УДК 543.432: 543.645.6

Application of digital colorimetry in control of phenols in building materials

Rudakov O.B., Khorokhordina E.A., Tran Hai Dang

Department of chemistry of Voronezh State University of Architecture and Civil Engineering, Russia

The opportunities of digital colorimetry in control of phenols in building materials using color tests are considered.

Key words: digital colorimetry, chromaticity parameters RGB, building materials, quality and safety control, phenols.

The opportunities of application of digital colorimetry in quality control of colored building production are used more and more often because of the development of digital techlogies [1-6]. From various models of representation of color we chose the color scheme of RGB. The registration of a video signal by means of a digital camera (DC) and by means of the flatbed scanners (FS) is used for the development of colorimetric techniques. It is more convenient to use DC in the field conditions, but too dark colors of coverings or solutions (if it is a question of color tests for existence of ekotoxiсants) are undesirable to photoregistration. For DC it is expedient to use a special box which allows unifying the photographing conditions. At the same time, in order to obtain the images of solutions it is possible to use not only ditches but also test-tubes with the help of DC. DC allows photographing firm materials and products.

As for FS, a scanner with a slide adapter is necessary for a registration of the diluted solutions in big ditches. There are some restrictions on the sizes and mass of a registered firm sample. A definite advantage of FS over DC are a built-in system of lighting with automatic calibration of white balance and sensitivity in each cycle of scanning.

The image file received from DC or FS with the slide adapter, can be automatically analysed by means of the standard software, both according to chromaticity characteristics, and reflecting ability or lightness.

In building industry phenols and their derivatives are used as antiseptics, stabilizers and antioxidants, monomers and the precursors, though the lowest phenols are ekotoxicants. Definition of their contents is particularly relavant for a quantitative assessment of ecological safety of production. The most selective control method of phenols is high performance liquid chromatography (HPLC), and for an express control a thin-layer chromatography is also used [7]. In some cases when a selective component wise definition is not required, the total content of phenols (a phenolic index) is supervised by means of a spectrophotometric method [7].

A number of color reactions for qualitative and quantitative definition of phenols is known. They are applied in the analytical practice and the diagnostics of materials [8]. The digital technologies give additional opportunities for effective application of color reactions in the analytical practice for a definition of phenols in water solutions.

For the analysis of building materials a sample of polymer was crushed on the special crusher station (the sieve size 5×5 mm) and the mass of a hinge plate was weighed on the analytical scales ~ 1,0-1,5 g with an accuracy of ±0,0002 g. Hinge plate was placed into a conic flat-bottomed flask, 30 ml of the mixed solution an acetonotrile - water in a volume ratio 4:1 were flowed with a graduated cylinder and stirred up with a vibratory blender within 15 min. 1,0 ml of the received water-acetonotrilic extract was selected with a help of a pipette and then was placed in a separating funnel, with a capacity of 50 ml.

10 ml of distilled water was added to this extract with a help of a graduated cylinder, then we alkalinized with NH₃ solution (C =1,6 mol/l) up to рН 9 and saturated this solution of sulfate of ammonium (NH₄)₂SO₄ (~6,3 g). The received extraction system was stirred up with a vibratory blender within 15 min. After the phase immiscibility (5 min) the photometric analysis with use 4 nitroanilines was carried out.

For carrying out the reaction with chloride of iron (III) a reagent was prepared, dissolving 5 g of waterless ferrous chloride (III) or 8,25 g of ferric chloride hexahydrate (III) in 100 ml of distilled water. Extract of phenols (0,5 ml) was added into 100 ml of water and then the reagent at the rate of 0,1 g of a specimen on 0,05 ml of iron solution (III) was added. At the time of addition of the reagent to the solution there was a violet coloring [9].

After that there was a definition of chromacity by means of DC as follows. To calculate the chromaticity coordinates and construct graduated charts digital images of the analysed objects were received with the camera Olympus SP-500 UZ in a special box which allows standardizing the lighting conditions. A cuvette, filled with an experimental sample, was installed into a box, and then the image was recorded. In order to reduce flare lights the inside of the box was covered with matte black paint. A rear white side served as a diffusion screen. For its lighting two halogen lamps with a total capacity of 80 Watts were used. In the box there was a base for optical cells with optical thickness from 10 to 100 mm. The optical scheme of the box allowed, thanks to a turning mirror, having DC horizontally, making work of an operator more convenient. Shooting conditions: save format graphical information – JPEG, the image size is 1 MB, Flash-off, light sensitivity ISO 100 – or "Auto", white balance settings – "halogen lamps".

Using FS, the registration of digital images of the solutions was performed with a scanner HP ScanJet 3500 with slide adapter in a specially constructed flask clamp. The resultant images were transmitted to a personal computer using the software FS. While scanning a mode using the slide adapter was entered. The cuvette, filled with the analyzed solution, was placed into the flask clamp, which consists of a light-tight box and a system of mirrors and allows you to change the direction of illumination from the slide adapter through the cuvette with a sample to an optical sensor of FS. The cuvettes were with optical thickness l from the range of 5 to 100 mm. Scanning conditions: TrueColor color mode (16.5 million colors) color scale RGB, 200 dpi optical resolution.

The image file produced on a digital device (DC or FS) was analyzed using its own program developed in an environment of mathematical package Mathcad or electronic image editor Adobe Photoshop.

In the paper [9] we present a method for determining the content of individual phenols or the total content of their mixtures in the aqueous solutions by analyzing the digitized images of the solutions after two color reaction. The azocoupling reaction of phenols with chromogenic agent was used as a color test. It was received by diazotizing para-nitroaniline (reaction 1), and the reaction with FeCl₃ (reaction 2). To obtain an analytical signal DC was used.

The objects chosen for a research were phenol, ortho-, meta-, para-cresols, ortho-, meta-, para-dihydroxybenzenes and ortho-tret-butylphenol. The generalized colorimetric data were presented in the form of the radar chart (RC). On its axes there are F_i color coordinates in the RGB model in a sequence of R₁,G₁, B₁, R₂, G₂, B₂ where the indexes 1 and 2 are related to one of color reactions.

As can be seen from fig. 1, RC form an individual profile ("a visual print"), which is characteristic for each phenol which can be quantitatively described using the RC geometrical parameters – area (S), perimeter (P), fractality (D) and closeness value of vector arrays ε [8].

RC area and perimeter can be considered as a factor considering reactionary ability and structure of connection in case of approximately identical concentration. The more intensive the received coloring of solution is, the less the geometrical parameters of RC are, the better the color reaction was carried out, or the stronger the chromophoric effect is which will depend on the balance of mesomeric and inductive effects in the colored complex. So, if diatomic phenols have 2 group -OH, both of them will react with a chromogenic reagent.

Alkyl substituents, especially with a branched carbon skeleton, can complicate sterically a color reaction in the ortho-position, even a group –OH in the ortho-position, forming a hydrogen bond with the adjacent group –OH, can impede the target reaction.

For identification (PD/RC image recognition) fractality values (D) and a closeness value of vector arrays (ε) were tested [9]. Values D and value ε to a lesser degree have to depend on the concentration of analyte, and more characterize profile identity of a figure. The closeness value of vector arrays ε appeared to be the most informative.

In tab. 1 the RC geometrical parameters are sorted by the magnitude of the hydrophobicity of phenols N which is equal to a logarithm of phenol distribution between n-oktanol and water. The discovered trend – the higher the hydrophobicity of phenol is, the more the area and the RC perimeter.

Table 1. Parameters of phenols hydrophobicity (H), and geometric parameters (area S, perimeter P, fractality D and closeness value of vector arrays ε) of colorimetric RC for different phenols, C = 0,15-0,19 g/l

Compound	Н	S	P	D	ε
meta- dihydroxybenzene	0,80	19050	623	1,67	0,260
para- dihydroxybenzene	0,56	27250	657	1,78	0,215
ortho- dihydroxybenzene	0,91	27940	740	1,81	0,248
meta-cresol	2,00	32370	775	1,66	0,430
para-cresol	2,13	33010	778	1,27	0,302
phenol	1,64	36270	813	1,82	0
ortho-cresol	2,13	39820	866	1,41	0,170
ortho-tret-butylphenol	3,35	53260	894	1,66	0,472

The minimal sizes of RC have dihydroxybenzenes, it is the effect of two hydroxyls, and moreover ortho-dihydroxybenzene is more similar to the phenol, it gives less dark color that confirms the assumption of the decrease of the OH-group reactivity capacity in ortho-position.

Fig. 1. RD of different phenol: 1) phenol; 2) meta- dihydroxybenzene, 3) para- dihydroxybenzene 4) meta-cresol, 5) ortho-cresol, 6) para-cresol, 7) ortho-tret-butylphenol, 8) ortho- dihydroxybenzene; 1,5£С£1,9 g/l

RC of para- and meta-cresol are slightly different from each other, but the ortho-cresol and ortho-tret-butylphenol, do provide the palest coloring of the solutions that can be easily explained by the steric effect of the substituent.

Thus, the color reactions of phenols because of the difference in a structure do not lead to identical, but to various parameters of chromaticity, and the coefficient characterizes these differences quantitatively.

In the examined comparison the fractality D, due to their simple configuration of comparable figures, does not let to identify the geometry peculiarities of RC and is not suitable as a as a generalizing identification indicator.

Photometric and colorimetric methods of analysis of the solutions, not differing in stages of sample preparation, differ only in the way recording the response. In the first case the optical density of absorbed light is detected with the photocolorimeter, and in the second – on FS or DC, total color distinction of the samples in unities accepted in a particular color model. Having a database of standard color samples, it is possible to exclude the subjectivity of a color evaluation, which is characteristic for visual examinations. For the analysis of color parameters both transparent and opaque samples, in a solid or liquid state are suitable.

Thus, the developed colorimetric method for determining the concentration of phenol in the aqueous solution has several advantages compared with photocolorimetry and HPLC, to be exact, at comparable precision of the definitions it is realized using less expensive equipment, it allows analyzing more concentrated solutions, is more informative than photocolorimetry, due to the increasing number of the registered analytical signals and demand smaller number of operations to carry out. In addition, digital colorimetry is implemented using low-cost digital devices and standard software.

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