Method and apparatus for measuring ph of low alkalinity solutions

(54) Title: METHOD AND SYSTEM FOR PH MEASUREMENT (51) Int. Cl .: G01N 21/27; G01N 21/80 (30) Unionist Priority: 07/05/2007 US 11 / 800,746 (73) Holder (s): GENERAL ELECTRIC COMPANY (72) Inventor (s): CAIBIN XAIO; BINGZHI CHEN; WEIYI CUI; LI ZHANG (74) Attorney (s): ARTUR FRANCISCO SCHAAL (86) International Request: PCT US2008060321 of 15/04/2008 (87) International Publication: WO

2008/137260 of 11/13/2008

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10 ppm PO4, Na / P = 21 10 pprrt PO4, Na / P = 2.5

Δ 5 ppm P04, Na / P = 2.5 @ 5 ppm PO4, Na / P = 2 □ 50 ppm PO4, Na / P ~ 2 • 500 ppnt PO4, Na / P = 2 © 1 ppm PO4, Na / P “2.5 - pc. (10 ppm PO4, Na / P = 21)

--- ροή. (10 ppm PO4, Na / P = 2.5)

---- per. (5 ppm PO4, Na / P = 2.5)

..... poii. (500 ppm PO4, Na / P = 2)

- peir. (1 ppm PO4, Na / P = 2.5)

- poii. (50 ppm PO4, Na / P = 2) “METHOD AND SYSTEM FOR PH MEASUREMENT” Background of the Invention

Field of the Invention

The present invention generally refers to a system for measuring pH and, more particularly, it relates to an improved method and apparatus for measuring the pH of low alkalinity solutions by extrapolating spectrophotometric measurements from a plurality of pH indicator sensors.

Description of the Related Art

A wide variety of systems and methods have been employed to measure the pH of water systems. For example, a glass electrode is commonly used for pH measurement in both a laboratory and industrial environment. Alternatively, it is known that spectrophotometric techniques can be used for pH measurement. Exemplary systems and methods for measuring pH have been described in the US patent application serial number 11 / 507,689, deposited on August 22, 2006, which is attributed to the same assignee of this application, the description of which is incorporated by reference in this document.

Although prior art devices and systems have provided useful products, they are not entirely satisfactory for providing a quick, simple and accurate measurement of low alkalinity water samples in a relatively simple and user-friendly manner. One of the challenges associated with measuring the pH of low alkalinity solutions is the fact that the pH disturbance induced by the introduction of indicators in the sample solution is not negligible. This is true due to the fact that the indicators themselves are weak acids or bases. Otherwise determined, the pH of a weakly buffered solution (ie, of low alkalinity) can be severely disturbed due to the fact that the amount of indicator concentration introduced into the sample is significant in relation to the amount of acid or base in the solution. .

Attempts have been made in the prior art to minimize or correct indicator-induced disturbance in the aqueous phase by: (1) adjusting the pH of the indicator stock solution close to the pH of the samples; (2) reduce the proportion of indicator addition to the sample volume; and (3) observe the pH disturbances induced by the indicator through gradual indicator additions and then use the linear extrapolation methods to obtain the pH of the sample. Such prior art methods provide useful results, however, they are typically time consuming and difficult to use. Therefore, there remains a strong need for an improved method and system that provides a precise, accurate and rapid pH measurement for low alkalinity samples at a relatively low cost and easy to use.

Brief Description of the Invention

One of the challenges associated with measuring the pH of low alkalinity solutions is the fact that the disturbance in the pH values ​​induced by the introduction of indicators in the sample solutions is not negligible. As a result, pH measurements can be severely disturbed due to the fact that indicator concentrations are introduced into a weakly buffered (ie, low alkalinity) solution. To meet this challenge, the present invention describes systems and methods that comprise a set of sensors that comprises a plurality of pH indicators, with each indicator having a different indicator concentration. The sensor set is calibrated by applying the sensor set to a sample solution that has a known pH. The response of each pH indicator is simultaneously recorded, and a calibration function (that is, the calibration curve) is that generated represents the pH response versus the indicator concentration for each indicator concentration. Once calibrated, the sensor set can then be applied to low alkalinity sample solutions that have an unknown pH. The results of the pH values ​​of each pH indicator are compared with the calibration curve, and an adjustment function (ie, adjustment equations) that represents the pH response of each indicator concentration is generated. The adjustment equations are then generated and extrapolated to determine the points of interception (that is, when the indicator concentration is zero) to obtain the original (ie, real) pH of the unknown sample.

Other aspects of the present invention relate to the use of such systems and methods, and to exemplary methods for measuring the pH of low alkalinity solutions. The additional aspects of the present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the attached claims with reference to the attached drawings.

Brief Description of the Figures

Figure 1 is a graphic illustration showing changes in pH after the introduction of different amounts of thymol blue;

Figure 2 shows a series of graphs showing the pH values ​​of different solutions before and after adding an indicator;

Figure 3 is a graph that illustrates the relationship between the measured pH versus the amount of phenol red added;

Figure 4 illustrates the calibration curves generated at four different indicator concentrations; and

Figure 5 is a graph that illustrates a result of an exemplary linear extrapolation method of the present invention.

Detailed Description of the Invention

The present invention describes systems and methods comprising a set of polymer film-based sensors to quickly and precisely measure the pH of low alkalinity solutions, for example, low alkalinity water samples. It is known that alkalinity or buffering capacity is one of the basic characteristics of water samples. Alkalinity is a measure of a solution's ability to neutralize acids. Lower alkalinity means the lower ability to resist pH change when an acid is added to the solution.

The concept of the present invention is based on the recognition that in low alkalinity solutions, the pH disturbance induced by the introduction of indicators in the sample is not negligible. This is true due to the fact that the indicators themselves are weak acids or bases. As a result, the pH of a solution can be severely disturbed due to the fact that the amount of indicator concentration introduced into the sample is significant in relation to the amount of acid or base present in the weakly buffered solution (ie, of low alkalinity) . This disturbing effect is even more pronounced in the film loaded with pH indicator.

To address this challenge, an aspect of the present invention describes an extrapolation process to quickly and accurately measure the pH of low alkalinity samples. Preferably, the method uses, however, is not limited to a set of sensors constructed in accordance with US patent application serial number 11 / 507,689 previously incorporated by reference in this document. Such a set of sensors is configured to comprise a plurality of indicator portions, each with different indicator concentrations. Once built, the set of sensors is used to spectrophotometrically measure the pH of the sample, through which each indicator provides a different absorbance measurement of pH simultaneously. The measured pH values ​​of each indicator portion are plotted against their respective indicator concentrations, and an adjustment function (ie, adjustment equation) that represents the measured pH values ​​is extrapolated to determine the points of interception when the concentration indicator value is zero to obtain the initial pH (that is, actual pH) of the sample. The systems and methods of the present invention provide an advantage over the known methods because instead of trying to minimize the pH disturbances caused by the indicator additions, the present invention takes advantage of the relationship between pH disturbances of different indicator concentrations to calibrate the set of sensors providing, thus a baseline benchmark for determining pH measurements of low alkalinity samples that have an unknown pH.

As described in the present document, the systems and methods of the present invention are particularly well suited for quickly and precisely determining the pH of low alkalinity solutions. The pH measurement of low alkalinity solutions is not trivial due to disturbances induced by the addition of weak acids or base indicators in the solution, especially when the concentration of indicator (which is typically a weak acid or base) is significant in relation to the amount of acid or base in the sample solution. The pH response can be measured by colorimeter, spectrophotometer or fluorescent spectrometer.

According to an exemplary embodiment of the present invention, a set of pH sensors was constructed with a set of four films, although it is understood that more or less films can be used without departing from the scope of the present invention. Each sensor film contained a different pH indicator concentration which will be denoted as lni, ln₂, ln₃ and ln₄, respectively. For the purposes of the examples in this document, the indicator concentration of each film ranged from about 0.01 to 10%.

Solid films are typically prepared from water-soluble polymers, cellulose acetate or Poly 2-Hydroxyethyl Methacrylate (pHEMA). The indicators can be colorimetric pH indicators, fluorescent pH indicators or other suitable pH indicators known or later developed in the art. Preferably, colorimetric pH indicators are selected from a group consisting of phenol red, cresol red, m-cresol purple, thymol blue, bromochlorophenol W blue. S., bromocresol green, chlorophenol red, bromocresol purple, bromothymol blue, neutral red, phenolphthalein, o-cresolphthalein, Nile A blue, thymolphthalein, bromophenol blue, metacresol purple, malachite green, bright green, violet crystal, methyl green, methyl violet 2B, picric acid, yellow naphthol S, yellow methanyl, basic fuchsin, phloxin B. methyl yellow, methyl orange, alizarin.

In order to demonstrate the concepts of the present invention, a theoretical calculation of pH change (i.e., disturbance) was performed in low alkalinity solutions due to the addition of different amounts of indicator material in a sample solution. Although the examples described in this document are included to demonstrate the wide applicability of the present invention, it should be assessed by those skilled in the art that the techniques described in the examples in this document represent the techniques discovered by the inventors and, therefore, can be considered for constitute the exemplary modes for their practice. However, those skilled in the art, considering the present description, must evaluate that many changes can be made in the specific modalities described and still obtain an equal or similar result without departing from the scope of the invention. The calibration and extrapolation methods described in this document can be used to determine the pH of low alkalinity samples with pH responses measured by colorimeter, spectrophotometer or fluorescent spectrometer.

As shown in Figure 1, a graphic illustration shows how changes in pH are made after introducing different amounts of thymol blue into the solution. The results in Figure 1 indicate that the delta pH (ie, actual pH - measured pH) becomes higher and higher with increasing additions of indicator concentration in the solution. This result clearly illustrates that weakly buffered solutions (ie, low alkalinity) can be severely disturbed by indicator additions.

Still referring to Figure 1, the theoretical calculation of pH disturbance shows that the lower the alkalinity, the greater the pH delta. Therefore, it can be concluded that the greater the indicator addition, and the lower the alkalinity, the more the pH of the solution will be altered or disturbed.

To prove this conclusion, a first experiment was conducted, in which a series of 100 ppm carbonate buffers was implemented, and the pH value of different solutions was measured before and after the indicator additions. The results of this first experiment are shown in Figure 2. As shown in Figure 2, a series of graphs illustrates the pH values ​​of different solutions measured before and after the indicator additions. Based on these results, it becomes apparent that when 20 ppm of phenol red (acid form) was added to the solution, the pH measurement decreases slightly. Figure 2 also illustrates that a gradual reduction in pH was observed as the amount of phenol red reduced from 0 ppm (diamond points) to 100 ppm (square points). When another 100 ppm of phenol red was added, the pH was greatly disturbed. As shown in Figure 2, with 100 ppm of phenol red * added, solutions with a pH higher than about 8.0 become basically indistinguishable. Based on these results, it became apparent that a correction in the delta pH induced by indicator additions can be taken into account to obtain the current pH (real pH) of the solution.

Consequently, a second experiment was conducted to show that an extrapolation method can be useful to determine the pH. In this second experiment, two 100 ppm carbonate buffers with an original pH of 8.12 and 8.53 were chosen. The phenol red indicator which was a pH response range of about 6.8 to 8.2 was used. When an acid form of phenol red was gradually added to the weakly buffered carbonate solution, a pH meter was used to monitor the pH of the solution.

As shown in Figure 3, a linear ratio of the pH measured for 100 ppm of indicator addition was plotted for each of the carbonate buffers at 100 ppm. The linear functions that represent the measured pH of each type of indicator were extrapolated when the percentage of indicator was zero to obtain the points of interception. As shown in Figure 3, the intercept points, that is, 8.13 and 8.46, represent the pH of the solution when the indicator concentration is zero.

In this way, the intercept points represent the original pH of the solution before indicator additions. It is readily apparent that the intercept points are very close to the initial pH values, that is, 8.12 and 8.53, of the carbonate buffers, respectively. Consequently, the experiments show that the pH disturbance due to the indicator condition is not negligible when the alkalinity is very low. In addition, the experiments demonstrate that the exemplary linear extrapolation technique of the present invention is very useful to obtain the original pH of the sample. The algorithms used in the exemplary extrapolation technique are described in more detail below.

To correct the changes in pH induced by indicator additions, a calibration curve was fitted using a standard synthetic cooling solution with sufficiently high alkalinity versus a solid pH sensor with a series of indicator concentrations. In this third experiment, the pH of the samples was measured with the same solid pH sensor, and the pH measured for each indicator concentration was calculated. The measured pH versus the indicator concentration was then plotted and an adjustment equation was generated and extrapolated when the indicator concentration is zero, to obtain the initial pH (ie, real pH) of the unknown sample.

As shown in Figure 4, a calibration curve was generated in four (0.5%, 1.0%, 1.5%, 2.0%) indicator concentrations. The pH value of an unknown sample with low alkalinity (less than 100 ppm) was measured.

Figure 5 is a graph illustrating the results of an exemplary linear extrapolation method of the present invention. As can be seen from Figure 5, the point of interception of the equation (that is, when the indicator concentration is zero) is 9.18. Since the intercept point represents the pH before the indicator additions, the extrapolation method demonstrates that the 9.18 intercept point is a very good approximation of the actual pH value 9.07 measured by a pH meter.

In order to obtain the results illustrated in Figures 4 and 5, a set of pH sensors was built with a set of four films, in which each sensor film contains a different pH indicator concentration, such as Im, Ιη₂, ln₃ and ln₄, respectively. Next, an absorbance response was measured for each pH sensor film from a series of standard pH solutions that have a known and fixed alkalinity value.

Then, a calibration curve was generated for each pH sensor film from the data measured from the second previous step. The calibration functions are denoted fi, f₂, f₃ ef₄ for the purposes of the calculations shown below.

Next, a sample of unknown pH was applied to the set of pH sensors, and the absorbance values ​​were measured from each film. For the purposes of the calculations shown below, these absorbance values ​​are denoted Ai, A₂, A₃ and A4 for films 1, 2, 3 and 4, respectively.

Next, preliminary pH values ​​are calculated for each film of each calibration equation and corresponding absorbance value. For example, 0 pH for films 1 to 4 are represented as: pHi = fi (Ai), pH₂ = f₂(THE₂), pH₃ = f₃(THE₃) and pH₄ = f₄(THE₄), respectively. Note that these pH values ​​can be equal if the alkalinity value of the unknown sample is equal to that of the standard calibration solution. However, pHv pH₂, pH₃ and pH₄ will have different values ​​if the alkalinity value of the unknown sample is not the same as that of the standard calibration solution.

In the final step, the actual pH value for the unknown sample is calculated from the preliminary pH values ​​pHi, pH₂, pH₃ and pH₄ based on the extrapolation algorithm provided below:

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on what:

i is the film index;

Inj represents the indicator concentration in film i^th; pHj; is the apparent pH value calculated from the absorbance of film i^th and the corresponding calibration equation fi; and N is the number of pH films.

Figure 5 is a graphic illustration of the exemplary extrapolation algorithm. The calculations for the results shown in Figure 5 and the corresponding mathematical procedure are shown below:

Equation 2:

N = 4, i = 1,2, 3 and 4

Equation 3:

Σ (ΙΠί)² = 2,02 + 1,52 + 1,02 + 0,52 = 7,5

Equation 4:

ΣρΗί = 8.38 + 8.60 + 8.75 + 9.00 = 34.73

Equation 5:

Σΐηι = 2.0 + 1.5 + 1.0 + 0.5 = 5.0

Equation 6:

PHί PHj = 2.0 x 8.38 + 1.5 x 8.60 + 1.0 x 8.75 + 0.5 x 9.00 = 42.91

Equation 7:

pH sample = (34.73 x 7.5 - 5.0 x 42.9) / (7.5 x 4 - 5.0 x 5.0) =

9,18

Based on the results described above, the present invention thus provides a system for directly measuring the pH of low alkalinity samples by providing a set of sensors that have a plurality of indicator concentrations, and calibrates the measured pH of a sample unknown in the calibration curve generated from a known sample to obtain the pH of the unknown sample. In accordance with the present invention, these measurements are recorded simultaneously in a timely manner to avoid tedious and prolonged measurements and calculations involved in gradual indicator additions. As an example, an exemplary solid-film sensor of the present invention demonstrated a rapid response to the target, the results being obtained in approximately minutes for on-site (field) testing.

As described herein, the systems and methods of the present invention incorporate a solid polymer based pH sensor film assembly comprising a number of different indicator concentrations. Once built, the sensor array is applied to a sample solution that contains a known pH and alkalinity. The pH response of each indicator concentration is simultaneously measured and recorded. Next, a calibration function (ie, calibration curve) is generated by representing the measured pH versus each indicator concentration. The calibration curve thus represents a graph of the measured pH versus the indicator concentration. Next, an adjustment function (ie, adjustment equation) that represents each pH measurement is generated. The adjustment equation is extrapolated to determine the points of interception when the indicator concentration is zero, thus obtaining an accurate indication of the original pH of the sample before indicator additions. In this way, the calibration curve represents a baseline reference function that can be used to calibrate the different results of each indicator portion to easily take advantage of the pH disturbance of different indicator additions, in order to extrapolate the pH of samples low alkalinity.

Although the description has been illustrated and described in typical exemplary modalities, it is not intended to be limited to the details shown, since various modifications and substitutions can be made without departing from the scope and spirit of the present description. As such, additional modifications and equivalents of the description presented in this document may occur for those skilled in the art using nothing more than routine experimentation, and it is believed that all such modifications and equivalents are within the scope of the description, as defined by the appended claims.