Research Into Optical PH Sensor Based On Spectral Analysis

pH is a well-known measurement to evaluate the hydrogen ion concentration in a solution,and it is also used to judge the acidity or alkalinity of a solution.In many fields including the environmental protection,industrial,pharmaceutical and food industries,pH is a very important parameter.Currently pH glass electrode is widely used for pH measurement which has the advantages of wide measuring range and high precision,but also has the shortcomings such as short life and being susceptible to electromagnetic and radio frequency interference.It is easily contaminated or poisoned while being used in harsh environments(high temperature,high pressure or high corrosion,etc.)which leads to unstable even failed results.Therefore,various non-glass pH sensors are becoming a hot spot,among which the optical pH sensor gradually attracts many researchers' and users' attentions due to its superior performance in recent years.The optical pH sensor is composed of the pH indicator and the optical fiber.Compared with the traditional measurement technology,the optical pH sensor has the advantages such as stable measurement signal,being easy to carry,good anti-electromagnetic interference,pollution-free and so on.Many optical pH sensors used the single pH indicator for measuring the variations of optical parameters to judge the pH of the solution.,which were easily affected by environmental factors and had limited measurement range and accuracy.Based on the current research status of optical pH sensors,a sol-gel optical pH sensor based on spectral analysis is proposed in this paper.The main contents of this paper include the following four aspects:(1)According to the detection principle of the existing optical pH sensor,a theoretical model of an optical pH sensor with sol-gel was constructed based on the evanescent wave theory.A spectral acquisition device composed of light source,optical fiber,and USB2000 +spectrometer was designed and implemented.(2)Four kinds of indicators were embedded in the organic carrier which was cross linked by TEOS and cellulose acetate by a sol-gel method to prepare an optical pH sensor with a wide measurement range.The used four kinds of indicators include Congo red,bromophenol blue,cresol red and chlorophenol red.(3)The optimal ratio of tetraethoxysilane,absolute ethanol,deionized water and hydrochloric acid in the sensor-sensitive membrane glue precursor was explored by experiment.The optimal amount of materials including cellulose acetate,DMF,indicator,hydrochloric acid and precursor was determined by the orthogonal experiment.Then,the optimized sensitive membrane was characterized by Fourier transform infrared spectroscopy and field emission scanning electron microscopy.The linearity,measurement accuracy,repeatability and stability and the response time of the pH sensor were tested.(4)The spectrum-pH model based on the improved nu-SVR model and the RBF kernel function was established.The full spectral information acquired by the designed pH spectrum acquisition device was used to train and predict the spectrum-pH model.The experimental results show that based on full spectral analysis,the sol-gel optical pH sensor's measurement accuracy is 0.1pH.Compared with the traditional pH glass electrode,the designed sol-gel optical pH sensor has better stability and repeatability.