results it is time-consuming :mtl can only be l~andlccl by cspcriencccl analysts. Furthermore, much time is required for the rccrystallisation of permanganate and oxalic acicl. For these reasons a simpler mctllod was sought in wliicli rlistillntion coulcl lx omitted and the recrystnllisntion of chemicals avoiclccl.

Very few mctllocls have been clcscribccl in tlic literature for the analysis of iodine in crops. Distillation after acid digestion has been usccl by both JOHNSON ANI) l3u*rte1~~ and kIOUS’rON3. Numerous mcthocls for the clctermination of iodine in blood have been publisliccl (for reviews, see 415) but none of thcsc mcthocls appcarccl sufficiently attrac- tive for application in a routine clctcrinination of ioclinc in crop samples. In the mctli- ocls clc1xznding on alkalitic nsliing, tlic temperature ant1 the duration of the dccoinpo- sition arc very critical, wliercas tliosc lmsccl on an acid cligestion followccl by distil- lation require too much time. Some methods in which tl~c~distillation is rcplacccl by removal of iodine in a current of air’) or in wllicli iocliiic is cliffusccl at 0o07 are also rxtlier laborious despite a certain simplification.

l’hc method of ZAIC”, in which blood samples ilre osiclisccl with chloric acid in the prcsciice of chromate and the iodine is directly cletcrniinecl in the clilutccl digest, is a very rapid mctliod which might be appliecl i1S ;L routine method for crop analyses. It leas, liowevcr, the disadvantage that tllc cliloric acicl rcagcnt has to be freshly prepared regularly. We 1lilVC sought for a digestion misture wllicli is easily ol~t:iinablc and clots not yielcl any substances which co~~lcl intcrfcre with the colorimctric cletermination following the decomposition. For this reason the so-cnllccL Neumann acid was chosen; this consists of a misturc of cclunl volumes of sulpliuric and nitric acid, and is appliecl -.- ..---__.. .- _ ..__ * l’rcncnt atltlrcss: I~:rlmratory for Cytochcmistry. IhGchuixcrw~g zoo, 1C.C. Cinivcrsity, Sijmcgcn, ‘I’lw Nothwlands.

/Itral. C/rim. Acta, 26 (1962) 532-540

L)ETERhIIXATION 0FIOI‘)INELN I'LANThIATERIALS 533

in the analysis of several crops. As the decomposition is often more successful when some perchloric acid is present we followed the latter procedure.

For the investigation of the reliability of the method, radioactive iodine (1311) was used as a tracer.

Reagents

Unless otherwise mentioned analytical &Tade chemicals from Merck were used, Neacmann acid: ZL misture of I volume of concentrated sulphuric acid (s.g. 1.84) and

I volume of concentrated nitric acid (s.g. 1.40). Pcrddoric acid: (s.6. 1.67). Acid arse,Gc solrrtiorl: 9.8 c: of arsenic triosicle was dissolved in 14 ml of IO N sodium

hydroxide; 600 ml of water was added and the misture neutralized with IO N sulphur- ic acicl; 42 ml of concentrated sulphuric acid was added followed after shaking by IO ml of concentrated hydrochloric acid. The misture was diluted to I 1.

Dilrrtion liquid: 3.0 N sulphuric acid. Ccric solution: 5 6 of cerium ammonium sulphate (British Drug Houses Ltcl; low in

other rare earths) was clissolvecl in 70 ml of 5 N sulphuric acid by gentle heating. After filtering the solution was diluted with distilled water to IOO ml. For USC the solu- tion is dilutecl with 3 volumes of 3.5 N sulphuric acid. This solution can be kept for a few weeks. The ccriurn ammonium sulphate is previously purified by washing 15 g of the salt with 75 ml of distilled 96% ethanol.

Standard iodine sohtion: I mg of iodine as ICI03 in I liter of water.

Method of clreclkng with radioactive iodijte

0.1-0.2 pC 1311 (carrier free) as NaI is pipctted into a 50-m! measuring flask in which zoo rn6 of crop sample have been placecl. After the decomposition I ml of the radio- active solution in a porcelain crucible (Wcta no. 3127) was placed in a lead castle; it was measured by means of a Philips scintillation counter no. 4111 and the rccovcry was calculated.

Procedure /OY detcmtinatiort

200 mg of crop sample was dried at 105~ and placed in a go-ml measuring flask. After this 7.5 ml of Neumann acid was added with shaking to moisten the sample complctcly ant! then I ml of perchloric acid was pipetted in. The digest was then heated over a period of I h to 16o-1go~ by means of an electric hot plate (type Invcntum no. 224122 ; first position with a surface temperature of 200~ ). The surface temperature was then raised to 275” (second position) until the first occurrence of a yellow colour (the temperature of the digest was rg5-2x5O). The total duration of the clecomposition was about go min.

After being cooled, the digests wcrc diluted with water to 40 ml with constant shaking. Then I ml of acid arsenic solution was added; the solution was then diluted to 50 ml, and shaken again.

After one or two days of standing at room temperature (18-24~) the solutions were shaken a few times; 5 ml of each solution was then pipetted into a colorimcter tube, into which 0.4 ml of acid arsenic solution had been pipetted beforehand. The contents

. .I

.4uaI.Cltitrr. Aclu, 2G (rg6z) 532-540

534 G. W. 1;. II. f%. I’AUWELS, J. C. VAN WESEMAEL

of the colorimctcr tubes wcreshnken .twicc and then placccl in a waterbath, adjustecl to exactly 3o”*

After about 20 min I ml of the ceric solution was adclccl in succession to each tube at intervals of I min. The transmittance at 420m,u was measured by a calorimeter (Lumetron) 30 see after the addition of the ccrium. The transmittance was again meas- u1cd20 min after the first determination. It is thus possible to carry out one series of 20 determinations at a time if one measurement is executed every minute. The clif- fcrcncc in transmission between the two calorimeter rcaclings less the value of the. ” blank decomposition is a measure of the ioclinc content. With cvcry series of x8 samples two blanks are includccl. ‘The iodine contents can bc read from a calibration curve. The points of this curve arc cstablishecl by cligcsting amounts from 0 to 0.5 ml of the standard iodine solution in the same way as for the crop samples.

When the iodine content is so low that the cliffercnce in transmission less the blank value, is not more than 4, EL thircl measurement is carried out after IZO min. The iodine contents are then read from a test curve which has been preparecl in a similar way.

When the iodine content is so high that the second rending of the transmission is greater than 75% within 20 min, the solution must be di1utcc.l. The dilution can be cffcctccl by aclcling dilution liquicl. In this cast the iodine contents shoulcl be read from ;L test curve which has also been prcparecl by application of a corresponcling clilution.

DISCUSSION

Digestion with Neumann acicl alone lccl to great losses of ;~lclcd raclioactivc iodine. In two cliffcrcnt measurements the rccovcry was only 20 md 51% rcspcctivcly. However, addition of perchloric acid clccrcasecl the losses of iodine consiclcrably (see Table I). Even 0.1 ml of pcrchloric acid was nearly sufficient for a quantitative rc- covery. It appeared safer, however, to a&l I ml of pcrchloric acicl to each cligcst.

Amount of snntfile tnken

The rccoverics of inactive iodate aclclcd clicl not appeal to cliffcr significantly on varying the amount of sample from 50 to 200 mg. With 300 mg, however, 5 to 10% of the iodine was not rccovcrccl while with 400 mg as much as 15 to 30% was lost. zoo mg of crop sample is therefore the most suitable amount for reliable clecomposi- tion.

I‘)ETERhIINATION OF IODINE IN PLANT hIA’l-ERIAI,S 535

T&~fierature of digestion

Fig. I shows the influence of the final tekrqxrature of the digestion on the acidity of the digest diluted to 50 ml. The decrease of aciclity in the temperature range of 150- zoo0 can probably be ascribed to volatilization of nitric acid which is still present. The xcitlity &creases very sharply in the range of ZIO--2~5~. The liquid becomes deep yellow in this range; it is likely tllat this further decrease of aciclity is caused by de- composition of the pcrcliloric acicl present. At 240~-270° the licluicl turns colourlcss

Normality

2.4 150 200 250 300 350°C

FiK. I, Rchtion Ijc*twc*cn tlic nornwlity of the tliKcst after tlilution tc~ 50 1111 ancl tlw final tcmpcra- turc of tligcstirm.

agai 17. Apparently all tlic pcrcliloric acid has then tlisappcarecl. The amount of re- covery of ddcd ‘:“I \V;LS independent of heating up to at least 2&P. No significant clif- fcrcnccs in ioclinc contents were cstablisliccl wlicn samples of known ioclinc content were cligestccl at tcmpcraturcs in the range of rGo to 250’. On digestion at tcmpcrxtia- rcs l~clow do0 only lxirt of the ioclinc wns rccovcrccl. Presumably the clccornposition was incomplete.

‘The initial ~pp~ilKXllCC2 ol the yellow colour the digestion, for it was not necessary then to tcmperaturc wns about 195-215~.

Method of dilrltiort

After clilution of to tlCpCnc1 strongly

the digest the ioclinc vdlatilixcs slowly. ‘Hit volatilization appears on the tcmpcraturc nt which tile liquicl is kept (SW Kg. 2). Acldi-

was used to detcrminc the end-point of place a thcrmometcr in the liquid. This

%

‘OOb z\x )(-x-x OT 0

80,

60: \ 40 ’ b 0 room temp.

20X 2 4 6 6 10 72 lldays .

Fig. 2. Rclntion IxAwccn the pcrccntngc of iotlinc rcmaiuing in tlic dilutctl rligcst ancl the time of storngc crt o” nnd room tcnlpcrnturc rcspcctivcly (no arscnic(Il1) was added).

536 G. W. 1’. H. B. PAUWDLS, J. C. VAN WESEXIAEL

tion of I ml of acid arsenic solution made the losses &crease sharply. Possibly the iodide is slowly converted to iocline and then reduced to the less volatile ioclide by the arsenic(III), so that the risk of volatilization clecreases again. It should, however, be noted that on addition of greater amounts of arsenic the loss of iodine increases again (see Fig. 3)*.

% A noAs+HCl o I ml As l HCI

100*4-h. . -._. *z -9.-._._ o

x 4mlAc,*HCl . 10ml As+ HCL

60*\ *-. _~ “*-&.._, 0

-.4. b

\

-c -e-X

60, -_

---a__ ---_

40. \

-0

20. d-O d

I 3 6 9 12 15 16 21 day3

L:iK. 3, Rclntion bctwccn the pcrccntxgc of iotlinc in the clilutcci cligcst and the tirnc of storngc on adding cliffcrcnt quantities ot acid arsenic solution. ‘I’hc tctnpcraturc of storage wxi nbout 20”.

‘L’lw find volurnc was 50 ml.

Though the losses could not be complctcly suppt-essccl by dclition of acid arsenic solution the clecrcasc in the iodine content dtcr I clay was never more than IO”/~. It was not necessary to malce a correction for thcsc losses lxcausc the measuring points of the calibration curve were subject to identical losses after cLilution.

Finally it shoulcl bc remarked that the acid arsenic solution should ncvcr be aclclccl directly to the undiluted digest because this would lcacl to very scvcrc losses of iodine. As a matter of fact conccntratcd sulphuric acid reosiclizcs the liyclriodic acicl formed by rccluction. At an acidity of 3.5 N the losses by this reaction arc not very great. For this reason the digest slioulcl lx diluted to at lcnst 40 ml before the nrscnic can be XL- &xl. On further clilution to 50 ml the concentration of the sulphuric acicl decreases still more.

Occasionally the catalytic reduction of ccrium(IV) by arsenic(II1) seemed to bc rctnrcled, so that a clctcrmination completccl shortly after the digestion period gave incon-cct results. In some cases, this retardation was still perceptible one clay aEter the cligcstion. It sccmccl that tlic tcmpcrature of storage of tlic digest after clilution to 50 ml had. consiclcrablc influence on the cstcnt of retardation.

A given sample of grass was cligcstcd with the acid misture and the solutions ob- tained after dilution and nclclition of 1 ml of acid arsenic solution were left to stand for a night at cliffcrcnt temperatures. The next clny the iodine content wus determined cololimetrically (see Fig. 4). The results obtained suggest that sometimes an inter- fering substance remains after the c&c&ion; this gradually hydrolyzes or volatilizes if the solution is allowed to stand for some time at a suitable temperature. ‘The

* If the cligcstion is carriccl out at 250~ then tlic ioclinc content tannins fairly constant on addition of 4 ml of acid arsenic solution.

.4 11~11. Cliim. Actn, 2G (1962) 532-540

DETERMINATION OF IODINE IN PLANT >lATERIALS 537

interference can be practically eliminated by stancling for one day at room temperature (18-24”). Standing for another day had no effect on the results.

40, /

.A* 20.

10 20 3ooc

I:ig. .I. Rcl;ltiwi t~ctwccn the iotliric nIcwurc’c1 in the tlilutctl tligcst (as u pwxcnti~gc of the total iodine prcscnt) ilntl the tcnlpcriltllrc of stc)lYlg:c tlurinC: the first tl;ly nftcr tlcconqwsition nnd dilu- tion. l’hc! snrnplcs invcstig;~tccl showed iLn initiill arrest of the c;lt:llytic rctluction of ccriunl.

Apart from the retardation descrilxtl abovc WC have also sometimes found a dimin- ished sensitivity, which did not disappear after standing for one day at room tempcra- turc. Normally the tliffcrcncc bctwccn the transmission reaclings before and after the reaction (20 min) for 0.1 ml of the standarcl iodine solution amounted to 6 to 7, During a particular period the diffcrcnccs wcrc only 3.5-4.5. This climinishcd scnsitiv- ity was also found for the other points in the standard curve. l’hc factor rcsponsiblc for the rcduccd sensitivity was the sulphuric acicl usccl. The low cliffcrcnccs in trans- mission were found with a particular batch of sulphuric acid, while two other batches produced normal values.

Culihation czwvc

Fig. 5 represents some typical stanclard curves. Diffcrenccs in transmission dcter- minctl after 20 and 120 min were plotted for ioclinc contents vilr>*ing from o to 3000 /46 per kg of crop sample. ‘l’lic values were Ol~tiLillCtl by means of “destruction” of

d transmlsslon

Fig. 5. Iiclation bctwccn the tliffcrcncc in trxnsmission obscrvcd after 20 or I +o tnin (after dctluc- tion of the blank value) and iodine contents corrcsponcling to fixed points of the standard curve

and cxprcsscd in pg I per kg clry weight.

538 ti. W. I:. H. 1%. I’AUWEM, J. C. VAN WESEMAEL

known quantities of potassium ioclate. In several cases the curve drawn by the check points was not completely straight, being slightly less steep at lower contents. Owing to the effect of dilution the standard curves for samples with a high iodine content are situatccl below the average “normal standard curve”. This can be ascribed to the fact that the arsenic concentration diminishes on dilution with 3.0 N sulphuric acid so tllat the catalytic reduction of ccrium(IV) proceeds more slowly.

Contyhhwn of the reszr1t.s of the m.w melhod with those of the distillation w&hod

In ‘L’ablc 11 arc shown the results of iodine determinations in grass samples varying

‘l’lii~ ;tnidyscs wc’rc partly rclw;dxtl on tliffcrc~nt: tlays. - -- --_. .-.. ._.___ --__.-. .- . _._-___ _.._ - -... .- .._._ --___.. -._- ._...- -.. . -_-._ -

30 I qo LOI ‘1 I I

4 4 5 571 729 6.13

4.57 81‘1

1310 1862

2122 23.12 3 2 8 .? .2C,5H

6s -I- 3.5 too ---.3’, a12 ---SO 381 -30

363 ---Hz

5 1 (J -.5s GC, I --f,H (ifi I -1. 1s

5 ‘ 4 -1.. 57 78‘1 --30

1.271 -4s I Ho6 -9’

ZOGS --57 2147 -IO.5 237'1 -I- 0.1 22CJH -3fiO

II2 gc, 07 97

37 ‘JZ

I0.I N(,

0.5 2.4

0 * 5 3.H

SO 26

4 ’ 3" S3 5'1

12'0 b1

7’ 55 .5x

III I

.!I I 2.10 LO I ‘47

36

SJ 3: I lS0

138 6-i

‘2 7 2 I.52

H

7 ‘7

8

7 3

I.? b

32 5 ’ ‘I 5 ‘5 5

x 5

‘3 5 I, .5 fJ 5

’ 4 5

II 5 5 5 3 5 f.J 5

x 5 ‘J 5

IO 5 #I 5

H.1‘ 7.9’

I\ Values obtninccl I>\? tlw di.stillntion nwtliocl. 13 \~olucs obtainctl l.+ the new mcthrd. 0 Loclinc contents ilS rlcLcrrninct1 by tlic iicw inctllotl amI tmprcsxccl as a lxmxiit;r~c ol tlic icltlinc contents bv the olcl nlctlwtl.

d Stllclcl1t’s I = tiiffcrcncc _-.__

,/=A _ S(l”

R ‘I’lic viiluc 201 hiis hen IcfL out of consitlcr;~Lion. h ,/r

V,,(,,_,) . * Avcrngc of the pcrccntngcs of the 3.50 p&kg dry IllilttCr.

Yr(cf/~) V;~UCH of tlw sanlplcs with ail ioclinc ccmlcnt: liiglicr thail

..I ,ttrl. Chitu. .4c/tr, 26 (1961,) S32-540,,

DE-I-EI~hlINATION 01’ IODISE IN PLANT XIATERIALS . 539

in iodine contents between 30 and 3000 ,ug per kg clry matter. The same samples were also analysed according to the distillation method previously used in this laboratory. It appears that the values of vX’&/(n- I) and of Z(d/n) are approsimately qua1 for both methods. The average deviation of mean values for samples with an iodine con- tent higher than 350 pg per kg appeared to be about S’y., both for the distillation meth- od and for the new method. When all the determinations were made on the same day the average deviation with the new method did not appear to bc higher than 3.2% for a grass sntnple with a mean iodine content of 1710 ,uug per kg.

From Table II it can IJC seen that the values by the new method are on averngc 6% lower than those dctcrminccl according to the distillation method. Tnblc III contains ;ulditional observations: in scrics nnalyscs of ioclinc colitcnts lower than 400 pug per kg, the new method producecl values which were cithcr cclunl to or higher than the values obtained by the distillation method. For snmplcs of ioclinc contents higher than 400 pg the new method produced values which wcrc relatively lower, which is in qrcement with the data of 'I'nl~lc I I.

‘I’:\ 111.13 1 I I

__________ _ _. ._._ .- .__...__ -_.-- _..... -.- -_.-. _ .I_--- -_.-..-..---.-.---.-~ --.__-._-.- _.

N,w #IId kld ,‘# //kc ._._--. ,a”:; .Vl~llllk~r II/ srrrllpll~.S .V rcmbrr uf rcpliictt ions

K.lluf~, m~tkrut ___.._. ._.__. -- ..- ..-.... ..- ___..__..___~ _.__.. _ ___ ___. __.. ___. -_- _..._ _._ .-- _-__ _^. --__

I oo- ‘5” I a):& I I II 2

I oo- 250 I OH rf: fJ.S IS L

‘!j”- 40” LOI A_ 5.5 I ‘1 .?

**w>- Ci_j” 0.1 2: 3.0 ‘7 L I .jO--31100 0.1 A.. 2.3 ‘5 SCL’ ‘I’nblc I1

Io,cmcl-- 110.000 07Az 3.3 7 3 c)r 4 _ ____ _ __ ___..._ -_. -. _ _.._._ _-~- .-- ,.. ___.. ---._.--------..-- .---- _.-

Thus the absolute values of the iodine contents cletcrminecl according to the new m&hod may be somewhat on the low side. We also obscrvctl that ‘the rccovcry of iodine aclclccl in the form of potassium iodatc to grass samples and corresponding to increases of the iodine content between 600 and 2500 ~6 per kg, was only 92.6&2.5O&.

It is very probable, on the other hand, tllitt for values lower than 400 ,q per kg the distillation method gives values which arc to low.

GI:NERAI. I>ISCUSSION

The new method seems as good as the distillation method previously used in this laboratory, as regards reproducibility. At low contents (below 350 pg per I kg) the reproducibility is even better. Diffcrcnccs from the nbsolutc values arc certainly not grentcr tlliltl 7 to S%.

For practical purposes, such as cases where the iodine content of grass should be known in relation to possible iodine clcficiency of cattle, the new method can be deemed sufficiently accurate. A very important factor is that the number of analyses which can be carried out by one analyst is at least 3 times greater than that possible by the

And. Claim. Actn, 2G (iqG2) 532-540

540 G. W. I:. H. 13. I’AUWELS, J. C. VAN WESEMAEL

distillation method. Moreover, the equipment is very simple and the procedure can be quickly learned. In contrast, the cliStillation method requires complicatccl apparatus and cnn only be clone reproducibly after much training. The most important cliffcr- enccs hctwecn the two mcthocls are as follows:

____. -_.- . . . . _.._ I__----. _ -..- - ..__- .-- ___- . _---_. Ntw mclld Dicf illrcf iw 9wlhrd

_._._._--_.._..-__..I---.-.___._ .-._ ._-- _.._ -- _.__..... -_--.-

Xlcnn w-rrWL 7.9% 8. I ‘Y” Ntrnlbcr of snn~plcs/clay per person (tluplicatc) IO 3 _._- -__- -.- ..__. ------- -_._.- - . ..--.---- -...-_- _.-.. - -_.. ..-_ -. I* for iotlinc ccmtcnts higlwr tlian 350 pg per kg

‘UIC authors arc: inclcbtccl to Prof. Dr. A. C. SCHUITELEN of the Lahotxtory ot Agri- cultural Chemistry nncl to Dr. Ir. D. nr: %IXUW of the Institute of Applied Nuclear Energy in R~~iculturc for permission to cjn-ry out espcrimental work in their IAora- tories. ‘I’hcy arc nlso grateful to Dt . _J. J. LEHIC for his interest in the work :mcl for nssistancc in translation. ‘I’hnnks :\rc also clue to Mr. J. \~ASSINIC for technic~d help.

Uric n011vc1lc inJtlioclc siniplo ct rapiclc cst prq30dc p~~~ir Ic tlosajic clc l’iotlc clans Lcs pli;ntcS. I.‘~cliimlillon ?r analyscr cst tl&ompcw? illi nioycn tics mzitlcs sulfuricliic, iiitriquc ct p~rcliloriqiw. Alwhs tlilution, I’ich cst tlosd coloriii~~tric~iicrtlc~~t.

13cschrcilmJig cinw cinlwhcti uric1 rnsclicri Ncthotlc air IjcstininiunC: tics Jrrtlgctinltcs von I’flan- 7x11. hX ~\~lfscliluss crfdgt lnit %liwcfcls!li~rc, Sdpctcrsiiurc iincl l_Q2rclilors!Lurc ; nach Vcrtliiriiwti win1 tlas J ocl kolorirnctriscll bcstirnmt.