ISO 17319:2015
(Main)Fertilizers and soil conditioners — Determination of water-soluble potassium content — Potassium tetraphenylborate gravimetric method
Fertilizers and soil conditioners — Determination of water-soluble potassium content — Potassium tetraphenylborate gravimetric method
ISO 17319:2015 specifies a gravimetric method for the determination of the water-soluble potassium content of test solutions of fertilizers. It is suitable for use in arbitration and for reference purposes. ISO 17319:2015 is applicable to those fertilizers containing more than 1,0 % K2O or equivalent amount of K content.
Matières fertilisantes — Dosage de la teneur en potassium — Méthode gravimétrique au tétraphénylborate de potassium
General Information
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 17319
First edition
2015-05-15
Fertilizers and soil conditioners —
Determination of water-soluble
potassium content — Potassium
tetraphenylborate gravimetric method
Matières fertilisantes — Dosage de la teneur en potassium —
Méthode gravimétrique au tétraphénylborate de potassium
Reference number
ISO 17319:2015(E)
©
ISO 2015
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ISO 17319:2015(E)
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ISO 17319:2015(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Principle . 1
4 Reagents . 1
5 Apparatus and materials. 2
6 Test solution . 2
7 Procedure. 2
7.1 Aliquot portion of test solution . 2
7.1.1 Procedure in the presence of cyanamide and/or organic materials . 2
7.1.2 Procedure in the absence of cyanamide and/or organic materials . 3
7.2 Determination . 3
7.3 Blank test . 3
7.4 Periodic quality control . 3
8 Expression of results . 3
8.1 Calculation . 3
8.2 Precision . 4
8.2.1 Ring test . 4
8.2.2 Repeatability, r .4
8.2.3 Reproducibility, R .4
9 Test report . 4
Annex A (informative) Report of Method Accuracy . 5
Annex B (informative) Interlaboratory testing .13
Bibliography .19
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ISO 17319:2015(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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electrotechnical standardization.
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described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
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assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 134, Fertilizers and soil conditioners.
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INTERNATIONAL STANDARD ISO 17319:2015(E)
Fertilizers and soil conditioners — Determination
of water-soluble potassium content — Potassium
tetraphenylborate gravimetric method
1 Scope
This International Standard specifies a gravimetric method for the determination of the water-soluble
potassium content of test solutions of fertilizers. It is suitable for use in arbitration and for reference
purposes.
This International Standard is applicable to those fertilizers containing more than 1,0 % K O or
2
equivalent amount of K content.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 3696:1987, Water for analytical laboratory use — Specification and test methods
ISO 5317, Fertilizers — Determination of water-soluble potassium content — Preparation of the test solution
3 Principle
Precipitation of potassium ions present in an aliquot portion of the test solution (previously treated
with bromine water and activated charcoal if cyanamide and/or organic materials are present) by
sodium tetraphenylborate in a weakly alkaline medium in the presence of disodium ethylenediamine-
tetraacetatedihydrate (EDTA disodium salt) to eliminate interference by ammonium ions and other
metal cations.
Filtration of the precipitate, drying and weighing.
4 Reagents
WARNING — Sodium hydroxide is corrosive, bromine is corrosive, oxidative and toxic. The related
operations shall be performed in fume hood. This standard does not point out all possible safety
problems, and the user shall bear the responsibility to take proper safety and health measures,
and ensure the operations compliant with the conditions stipulated by the related laws and
regulations of the state.
During the analysis, use only reagents of recognized analytical grade, and water conforming to grade 3
of ISO 3696:1987
4.1 Sodium tetraphenylborate, approximately 15 g/L solution.
Dissolve 7,5 g of sodium tetraphenylborate [NaB(C H ) ] in 480 ml of water. Add 2 ml of the sodium
6 5 4
hydroxide solution (4.4) and 20 ml of a 100 gram/litre (g/L) solution of magnesium chloride hexahydrate
(MgCl ·6H O). Stir for 15 min and filter through the filter paper (5.2).
2 2
This solution may be stored in a plastics bottle for not longer than 1 month. Filter immediately before
use.
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ISO 17319:2015(E)
4.2 Sodium tetraphenylborate, wash solution.
Dilute 1 volume of the Sodium tetraphenylborate solution (4.1) with 10 volumes of water.
4.3 EDTA, disodium salt, 40 g/l solution.
Dissolve 4,0 g EDTA disodium salt in water in 100 ml graduated flask. Make up to the volume and mix.
4.4 Sodium hydroxide, 400 g/L solution.
4.5 Phenolphthalein, 5 g/L ethanolic solution.
Dissolve 0,5 g of phenolphthalein in 100 ml of 95 % (volume fraction) ethanol.
4.6 Bromine water, saturated solution, approximately 3,2 % (w/v) bromine; shelf life of less than 6
months.
4.7 Charcoal, activated, which does not adsorb or liberate potassium ions.
5 Apparatus and materials
During the analysis, use only glassware of grade A, and the following:
5.1 Gooch crucibles, having a sintered glass or porcelain disc of porosity grade P 10 or P 16 (pore size
index of 4 μm to 16 μm).
5.2 Quantitative filter paper (intermediate speed).
5.3 Drying oven, capable of being controlled at 120°C ± 5°C.
5.4 Usual laboratory equipment, including pipettes, beakers (250 ml), and volumetric flasks, etc.
6 Test solution
Use the clear solution prepared as specified in ISO 5317, also described as follows:
Weigh, to the nearest 0,001 g, 5 g of the prepared test sample to form the test portion. Transfer the
test portion to the flask with 400 ml of water. Bring to a boil and continue to boil for 30 min. Cool the
contents of the flask and transfer to the volumetric flask. Dilute to the mark, mix well and filter into a
dry beaker. Discard the first 50 ml of filtrate.
7 Procedure
7.1 Aliquot portion of test solution
7.1.1 Procedure in the presence of cyanamide and/or organic materials
Preparation of the test solution as specified in ISO 5317.
Transfer, by means of a pipette, an aliquot portion, V1, of the test solution (Clause 6), containing 50 mg to
100 mg of potassium oxide (K O), preferably about 80 mg, to a 250 ml beaker. Add 5 ml of the bromine
2
water (4.6) and boil the solution until all the bromine has been removed and then, if necessary, to reduce
the volume to less than 100 ml. Allow the solution to cool to ambient temperature and transfer it to a
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ISO 17319:2015(E)
100 ml volumetric flask. Add about 0,5 g of the activated charcoal (4.7) and shake vigorously. Dilute with
distilled water to the mark and mix well.
Filter the solution and use a pipette to transfer 50 ml of the filtrate into a 250 ml beaker.
7.1.2 Procedure in the absence of cyanamide and/or organic materials
Transfer, by means of a pipette an aliquot portion, V1, of the test solution (Clause 6) containing 25 to
50 mg (preferably 40 mg) of K O to a 250 ml beaker and adjust the volume to approximately 50 ml by
2
dilution or evaporation.
NOTE If the content of fertilizer sample cannot be specified, use the procedure described in 7.1.1.
7.2 Determination
Further treat the aliquot portion of the test solution (7.1) as follows.
Add 20 ml of the EDTA solution (4.3) and 3 to 5 drops of the phenolphthalein solution (4.5). Add, drop by
drop, the sodium hydroxide solution (4.4) until a pink colour appears and then add 1 ml of the sodium
hydroxide solution in excess.
Slightly boil for 15 min.
The solution should remain pink. If it does not, add 1 to 3 drops of the phenolphthalein solution (4.6)
and, if necessary, restore the pink colour by adding, drop by drop, the sodium hydroxide solution. (4.5).
Remove the beaker from the steam bath and immediately add, drop by drop, while stirring, 30 mL of the
tetraphenylborate solution (4.1).
Continue stirring for 1 min then cool rapidly to ambient temperature in running water and allow to
stand for 15 min.
Weigh the filter crucible (5.1) previously dried in the oven (5.3) controlled at 120 ± 5 °C, and cooled in a
desiccator and then weighed to the nearest 0,000 1 g.
Decant the supernatant liquid through the crucible. Rinse the beaker and precipitate 3 to 5 times with a
total of about 40 ml of the wash solution (4.2), followed by 5 ml of water.
Dry the crucible and precipitate in the oven (5.3) controlled at 120 ± 5 °C for 90 minutes, allow to cool in
a desiccator and weigh to the nearest 0,000 1 g.
7.3 Blank test
Carry out a blank test at the same time as the determination.
7.4 Periodic quality control
Carry out a quality control test periodically as the determination using K SO standard material.
2 4
8 Expression of results
8.1 Calculation
The potassium content, expressed as a percentage by mass as potassium (K) or as potassium oxide
(K O), is given by Formula (1) and Formula (2).
2
a) if cyanamide and/or organic materials are present
mm− −−mm ××f V ×200
() ()
21 43 0
(1)
mV×
01
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ISO 17319:2015(E)
b) if cyanamide and/or organic materials are absent
mm− −−mm ××f V ×100
() ()
21 43 0
(2)
mV×
01
where
m is the mass, in grams, of the test portion;
0
m is the mass, in grams, of the filter crucible;
1
m is the mass, in grams, of the crucible and precipitate;
2
m is the mass, in grams, of the crucible used for the blank test;
3
m is the mass, in grams, of the crucible used for the blank test and the corresponding precipitate;
4
V is the volume, in millilitres, of the test portion (volume in which the test portion was dis-
o
solved;
V is the volume, in millilitres, of the aliquot portion of the test portion taken for the determina-
1
tion;
f is a factor which, if the potassium content is expressed as potassium (K), is equal to 0,1091 or
if the potassium content is expressed as potassium oxide (K O), is equal to 0.,1314.
2
8.2 Precision
8.2.1 Ring test
Details of Ring test on the precision of the method are summarized in Annex A.
8.2.2 Repeatability, r
Repeatability limit r: 0,19 %
8.2.3 Reproducibility, R
Reproducibility limit R: 0,006 2 m + 0,305 2, in which m represented the concentration of K O.
2
9 Test report
The test report shall contain at least the following information:
a) all information necessary for the complete identification of the sample;
b) test method used with reference to this International Standard (i.e. ISO 17319);
c) test results obtained;
d) date of sampling and sampling procedure (if known);
e) date when the analysis was finished;
f) whether the requirement of the repeatability limit has been fulfilled.
All operating details not specified in this standard, or regarded as optional, together with details of any
incidents that occurred when performing the method, which might have influenced the test results.
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ISO 17319:2015(E)
Annex A
(informative)
Report of Method Accuracy
A.1 Overview
Review of ISO standard on determination of potassium content in fertilizer
The current effective ISO standard on determination of potassium content in fertilizer is ISO 5310:1986.
ISO 5310:1986 specifies a titrimetric method (volumetric method) for the determination of the potassium
content of fertilizers.
The principle of ISO 5310:1986 is as follows:
— Prepare a test solution as specified in ISO 5317 or ISO 7407;
— Precipitate the potassium ions present in an aliquot portion of the test solution (previously treated
with bromine water and activated charcoal if cyanamide and/or organic materials presented) by
excess amount of sodium tetraphenylborate (NaTPB) in a weakly alkaline medium;
— Filter and determine the unreacted NaTPB by back-titration with cetyltrimethylammonium bromide
(CTAB) solution, using thiazol yellow (Titan yellow) as the indicator.
As a kind of volumetric method, this method would be expected to have an increased possibility of
deviation when it is used for analyzing high concentration potassium samples due to the higher dilution
ratio.
Abolished ISO standard: ISO 5318:1983 (withdrawn in 2008).
ISO 5318:1983 specifies a gravimetric method for the determination of the potassium content of a test
solutions of fertilizers. It is suitable for use in arbitration and for reference purposes.
The principle of ISO 5318:1983 is as follows:
— Preparation of a test solution as specified in ISO 5317;
— Precipitation of potassium ion present in an aliquot portion of the test solution (previously treated
with bromine water and activated charcoal if cyanamide and/or organic materials presented) by
NaTPB in a weakly alkaline medium in the presence of ethylene diaminetetraacetic acid (EDTA)
disodium salt and formaldehyde (HCHO) to eliminate the possible interference by ammonium ions;
— Filtration of the precipitate, drying and weighing.
The method mentioned in ISO 5318:1983 was proposed by Ford in 1956 and it is one of the classic methods
for determination of potassium content. It has been recognized as the official method for determination
of potassium content by AOAC (Association of Official Analytical Chemists, US), CEN (ComiteEuropeen
de Normalization, EU), and SAC (Standardization Administration of the People’s Republic of China).
This method calls for precise concentration control of the concentration of alkali (NaOH) solution, it may
have an increase of the possibility of deviation by badly control of the concentration of alkali (NaOH)
+
solution. Since the reaction between formaldehyde with ammonium ion (NH ) is reversible:
4
64HCHO++NH Cl 44NaOH⇔ CaHN ++N Cl 10H O (A.1)
()
42 42
6
This reaction will form hexamethylenetetramine, namely, urotropin.
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ISO 17319:2015(E)
+
In this reversible reaction, the ammonium ion (NH ) can be well screened under the concentration
4
of NaOH higher than 0,05 mol/L; Otherwise, namely, when the concentration of NaOH is lower than
+
0,05 mol/L, Formula (A.1) will turn back to its left side to form more ammonium ions (NH ), which may
4
lead to an unexpected precipitation of ammonium tetraphenylborate (NH TPB) in the solution, by the
4
+
combination of sodium tetraphenylborate (NaTPB) with ammonium ions (NH ).
4
+
On the other hand, in order to maskall the ammonium ions (NH ) under the alkali condition, some
4
other cations such as Ca(II), Mg(II), Fe(III) will form precipitation of hydroxide and/or phosphate, so
ethylene diaminetetraacetic acid (EDTA) is needed here to mask all the other cations may exist in the
solution. Data has shown that the Fe(III) in the solution will form precipitation of iron hydroxide under
the concentration of NaOH higher than 0,16 mol/L.
Considering all the restrictions mentioned above, the standard ISO 5318:1983 has a strict requirement
on alkali (NaOH) concentration which should be controlled between 0,05 mol/L~0,16 mol/L, otherwise,
there will be significant deviation on test results.
On the other hand, from the EH&S point of view, there exists a common view that formaldehyde (HCHO)
is definitely not an environment-friendly substance. HCHO will undergo a polymerization process under
a long period of reservation; it calls for a pretreatment process of filtrating before taken as the reagent,
these processes will cause tears of the operators. As a Group I carcinogenic substance (see MSDS sheet
of HCHO aq. from Sigma-Aldrich), formaldehyde causes sensitization to eyes, skin as well as respiratory
tract. All these shortcomings will definitely do harm to the health of operators, and bring about difficulty
to this method practically.
China’s effort towards solving existing problems
In order to solve those as-mentioned problems, we have established a method which can eliminate
+
the interference of ammonium ion (NH ) without adding formaldehyde. The test for determining
4
the accuracy and precision of our newly-established method can be found as follows and also in our
international ring test report (Annex to the CD2).
During a long-term of practices, we have confirmed that the potassium content determined without
adding formaldehyde after heating in alkali condition is almost the same as the one with formaldehyde
+
added, even though there is a large amount of ammonium ion (NH ) existing in the sample. A preliminary
4
+
speculation is that ammonium ion (NH ) may form ammonia (NH ) in alkali solution and then be totally
4 3
evaporated under constant heating.
heating
+−
NH +OH ↑→NH +HO (A.2)
43 2
A.2 Detailed Accuracy Test
A.2.1 Test methods of ISO 17319
A.2.1.1 Principle
Heating the test solution in an alkaline medium (NaOH) to eliminate the interference of ammonium ion
+
(NH ), adding disodium ethylenediamine-tetraacetate dehydrate (EDTA, disodium salt) to eliminate
4
interference of other metal cations, then precipitation of potassium ions in an aliquot portion of the
test solution by sodium tetraphenylborate in an alkaline medium to get potassium tetraphenylborate
precipitate. Filtration of the potassium tetraphenylborate precipitate, drying and weighing.
A.2.1.2 Test methods
The procedures are basically the same as the one specified in ISO 5318, the key difference here is no
formaldehyde will be added in the test solution.
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ISO 17319:2015(E)
A.2.2 Results and Discussion
A.2.2.1 Test samples
Sixteen typical samples of potassium sulfate from China, USA and Germany, 5 different formulas of
compound fertilizers, 1 organic-inorganic compound fertilizer, 2 standard reference materials of
potassium chloride and potassium sulfate have been selected as test samples, which are of different
specifications and prepared by various technique to guarantee a wide range of representatives of the
samples.
A.2.2.2 Comparative test by 2 analysis methods of potassium sulfate (K SO ) sample (with vs.
2 4
without HCHO)
In order to validate the practicability of our test methods without HCHO, we carried out comparative
test by 2 analysis methods of potassium sulfate (K SO ) sample (with vs. without HCHO), the test results
2 4
are as follows (Table A.1):
Table A.1 — Results of K O concentrations with and without HCHO, in the unit of %
2
Difference Difference
No. of With Without No. of With Without
between two between two
sample HCHO HCHO sample HCHO HCHO
methods methods
001 51,53 51,40 0,13 009 52,16 52,15 0,01
002 50,83 51,07 −0,24 010 49,52 49,72 −0,20
003 51,69 51,85 −0,16 011 28,65 28,92 −0,27
004 48,60 48,79 −0,19 012 50,92 51,04 −0,12
005 51,46 51,19 0,27 013 51,70 51,46 0,24
006 50,92 50,99 −0,07 014 50,94 50,98 −0,04
007 48,20 48,20 0,00 015 37,28 37,50 −0,22
008 46,24 46,24 0,00 016 26,42 26,28 0,14
Table A.1 has shown clearly that the absolute difference between the two methods is between
0,00 % approximately 0,27 %, which lies in the range of absolute difference between parallel tests
specified in ISO 5318 (no more than 0,39 %), which indicates that the test results of the two methods
(with vs. without HCHO) are highly comparative.
To further validate that there is no significant difference between the two methods (with vs. without
HCHO), we have chosen No.001 sample to carry out 5 parallel tests by two methods (with vs. without
HCHO), the results are as follows (Table A.2):
Table A.2 — Replicated 5 test results of K O concentrations of sample 001: methods
2
with/without HCHO, in the unit of %
No. of sam- with HCHO without HCHO
ple
001 52,45 52,40 52,24 52,04 52,28 52,41 52,37 52,31 52,22 52,08
Data from Table A.2 are taken for F-tests, variations of both the two set of data from the two methods
(with vs. without HCHO) are compared firstly.
n =5, f =4, s =0,160
1 1 1
n =5, f =4 s =0,132
2 2 2
2 2
∴F =s /s =1,47
calculation 1 2
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ISO 17319:2015(E)
Referring to the one side numerical table of F-value (with confidence coefficient = 95 %), we found
F = 6,39. Since F = 1,47 < F = 6,39, it indicates that the variations of both the two
critical calculation critical
sets of data have no significant difference, which also means the precision of both the two methods have
no significant difference.
Combined standard deviation, s, can be calculated as follows:
2 2
ΣΣxx− +−xx
() ()
ii1 122
s= =0,206 (A.4)
nn+−2
12
Then we use t-tests on the two sets of data from the two methods (with vs. without HCHO), to verify if
there is any systematic difference between the two sets of data:
||xx− nn−
12 12
t = ⋅ ==00,,30P,90,f =8 (A.5)
calculation
s nn+
1 2
Referring to the two side numerical table of t -value, we found t = 1,86, since
a,f 0.10,8
t = 0 03 < t = 1 86, it indicates that there is no significant difference between the data from
calculation 0.10,8
both methods (with vs. without HCHO).
All the statistical work here illustrates that both the two methods (with vs. without HCHO) are equally
effective for the determination of the potassium content of potassium sulfate (K SO ) sample. Both
2 4
methods share a high quality of accuracy and precision. We will show more details on applying our new
method (without HCHO) below.
A.2.2.3 Precision test of the new method (without HCHO) of potassium sulfate (K SO ) sample
2 4
To further validate the precision of the new method (without HCHO), we choose 3 representative samples
to carry out 5 parallel tests, and a Dixon test was carried out to validate the precision of data from the
new method (without HCHO). The results are as follows (see Table A.3).
Table A.3 has shown clearly that the range of data from parallel tests is between 0,11 % approximately
0,33 %, which lies in the range of absolute difference between parallel tests specified in ISO 5318 (no
more than 0,39 %) and shows a good quality of precision of the new method (without HCHO).
Referring to the numerical table of Q -value, we found Q = 0,642 since
(0.05,5) (0.05,5)
Q < Q = 0,642, we also found a good quality of precision of the new method (without
calculation (0.05,5)
HCHO).
Table A.3 — Precision test results of the new method (without HCHO)
Standard
No. of Arithmetic Range
Test results (%) Data for Dixon tests Deviation
sample average (%) (%)
(%)
46,21 46,24 Q = 0,379
1
008 46,31 46,10 46,25 0,29 Q = 0,276 0,11
5
46,39 Q = 0,642
(0.05,5)
52,41 52,37 Q = 0,424
1
009 52,31 52,22 52,28 0,33 Q = 0,121 0,13
5
52,08 Q = 0,642
(0.05,5)
37,29 37,62 Q = 0,151
1
015 37,34 37,37 37,45 0,33 Q = 0 0,16
5
37,62 Q = 0,642
(0.05,5)
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ISO 17319:2015(E)
A.2.2.4 Potassium oxide content of potassium sulfate (K SO ) sample by new test method
2 4
(without HCHO) (see Table A.5)
The parallel differences lie between 0,02 % and 0,38 %, all within the allowable deviation and indicate
the good parallelism of the new method (without HCHO).
Table A.4 — The K O content of the sample, in the unit of %
2
parallel parallel
No. of No. of
Replicate results Average differ- Replicate results Average differ-
sample sample
ence ence
001 51,47 51,32 51,40 0,15 009 52,22 52,08 52,15 0,14
002 51,01 51,13 51,07 0,12 010 49,71 49,74 49,72 0,03
003 51,98 51,72 51,85 0,26 011 28,86 28,97 28,92 0,11
004
...
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