Acetic acid sensors

Absorption spectroscopic sensing, general discussion, 271 Acetic acid sensors, 334 Activities of ions, relation to interfacial potential difference, 369... 

Sensors for acidic (HC1, SO2, CO2, acetic acid) or basic gases (NH3, amines) often make use of pH indicator dyes immobilised in polymers. Lipophilic Bromophenol Blue (see section 4.2) dissolved in silicone is... 

The sensors were exposed to the following VOC n-pentane, methanol, benzene, triethylamine and acetic acid. 

Reflectance measurements provided an excellent means for building an ammonium ion sensor involving immobilization of a colorimetric acid-base indicator in the flow-cell depicted schematically in Fig. 3.38.C. The cell was furnished with a microporous PTFE membrane supported on the inner surface of the light window. The detection limit achieved was found to depend on the constant of the immobilized acid-base indicator used it was lO M for /7-Xylenol Blue (pAT, = 2.0). The response time was related to the ammonium ion concentration and ranged from 1 to 60 min. The sensor remained stable for over 6 months and was used to determine the analyte in real samples consisting of purified waste water, which was taken from a tank where the water was collected for release into the mimicipal waste water treatment plant. Since no significant interference fi-om acid compounds such as carbon dioxide or acetic acid was encountered, the sensor proved to be applicable to real samples after pH adjustment. The ammonium concentrations provided by the sensor were consistent with those obtained by ion chromatography, a spectrophotometric assay and an ammonia-selective electrode [269]. 

High cell densities are not only a prerequisite for high productivity additionally an effective on-line control and modeling of the bioprocesses is necessary. For industrial applications, optical measurement methods are more attractive because they are non-invasive and more robust. The potential of the BioView sensor for on-line bioprocess monitoring and control was tested. For high-cell-density cultivation of Escherichia coli, maintaining aerobic conditions and removal of inhibitory by-products are essential. Acetic acid is known to be one of the critical metabolites. Information about changes in the cell metabolism and the time of important process operations is accessible on-line for optimization... 

This problem is overcome by the Bio View sensor, which offers the possibility to monitor the whole spectral range simultaneously, and by using suitable data analysis and mathematical methods like chemometric regression models 11061. Real-time fluorescence measurement can be used more effectively comparing time-consuming off-line methods. Partial least squares (PLS) calibration models were developed for simultaneous on-line prediction of the cell dry mass concentration (Fig. 5), product concentration (Fig. 6), and metabolite concentrations (e. g., acetic acid, not shown) from 2D spectra. 

Figure 14 shows the variable-pH kinetic (VpHK) profile obtained spectrophoto-metrically for the reaction of hydrolysis of aspirin with pH varying in the range 2-10 at T = 342.5 K. The variable-concentration conditions were realized by adding a concentrated solution of NaOH (0.6 M) to the thermostatted reaction vessel containing the aqueous solution of acetylsahcylic acid and a buffer composed of acetic acid (0.01 M), fosforic acid (0.01 M), and boric acid (0.01 M). In this way an almost linear increase of pH was generated. The absorbance was read by an optical fiber cell and stored in a computer. The pH was monitored by a pH sensor connected to a computer. 

Another fluorescence-based method for assaying activity and enantioselectivity of synthetic catalysts, specifically in the acylation of chiral alcohols, was recently reported [27]. The idea is to use a molecular sensor that fluoresces upon formation of an acidic product (acetic acid). Adaptation to high-throughput evaluation of enantioselective lipases or esterases needs to be demonstrated. 

The pattern of each taste substance is different, and hence each taste substance can be easily discriminated The reproducibility is very high, because the standard deviations are smaller than 1 % or so. The taste sensor shows similar response patterns to the same group of taste. As examples of sour substances, HC1, citric acid and acetic acid show similar response patterns. Bitter substances such as quinine, MgS04 and phenylthiourea show similar patterns. 

Sandwich casting permits one to prepare an MIP film with uniform thickness [28, 106, 108, 109]. In this procedure, a drop of the solution containing a monomer, cross-linker, template and initiator is dispensed on the surface of a PZ transducer and covered with a microscope quartz slide. Then this assembly is exposed to UV light in order to initiate polymerization that results in a thin MIP film. The polymerization can be performed either on the activated immobilized initiator PZ transducer surface or on the bare transducer surface. For example, sialic acid has been determined with an MIP film immobilized on a platinum-film electrode of the quartz resonator using the former procedure [57]. That is, 1-butanethiol has been used for modification of the Pt surface. An indole-3-acetic acid plant hormone served as the template. An MIP-PZ chemosensor prepared that way operated reproducibly. That is, the coefficient of variation of the chemosensor performance was 9% for three different sensors. 

The sensor did not respond to volatile compounds such as methyl alcohol, formic acid, acetic acid, propionic acid, and other nutrients for microorganisms such as carbohydrates, amino acids, and ions. The selectivity of the microbial sensor for ethyl alcohol was satisfactory. 

Acetic Acid. On-line measurement of acetic acid concentrations is required in fermentation processes. The microbial sensor for ethyl alcohol described above could be used for the determination of acetic acid(8). 

The calibration graphs obtained showed linear relationships between the current decrease and the concentration of acetic acid up to 72 mg l-. The minimum concentration for determination was 5 mg of acetic acid 1". The current difference was reproducible within 6 % for an acetic acid sample contaning 54 mg l 1. The standard deviation was 1.6 mg 1 in 20 experiments. The sensor did not respond to volatile compounds such as formic acid and methanol or to nonvolatile nutrients such as glucose and phosphate ions. 

The microbial sensor for acetic acid was applied to a fermentation broth of glutamic acid. The concentration of acetic acid was determined by the microbial sensor and by a gas chromatographic method. Good agreement was obtained the regression coefficient was 1.04 for 26 experiments. 

The sensor did not respond to volatile compounds such as acetic acid, ethyl alcohol, and amines (diethylamine, propylamine, and butylamine) or to nonvolatile nutrients such as glucose, amino acids, and metal ions (potassium and sodium ions). Therefore, the selectivity of this microbial sensor was satisfactory in the presence of these different substances. The current output of the sensor was almost constant for more than 21 days and 400 assays. The microbial sensor can be used to assay sodium nitrite for a long period. In the same experiments the concentration of sodium nitrite was determined by both the sensor proposed and the conventional method (dime-thyl-a-naphtylamine method). A good correlation was obtained between the sodium nitrite concentrations determined by the two methods (correlation coefficient 0.99). 

Figure 34 shows the results for alcohol (methanol, ethanol, 1-propanol and 1-butanol), ketone (acetone and diacetyl), terpene (pinene and linalool), aldehyde (n-nonyl aldehyde) and ester (acetic acid n-amyl ester and n-butyric acid ethyl ester) of various concentrations. Because of the linear characteristics of the CTL-based sensor, the plots are located in a similar region for a certain type of gas of various concentrations where the Henry-type adsorption isotherm holds. Thus, we can identify these gases with various concentrations by simple data-processing. 

Hu et al. [58] reported a well defined peak at 0.68 V for 5.0 x 10 M indole-3-acetic acid (an important hormone present in plants) in pH 2.0 phosphate buffer at GCE modified with MWCNT dispersed in DHP [58]. The oxidation peak current of the indole-3-acetic acid increased gradually with the amount of MWCNT-DHP dispersed at the GCE up to a volume of 15 pL. For higher volumes the thickness of the layer blocked the electron transfer. The oxidation peak current presented a linear relationship with the concentration of the hormone from 1 X 10 M to 5 X 10 M, with a detection limit of 2 x 10 M and a reproducibility of 4.3 % for 36 measurements of 5 x 10 M. The authors extended the use of this sensor to the determination of the hormone in gladiola, apple and phoenix leaves showing a very good agreement with HPLC determinations. 

Extensive research and development of microbial sensors has been carried out by Suzuki et al. (89-94) and Rechnitz et al. (95-97) (see Table III). Microbial sensors consisting of membrane-bound whole cells and an oxygen electrode were constructed for the determination of substrates such as assimilable sugars, acetic acid, alcohols and ammonia, and for the estimation of biochemical oxygen demand (BOD) (98-104). Glutamic acid was determined with a microbial sensor which consists of membrane-bound whole cells containing glutamate decarboxylase and a carbon dioxide gas electrode. These microbial sensors have been applied and evaluated for on-line measurements in fermentation processes (105,106). 

A microbial FET for the determination of alcohol has been constructed by Tamiya et al. (1988). The cell membrane of Gluconobacter suboxydans, which converts ethanol to acetic acid, was attached in calcium alginate to the gate of a pH-FET and covered by a nitrocellulose layer. The differential output versus a membrane-free reference gate was linearly related to the logarithm of the ethanol concentration up to 20 mg/1. The sensor responded to propanol and butanol with similar sensitivity, but not to methanol. The response time was 10 min. Below 30°C the sensor was stable for 40 h. 

Theophylline-imprinted polymers prepared with MAA and EDMA were utilized for SPR sensors, in which the particles were immobilized on the silver film on the SPR sensor chip by evaporation from acetonitrile-acetic acid (99 1 v/v) containing the particles [20]. The detection limit was reported to be 0.4 mg/mL of theophylline in aqueous solution. 

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