Contenido

El empleo de este procedimiento de análisis colorimétrico, basado en tubos Nessler era largo y engorroso. El procedimiento se simplificó mediante el empleo de los colorímetros, nombre que recibieron los dispositivos utilizados para determinar la concentración de sustancias coloreadas en disolución. El primer dispositivo de medida que puede ser considerado como un colorímetro, fue el diseñado por el óptico francés fabricante de instrumentos, Jules Duboscq") (1817-1886), considerado el primer colorímetro de la historia del análisis químico. Duboscq desarrolló un instrumento para estimar la concentración de la disolución que se basaba en la modificación del camino óptico recorrido por la luz a través de la muestra. Mediante un dispositivo se podía variar la altura de las muestras, y según fuera la relación entre la patrón y la muestra desconocida, se podía establecer la concentración de la misma. La luz necesaria era de origen natural y dirigida hacia la muestra mediante un espejo.[5]​ Varios instrumentos se desarrollaron según ese principio, desde simples comparadores ópticos hasta complejos instrumentos de medición.

Las medidas en esto colorímetros se llevaban a cabo utilizando una sola disolución patrón y empleaba el denominado procedimiento de compensación, que consistía en ir variando la distancia óptica (camino óptico) de la muestra hasta que la luz situada bajo los tubos, observada desde arriba, se aprecien con igual intensidad; es decir presenten la misma trasmisión. El colorímetro de Duboscq permite hacer esta operación de forma sencilla gracias al ocular de campo partido con el que está dotado. Considerando la Ley de Beer-Lambert y llamando A a la absorbancia de la muestra y A a la del patrón:.

De esta manera y de forma sencilla, conocida la concentración de la disolución de referencia y las longitudes de los caminos ópticos en cada uno de los tubos, se podía establecer la concentración del analito en el tubo de muestra.

Photocolorimeters

With the appearance of electronics and photovoltaic cells, capable of producing an electric current proportional to the intensity of the light that reaches it, more precise colorimeters began to be designed, sometimes also called photocolorimeters or photometers. In these devices, the human eye is replaced by a less subjective electronic detector, such as the photocell, and as the lighting source, a small tungsten filament lamp, whose behavior resembles a black body emitter, which at the incandescence temperature emits visible and infrared light. Since the laws of radiation absorption (Beer's Law) are only fulfilled when monochromatic light is used, the simplest photocolorimeters are usually equipped with filters or sets of filters that are placed between the visible light source and the sample, eliminating most of the unabsorbed radiation, allowing only the transmission of a narrow range of wavelengths.[6]​ In this type of devices, the determination of the concentration of analyte in the test sample is done by measuring the amount of light that passes through the solution, comparing the incoming quantity and the reading of the absorbed quantity. The operation is repeated for a series of standards of known solute concentration and the results are represented as measures of absorbance versus concentration. By interpolating the absorbance on the graph, the value of the unknown concentration of the sample can be found.

After the Second World War, instruments began to be developed that allowed the selection of wavelength with greater precision by having optical prisms or diffraction gratings. These new instruments were also equipped with emission sources that covered both ultraviolet, visible and near infrared (from 190 nm to 1800 nm) and with detectors much more sensitive than the photocell, such as, for example, the so-called photomultiplier tubes. These more sophisticated and precise instruments are capable of recording both complete spectra and performing absorption measurements at specific wavelengths, which is why they are commonly called spectrophotometers. Consequently, the analysis technique, as it is not limited exclusively to the visible radiation range, is called ultraviolet-visible molecular absorption spectrophotometry or more commonly uv-vis spectrophotometry, although still, today, the term colorimetry has not completely fallen into disuse.

Absorption measurements in the ultraviolet and visible regions are widely used to identify and determine an enormous number of inorganic and organic species, being one of the most useful tools available to the scientist. It is widely applied in general analysis, environmental, forensic and clinical laboratories throughout the world.[7] Many of the spectroscopic methods of analysis based on ultraviolet-visible radiation have their origins in the first colorimetric methods developed by chemists, especially those intended to analyze non-absorbent species that can be chemically treated to convert them into colored species.

Among the important characteristics of colorimetric or photometric methods are:[7]​.

There are numerous substances, both of organic and inorganic origin, that are naturally colored and that are therefore susceptible to being determined by colorimetric procedures. Many of the transition metal ions have characteristic colors when they are in solution, as they form "Complex (chemistry)") complexes with water. In other cases, for those species that do not show color naturally or in solution, it is possible to use reagents that undergo a measurable color change in the presence of the analyte. There are numerous reagents that react more or less selectively with non-absorbent species and generate strongly colored products. They are also often used to determine absorbing species, such as transition metal ions, as they form products with molar absorptivity several orders of magnitude higher than that of the species before the reaction, thereby achieving significant improvements in the selectivity of the analysis. Among the frequently used inorganic reagents are the thiocyanate ion for iron, cobalt and molybdenum; hydrogen peroxide for titanium, vanadium and chromium; and the iodide ion for bismuth, palladium and tellurium. There are also organic chelating agents, which form stable colored complexes with various cations, such as diethyldithiocarbamate for the determination of copper, dithizone for lead and dimethylglyoxime for nickel.[7]​.

Colorimetric methods are also used in biochemistry to test for the presence of enzymes, specific compounds, antibodies, hormones, and many more analytes. For example,.

Contenido

El empleo de este procedimiento de análisis colorimétrico, basado en tubos Nessler era largo y engorroso. El procedimiento se simplificó mediante el empleo de los colorímetros, nombre que recibieron los dispositivos utilizados para determinar la concentración de sustancias coloreadas en disolución. El primer dispositivo de medida que puede ser considerado como un colorímetro, fue el diseñado por el óptico francés fabricante de instrumentos, Jules Duboscq") (1817-1886), considerado el primer colorímetro de la historia del análisis químico. Duboscq desarrolló un instrumento para estimar la concentración de la disolución que se basaba en la modificación del camino óptico recorrido por la luz a través de la muestra. Mediante un dispositivo se podía variar la altura de las muestras, y según fuera la relación entre la patrón y la muestra desconocida, se podía establecer la concentración de la misma. La luz necesaria era de origen natural y dirigida hacia la muestra mediante un espejo.[5]​ Varios instrumentos se desarrollaron según ese principio, desde simples comparadores ópticos hasta complejos instrumentos de medición.

Las medidas en esto colorímetros se llevaban a cabo utilizando una sola disolución patrón y empleaba el denominado procedimiento de compensación, que consistía en ir variando la distancia óptica (camino óptico) de la muestra hasta que la luz situada bajo los tubos, observada desde arriba, se aprecien con igual intensidad; es decir presenten la misma trasmisión. El colorímetro de Duboscq permite hacer esta operación de forma sencilla gracias al ocular de campo partido con el que está dotado. Considerando la Ley de Beer-Lambert y llamando A a la absorbancia de la muestra y A a la del patrón:.

De esta manera y de forma sencilla, conocida la concentración de la disolución de referencia y las longitudes de los caminos ópticos en cada uno de los tubos, se podía establecer la concentración del analito en el tubo de muestra.

Photocolorimeters

With the appearance of electronics and photovoltaic cells, capable of producing an electric current proportional to the intensity of the light that reaches it, more precise colorimeters began to be designed, sometimes also called photocolorimeters or photometers. In these devices, the human eye is replaced by a less subjective electronic detector, such as the photocell, and as the lighting source, a small tungsten filament lamp, whose behavior resembles a black body emitter, which at the incandescence temperature emits visible and infrared light. Since the laws of radiation absorption (Beer's Law) are only fulfilled when monochromatic light is used, the simplest photocolorimeters are usually equipped with filters or sets of filters that are placed between the visible light source and the sample, eliminating most of the unabsorbed radiation, allowing only the transmission of a narrow range of wavelengths.[6]​ In this type of devices, the determination of the concentration of analyte in the test sample is done by measuring the amount of light that passes through the solution, comparing the incoming quantity and the reading of the absorbed quantity. The operation is repeated for a series of standards of known solute concentration and the results are represented as measures of absorbance versus concentration. By interpolating the absorbance on the graph, the value of the unknown concentration of the sample can be found.

After the Second World War, instruments began to be developed that allowed the selection of wavelength with greater precision by having optical prisms or diffraction gratings. These new instruments were also equipped with emission sources that covered both ultraviolet, visible and near infrared (from 190 nm to 1800 nm) and with detectors much more sensitive than the photocell, such as, for example, the so-called photomultiplier tubes. These more sophisticated and precise instruments are capable of recording both complete spectra and performing absorption measurements at specific wavelengths, which is why they are commonly called spectrophotometers. Consequently, the analysis technique, as it is not limited exclusively to the visible radiation range, is called ultraviolet-visible molecular absorption spectrophotometry or more commonly uv-vis spectrophotometry, although still, today, the term colorimetry has not completely fallen into disuse.

Absorption measurements in the ultraviolet and visible regions are widely used to identify and determine an enormous number of inorganic and organic species, being one of the most useful tools available to the scientist. It is widely applied in general analysis, environmental, forensic and clinical laboratories throughout the world.[7] Many of the spectroscopic methods of analysis based on ultraviolet-visible radiation have their origins in the first colorimetric methods developed by chemists, especially those intended to analyze non-absorbent species that can be chemically treated to convert them into colored species.

Among the important characteristics of colorimetric or photometric methods are:[7]​.

There are numerous substances, both of organic and inorganic origin, that are naturally colored and that are therefore susceptible to being determined by colorimetric procedures. Many of the transition metal ions have characteristic colors when they are in solution, as they form "Complex (chemistry)") complexes with water. In other cases, for those species that do not show color naturally or in solution, it is possible to use reagents that undergo a measurable color change in the presence of the analyte. There are numerous reagents that react more or less selectively with non-absorbent species and generate strongly colored products. They are also often used to determine absorbing species, such as transition metal ions, as they form products with molar absorptivity several orders of magnitude higher than that of the species before the reaction, thereby achieving significant improvements in the selectivity of the analysis. Among the frequently used inorganic reagents are the thiocyanate ion for iron, cobalt and molybdenum; hydrogen peroxide for titanium, vanadium and chromium; and the iodide ion for bismuth, palladium and tellurium. There are also organic chelating agents, which form stable colored complexes with various cations, such as diethyldithiocarbamate for the determination of copper, dithizone for lead and dimethylglyoxime for nickel.[7]​.

Colorimetric methods are also used in biochemistry to test for the presence of enzymes, specific compounds, antibodies, hormones, and many more analytes. For example,.