How to Use Colorimeter to Measure the Concentration of Heavy Metals?

The detection and determination of heavy metals in the area of analytics and drug control are important and demanding tasks, and, therefore, drug analysis and quality control require increasingly better analytical methods and procedures to solve specific problems in pharmaceuticals and in industrial production of medicines. A recent trend to obviate the difficulties of obtaining single analysis sensors based on specific receptors – problems of selectivity – is to combine nonspecific sensors in an array that produces a set of analytical signals (electronic tongue for gaseous species/electronic nose for species in solution). Different principles have been used for the preparation of sensor arrays for electronic tongues, with the most widely used being: tin oxide catalytic sensors, conducting polymer sensors, acoustic wave sensors, quartz crystal microbalance sensors, sensors based on MOSFET technology, systems based on ion mobility spectrometry as well as mass spectrometry techniques like API and PTR, and lastly, optical techniques, principally fiber optics and fluorescence. The basic requirements for the optical tongue systems, in reference to the sensing array, which is the key of the system, is that sensors should present low selectivity or high cross-sensitivity, and that they have reproducible analytical characteristics. Moreover, it is necessary to look for sensors to be more accurate and more robust, which is contradictory to a certain degree, since usually the more accurate a sensor is, the less robust it becomes. One solution is to use disposable sensors which are, therefore, not integrated into the device. Additional requirements refer to cost and portability.

Preparation of the sensing array

Different chromogenic reagents for heavy metals were tested for immobilization using different strategies such as ion exchange, adsorption, and entrapment using as criteria for selection of the conditions of no leaching, change in tone color coordinate by reaction, and non-selective behavior.

Description of the instrument

The portable instrument is a microcontroller-based system that allows one to calculate theH component of the HSV colour space from an image of a colorimetric sensor array obtained with amicro-camera. The colour information is used to determine the concentration of metals in a solution.

The block diagram of the prototype

In the picture, photography of the prototype described is presented. The micro-LCD screen shows an image of the sensor array in which different membranes are presented. The user can select, by touching the screen, the individual sensors to calculate the H coordinate and metals concentration so that a defective sensor can be ignored. The sensor array is placed at the bottom of the instrument, in whole built facing the micro-camera, covered by a transparent Mylar polyester sheet, to allow to capture the picture of the membranes which is processed in hindsight.

Micro-LCD screen shows an image

From this it can be seen,a handheld colorimeter that uses the hue component of HSV color space for a colorimetric sensor array to calculate the concentration of metals in solutions such as zinc and copper is presented. The analytical procedure consists of determining the color changes in two different kinds of sensing membranes, PAN or ZIRCON, and relating them to variations of the metal concentration.

The H component of the colorimetric sensor is obtained from an image captured with an integrated micro-camera and displayed on a touch-micro LCD screen. The main advantage of the direct communication between the micro-camera and the LCD screen is to avoid the necessity of using external memory to save the image since the color information of each pixel can be directly read from the screen. The prototype works in an ambient light environment; therefore there is no need for a specific light source integrated into the prototype. Because of all of this, the result is a simple electronic design with a reduced number of electronic components.

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