AQUEOUS SOLUTION

Field of the Invention

The present invention relates to a device and a method for measuring the ion concentration in an aqueous solution, and in particular the metal ion concentration in the aqueous solution.

Background of the Invention

The concentration of metal ions such as potassium, sodium, calcium, magnesium and zinc ions in aqueous solutions plays an important part in many areas such as washing, personal care, cooking and brewing. For example, the hardness of water is determined by the concentration of multivalent cations in the water, such as calcium, magnesium, and hard water can form a complex with detergent and thereby lower the detergency. As a result, blots will be left on hard surfaces such as dishes and glass after cleaning with water containing more metallic ions. Hard water can also cause a decrease of the dissoluble protein content during soymilk brewing.

Some methods have been developed for measuring metal ions in water. For example, titrations can be used to detect Ca ²⁺ and Pb ²⁺ ions. However, titration has to be carried out in a skillful manner, which makes it unsuitable for domestic use. Other detection methods such as atomic absorption spectrometry and ionic chromatography require expensive equipment.

Moreover, these detection methods are all off-line detections. Water samples are first collected and then it takes a period of time before the test results are available.

Cyclic voltammetry can be used to measure the concentration of electro-active species, such as Fe(CN)6 ^3" and Ru(byp)3 ²⁺ ' in aqueous solution. However, since many metal ions, such as potassium, sodium, calcium, magnesium ions, are electro -inactive in aqueous solution and yield no redox peaks during the cyclic voltammetry scan, this technology cannot be applied in the detection of these metal ions.

Object and Summary of the Invention

To better address one or more of these concerns, in a first aspect of the invention, a device for measuring metal ion concentration in an aqueous solution comprises a reference electrode to be immersed in the aqueous solution and a working electrode to be immersed in the aqueous solution, the working electrode comprising a conductor, a first portion of the conductor being coated with a polyvmylchloride membrane comprising anionic surfactant for attracting the metal ions. The device also includes a voltmeter connected to the working electrode and the reference electrode for measuring a potential difference between the working electrode and the reference electrode; and a data processor for determining the metal ion concentration according to the potential difference obtained by the voltmeter. Advantageously, anionic surfactant has a hydrophilic head carrying negative charges which can attract metal ions effectively on the membrane, which increases the efficiency of measurement of metal ions.

According to one embodiment of the invention, the first portion of the conductor is the bottom surface of the conductor.

According to one embodiment of the invention, the first portion of the conductor is the portion to be immersed in the aqueous solution..

Advantageously, since the membrane coated on the electrode prevents contact between the conductor and the solution, the accumulation time of metal ions on the membrane is decreased, which reduces the detection time and improves the efficiency of detection.

According to one embodiment of the invention, a second portion of the conductor is coated with an electrical insulator for electrically insulating the conductor portion which is not coated with the membrane. The second portion of the conductor can be the portion that is immersed in the aqueous solution and that is not coated with the membrane. The second portion of the conductor also can be the portion of the conductor other than the first portion.

According to one embodiment of the invention, the polyvmylchloride membrane comprises surfactant and plasticizer. Since the plasticizer can increase the flexibility of the membrane, the membrane can be tightly attached on the electrode.

According to one embodiment of the invention, the ratio of anionic surfactant to polyvmylchloride in the membrane ranges from 1 : 1000 to 1 :10.

According to one embodiment of the invention, the anionic surfactant is selected from the group consisting of sodium dodecylbenzene sulfonate, sodium stearate, sodium alkane sulfonate and sodium alkane sulfate. Advantageously, since these chemicals are inexpensive, the cost of the device is decreased. According to another aspect of the invention, a method of measuring the metal ion concentration in an aqueous solution comprises the steps of immersing a reference electrode in the aqueous solution and immersing a working electrode in the aqueous solution, wherein the working electrode comprises a conductor, a first portion of the conductor being coated with a polyvinylchloride membrane comprising anionic surfactant for attracting the metal ions, and subsequently measuring the potential difference between the working electrode and the reference electrode by a voltmeter and determining the metal ion concentration in the aqueous solution by a data processor according to the potential difference obtained by the voltmeter.

Brief Description of the Drawings

The above and other objects and features of the present invention will become more apparent from the following detailed description considered in connection with the accompanying drawings, in which:

FIG. 1 shows an exemplary device for measuring metal ion concentration in aqueous solution according to one embodiment of the invention;

FIG. 2 shows an exemplary working electrode according to one embodiment of the invention;

FIG.3 shows an exemplary working electrode according to another embodiment of the invention;

FIG.4 shows an exemplary working electrode according to another embodiment of the invention;

FIG. 5 shows a flow chart of a method of measuring metal ions in the aqueous solution according to one embodiment of the invention.

Detailed Description

FIG. 1 shows an exemplary device for measuring metal ion concentration in aqueous solution according to one embodiment of the invention. The device includes a reference electrode 11 and a working electrode 12. The reference electrode can be a saturated calomel electrode or a silver/silver-chloride electrode. The working electrode comprises a conductor, wherein a first portion of the conductor is coated with a polyvinylchloride membrane comprising anionic surfactant. Both electrodes are immersed into the aqueous solution 13 that includes the metal ions to be detected, such as potassium, sodium, calcium or magnesium. A voltmeter 14 is connected to the two electrodes to measure the potential difference between the two electrodes. The potential signal is determined by the charge distribution on both sides of the membrane. And a data processor 15 is used to determine the metal concentration according to the potential difference obtained by the voltmeter. The determination of the metal concentration is based on the electrode calibration curve. The calibration curve shows a function relationship between potential difference and logarithmic concentration of standard solutions of specific ions. The potential difference E of the working electrode varies approximately linearly with the logarithm of metal concentration in aqueous solution and the potential difference can be expressed by means of the following equation:

E=E ₀ +SlogC

wherein:

E= measured potential difference between the working electrode and the reference electrode,

Eo= measured potential difference between the working electrode and the reference electrode at a C=l concentration,

S=the slope of the calibration curve,

C=the concentration of the ion(s) to be measured.

The slope S of the calibration curve can be calculated by means of measurement data of the standard solutions of specific ions. Table 1 and Table 2 show the measurement results of standard solutions of sodium and calcium ions.

Then, the metal ion concentration of a solution is calculated by means of the equation:

The anionic surfactant can be selected from the group consisting of sodium dodecylbenzene sulfonate, sodium stearate, sodium alkane sulfonate and sodium alkane sulfate. Generalized structures of these chemicals are shown below.

sodium dodecylbenzene sulfonate sodium stearate O

CH _s iCH

sodium alkane sulfate sodium alkane sulfonate

The anionic surfactants can be used as substances for attracting metal ions due to electrostatic interactions between the negatively charged anionic surfactant and the positively charged metal ions. The hydrophobic tails of the surfactant, such as CH ₃ (CH ₂ )ioCH ₂ - of SDS, make these molecules bind tightly with the polyvinylchloride membrane due to hydrophobic interactions. The polar "heads" of the surfactant, such as -CeH^SC of SDS, due to favourable interactions with the solution, form a hydrophilic outer layer that in effect contacts the solution. The hydrophilic head of anionic surfactant carries a negative charge in aqueous solution, so the metal ions which are positively charged will be attracted to the anionic surfactant membrane by electrostatic adsorption, which results in a change of the electric equilibrium on the working electrode. And the change of the electric equilibrium results in a potential change on the working electrode.

FIG. 2 shows an exemplary working electrode according to one embodiment of the invention. The working electrode includes a conductor 21 and an insulator 22. The bottom surface of the conductor is coated with the polyvinylchloride membrane 23 comprising anionic surfactant such as sodium dodecylbenzene sulfonate, sodium stearate, sodium alkane sulfonate or sodium alkane sulfate. A second portion of the conductor is coated with the insulator made of polytetrafluoroethylene (PTFE).The second portion of the conductor can be the remaining part of the conductor excepting the bottom surface. Or the second portion can be the portion of the conductor that is immersed in the solution excepting the bottom surface of the conductor. Or the second portion of the conductor is a combination of an immersed portion, excepting the bottom surface, and a non-immersed portion in the solution.

FIG.3 shows an exemplary working electrode according to another embodiment of the invention. The working electrode includes a conductor 31 and an insulator 32. A first portion of the conductor includes the bottom surface of the conductor and another part of the conductor to be immersed in the solution. The first portion is coated with membrane 33 and the second portion of the conductor, excepting the first portion, is coated by the insulator made of polytetrafluoroethylene (PTFE). The second portion can be the portion of the conductor that is immersed in the solution, excepting the bottom surface of the conductor. Or the second portion of the conductor comprises an immersed portion, excepting the bottom surface, and a non- immersed portion in the solution.

FIG.4 shows an exemplary working electrode according to another embodiment of the invention. The working electrode includes a conductor 41 and an insulator 42. An immersed portion of the conductor excepting the portion coated by the membrane 43 is coated by an insulator made of polytetrafluoroethylene (PTFE). Besides the immersed portion of the conductor, part of the non- immersed portion can also be coated with an insulator made of polytetrafluoroethylene (PTFE).

FIG. 5 shows a flow chart of a method of measuring metal ions in the aqueous solution according to one embodiment of the invention. The method comprises step S51 of immersing a reference electrode 11 in the aqueous solution 13; and step S52 of immersing a working electrode 12 in the aqueous solution, wherein the working electrode comprises a conductor, a first portion of the conductor being coated with a polyvinylchloride membrane comprising anionic surfactant for attracting metal ions. The metal ions which are positively charged will be attracted to the anionic surfactant membrane by electrostatic adsorption, which results in a change of the electric equilibrium on the working electrode. And the change of the electric equilibrium results in a potential change on the working electrode. The method further comprises step S53 of measuring the potential difference between the reference electrode and the working electrode by a voltmeter 14; and step S54 of determining the metal ion concentration in the aqueous solution by a data processor 15 according to the potential difference obtained by the voltmeter.

Example of Preparing the Working Electrode:

According to this example, sodium dodecylbenzene sulfonate (SDS) is used as surfactant in the polyvinylchloride membrane that is coated on the working electrode. The process of preparing the working electrode is as follows.

1) generating polyvinylchloride (PVC) solution

dissolving 120mg PVC powder in 2ml tetrahydrofuran (THF) to generate a 6% (m/v) polyvinylchloride (PVC) solution.

2) making a mixed solution of polyvinylchloride and sodium dodecylbenzene

sulfonate (PVC/SDS) dissolving 12 mg SDS in 2 ml 6% PVC solution to make the ratio of SDS to PVC equal to 1 :10. The ratio of SDS to PVC can be varied in a range of 1 :1000 to 1 : 10, according to the amount of SDS added into the solution. Plasticizer, such as o-NPOE, could be added to improve the flexibility of the membrane. The mass content of plasticizer could be ranged from 30%-70%.

3) polishing and cleaning alumina on glassy carbon electrode surface

polishing a glassy carbon (GC) electrode with 0.05 μηι alumina powder and washing it with water. Subsequently, sonication is applied to wash GC for 10 min in each of water, alcohol and water. After washing, the electrode is dried in air.

4) coating PVC/SDS membrane on the electrode

In order to ensure that the membrane is tightly immobilized on the electrode surface, the cleaned glassy carbon electrode is immersed in the mixed PVC/SDS solution for 1 second to wet the electrode. And then 30 μΐ of the mixed solution is provided on the electrode surface by dipping. Finally, the PVC/SDS membrane is formed on the electrode surface after evaporating the tetrahydrofuran(THF) for one hour.

Example of Measuring Metal Ion Concentration in Aqueous Solution:

According to the example, the calcium ion concentration is tested in a series of solutions of known calcium chloride concentration in mol per liter and the sodium ion concentration is tested in a series of solutions of known sodium chloride concentration in mol per liter. The working electrode used in the test is prepared according to above example, wherein the mixed PVC/SDS solution comprises 12mg SDS, 120mg PVC, 2ml THE Table 1 and table 2 are the test result of calcium ions and sodium ions. It shows the calcium ion concentration and sodium ion concentration are well detected and reflected by the potential difference.

Table 1

CaCl ₂ solutions

Concentration

1.00E-05 1.00E-04 1.00E-03 1.00E-02 l.OOE-01

(M)

E (mV vs.

177.3 200.4 222.8 241.8 261.4

SCE) Table 2

NaCl solutions

It should be noted that the above described embodiments are given for describing rather than limiting the invention, and it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the scope of the invention and the appended claims. The protective scope of the invention is defined by the accompanying claims. In addition, any of the reference numerals in the claims should not be interpreted as a limitation to the claims. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The indefinite article "a" or "an" preceding an element or step does not exclude the presence of a plurality of such elements or steps.