U.S. patent application number 14/580497 was filed with the patent office on 2016-06-23 for detecting stannous ions.
The applicant listed for this patent is Colgate-Palmolive Company. Invention is credited to Stanislav Jaracz, Michael Prencipe.
Application Number | 20160178595 14/580497 |
Document ID | / |
Family ID | 56027786 |
Filed Date | 2016-06-23 |
United States Patent
Application |
20160178595 |
Kind Code |
A1 |
Jaracz; Stanislav ; et
al. |
June 23, 2016 |
DETECTING STANNOUS IONS
Abstract
A method of detecting stannous ions that includes providing a
zincon-containing indicator having a colour, contacting the
indicator with a sample, and detecting any resulting change in the
colour of the indicator, wherein a change in the colour of the
indicator from the colour to colourless indicates that stannous
ions are present in the sample. The method is useful for the
analysis of oral care compositions. Also provided is the use of
zinc on as an indicator for detecting stannous ions.
Inventors: |
Jaracz; Stanislav;
(Somerset, NJ) ; Prencipe; Michael; (Princeton
Junction, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Colgate-Palmolive Company |
Piscataway |
NJ |
US |
|
|
Family ID: |
56027786 |
Appl. No.: |
14/580497 |
Filed: |
December 23, 2014 |
Current U.S.
Class: |
436/81 ;
436/73 |
Current CPC
Class: |
G01N 21/78 20130101;
G01N 21/79 20130101; G01N 21/77 20130101; A61Q 11/00 20130101 |
International
Class: |
G01N 33/20 20060101
G01N033/20 |
Claims
1. A method of detecting stannous ions in a sample, which method
comprises: (i) providing an amount of an indicator comprising
zincon or a salt thereof having a UV/visible absorbance spectrum;
(ii) contacting the indicator with a sample such that if stannous
ions are present in the sample, stannous ions react with the
indicator; and (iii) detecting a change in the UV/visible
absorbance spectrum of the indicator to determine whether stannous
ions are present in the sample.
2. The method of claim 1, wherein step (iii) comprises detecting a
change in the colour of the indicator.
3. The method of claim 1, wherein the sample comprises an oral care
composition.
4. The method of claim 3, wherein the oral care composition
comprises a toothpaste.
5. The method of claim 1, wherein step (ii) comprises forming a
liquid comprising the indicator, the sample, and water.
6. The method of claim 5, wherein the amount of the indicator
present in the liquid is no greater than 500 ppm by weight of the
liquid.
7. The method of claim 6, wherein the amount of the indicator
present in the liquid is in the range of 30 to 60 ppm by weight of
the liquid.
8. The method of claim 5, comprising titrating the sample with a
solution of the indicator.
9. The method of claim 5, wherein the Liquid has a pH in the range
of 5 to 10.
10. The method of claim 9, wherein the liquid has a pH in the range
of 6.3 to 7.3.
11. The method of claim 9, wherein the liquid has a pH in the range
7 to 10.
12. The method of claim 11, wherein the sample includes zinc
ions.
13. The method of claim 12, wherein stannous ions are present in
the sample, and wherein the method further comprises: oxidizing the
stannous ions after step (iii) to form stannic ions; and allowing
the zinc ions to react with the indicator.
14. The method of claim 13, subsequently comprising measuring the
concentration of the zinc by colourimetry.
15-17. (canceled)
Description
TECHNICAL HELD
[0001] The present invention generally relates to methods for
detecting stannous ions, particularly to methods for detecting
stannous ions in oral care compositions, and to the use of an
indicator to detect stannous ions.
BACKGROUND
[0002] Stannous ions (Sn.sup.2+) are used in oral care compositions
because they display biocidal activity and are effective in
preventing dental erosion (Rakhmatullina et al, 2013. "Inhibition
of enamel erosion by stannous and fluoride containing rinsing
solutions", Schweiz Monatsschr Zahnmed, vol. 123, pp. 192-197).
Aqueous solutions comprising stannous ions have been reported to be
unstable (Nevitt et al, 1958. "Topical applications of sodium
fluoride and stannous fluoride", Public Health Rep, vol. 73, no 9,
pp. 847-850). Stability is an important property to assess when
formulating an oral care composition. There is therefore a need in
the art for methods of testing oral care compositions for the
presence of stannous ions.
[0003] Existing methods for analysing oral care compositions
include the measurement the total amount of tin present in the
composition. However, not all forms of tin are therapeutically
active. In particular, stannic (tin (IV)) ions are believed to be
inactive. The existing methods do not discriminate between the
various forms of tin. There is therefore a need in the art for a
method for the selective detection of stannous ions.
[0004] WO2008/041055 discloses oral care compositions comprising a
stannous ion source, a polyvalent cation source and a mineral
surface active agent. A method of investigating the binding of
stannous is also outlined. In this method, the stannous is provided
in the form of stannous fluoride. The binding of stannous is
estimated by potentiometric detection of available ionic fluoride.
Hence, this method does not directly detect the presence of
stannous ions.
BRIEF SUMMARY
[0005] In one aspect, the present invention provides a method of
detecting stannous ions in a sample, which method comprises (i)
providing an indicator having a UV/visible absorbance spectrum;
(ii) contacting the indicator with a sample, such that if stannous
ions are present in the sample, stannous ions react with the
indicator; and (iii) detecting a change in the UV/visible
absorbance spectrum of the indicator to determine whether stannous
ions are present in the sample; wherein the indicator is zincon or
a salt thereof. Zincon (o-[.alpha.-(2-hydroxy-5-sulfophenyl
azobenzylidene hydrazino] benzoic acid, CAS no. 62625-22-3) has
surprisingly been found that zincon reacts rapidly and highly
selectively with stannous ions. The resulting complex has a
different UV/vis absorbance spectrum to free zincon. The present
method thereby provides a simple, rapid method of detecting
stannous ions.
[0006] Step (iii) may comprise detecting a change in the colour of
the indicator. A change in the colour of the indicator corresponds
to a change in the absorbance of the indicator in the region of the
spectrum visible to the human eye (i.e. at wavelengths in the range
390 to 730 nm). Zincon is a brightly coloured dye which appears red
when it is present in an aqueous solution at a pH in the range 7 to
10. The complex of zincon with stannous appears colourless under
these conditions. The presence of stannous ions may therefore be
readily detected by simple visual inspection. The use of detection
means, such as a UV/visible spectrometer or a colourimeter, is also
contemplated herein.
[0007] The detection of stannous ions may be qualitative, and may
comprise observing the colour of the indicator. Qualitative
detection is generally rapid, simple, and typically requires only a
small amount of indicator. Alternatively, the detection may be
quantitative. Quantitation may be achieved by titration. The
skilled artisan will be familiar with titration methods.
[0008] The nature of the sample is not particularly limited. The
sample may comprise an oral care composition. Oral care
compositions may comprise stannous ions and/or zinc ions as active
ingredients, and the methods described herein provide a convenient
way of detecting the presence of these ions. The method may be
used, for example, during the formulation of new oral care
compositions and for shelf-life determinations. Further
applications include quality control and quality assurance.
[0009] The nature of the oral care composition is not particularly
limited. The oral care composition may be, for example, a
toothpaste, a tooth gel, a tooth powder, or a mouth rinse. The oral
care composition may comprise toothpaste; in this arrangement, the
sample may be formed by dispersing the toothpaste in water.
[0010] Step (ii) may comprise forming a liquid comprising the
indicator, the sample, and water wherein the zincon is dissolved in
the liquid. The liquid may comprise a solution. The liquid may
include suspended material. Zincon is soluble in water, and
providing a liquid in this way is a straightforward way of
achieving good contact between the zincon and the sample.
[0011] The amount of the indicator present in the liquid is
advantageously no greater than 500 ppm by weight of the liquid, and
may be in the range of 30 to 60 ppm by weight of the liquid. Dilute
solutions of zincon provide good sensitivity, because fewer
stannous ions are required to produce a colour change.
[0012] In the arrangements where step (ii) comprises forming a
liquid, the method may comprise titrating the sample with a
solution of the indicator. Titration allows quantitative
measurement of the amount of stannous present in the sample, but
may require a large amount of zincon to be used.
[0013] The liquid may have a pH in the range of 5 to 10. The
reaction between stannous ions and zincon was found to proceed well
under these pH conditions.
[0014] The pH of the liquid may be in the range 6.3 to 7.3. This pH
range approximates the pH range found in the oral cavity (Afraiman
et al, 2006. "The distribution of oral mucosal pH values in healthy
saliva secretors", Oral Dis, 12, 4, pp 420-423). Thus, the delivery
of stannous ions under biorelevant conditions may be
investigated.
[0015] The pH range may alternatively be in the range of 7 to 10.
This is representative of the pH ranges used in typical oral care
compositions, which are often mildly alkaline. Tests which operate
under these pH conditions are therefore particularly advantageous.
The pH range of 7 to 10 is also preferable in the arrangements
where zinc ions are to be detected, because the reaction between
zincon and zinc ions is sensitive to pH. In the arrangements where
zinc ions are to be detected, the is most preferably in the range 9
to 9.5.
[0016] The sample may include zinc ions. Zinc has been reported to
form a dark blue complex with zincon (Yoe and Rush, 1952. "A new
colorimetric reagent for zinc", Anal. Chim. Acta, vol. 6, pp
526-527). Surprisingly, zincon reacts selectively with stannous
ions. The colourless stannous complex is formed even in the
presence of zinc ions. Zinc ions are often included in oral care
compositions. It is therefore useful to be able to detect stannous
ions in the presence of zinc ions.
[0017] The method may comprise, if stannous ions are detected in
the sample, oxidizing the stannous ions to form stannic ions
(Sn.sup.4+); and allowing the zinc ions to react with the
indicator. It has been found that zincon binds zinc ions in
preference to stannic ions. By oxidizing stannous ions to stannic
ions in situ, the zincon may be used to detect both zinc ions and
stannous ions in a single assay. This simplifies analysis of the
sample.
[0018] The stannous ions may be oxidized to stannic ions using any
conventional method. An electrochemical technique or an oxidizing
agent may be used. The preferred oxidizing agent is hydrogen
peroxide. Hydrogen peroxide is readily available commercially, and
reacts rapidly with stannous ions. For safety and ease of handling,
the hydrogen peroxide may be provided in the form of an aqueous
solution. The aqueous solution may comprise hydrogen peroxide in an
amount in the range 1% to 5% by weight of the solution.
Alternatively, the oxidizing agent may be oxygen from the
atmosphere. Allowing a solution comprising stannous ions to stand
in air may allow the stannous ions to oxidise. This has the
advantage of not requiring the use of any further reagents.
[0019] In the arrangements wherein zincon zinc is formed, the
concentration of the zincon zinc may be measured by colorimerty. It
is often desirable to quantify the amount of zinc ions released by
an oral care composition. Because zincon binds to zinc weakly, only
the free zinc ions will be detected. This allows the
bioavailability of the zinc to be estimated.
[0020] In another aspect, the present invention provides the use of
zincon as an indicator for detecting stannous ions. Zincon forms a
colourless complex with stannous ions, and binds to stannous ions
selectively. Zincon is therefore useful for detecting stannous ions
in complex mixtures, such as oral care compositions.
[0021] The stannous ions may be detected even in the presence of
zinc ions. Although zincon has been reported to form a blue complex
in the presence of zinc ions, zincon binds stannous ions in
preference to zinc ions. Oral care compositions may comprise both
stannous ions and zinc ions. It is therefore useful to be able to
detect stannous ions in the presence of zinc ions.
[0022] The use may be additionally for detecting zinc ions.
Contacting zincon with stannic ions does not produce a colour
change, and it is therefore believed that zincon does not form a
complex with stannic ions. By oxidizing stannous ions to stannic
ions subsequent to the detection of the stannous ions, any zinc
ions which are present may be detected.
[0023] Further areas of applicability of the present invention will
become apparent from the detailed description provided hereinafter.
It should be understood that the detailed description and specific
examples, while indicating the preferred embodiment of the
invention, are intended for purposes of illustration only and are
not intended to limit the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The present invention will become more fully understood from
the detailed description and the accompanying drawings, in
which:
[0025] FIG. 1 shows a photograph of vials containing the test
solutions described in Example 1.
DETAILED DESCRIPTION
[0026] The following description of the preferred embodiments is
merely exemplary in nature and is in no way intended to limit the
invention, its application, or uses.
[0027] As used throughout, ranges are used as shorthand for
describing each and every value that is within the range. Any value
within the range can be selected as the terminus of the range. In
addition, all references cited herein are hereby incorporated by
reference in their entireties. In the event of a conflict in a
definition in the present disclosure and that of as cited
reference, the present disclosure controls.
[0028] Unless otherwise specified, all percentages and amounts
expressed herein and elsewhere in the specification should be
understood to refer to percentages by weight. The amounts given are
based on the active weight of the material.
[0029] As used herein, the term "zincon" refers to
o-[.alpha.-(2-hydroxy-5-sulfophenyl azo)-benzylidene hydrazino]
benzoic acid, which has CAS number 56484-13-0. The structure of the
sodium salt of zincon is shown below:
##STR00001##
[0030] The term "stannous ions" refers to tin (II) ions, i.e.
Sn.sup.2+. The term "stannic ions" refers to tin (IV) ions, i.e.
Sn.sup.4+. The term "zinc ions" refers in particular to zinc (II)
ions, i.e. Zn.sup.2+.
[0031] Zincon has been reported to form complexes with copper,
cobalt, nickel and iron (Yoe and Rush, 1952, "A new colorimetric
reagent for zinc". Anal. Chim. Acta, vol, 6, pp 526-527). The use
of zincon to detect cupric (copper (II)) ions in immersion tin
plating operations has also been reported (Wilcox, 2009, "The black
and white of immersion tin: keeping an eye on cupric ions can
eliminate black tin". Printed Circuit Design & Fab, 1 Jul.
2009).
[0032] The electrochemical synthesis of a tin zincon complex has
also been reported (Mabrouk et al, 1992, "Direct electrochemical
synthesis of cobalt, nickel, copper, zinc, cadmium, tin and lead
zincon complexes", Trans. Met. Chem., 17, pp 1-4). The complex
obtained by this method is described as red-brown. The colourless
complex formed in the methods of the present invention is not
identified.
[0033] The present invention is based on the surprising finding
that zincon reacts rapidly and specifically with stannous ions to
produce a colourless complex. As shown in FIG. 1 and discussed in
detail in the Examples, the colourless complex is formed even in
the presence of strong chelating agents and other metal ions such
as zinc. Zincon is therefore useful for detecting stannous ions
even in complex mixtures, such as oral care compositions.
[0034] Notably, zincon does not undergo a colour change when
contacted with stannic ions. Without wishing, to be bound by
theory, it is believed that stannic ions do not react with zincon,
at least under the conditions used in the Examples. If such a
reaction does occur, then the binding of stannic ions by zincon is
weak, because the presence of stannic ions did not prevent the
formation of a zinc-zincon complex. The selective detection of
stannous ions is important. This is because, unlike stannous ions,
stannic ions are not believed to display useful therapeutic
activity. The selectivity of zincon for stannous ions renders
zincon particularly useful for the analysis of oral care
compositions.
[0035] Accordingly, in one aspect the present invention provides a
method of detecting stannous ions in a sample, which method
comprises: (i) providing an indicator haying a UV/visible
absorbance spectrum; (ii) contacting the indicator with a sample
such that if stannous ions are present in the sample, stannous ions
react with the indicator; and (iii) detecting a change in the
UV/visible absorbance spectrum of the indicator to determine
whether stannous ions are present in the sample; wherein the
indicator is zincon or a suitable salt thereof.
[0036] Zincon reacts rapidly and selectively with stannous ions.
This reaction produces a characteristic colour change. The presence
of stannous ions may therefore be detected. The present method
therefore provides a cost effective technique for directly
detecting the presence of stannous ions.
[0037] The indicator is zincon or a suitable salt thereof. Zincon
is an ionizable compound. Zincon may therefore be provided in the
form of a salt. The counterion present in the salt is selected such
that the counterion does not interfere with the detection of
stannous ions. Suitable salts may be identified for example by
contacting the salt with a solution known to contain stannous ions.
If a colourless complex is formed, that is, if a colour change to
colourless occurs, then the counterion is suitable. A preferred
salt is zincon monosodium, which is commercially available. The
copper (II) salt of zincon is generally not a suitable salt.
[0038] The indicator may be supplied in the form of a composition
which is a solution comprising the zincon, a solvent and optionally
one or more additives. Zincon is poorly soluble in many organic
solvents. The solvent therefore preferably comprises water.
Examples of suitable solvents include water and aqueous cosolvent
mixtures, for example mixtures comprising water and an alcohol or a
ketone. Useful alcohols include C1 to C4 linear or branched alkyl
alcohols, such as methanol, ethanol, and isopropanol. Useful
ketones include acetone and the like.
[0039] In the arrangements where the solvent comprises water, the
pH of the solution may be in the range of 5 to 10, preferably 7 to
10, more preferably 9 to 9.5. Zincon has acidic functional groups
which become ionized at alkaline pH. Providing a solution which is
mildly alkaline therefore assists in dissolving the introit and
facilitates the reaction of zincon with positively charged metal
ions.
[0040] The solution may comprise a buffer for maintaining the pH
within the desired range. One of skill in the art will be familiar
with aqueous buffer solutions. The buffer preferably does not
comprise a chelating agent. In some arrangements, zincon is used
additionally to detect zinc ions. Zinc ions are bound by zincon
weakly, and the presence of chelating agents may therefore
interfere with their detection. Examples of useful buffers include
ammonium buffer, suitably at a pH in the range of 9 to 9.5; TRIS,
suitably at a pH in the range of 7 to 8.5; and HEPES, suitably at a
pH in the range of 7 to 7.5.
[0041] The amount of zincon present in the indicator composition
preferably does not exceed 2% by weight of the indicator
composition. For example, zincon may be present in the indicator
composition in an amount in the range 0.01% to 1% optionally 0.1%
to 1% by weight of the indicator composition. As will be discussed
below, it is advantageous to use concentrations of zincon below 500
ppm for detecting stannous ions. Indicator compositions comprising
zincon in an amount in the range of 0.01% to 1% by weight may be
conveniently diluted by addition to a sample to obtain a
concentration of zincon below 500 ppm.
[0042] Other components may be included in the indicator
composition. Examples of such components include preservatives for
extending shelf life. Viscosity modifiers, such as xanthan gum, are
also tolerated but are preferably absent.
[0043] The method further includes the step of contacting the
indicator with a sample. The nature of the sample is not
particularly limited.
[0044] The sample may comprise an aqueous solution. The aqueous
solution may have a pH in the range of 5 to 10, optionally 7 to 10,
and preferably 9 to 9.5. It has been found that the reaction
between stannous ions and zincon proceeds rapidly under these
conditions. The pH range of 7 to 10 is also representative of the
conditions present in a typical oral care composition.
[0045] The pH of the aqueous solution may alternatively be in the
range of 6.3 to 7.3. This approximates the pH range typically
encountered in the oral cavity.
[0046] The sample preferably comprises an oral care composition.
The oral care composition may be, for example, a toothpaste, a
tooth gel, a tooth powder or a mouth rinse. If the oral care
composition is a toothpaste, a tooth gel, or a tooth powder then
the oral care composition is suitably dispersed in a solvent. The
solvent is suitably water. For example, the sample may be an
aqueous dispersion comprising the oral care composition in an
amount in the range 10% to 30% by weight of the dispersion.
Preferably, the sample is colourless and/or is substantially free
of colourants. This allows the colour change of the indicator to be
observed more easily. However, the presence of colourants is
tolerated because free zincon is strongly coloured. This means that
the colour change of the zincon indicator is detectable even if a
background colour is present. In the arrangements where a colourant
is present, the change in the UV/visible absorbance of zincon is
advantageously detected by visual inspection.
[0047] The sample is advantageously substantially free of cupric
(copper (II)) ions. Cupric ions have been found to bind zincon even
more strongly than stannous ions, forming a bright blue complex.
The term "substantially free" is used to mean that the
concentration of cupric ions is such that the cupric ions do not
interfere with the detection of stannous. Cupric ions are
preferably completely absent, but may be present in an amount no
greater than 5 ppm. Oral care compositions generally do not
comprise cupric ions.
[0048] The sample and the indicator are contacted such that the
zincon present in the indicator is capable of reacting with
stannous ions present in the sample. Typically, the sample
comprises a liquid and/or the zincon is supplied in an indicator
composition which is a liquid.
[0049] A liquid indicator composition may be contacted with a
sample by dropwise addition of an indicator composition to the
sample. Alternatively, an indicator composition may be sprayed onto
the sample.
[0050] The sample may include zinc ions. The strong binding of
stannous ions by zincon allows the detection of stannous ions even
in the presence of zinc ions. In this arrangement, the method
optionally further comprises detecting the zinc ions, for example
by colourimetry. Detecting the presence of both zinc. ions and
stannous ions using a single indicator allows for the rapid
analysis of an oral care composition.
[0051] The zinc ions may be detected by oxidizing the stannous ions
to form stannic ions. This allows the zinc ions to react with the
zincon to form a zincon-zinc complex. The zincon zinc complex is
blue in colour. The colour change to blue allows the detection of
the zinc ions. As will be shown in the Examples, zinc ions may be
detected in the presence of stannic ions using zincon.
[0052] The method advantageously comprises adding an additional
amount of indicator to the sample after the stannous ions are
oxidized to stannic ions. Zincon may be degraded by oxidising
agents over time. Adding further zincon therefore allows zinc to be
detected more reliably.
[0053] The stannous ions may be oxidized to form stannic ions using
any conventional technique. Methods for oxidizing metal ions
include electrochemical techniques and the use of chemical
oxidizing agents. Examples of oxidizing agents useful in the
present methods include atmospheric oxygen; peroxides such as
hydrogen peroxide; peracetic acid; persulfates such as sodium
persulfate and potassium persulfate; hypochlorites such as sodium
hypochlorite; chlorates; perchlorates; iodates; periodates;
bromates; perbromates; iodine; and bromine.
[0054] A preferred oxidizing agent is hydrogen peroxide. Hydrogen
peroxide reacts rapidly with stannous ions, and is readily
commercially available. For safety and ease of handling, the
hydrogen peroxide may be supplied as an aqueous solution, for
example an aqueous solution including 1% to 5 hydrogen peroxide by
weight of the aqueous solution.
[0055] A further preferred oxidizing agent is atmospheric oxygen.
Atmospheric oxygen typically produces a slow oxidization of
stannous ions to stannic ions, but has the advantage that no
additional reagents are required. For example, stannous ions may be
oxidized to stannic ions by allowing the stannous ions to stand in
air for 24 hours or more.
[0056] Since the oxidation of stannous ions to stannic ions by
oxygen from the air is slow, the methods described herein may be
carried out in air. If desired, the methods may of course be
carried out under an inert gas. Examples of useful inert gases
include nitrogen and argon.
[0057] The method includes detecting, a change in the UV/visible
absorbance spectrum of the indicator.
[0058] The change in the UV/visible absorbance spectrum arising
from the reaction between zincon and stannous ions is such that it
produces a colour change which is visible to the human eye. The
change is perceivable as a change from red to colourless. Thus,
step (iii) most conveniently comprises detecting a change in the
colour of the indicator, for example, by visual observation. This
has the advantage of not requiring any particular
instrumentation.
[0059] The use of a spectrometer or colourimeter to carry out step
(iii) is also contemplated herein. The precise nature of the
changes to the UV/visible absorbance spectrum may be readily
determined by routine experimentation and will not be set out in
detail here. Reductions in absorbance readings within the visible
region (390 to 730 nm) are to be expected in the presence of
stannous ions. One of skill in the art will be familiar with the
use of spectrometers and colourimeters, and with methods of
preparing samples for these instruments. The detection may be
quantitative or qualitative. Qualitative detection generally
comprises simple visual inspection of the indicator. Quantitative
detection may be achieved, for example, by titration. The skilled
artisan will be familiar with suitable titration methods.
[0060] In the arrangements where the presence of zinc is
investigated, the method may comprise measuring the concentration
of the zincon zinc complex by colourimetry. Since the binding of
zinc by zincon is relatively weak, the presence of chelating agents
which render the zinc unavailable and would lead to a reduction in
the measured zinc concentration. In this way, determining the
amount of zinc present by using zincon provides an estimate of the
bioavailable zinc present in the cam position.
[0061] In another aspect, there is provided the use of zincon as an
indicator for detecting stannous ions. As noted above, zincon
undergoes a colour change to colourless when it reacts with
stannous ions. This allows the detection of stannous ions using
zincon. Stannous ions may be detected in the presence of zinc ions.
In this arrangement, the zincon may be further used to detect the
zinc ions. Generally, the stannous ions will be detected using the
zincon, and then the stannous ions will be oxidized to stannic ions
to allow the detection of the zinc ions.
EXAMPLES
[0062] The present invention will now be explained by reference to
the following non-limiting Examples.
Example 1
Detection of Stannous Ions
[0063] The behaviour of zincon in the presence of zinc ions,
stannous ions, and stannic ions was investigated. Contacting a
zincon indicator solution with stannous ions resulted in a colour
change from red to colourless. This colour change was observed even
in the presence of zinc (II) ions, which have been reported to
produce a blue complex with zincon. The inclusion of EDTA in a
solution of zincon and stannous ions did not prevent the colour
change from occurring. this indicates that zincon may be used to
detect stannous ions, even in the presence of other materials.
[0064] An indicator was prepared by dissolving zincon sodium salt
in a 0.2 M ammonia buffer. The pH of the buffer was 9.25. The
amount of zincon sodium salt present in the indicator was 0.3% by
weight. A series of test solutions was then prepared Each test
solution included 2 to 3 mL of deionised water, 1 mL of ammonia
buffer, and 40 .mu.L of the indicator solution. The remaining
components of the solution are set out in Table 1:
TABLE-US-00001 TABLE 1 solutions investigated Additive Solution 1
None Solution 2 20 .mu.L of 0.25M SnF.sub.2 Solution 3 20 .mu.L of
0.25M SnF.sub.2 and excess EDTA Solution 4 20 .mu.L of 0.25M
SnF.sub.2 and 20 .mu.L of 0.25M Zn(NO.sub.3).sub.2 Solution 5 20
.mu.L of 0.25M SnCl.sub.4 Solution 6 20 .mu.L of 0.24M SnCl.sub.4
and 20 .mu.L of 0.25M Zn(NO.sub.3).sub.2
[0065] The solutions were stirred to mix the components. A
photograph of the resulting solutions is shown in FIG. 1. Vials
containing solutions 1 to 6 are shown from left to right.
[0066] Solution 1, zincon in the absence of metal ions, was found
to have a red colour. The addition of stannous ions, in the form of
stannous fluoride, to zincon resulted in the formation of a
colourless solution (solution 2). Mixtures of stannous ions with
either the strong chelating agent EDTA (solution 3) or zinc (II)
ions (solution 4) also resulted in the formation of a colourless
solution. This shows that stannous ions are bound strongly by
zincon. Accordingly, zincon may be used to detect the presence of
stannous ions even when further materials are present.
[0067] No colour change was observed when stannic ions were added
to zincon (solution 5). A colour change from red to blue was
observed when zincon was contacted with a mixture of stannic ions
and zinc (II) (solution 6). This colour change is consistent with
the formation of a blue zincon zinc complex, as reported by Yoe and
Rush (Yoe and Rush, 1952. "A new colorimetric reagent for zinc",
Anal. Chim. Acta, vol 6, pp 526-527). This indicates that zincon
does not react with stannic ions.
Example 2
Detection of Stannous Ions in the Presence of Zinc Ions
[0068] The stannous ion and zinc ion content of a series of
commercially available toothpaste compositions were investigated
using Zincon. As shown in Example 1, Zincon binds stannous ions in
preference to zinc ions but does not bind stannic ions. It has been
found that the zinc content of a composition comprising zinc ions
and stannous ions may be investigated by oxidising the stannous
ions to stannic ions in situ. Hence, both zinc ions and stannic
ions may be detected in a single using a single indicator.
[0069] A Zincon indicator was prepared in accordance with Example
1. Three commercially available toothpaste compositions were then
investigated using the following method:
1 Disperse completely 0.5 g of the toothpaste in 2 mL of deionised
water. 2. Add 1 mL of 0.2 M ammonia buffer (pH 9.25). 3. Add 40 mL
of zincon indicator solution and stir briefly. If the solution
becomes colourless, dissolved stannous ions are present, if the
solution turns blue, stannous is absent and dissolved zinc is
present, and if the solution remains red, there is neither
dissolved stannous nor dissolved zinc. 4. If the solution is
colourless, add 100 .mu.L of 3% hydrogen peroxide and stir for 2
minutes to oxidize stannous to stannic. 5. Add 40 .mu.L of 0.3%
zincon. If the solution turns blue, zinc is bioavailable, if red,
zinc is not bioavailable (or absent). Any shade in between means
partially bioavailable zinc.
[0070] The toothpastes which were investigated are listed in Table
2:
TABLE-US-00002 TABLE 2 toothpaste compositions investigated
Composition Active metal ion source Composition A 0.454% SnF.sub.2
Composition B 1.07% SnCl.sub.2 Composition C 0.439% SnF.sub.2, 0.5%
zinc lactate dehydrate, high water All percentages given in the
table above are by weight of the composition.
[0071] Stannous ions were detected in Composition A and Composition
B. No stannous ions were detected in Composition C, indicating that
this formulation did not provide soluble stannous ions or that the
stannous ions had oxidised during storage to yield stannic ions.
The zinc was detected in Composition C.
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