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Transkript

@Metrohm
No. 133/2 e
Application Bulletin
General analytical laboratories
Of interest for:
A 1, 2, 7, 8, 11
Determination of ammonium with the ion-selective
electrode
Summary
Although the known photometric methods for the determination of ammonia are
accurate, they require a considerable amount of time (Nessler method 30 min, indophenol method 90 min reaction time). A further disadvantage of these methods
is that only clear solutions can be processed. Cloudy solutions must first be clarified by time-consuming procedures.
These problems do not exist with the ion-selective ammonia electrode. Measurements can be easily performed in drinking water, ground water and surface water
as well as in waste water, soil extracts and Kjeldahl digestion solutions (without
distillation).
The determination of ammonia in ammonia salts, of the nitric acid content in nitrates and of the nitrogen content of organic compounds is based on the principle that
the ammonia ion is released as ammonia gas upon addition of excess caustic
soda:
NH 4 + +OH--.. NH 3 t +H 2 0
The outer membrane of the electrode allows the ammonia to diffuse through. The
change in the pH value of the inner electrolyte solution is monitored by a combined glass electrode.
If the substance to be measured is not present in the form of an ammonia salt, it
must first be converted into one. Organic nitrogen compounds, especially amino
compounds are digested according to Kjeldahl by heating with concentrated
sulphuric acid. The carbon is oxidized to carbon dioxide in the process while the
organic nitrogen is transformed quantitatively into ammonium sulphate.
Numerous examples are given. In the appendix, various sample preparations and
an example for a Kjeldahl decomposition are described.
Instruments and accessories
•
1x2.692.0010 pH/Ion Meter
•
1x2.665.0030 or 1x2.725.0010 Dosimat titration stand with
1x2.728.0040 Magnetic Stirrer and
1x6.3012.153 (5 ml) or 1x6.3012.213 Exchange Unit (10 ml) and
1x6.2124.000 Keypad (required for 665 only)
•
1x6.2138.010 Connecting cable 692 - 665n28
@Metrohm
No. 133/2 e
Determination of ammonium with the ion-selective electrode
•
Printers and connecting cables see 692 Instructions for Use, chapter
«Connection possibilities»
•
1 x6.0506.01 O NH3 electrode with fixed cable
•
1 x6.1418.220 Titrating vessel with thermostatic sleeve and
Page2
1x6.1414.010 Titrating head and thermostat or
1x6.1110.100 Temperature sensor Pt 1OOO with
1x6.2104.080 Cable
Reagents
•
Caustic soda :
c(NaOH) = 1O mol/L
•
Standard solution:
2.97 g pure NH4CI are dissolved in distilled water and the
solution filled up to 1 L; p (NH 4) = 1OOO mg/L
•
Conditioning solution: c(NH 4CI) = 0.1 mol/L (can be kept during 1 to 5 days)
Handling of the electrode
To achieve optimal measurements, observe the following points:
•
Before the first use or after a longer storage in the dry state, the inner glass
electrode must be soaked for 6 to 12 hours in a pH= 7 buffer solution. Rinse
with distilled water afterwards and dry by dabbing with a soft paper towel.
The electrode should be fitted with the membrane as described in the Instructions for Use and 2.5 ml of inner electrolyte solution added. After it is put together the electrode should be shaken like a fever thermometer to remove any
air bubbles from the membrane. Then the electrode is placed for approximately
1O min in distilled water.
•
Rinse the electrode after every measurement with distilled water and remove
any adhering water droplets by dabbing with a soft paper towel. Before every
new measurement the electrode should be conditioned in distilled water for at
least 1O min. For determining concentrations under 1 mg/L, the electrode must
be conditioned for approximately 30 minutes. For storage times of 1 to 5 days
the electrode is placed in c(NH 4CI) = 0.1 mol/L. For longer periods, store dry.
•
An excessive slope in a standard addition is usually due to conditioning times
that are too short or to an error in the preparation of the electrode. However,
slopes that are not more than 3 mV above the theoretical value can be tolerated.
•
The measurements should be performed under uniform stirring conditions and
at constant temperature. Stirring should be continuous (stirring parameter 692:
«ON»), because bubbles can form on the membrane during an interruption in
stirring, which can cause false readings. Make sure that the stirrer is switched
on and that a stirring rod is in the measuring vessel. If additions are made without stirring, erroneous results are obtained!
•
Interferences can have the following causes:
Volatile amines:
Simulate an elevated concentration of ammonia. Ethylene amine gives almost
the same difference in potential as ammonia. Longer C chains cause less interference.
@Metrohm
No. 133/2 e
Page 3
Hydrazine:
Causes the same interference as described above.
Complexing ions:
Ag, Hg, Cu, and Ni among others simulate a lower ammonia concentration.
The interference caused by 20 ppm Hg can be eliminated by using a solution
with an additional 0.46 g Nal per 100 ml NaOH instead of the nonnal NaOH
solution.
Concentration:
Should not exceed 1 mol/L. At higher concentrations the sample must be diluted with distilled water (Kjeldahl digestion). However, in the lower concentration ranges the electrode becomes less sensitive and requires a longer time to
reach a stable reading (up to 15 min).
Proteins:
A high protein concentration (1 g/L) simulates higher ammonia concentrations.
The calibration curve becomes linear only above 10 mg/L of ammonium.
•
If more than 5 to 6 minutes are required to obtain a stable reading, the membrane and inner electrolyte must be replaced.
•
If samples with greatly varying ammonia concentrations are measured frequently, the time required to reach a stable reading becomes longer for the
samples with lower concentrations.
•
After the addition of NaOH the pH-value must be at least 12. Acidic samples
must be neutralized before the addition of NaOH. Add the NaOH and begin the
measurement immediately.
•
Choice of Procedure:
General hints
The standard addition is a quick and reliable method especially for undefined
sample matrices. A direct measurement is indicated when many measurements
of the same solutions are to be carried in a defined and unproblematic sample
matrix.
•
Choice of Standard Concentrations:
In order to ensure an accurate evaluation of the standard addition by the 692
pH-lion Meter, the standard concentrations (Cstd) for the different burette volumes (Vburettefml) must be chosen as a function of the sample concentration
(Csmp1) according to the table below
Vburettefml
Cstd : Csmp1
5
40: 1
10
20: 1
20
10: 1
50
5: 1
If the sample has been greatly diluted with ISA or TISAB, this ratio must be
considered.
@Metrohm
No. 133/2 e
Example:
sample concentration (Csmp1)
burette volume CVburette)
sample size
ISAITISAB
total volume (Vtotal)
Factor from table (Cstd : Csmpt)
Page 4
5 mg/L
10 ml
10 ml
10 ml
20ml
20
The result is a sample concentration of 2.5 mg/L in the initial solution. The optimal concentration of the standard is therefore 2.5 mg/L x 20 = so mg/L.
Note, however, that this is only a general guide for the standard concentration.
Exact measurements are also possible if the above recommendations are not
strictly adhered to.
•
The following fonnulae are used to calculate the results (see also the Instructions for use, paragraphs 3.1 and 3.2):
For csample size
OFF ml»: Result = ci x factor;
for csample size
XX.X ml»: Result= ci x cVtotal»/«sample size»;
for «sample size
XXX.X g»: Result= ci x «Vtotal»/«sample size» x 1 g/ml
Example using «sample size X.XXX g»:
A precisely weighed amount of substance, e.g. 5.153 g, is dissolved and the
solution filled up to 100 ml. This solution is diluted 100 times by taking 1O ml
and topping it off to 1OOO ml, for example. 20 ml of this dilute solution plus
ISAITISAB, etc. are put into the measuring vessel. These 20 ml contain 500
times (100 mU20 ml x 100) less substance than the original sample. The corresponding portion (aliquot) is entered as «factor 500». With every new weighing, the exact sample weight is entered as «sample size X.XXX g». If the result is expressed in g/1 OOg (percent) or mg/kg (ppm) there is no correction for
specific gravity. If, however, a volume-related unit is used, such as mol/L, g/L,
µg/L, etc., the specific gravity of the original sample must be taken into account. If the specific gravity is, for example, 2.01 g/ml, the factor referred to
above must be multiplied by 2.01 (500 x 2.01 = 1005).
•
Low ammonia concentrations:
For concentrations < 1 mg/L the standard solutions must have the same ionic
background as the sample solutions. The adjustment time becomes distinctly
longer. The blank values of the chemicals are to be taken into account.
•
All measurements should be perfonned at constant temperature (for example,
at 25 °C).
•
Refer to the hints given in the Instructions for Use of the 692 pH/Ion Meter.
No. 133/2 e
Page 5
Direct Measurement:
Please consider the hints given in the Instructions for Use of the 692 pH/Ion Meter.
The following calibration is an example for a sample with an unproblematic
matrix:
Initial solution:
20ml
ISA solution:
1 ml
Standard:
1OOO mg/l in 1O ml Exchange Unit
Concentration range:
1 ... 200 mg/l
No. of calibration points:
6
93-11-17 time 1e:2'3:25 ·
idl ff>p!ik.Nr.13311.::1
id2 ltl3-!.~
date
93-11-16. time 16:23:34
cone.calibration
date
Eas. inPUt: ·
t1
HH4(+1)
ion tlft
lfl3.
electr. id
teweraturi Rl.
25.0 C
cal.date
93-11-16 16:23
Ol.V
cone/PPS
93-11-17 10:29:25
OConcl'~
sid. 1
9.99E-81
-73.6
0.2
std. 2
std. 3
2.89£+98 -85.2
S.l3Eiillll .,.183.5
2.48E'**l1 -126.8
6.93E+01 -152.6
2.08:+92 -179.2
-e.s
std. 4
std. 5
:;td. 6
8.3
e.1
-9.1
e.e
U/..U
-68r
l
-Sir
-1-~
-121l
~
-1411
-16~
-111t
r - 1 - - - 1 - - . 1 " - - . . . . J 'log c
-288'-.
-1
variance
slOPe
EC8>
c<blank)
I
1
2
8.883
-59.2 .v
-42.711.J
2.33E+fle PPI
===
109 PPI
3
H!i4(+1)
@Metrohm
No. 133/2 e
Page6
Direct measurement in an aqueous ammonium standard:
date
93-11-16
t.i.me 16:28:42
Parameter
93-11-16
tilt 17;56:52
config
direct
INS.\fi:
}dU)(iHaries
>leasuriM Parwters
ion t':IPt:
cone.mi i:
date
ON
emiish
last disit:
lfi4(+1)
diaiM:
PPll
1eas.il'lf".Jt:
date
93-it-16
t.i.riie
17:56:57
electr. id:
NHS
tell'. ti'i~ i:
c
drift
twtrature
8.5 .v11in
!"l.(\
l'IUllber
t..we of Dosi.a,:
1
25.0
tfi4(+)
aetMd id
stirrer:
X:alculation Par.•ters
s.l size
IJ total
f.letor
wl size 111W
>calibration paraiiiters
drift
no.of standards
addition:
ain. conc.
aax.conc.
Vinit
;jeiJ h:e
{ff
delta aedSUretent:
WlftratiJre
c
Off ral
21.0 Ii
1.9
state:
>Plot Paraeters
left..,..
ri9"1t .ar.
left aan.T
riiht rwg. T
?rint header:
alwaYS
DH
da~t.:ue:
!ff Uk. tir .133/ld
tlH3-ISE
25.0 c
8.5 1\.llmin
Ml
6
auto
1. 9£+00 PPS
2.0E-t02 PPil
date&ti1e:
>RS232 seuir.95
baud rate:
data bit:
StOP Di t:
28.0 11
har1dshake:
HWs
RS control:
OH
l.0E+03
10
v Exchne lkli.t 1:
>anal0t ouiPUt
Hi•i ts Cone
state:
>l11its i
Ci tlzen
id2
)print mtas. value
no.oi Exchamt lklits
s~ie:
692
/t>rinter
::h<ir.acter set:
ldl
11
665
692.0010
Pfil9ra&l
01~
conc.1
select:
label
PPll
Cone
(fF
OFF
(fF
8. 9E+08
l.0E+30
28
39
Pflll
PPI
c
c
Print cri.t:
Pari t1:1:
drHt
00
2400
s
none
@Metrohm
No. 133/2 e
Page 7
Standard addition
In principle, there are three types of standard addition: - manual
-auto dos
-auto
Please consult the Instructions for Use for the characteristics of the individual
types. The quickest method is the automatic standard addition {«auto»). Note,
however, that in order to get exact results the difference in potential AU chosen
must be at least 12 mV per standard addition and that at least 3 standard additions
must be performed {total AU at least 30 mV). If exactly defined volumes of the
standard additions are required but maximum ease of operation is also desirable,
choose cauto dos» and enter the individual standard addition volumes (see
Instructions for Use).
Example for a concentration determination by standard addition:
Sample size:
ISA solution:
Standard:
No. of additions:
datt. 93-11-15
20 ml
1 ml
1OOO mg/l in 1O ml Exchange Unit
5
ti• 17:88:56
Par•wr
-.-:
sld.ldd
)Masuri111 l'il'•lfrs
1114<+!)
ion Sift!
cooc:.111itl
lltH. il'PU.:
tiectr.id:
1113
4rilt
t.5 lll!lsin
i-rawre
25.8 c
iM<•)
•Chad id
stirrer=
tll
lc.ilal'-'ian
21l.t •l
• l Slllt
U toW
21.1 Ill
,.,.
,.,..wrs
fil:lar
SIP! SUit lllill
1.e
Ii
93-11-15 time mee::s&
additiavsi.b1r;,clioo 11111110d
sld.ilid
INS.iwt:
llll
tlte~r.id
14
dlW
lfU.irAJU
25.8
i.eraturt Al.
cooc:.sld
c
1aa ....
u io~l
21.1 ai
-1ti4.l 11J
initial wl :.age
28.811
•1 i'IIt
93-11-15 11:•
claw
"'111
sld.1ncr.
std.incr.
std.incr.
sld.incr.
1
2
3
4
;Id. (Iler. 5
IYalJ
~
8.572 -169.4
-5.!
e.712
-m.~
-5.1
Ull -179.~
1.221 -191.4
1.668 -18'.2
-5.8
-4.~
-4.8
>sundard idlii lion
liffl
;:mc,s;;i.
nl'Cirll
aadit.illlll
dthaU
add
1&1!'1'11
iull
auio
5..,
dos.raw:
no.of addl:i.n
llldi.IJI
IJ EJCdlal1lt tilii:
18
'
UIMU
-1''r
-m~
-·1-mr
t\
-191t
\\
-1951
2
I
-181~
i
I
-185t
I
••• c
variru
sl.w
E<8>
lt!C(tl)
a
8.81!2
-S.1..,
-4i'
.s "'
111-
No. 133/2 e
Page8
Practical examples
1. Ammonium in drinking water using automatic standard addition
Sample preparation: none
ISAITISAB:
not required
Analysis:
20 ml sample is pipetted into the measuring vessel.
Shortly before beginning the measurement, 1 ml of
NaOH solution is added. The measuring vessel should
be closed after the addition.
93-11-18 tile 13:48:24
Paraeier
aeas. ~=
std.add
:wiSIJriM Parameters
icn l\if'i:
1'114<+t>
date
cone.unit:
ltiS.iOPUU
elecir. id: ·
drift
taffrasure
•ttm id
stirrer:
?Fii
1
lfl3
8.5 .U11in
25.8 c
lif4(+)
~
>calcvlatiOfl Piralll!WS
SIPl Si.zt .
tff cl
u total
21.0 11
factor
1.e
SIP I size mi t:
1l
>standird add.itioo
tlft:
add
ccnc.std.
report:
addition:
delta u
dos.rate:
oo.ot additions
I.I ExctlanM lkli t:
1eee.e ,,.
Ml
auto
lhlJ
lediUI
3
10
daie 93-11-!ll ti• 13:57:41
additioo/su'.ltracdon 1ttllod
1eas. tlffl
sw. add
electr. id
iti3
l
25.0 c
1000.e 1'1'61
21.0 •l
-35.6 llU
93-11-18 13:57
waaPerature .an.
cor.c.std
I.I total
initial uol ta9f
da~
<.lVal
8.844
0.829
std.lr11:r. 3 8.942
UlillU illl.O
-6S? -32.6
·-
sld.incr. 1
std. incr. 2
-78.2
-87.~
-!i:l.0
-9.?
U/..V
-3flr
j
I
-<WLI
I
t
I
-58~
I
t
I
-68t
l
!
\\
\
\\
'
\
\
-78t
•
-88~
\
\
f
-981
-1
vari..-.ce
slOPe
E<il)
lil4(+1)
8
log c
I
0.000
-59.3 llU
-48.7
0.822
~
Pftl
1
@Metroln
No. 133/2 e
Page9
2. Ammonium in
a waste water using automatic standard addition
Sample preparation: none
ISA/TISAB:
not required
Analysis:
20 ml sample is pipetted into the measuring vessel.
Shortly before beginning the measurement, 1 ml of
NaOH solution is added. The measuring vessel should
be closed after the addition.
Inlet of waste water treatment plant
da1e 9:5-11-15 lilll! 15:16:24
addi tiOIVSlb"ac:tion llEtl'lod
llNS.iwie:
std.acld
lllKtr.id .
Ii()
11eas. inPUi:
li.8 c
w.erawrt 11a11.
10119.8 PPll
an::.sld
IJ wial·
21.3 rd
-194.8 rJJ
ini Ual uol me
28.8 •l
Slll'l size
93-11-15 15:16
dase
date 93-11-15 tile. 15:36:12
iddili.rilsdltraciuin 11Ulod
meas. Wflt:
9
s1d.idd
1113
electr.id
11eas.inl'u1:
teRrawre w..
conc.sed
IJ iotal
inHial uol ta9e
1
25.8 C
11Bl.8 ..,.
21.8 Ill
-52.5 ~
29.9 •l
93-11-15 15: 36
!IJl.U
e.m -119.8 -14.2 .
9.117 -123.9 -4.9
-4.'3
8.186 -128.8
-133.
7
-4.9
8.1l8
8.159 -138.6 -4.9
;j)/11
st.d. incr. 1
sid.incr. 2
std.incr. 3
sid.incr. 4
std.incr. 5
8
Outlet of waste water treatment plant
Ui.U
d.1/11
std.inc:r.
s&ci. inc:r.
std.inc:r.
std. inc:r.
Utlil
dUllO
1
2
3
8.1141
3.815
0.818
-75.2 -21..7
-98. Z -s. e
-85.8
-4.8
4
8.022
-8'9.S
-4.S
std.mcr. 5
8.927
-94.7
-4.9
U/..U
-111
-51
\
-6
-71
\
-88
••
-141
l.lif'ianta
slOPt
EC8>
1114<+1>
log c
1
-1
-181
2
8.888
-57.9 rJJ
-45.5 .u
11.1 PPll
•
11arin:e
\
log c
1
'
8.882
SlOPt
-57.2 .u
E(8)
-46.8
!114(+1)
-
.u
1.36 PPI
@Metrohm
Parameter report for waste water:
date
93-11-15
iillle 16:42:56
parameter
std.add
ins Parameters
tti4(+D
ion i.Yf.e:
PPI
conc.ooii.:
1
aeas. il'lPUt:
tfi3
electr.id:
e.5 llJ/flin
drift
~r
25.0
~rawre
1ethod id
stirrer:
.~
I,,
NH-1(+)
~
>calculation Parameters
51Fl size
u totil
factor
S11Pl size unit:
>standard addi tioo
tl.:IPe:
conc.std.
fiP'Ort:
addition:
delta
u
dos.raw:
oo.of additions
IJ Exchan9e i))i i:
28.8 11
21.0 Ii
1.0
ml
<)jd
1000.0
F'Pfl
full
auto
S IV
lleciiUI
5
10
No. 133/2 e
Page 10
@Metrohm
No. 133/2 e
Page 11
3. Uquid fertilizer for ornamental flowers
Sample preparation: The sample is diluted by 1 : 100 (= sample solution,
factor 100)
=
ISAITISAB:
not required
Analysis:
20 ml sample solution and 1 ml 10% disodium tartrate
solution or 1 ml 10% tartaric acid are pipetted into the
measuring vessel. The NaOH solution is added just
before the start of the determination.
date
93-11-1'5
t.i• 16:26:38
Pa;•wr
•as.~=
s":l.add
>measur iM Para1eiers
ion "'":
conc.ooit:
•as.inPVU
lfM<+D
Pf'll
l
electr.idl
~
drift
0.5 I0/1ln
25.8 c
w..raiure
lfi4(+)
•&iwld id
~
stimr:
>cilcului.on Para1Hers
2'1.8 11
• l size
22.e 11
v iotal
i~.e
factor
511'1 Sitt llli t:
11
>siaidar'd addltior.
~
conc.std.
l'li!POrt:
addition:
delta
u
add
1000.e PPis
.
IJll\I dUlllV
d!Jlli
-4.9
-175.4
,;td. incr. 1
0.765
std. inc:r. 2 1.915 -188.7 -5.3
1.323 -18'5.6 -4.9
sid. iocr. 3
std. ir.:r. 4 1.812 -190.6 -5.t1
sui. i11er. 5 2.574 -1''5.5 -4.9
full
auw
5 Ill
no. of additions
aediUI
5
u Exd'lwt ij)i;:
19
dos.raie~
daie 93-11-15 ti• 13:42: 13
addi t.iwsi.btraction ll!Ukxi
INS. t..,.;
std.ai!
till3
electr.1d
.as. iriPUtl
.!
25.0 c
\ell'trature aan.
1000. 3 Pf'tl
cone.SW
22.8 11
i) total
-178.5 If.I
initial voi ta9t
100.0
factor
29.0 •l
s.l s.iu
93-11-15 13:42
date
U/MU
-165r
I
I
-171~
\
!\
-115t
\
-188tI \\
-185~
t
I
-19~
I
-ust
\
\<
\
\
'
-28811...._ _ _ _ _ ___.log c
2
3
Vifi~
8.007
!ilOPe
-~.l i;J
Em
-45.e td.J
lt14(tl)
1. 46E+94
Pf'SI
@Metrohm
No. 133/2 e
Page 12
Sample preparations for further application examples
1. Ammonium in soil samples
After weighing, the soil sample is dried and then boiled for 1 hour in c(HCI) =
2 moVL. The solution is filtered and the volume to be pipetted is adjusted to
pH= 12.
2. Ammonium in an aquarium
Same as for drinking water- see above, «Practical Examples».
3. Ammonium in urine
=
c(HCI) 0.1 mol/l is added to the sample, the resulting mixture stirred and
then filtered off. The volume to be pipetted is adjusted to pH 12.
=
4. Ammonium in boiler feed water
Same as for drinking water.
5. Total nitrogen in waste water
For measurements of total nitrogen content, a standard sulphuric acid digestion
according to Kjeldahl is carried out on a 1O ml sample. The resulting filtrate is
diluted with distilled water, topped off to 50 ml and adjusted to pH 12.
=
Example: Ammonium concentration determined by automatic standard addition: 38.6 mg/l
Calculation of total nitrogen content:
total nitrogen content (mg I L) =
ammonium content (mg I L) x molar mass N (g I mol)
molar massNH 4 + (g I mol)
38.6mg/Lx 14g/mol = 30 m /L
18g/mol
g
=
@Metrohm
No. 133/2 e
Page 13
Literature
Application Bulletin No. 79, Bibliography - Ion-selective electrodes
EPA 350.3 (1983), Nitrogen, ammonia (Potentiometric, ion selective electrode).
US Environmental Protection Agency
Hoge, J.H.C., Hazenberg, H.J.A., Gips, C.H., Determination of ammonia in urine
with an ammonia electrode and with a direct method.
Clin. Chim. Acta, (1974), 55 (3), 273-279
Ref.: Electroanal. Abstr. 13 (1975) 410
ISO 6778: (1984), Water quality - Determination of ammonium. Potentiometric
method.
Ito, K., Sunahara, H., Simple determination of ammonium and nitrate ions in activated sludge process waters with plastic-membrane ion selective electrodes
Bunseki Kagaku, (1987), 36, 123-125 (Japanese)
Ref.: Fresenius, Z. Anal. Chem. 328 (1987) 613
Love, M.D., Pardue, H.L., Pagan, G., Evaluation of transient response of ammonia-selective potentiometric electrodes for quantitative applications.
Anal. Chem., (1992), 64 (11), 1269-1276
Navarro, R.A., Rojas, P., Determination of ammoniacal and total nitrogen in fertilizers by ammonia-selective electrode
J. Assoc. Off. Anal. Chem., (1984), 67, 890-892
Rice, T.D., Sweeney, V., Semitekolos, R., Rhyder, G.J., Standard-addition determination of nitrogen in coal with an ammonia-sensitive electrode
Talanta, (1984), 31, 607-610

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