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1096 DOY & GUIJOSA, J. ASSOC. OFF. ANAL. CIIEM, (VOL. 64, NO. 5, J9RI)
FERTILlZERS
Ammonia-Selective Electrode Dctermillation oí Nitrogcll
in Fertilizers
VICTOR M. HOY and MANUEL GUljOSA
Fertilízl1Iltes M exicallos, S.A., SlIbgerencia de IllvesfigllciólI, Chiapas 184, México 7, n.r.
The official AOAC nldgnesium ollide method
(MOM), 2.065,fOfdeterminin¡; ammnni¡ll:ai nitrogen
in fertilizen; not containing ure,l was compared with
.In alternative anllllonia-se1ective electrode rnethod
(A5EM.1). likt-wise, the official AOAC Raney
pnwJer nlelhod (l{PMI,2.063.2.064, for Jelt"fmining
, total nitrogen in fertilizers, except nitric phosphates
cOlltaining nOllsulfate S, WolScompareJ with an am.mnnia-se1eclive electrode method (A5EM-2). Each
comparison includeJ 6 s
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BOY &: GUIJOS": J. ASSOC. OrFo ANAL CHEM. (VOL. 1>4,NO. 5, 19f1l)
(e) Tar/arie acid soll1lioll.-lO% aqueous solu-
tion.
(d) Sodilllll 1l.lIdroxidc solulioll. -ION.
(e) Ammolliacalllitrogcll so/uliolls.-Slock so/u-
lio/l.-1400 ~g N/mL. Dissolve 5.35 g Teagentgrade NH4Cl in water in 1 L volumetric flask and
dilute lo valume with water, Workillg 50/11-
1;0115.-( J) 700.ug N¡rnL. Transfer 100 mL stock
solution lo 200 ml volurnclric f1ask, add 20 ml
tartaric acid soiution, dilute lo volume wilh
water, and mix. (2) 140.ug N{mL. Transfer 50
rnL solution 1 lo 250 rnl volumetric flask. add 20
mL tartarie acid solution. dilute lo volume with
water, and mix. (3) 70 .ug N¡mL. Transfer 100
rnl solulion 210 200 ml volumetric flask, add 10
rnL lart
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1098 HOY & : GUIJOSA: /. ASSOC. OFF. ANAL. CIiEM. (VOL. 64, NO. S, 1981)
Table 2. Comparison 01results by olficial magneslum oxide and ammonla-5elective electrode methods on 6 samples
01 !ertilizers
Ammoniacal N. %
Average Std dev, Coelf. 01var,. %Magruder
Sample results MOM ASEM-l MOM ASEM.1 MOM ASEM-I
1 7.718 :r 0.3648 7.547 0.1084 1.4362" 2.7040:t 0.0430 2.717 2.593 0.0288 0.0557 1.060 2.1483" 11.035 :t 0.0786 11.088 11.213 0,0821 0.0393 0.740 0.3504" 18.24 :1:0.17 18,267 18.058 00907 0.0454 0.497 0.2515 b.c 21.20(theor.) 21.138 21.063 0.0417 0./224 0.197 0.5666~.d 17.50(theor.) 17.527 17.232 0.0698 0.0527 0.398 0.306
~ Significant dillerences in averages al 95% leyel, t-test.1 > Ammonium sulfate. reagent grade.
e Signilicant differences in stilndard deviation at 95% level, F-test.dAmmonium nitrate. reagent grade.
Thc ammonium sulfate and ammonium nitrate
samples were reagent grade.Mean per cenl nitrogen, standard deviations,
and relative standard deviations (CV,%)for each
of the samples by both methods are given in
Table 2. No value was rejccted at the 95%sig-
nificance level by the Dixon test (11), When
standard deviations were compared by means of
the F-test, differences were statistical1y signifi-
cant only for the ammonium sulfate samplc,
which indicatcs equivalent prccision for both
mcthods:although the l-test showed significant
statistical differences in the averages for 4 of the
5 samples studied. Therefore, MOM and
ASEM-l gave comparable prccision, but average
values for MOM are in bt'tter agreement with
refcrence values than are averages for ASEM-
1.
Total Nitragen
Six samples (7-12, Table 1) were .:malyzcd in
6 replica tes for lotal nitrogen by RPM and
ASEM-2. Samples 7905 B, 7907 B, 7807, 7709,
7717, and 7607were analyzed by 5, 5, 6, 7, 5, and
7 participant laboratories, respectively, in tlw
Magruder check sample program (lO).In a preliminar)' cxperiment with ASEM-2,
nitrogen was measured wilh the ammonia-se-
lective electrode directly a£ter digestion; howw
ever, values were very high. These high values
mal' be due to the increascd ionic strength of the
solution caused by solids forml'd during the ad-
dition of NaOH to evolve ammollia. Such re-
sults an' in accordanc{' with observations by
other authors (2, 7) who recommended that
samples ano standards have the same ionic
strength. Further studies are necessary to con-
sider this eHecL
Mean per cent nitrogen standard deviations,
and coefficienls of variation for t'ach ()f the
S Significant diflerences in standard deyiations at 95% level. F.test.
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BOY & GUIJOSA: J - ASSOC. OH. ANAL. CHEM. (VOL. (,4, NO. S, 1981) 1099
In general. the average valucs fOf arnmoniacal
and total nitrogen by MOM and RPM are in
better agreement with the referenee values than
are lhe average valucs ohtained byASEM-l and
ASEM.2. This means that the ac(uraey of the
magnesium oxide method and RalH'Y powder
rnethod are better Ihan those obtained by am-
monia-selective e¡c(trode tcchniques. 00 the
other hand, Ihe average vaJues ohtained by
ASEM.l and ASEM-2 are higher than those ob-
lained by MOM Uf RPM.
The accuracy and precision of Ihe ammonia-
selective electrode technique is suitable fOf
confine determination of ammoniacal nitrogen.
It simplifies Ihe normal procedure because Ihe
distillation step is omitted. In the determinatian
o C total nitrogen with the Raney powder method,
the final titration can be substituted by an elec-
trode measurement, hut with no improvemento C results.
REFERENCES
(1) Hoove-r, W. L., Colvin, B. M., Me!ton, J . R., Hanks,
A. R., & Howard, P. A. (1977) De-terminalion of
nilrogen in fertilizers and protein in feeds by the
Orion ammonia ('¡('ctrode, Bull. Prpl. AXri •. A/lul.
Srrvirrs, TeXilSA&M University, ColI('g(' Slation,
TX
(2) Thomas, R. F., & Booth, I{. L. (1973) £I1l'iro/l. Sci.
TccJl/Illl. 7,523-526
(3) Cilbert, T. R.,& Clay, A, " ' 1 . (1973) Aual. Clre1/!. 45,1757-1759
(4) Renfro, L.J., &Patel, Y.(I974) f. AI'I,I. PII.IISilll.37,
756-757
(5) Barbera, A., &Cant'pa, D. (1977) 1 - AS."Ilf. Off. Al/al.
OCIII. (,0,708-709
(6) Woodis, T. c., Jr, &Cummings, J. M., Jr (1973)'. Assoc. 0lf. Allal. Chem. 56,373-374
(7) Eagan, M. L., &Dubois, L. (1974) Al1al. CJ¡jm. Aria
70,157-167
(8) 0l/icial Metlwds o/ Al1/1I.II';S (1980) 13th Ed., AOAC,Arlington, VA, secs 2.065, 2.063-2.064
(9) 'Ilstmclioll Manual Irlr Ammon;a Elec/rode i\ltJdl'1
95-10 (1975) Orion Researeh, Ine., Cambríd~e,
MA
(10) Statistical Evalualion on 7607, 7709, 7712, 7807,7905 n, 7907 n , 7908, and 8003 M
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COROMINAS ET Al..: l. ASSOC. OFF. ANAL. CHEM. (VOl. 63, NO. 3. 1980)
Preliminary ComparisoH of Methods for Determining
Sodium in Fertilizers
LUIS F. COROMINAS, VICTOR M. BOY, and MANUEL GUIJOSA
i=erfilizllntes Mexicanos, S.A., Resellrcll Division, Zacatecas 80, México 7, D.1=.
551
The AOAC offidal hrst action method 2.147-
2.150 for the f1ame emission speclrophotometrk
(fES) determinalion of sodium in ferlilizers was
compared wHh the atomic absorption spectro-
photometric (AAS) method and Ihe sodium selec-
tive electrodc (SSE) method. Two synthetic NPK
samples, 3 commercial samples (urea, DAP, and
superphosphate), 2 Magruder NPK check samples,
and one Magruder muria te check sample were
used for the study. Slatistically significant differ-
ences were obtained in both averages and standard
deviations for all samples. In general, the preci-
sion for all methods was acceptable; the AAS
method seems to be the more accurale¡ the AOAC
method needs lo be revised; and the SS[ melhod
has the tendency lo give higher values. Addilional
experimental work is necessary to define which
method is the most convenient.
Previous experimental work (1-3) on the
detcrmination oE sodium in Eertilizers resulted
in the adoption o E the AOAC officia.l Hrst ac-
tion method 2.147-2.150 (4), which is based
on a fl.1mc cmission spectrophotomelric peoce-
dure (FES). It seems thal this method is not
used in many laboratorics. The Magruder re-
port for Samplc 7804 (5) showcd Ihat 23 Ia.bo-
ratories repoded valucs Eor sodium and o E
these. only 5 used the FES method .1nd 17 used
an atomic absorption spectrophotometric pro-
cedure (AAS). The availability o E a sodium
selective electroue (SSE) suggested the possi-
bility o E applying this tcchnique to the de-
termination of sodium in fertilizers, thus
greatly simplifying the analysis.
'"Ve dccided to perEorm a preliminary study
comparing the official AOAC procedure (FES
method) with the 2 aIternative methods. TheFES method was tcsted using 2 different types
o E equipment and flamesj the AAS rnethod
was adapted frorn a procedure for phosphate
rock (6)j and the SSE mcthod was established
Eollowing the recommendations from the elcc-
trode 5upplicrs (7, 8).
Comparative Sludy
Table 1 shows the 8 samplcs used E o r com-
parative evaluation. Thrcc samples were com-
rncrcial fertilizers of unknown sodium con-
centration, 2 samples were Magruder NPK fer-
tilizer check samples, 2 were synthetic NPK
samples, and one was 7804, Magrudcr muria te
check sample, mentioned earHee. Samples were
selected to test the applicability of the peocc-
dures to different types o E products and levels
of sodium content. Ten replica te determina-
Iions were performed on each sample for each
method.
METHODS
1. F1ame Emission Spectrophotomelric
Method 1 (FES Methud-l)
See 2.147-2.150 (4). Readings were made using
Model Mark 11flame photometer (available fraIn
Evans Eledroselenium Ltd), wilh air-propane
flame.
n . Flame Emission Spectrophotometric
Method 2 (FES Method-2)
See 2.147-2.150 (4). Readings were made using
Model AA6 atomic absorplion spectropholometer
(available from V••rian Techlron Ply Ud) in emis-
sioo mode. wilh air-acetylene flameo
111. Atomic Absorption Spectropholometric
Method (AAS Method) (5)
Apparatus and Reagents
(a) Atomic absorptioJl spectropllOfometer.-
Model AA6 (Varian Techlron Ply Ud), or equiv-
alent.
(b) SodiulIl so{utiolls.-(l) Stock SOIIl/íOlI.-
1000 IIg Na/mL Disso]ve 2.5421 g rure NaCI in
ca 3-5 rnl HCI (1 + 1). Di]ule lo 1 L (2) Workingstandard solutioIlS.-O, 20, 40, 100. and 200 M~
Na/ml. To 250 ml volumelric flasks, add O. 5.
10, 25, and 50 ml Na stock solulion. Adjusl each
standard to ca o.lN in HCI (aboul 0.8 ml HCII100 ml) and dilule lo 250 mL.
Preparation of Sample Solution
Weigh 1.00 g sample into 250 ml beaker. add
15 ml HCI (1+ 1) and digesl 15 min on hol plale.Filter Ihrough fas! paper inlo 250 ml volumelric
f1ask. washing paper and residue Ihoroughly with
waler. Measure absorption of soIution directly or
dilule with o.IN HCI to oblain soIulion withinrange of inslrumenl.
Rece;ved AUlI:u,1 30. 1979. Accet>led Dl'cl'mber U. 1979.
000"-'7'6/80/6303--0"1-03$01 ,00~ A••o(iolion of Offieiol Anolylicol Chemi,h, In(.
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552 COROt.UNAS ET AL., J . ASSOC. OFF. ANAl. CHEM. (VOL. 63, NO. J, 19~O)
Table l. Description 01lertili;ter samples studled
This "'po.1 01 the As~odale Referce wa, p.e,enled al lhe
9Jrd Annual Meeting of lhe AOAC, Oct. 1.5--1&,1979, ilt
W,)shin¡:lon, OC.
Determinationo
5et wavelength al 5890A. Adjust burner hcad
900 to bcam oc anolher position if preferred loreduce sensitivity, using air~acetylene flameo As-
pírate standards and unknown samples. PJol curve
from standard values and determine Na content of
unknown samples from plot of absorplion against
¡.¡g/mL
IV. Sodium Selective £Iectrode Method
(SSE Me.hod) (7, 8)
Apparatus and Reagents
See 2.091(a)-(b) and (e) pH meter.-Model 701 Digital (available
from Orion Research, lnc., Cambridge, MA
02139), or equivalent.
(d) Eleetrodes.-Sodium ion seleetive glass elec-
trode and reference eleclrode Models 39278 and
39402 (Bcckman Instrumcnts, Inc., Fullerton, CA
92634), or equivalent.
(e) Sodium solutjOllS.-Stock SOllaioll,-IOOO flg
Na/roL. Dissolve 2,5421 g pure NaCl in 800 ml
water. Adjust to pH 10 wilh concentrated am-
monium hydroxide and dilute lo 1 L. Workü¡g
solutiolls.-(l) 500 ¡.¡g Na/mL. Transfer 100 ml
stock solution to 250 ml beaker, add ca 50 ml
water, and adjust to pH 10 with coneentrated NH¡OH. Transfer to 200 ml volumetric flask,
dilute lo volume with waler, and mix. (2) 100 ¡.¡g
Na/mL. Transfer 50 ml 50lution 1 to 250 rol
beaker, add ca 150 ml water, and adjust to pH 10
with concen!rated NH40H. Transfer lo 250 rol
volumelric flask, dilute to volume with water, and
mix. (3) 50 ¡.¡gNa/roL. Transfer 25 ml Solution 1
to 250 ml beaker, add ca 175 ml water, and ad.
Sample Oescriplion
A rnurlate, 0.0.60, Magruder 1804cheCk s8mple
B DAP, 18.46.0,fertilizer grade dium.
monium phosphate
e superphosphate, 0.20-0,lertillzer grade
normal superphosphale
O 19'19-19,Magruder I¡quid check sample
E 12.}5-10,Magruder sol id check sample
F urea, 46.0-0,ferlilizer grade urea
G 20.20.20, synlhetic sample, prepd with
diammoniurn phosphate, potassium
chloride, and sodium chloride.
reagent grades
H same as Sample G
Approx.sodiumcontenl,
%N.
1.0
0.1
just to pH 10 with concenlraled NH,¡OH. Transfer
to 250 ml volumelric f1ask, dilule to volume wilh
water, and mix. (4) 10 ¡.¡gNa/roL. Transfer 25 rol
Solution 2 to 250 rol beaker, add ea 175 rol water,
and adjus! lo pH 10 with concentrated NH,OH.
Transfcr to 250 ml volumetric flask, dilule lo vol-
ume with water, and mix. (5) 5 ¡.¡gNa/mL. Trans-
fer 25 ml Solution 3 lo 250 ml beaker, add ca
175 ml waler, and adjust to pH 10 with concen-
!rated NH10H. Transfer to 250 ml volumetric
f1ask, dilute to volume wilh waler, and mix. (6) 1
¡.¡g Na/ml. Transfer 25 ml Solution 4 to 250 ml
beakcr, add ca 175 ml water, and adjusl to pH JO
wilh concentratcd NH,OH. Transfer to 250 ml
volumetric f1ask, dilute to volume wilh water, and
mix.
Preparation of Solulion
Prepare solution as in 2.093(a), using sample
containing ca 25 mg Na, in 250 ml beaker. After boiling adjust lo pH 10 wilh NH,OH. Transf('r lo
250 ml volumetric f1ask, dilute to volume wilh
waler, mix, ami pass through dry filter.
Determination
Conlled Na and reference elcctrodes lo pH
meter, place electrodes in low concentration Na
solution, and warm IIp pH meter. Transfer C.1 100
ml stock, working, and unknown sample solulions
into sep.1fale 250 ml I;.eakers. Place electrodes in
each solution and while stirring wilh magnetic
stirrer al constan! rate, read mV of the stock
working and unknown sample solutions. Rime
and blol c1ectrodes between solutions. Plot Ihe mVrcadings (linear axis) against concentralion (Iog
axis) on standard 3- or 4~cycle semilo¡¡;arithmic
paper. Determine Na content of unknown samples
from slandard curve.
Results and Discussion
The averages D E the 10 determinations madI"
on eaeh s.lrnple with eaeh method, and the cor-
rcsponding standard deviations and eoefficients
o E variation, are shown in Table 2. In general,
the standard deviations for the 4 methods
showed precisiom that are aeeeptahle, al-
Ihough the F-Iest showed significanl s!atisticaidiffcrenecs in 50% o E thc total pairs that can
be formed. Comparing the averages by mean"of the t-test showed signiricant statistical dif-
ferences in 70% of the total pairs.
Differences are evident between the aver-
ages for the 2 versions of Ihe FES method, anJ
this can only be attrihutcd to thc use of differ-
cnt equipment and different Aame Ch.1file-
teristics¡ thus, the AOAC official method need ••
to be revised. In the Magruder report for
Sarnple 7804 (Sample A), the avcrage for tite
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COROMINA5 f.T AL.: J. AS50C. orF. ANAL. CHEM. (VOL. 63, NO. J, 1980) 553
Table 2. Comparative results lor the det••rmlnatlon 01!!Iodlum(% Na) In 8 samples 01 ferUlizers
Method Melhod
Slatislic 1 1 1 1 1 IV 1 1 1 1 1 IV
Sample A, Muriate Sarnple B, Diamrnonium Phosphate
Average 1.538 1.447 1. 449" 1. 809 0.080 0.09l" 0.098 0.111Std dev. 0.030 0.010 0.013 0.014 0.007 0.004 0.006 0.009Coer!.of
var., % 1.951 0.691 0.897 0.774 8.750 4.301 6.122 8.109
Sample C, Normal Superphosphate Sample D. 19.19.9
Average 0.130 0.159 0.232" 0.211 0.602" 0.605 0.512 0.714Std dev. 0.008 0.008 0.004 o.on 0.007 0.009 0.008 0.027Coeff.ol
var.. % 6.1S4 5.0]1 1. 724 6.161 1.167 1. 488 1.56] ].782
Sarnple E. 1l.15.1O Sample F. Urea
Average 0.200' 0.230" 0.173 0.260 NO' 0.00246 0.00103 0.0079Std dev. 0.008 0.007 0.004 0.005 0.00040 0.00013 0.0010Coell.ol
var .•% 4.000 3.043 2.]12 1.9lJ 16.260 12.621 12.658
Sample G. lO-lO.lO. Synlhetle Sample H. lO.20.20. Synthetie
Average 0.861~ 1.312 1.108 0.790 0.133 0.117 0.119 0.084Std dev. 0.058 0.116 0.008 0.027 0.005 0.003 0. 000 0.004Coefl.ol
var .•% 6.736 8.841 0.712 3.418 3.759 2.564 0.000 4.76l