Význam tématu
Komplexní pyrithionáty zinku, mědi a sodíku představují široce používané fungicidy a baktericidy v kosmetice, farmacii a průmyslové ochraně proti řasám a biofoulingu. Spolehlivé stanovení jejich obsahu je nezbytné pro zajištění kvality surovin a finálních výrobků, bezpečnost spotřebitelů a dodržení regulačních norem.
Cíle a přehled studie
Cílem publikovaného bulletinâ je popsat univerzální potenciometrickou titrační metodu pro kvantitativní stanovení komplexů pyrithionu (ZnPT, CuPT, NaPT). Metoda využívá jodometrické titrace se standardními roztoky jódu a thiosíranu sodného a indikaci bodů ekvivalence pomocí Pt Titrode elektrod.
Použitá metodika a instrumentace
Metoda spočívá v rozpuštění vzorku ve vodě a kyselině (HCl nebo CH₃COOH), následné titraci roztokem jódu (c=0,05 mol/L) u ZnPT a NaPT nebo thiosíranu sodného (c=0,1 mol/L) u CuPT. Pro stanovení korekčního faktoru (titeru) se provádí kalibrace standardním thiosíranským nebo jodovým roztokem.
Použitá instrumentace
- Titrátor s DET módem
- Pt Titrode (Metrohm 6.0431.100)
- Volitelně autosampler
- Magnetický míchač
- Buret (20 mL nebo 50 mL podle metody)
Hlavní výsledky a diskuse
Ukázkové titrační křivky pro ZnPT a NaPT vykazují dobře definovaný první bod ekvivalence. Pro CuPT je kromě kyselého rozpuštění nutné přidání močoviny a amoniaku pro tvorbu komplexů, následně se titruje Na₂S₂O₃. Metoda je citlivá na přítomnost jiných thiolových sloučenin, proto je třeba dbát čistoty vzorku a reagencií. Stanovení prokázalo lineární závislost signálu v rozsahu 0,1–100 % obsahu pyrithionátu.
Přínosy a praktické využití metody
- Vysoká přesnost a reprodukovatelnost v širokém rozsahu koncentrací
- Možnost automatizace pomocí autosampleru
- Univerzální postup pro různé pyrithionátové komplexy
- Aplikace v QA/QC laboratořích v kosmetickém, farmaceutickém a průmyslovém sektoru
Budoucí trendy a možnosti využití
Očekává se rozšíření metodiky o on-line monitoring procesů a využití tenkých vrstvových elektrod pro snížení spotřeby vzorku. Dalšími směry jsou miniaturizace titrátoru, integrace do dálkově ovládaných systémů a adaptace na nové thiolové komplexy hypotetických biocidů.
Závěr
Popisovaná potenciometrická titrace s Pt Titrode poskytuje robustní, rychlou a efektivní metodu pro stanovení komplexů pyrithionátu. Díky jednoduché přípravě vzorku, standardním parametric�m a možnosti plné automatizace je vhodná pro rutinní analýzy kvality i výzkumné účely.
Reference
- Application Bulletin 441/1 e: Assay of pyrithione complexes – Reliable determination by potentiometric titration, Metrohm, July 2020
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Page 1 of 6
Application Bulletin 441/1 e
Assay of pyrithione complexes
Reliable determination by potentiometric titration
Branch
Chemical; Pharmaceuticals; Personal care & cosmetics
Keywords
Titration; potentiometric titration; pyrithione; pyridinethione;
zinc pyrithione; ZnPT; copper pyrithione; CuPT; sodium
pyrithione; NaPT; Pt Titrode; 6.0431.100; S01; S010; S013;
S04; S040; S12; S120; S122
Summary
Pyrithione complexes, such as zinc pyrithione (ZnPT), copper
pyrithione (CuPT), and sodium pyrithione (NaPT), are used
as fungicides and bactericides. ZnPT is used in the treatment
of skin conditions such as seborrheic dermatitis or dandruff.
Furthermore, ZnPT is sometimes used as an antibacterial
agent in paints to prevent algae and mildew growth. CuPT is
primarily in use as a biocide to prevent biofouling of surfaces
submerged in water. Meanwhile, NaPT is used as antifungal
agent for treatment of mycosis, such as athlete’s foot.
The different pyrithione complexes are determined by
iodometric titration using a maintenance-free Pt Titrode for
the indication.
Zinc pyrithione
Summary
ZnPT is dissolved in water, acidified with HCl, and then
titrated with iodine. During the titration, iodine is consumed by
the thiol groups present in ZnPT.
Instruments
•
Titrator with DET mode
•
Autosampler (optional)
•
Magnetic stirrer
•
50 mL buret
Electrode
Pt Titrode
6.0431.100
Reagents
•
Hydrochloric acid, w(HCl) = 37%
•
Ultrapure water
•
Iodine, c(I2) = 0.05 mol/L
•
Sodium thiosulfate, Na2S2O3, p.a.
•
Glacial acetic acid, CH3COOH, p.a.
Solutions
Titrant
c(I2) = 0.05 mol/L
Should be bought from a supplier.
Acetic acid solution
c(CH3COOH) = 2 mol/L
114.8 mL glacial acetic acid is
pipetted into a clean 1000 mL
volumetric flask containing 500 mL
ultrapure water. The flask is then
filled up to the mark with ultrapure
water.
Application Bulletin 441/1 e
Analysis of pyrithione complexes
Page 2 of 6
Standard
Sodium thiosulfate
Na2S2O3 was dried for 3 h in a
drying oven at 120 °C and allowed
to cool down in a desiccator
overnight.
Sample preparation
An appropriate amount of sample is weighed into a 250 mL
sample beaker and the exact weight is recorded. 10 mL
ultrapure water is added, and the beaker is swirled to
disperse the sample. In a fume hood, 20 mL of concentrated
HCl is added, and the beaker is swirled until the sample is
fully dissolved.
Analysis
Titer
60–80 mg of dry sodium thiosulfate is weighed into a 120 mL
beaker and dissolved in approximately 50 mL of ultrapure
water. After the addition of 5 mL c(CH3COOH) = 2 mol/L, the
solution is titrated with c(I2) = 0.05 mol/L until after the
equivalence point.
Sample
125 mL ultrapure water is added to the prepared sample in
order to submerge electrode and dosing tips. Then the
solution is titrated with c(I2) = 0.05 mol/L until after the first
equivalence point.
Parameters
The same parameters are used for the titer and sample
determination.
Mode
DET U
Stirring rate
10
Start volume
1 mL
Pause
10 s
Signal drift
25 mV/min
Measuring point density
4
Min. increment
20 µL
Max. increment
Off
Dosing rate
10 mL/min
Stop volume
40 mL
Stop EP
1
Volume after stop
3 mL
EP criterion
30
EP recognition
greatest
Calculations
Titer
f =
mS
VEP1 × 2 × cI2 × MStd
f:
Correction factor («titer») without unit
ms:
Sample size in mg
VEP1:
Titrant consumption until the first equivalence
point in mL
2:
Stoichiometric factor
cI2:
Concentration of the selected titrant in mol/L;
here c(I2) = 0.05 mol/L
MStd:
Molecular weight of Na2S2O3; 158.11 g/mol
Sample
ZnPT =
VEP1 × f × cI2 × MA
mS × 10
ZnPT:
ZnPT content in percent
VEP1:
Titrant consumption until the first equivalence
point in mL
f:
Correction factor («titer») without unit
cI2:
Concentration of the selected titrant in mol/L;
here c(I2) = 0.05 mol/L
MA:
Molecular weight of ZnPT; 317.72 g/mol
ms:
Sample size in g
10:
Conversion factor for %
Example determination
Fig. 1: Determination of ZnPT (blue = titration curve, pink = ERC)
Comments
•
Any thiol-containing impurities can interfere with this
analysis.
•
Anti-diffusion dosing tips can be fouled by the iodine
titrant, therefore capillary tips are recommended.
Application Bulletin 441/1 e
Analysis of pyrithione complexes
Page 3 of 6
Sodium pyrithione
Summary
NaPT is dissolved in water, acidified with HCl, and then
titrated with iodine. During the titration, iodine is consumed by
the thiol group present in NaPT.
Instruments
•
Titrator with DET mode
•
Autosampler (optional)
•
Magnetic stirrer
•
50 mL buret
Electrodes
Pt Titrode
6.0431.100
Reagents
•
Hydrochloric acid, w(HCl) = 37%
•
Ultrapure water
•
Iodine, c(I2) = 0.05 mol/L
•
Sodium thiosulfate, Na2S2O3, p.a.
•
Glacial acetic acid, CH3COOH, p.a.
Solutions
Titrant
c(I2) = 0.05 mol/L
Should be bought from a supplier.
Acetic acid solution
c(CH3COOH) = 2 mol/L
114.8 mL glacial acetic acid is
pipetted into a clean 1000 mL
volumetric flask containing 500 mL
ultrapure water. The flask is then
filled up to the mark with ultrapure
water.
Standard
Sodium thiosulfate
Na2S2O3 was dried for 3 h in a
drying oven at 120 °C and allowed
to cool down in a desiccator
overnight.
Sample preparation
An appropriate amount of sample is weighed into a 250 mL
sample beaker and the exact weight is recorded. 10 mL of
ultrapure water is added, and the beaker is swirled to
disperse the sample. In a fume hood, 20 mL concentrated
HCl are added and the beaker is swirled until the sample is
fully dissolved.
Analysis
Titer
60–80 mg of dry sodium thiosulfate is weighed into a 120 mL
beaker and dissolved in approximately 50 mL of ultrapure
water. After the addition of 5 mL c(CH3COOH) = 2 mol/L, the
solution is titrated with c(I2) = 0.05 mol/L until after the
equivalence point.
Sample
125 mL ultrapure water is added to the prepared sample in
order to submerge electrode and dosing tips. Then, the
solution is titrated with c(I2) = 0.05 mol/L until after the first
equivalence point.
Parameters
The same parameters are used for the titer and sample
determination.
Mode
DET U
Stirring rate
10
Start volume
1 mL
Pause
10 s
Signal drift
25 mV/min
Measuring point density
4
Min. increment
20 µL
Max. increment
Off
Dosing rate
10 mL/min
Stop Volume
40 mL
Stop EP
1
Volume after stop
3 mL
EP criterion
30
EP recognition
greatest
Calculations
Titer
f =
mS
VEP1 × 2 × cI2 × MStd
f:
Correction factor («titer») without unit
ms:
Sample size in mg
VEP1:
Titrant consumption until the first equivalence
point in mL
2:
Stoichiometric factor
Application Bulletin 441/1 e
Analysis of pyrithione complexes
Page 4 of 6
cI2:
Concentration of the selected titrant in mol/L;
here c(I2) = 0.05 mol/L
MStd:
Molecular weight of Na2S2O3; 158.11 g/mol
Sample
NaPT =
VEP1 × f × cI2 × MA
mS × 2 × 10
NaPT:
NaPT content in percent
VEP1:
Titrant consumption until the first equivalence
point in mL
f:
Correction factor («titer») without unit
cI2:
Concentration of the selected titrant in mol/L;
here c(I2) = 0.05 mol/L
MA:
Molecular weight of NaPT; 149.15 g/mol
ms:
Sample size in g
2:
Stoichiometric factor
10:
Conversion factor to obtain %
Example determination
Fig. 2: Determination of the NaPT (blue = titration curve, pink =
ERC)
Comments
•
This method is applicable to all in-process and final
samples that contain from 0.1% to 100% sodium
pyrithione by weight.
•
This method does not apply to sodium pyrithione
formulations where other oxidizable species are
present, or where other ingredients interfere with
equivalence point detection.
•
Glacial acetic acid can be used instead of concentrated
HCl for a more defined equivalence point in samples
with low concentrations of sodium pyrithione.
Copper pyrithione
Summary
Assay of copper pyrithione (CuPT) is determined by
iodometric titration with sodium thiosulfate as titrant using Pt
Titrode.
Instruments
•
Titrator with DET mode
•
Autosampler (optional)
•
Magnetic stirrer
•
20 mL buret
Electrodes
Pt Titrode
6.0431.100
Reagents
•
Sodium thiosulfate, Na2S2O3, p.a.
•
Glacial acetic acid, CH3COOH, p.a.
•
Potassium iodide, KI, p.a.
•
Potassium iodate, KIO3, p.a.
•
Sulfuric acid, w(H2SO4) = 98%
•
Nitric acid, w(HNO3) = 65%
•
Ammonia solution, w(NH3) ≈ 25%
•
Urea, CH4N2O, p.a.
•
Sodium thiocyanate, NaSCN, p.a.
•
Ultrapure water
Solutions
Titrant
c(Na2S2O3) = 0.1 mol/L
Should be bought from a supplier.
Urea solution
w(CH4N2O) = 5%
5 g urea is weighed accurately
and transferred into a clean
100 mL volumetric flask. After
dissolution, the flask is filled up to
the mark with ultrapure water.
Sodium thiocyanate
solution
w(NaSCN) = 20%
20 g NaSCN is weighed
accurately and transferred into a
clean 100 mL volumetric flask.
The flask is then made up to the
mark with ultrapure water.
Application Bulletin 441/1 e
Analysis of pyrithione complexes
Page 5 of 6
Potassium iodide
solution
w(KI) = 20%
20 g KI is weighed accurately and
transferred into a clean 100 mL
volumetric flask. Then the flask is
made up to the mark with
ultrapure water.
Sulfuric acid
solution
w(H2SO4) = 10%
10 mL concentrated sulfuric acid
is taken in a clean 100 mL
volumetric flask containing already
50 mL ultrapure water. The flask is
then made up to the mark with
ultrapure water.
Standard solution
Potassium iodate
Potassium iodate is dried in a
drying oven for 2 h at 110 °C and
allowed to cool down in a
desiccator for at least 1 h.
Sample preparation
Approximately 0.1 g sample is weighed accurately and
transferred into 150 mL beaker. 15 mL of ultrapure water and
6 mL concentrated nitric acid are added. The content is
heated and kept boiling for 1 min. The solution is then taken
from the hot plate and allowed to cool slowly to room
temperature. 40 mL of ultrapure water and 5 mL of urea
solution are added to the mixture, and it is heated again and
kept boiling for 2 min. The solution is again allowed to cool to
room temperature. Ammonia solution is then added dropwise
with stirring until a permanent pale blue precipitation occurs.
Immediately, 6 mL glacial acetic acid is added to the solution
and the solution is then allowed to cool down thoroughly.
Analysis
Titer
Approximately 50 mg of dry potassium iodate is weighed into
a 120 mL beaker, and dissolved in approximately 50 mL of
ultrapure water. Approximately 1 g KI and 25 mL w(H2SO4) =
10% are added. Then, the solution is immediately titrated with
c(Na2S2O3) = 0.1 mol/L until after the equivalence point.
Sample
To the cooled solution, 20 mL KI solution and 10 mL sodium
thiocyanate solution are added. The beaker is then covered
with a watch glass and kept in the dark for 15 min. It is then
titrated with c(Na2S2O3) = 0.1 mol/L until after the equivalence
point.
Parameters
Mode
DET U
Stirring rate
8
Pause
30 s
Signal drift
50 mV/min
Min. waiting time
0 s
Max. waiting time
26 s
Min increment
10 µL
Dosing rate
max mL/min
Stop volume
10 mL
EP criterion
30
EP recognition
greatest
Calculations
Titer
f =
mS × 6
VEP1 × cNa2S2O3 × MStd
f:
Correction factor («titer») without unit
ms:
Sample size in mg
6:
Stoichiometric factor
VEP1:
Titrant consumption until the first equivalence
point in mL
cNa2S2O3: Concentration of the selected titrant in mol/L;
here c(Na2S2O3) = 0.1 mol/L
MStd:
Molecular weight of KIO3; 214.001 g/mol
Sample
CuPT =
VEP1 × f × cNa2S2O3 × MA
mS × 2 × 10
CuPT:
CuPT content in percent
VEP1:
Titrant consumption until the first equivalence
point in mL
f:
Correction factor («titer») without unit
cNa2S2O3: Concentration of the selected titrant in mol/L;
here c(Na2S2O3) = 0.1 mol/L
MA:
Molecular weight of CuPT; 315.86 g/mol
ms:
Sample size in g
2:
Stoichiometric factor
10:
Conversion factor to obtain %
Application Bulletin 441/1 e
Analysis of pyrithione complexes
Page 6 of 6
Example determination
Fig. 3: Determination of the CuPT (blue = titration curve, pink =
ERC)
Comments
•
Before every reagent addition, the content should be
cooled to room temperature.
•
Urea solution is added and boiled to remove excess of
nitric acid in solution.
•
During the ammonia addition, Cu(II) ammonia complex
will be formed, giving a light blue color. When the
precipitation occurs, heat the solution again to remove
excess ammonia and continue the analysis.
•
Liberated iodine will be adsorbed onto the surface of
the CuI precipitate. To negate that interaction, enough
sodium thiocyanate should be added to the solution.
Date
July 2020
Author
Competence Center Titration
Metrohm International Headquarters
Document Outline
- Application Bulletin 441/1 e
