Spectrophotometric Determination of Carbon Disulphide in the Workplace

is papre describes a simple method of carbon disulphide determination in the air of working environment in the chemical companies and plants aer its absorption into aprotic N,N-dimethylformamide solvent. Carbon disulphide absorbed into aprotic solvent was transformed by using ammonium hydroxide on sulphides which were determined by spectrophotometry. 5,5Dithiobis(2-nitrobenzoic acid) and blue tetrazolium chloride were used as chromogenic sensing reagents. Colour-reducing products were measured at the wavelength of 500, respectively 520 nm. Detection limits for determination of carbon disulphide in the air are 0.2, respectively 0.4mg⋅m.


Introduction
Carbon disulphide (CS 2 ) is a strong neurotic poison with broad toxic in�uence on the human organism [1].It belongs among important industrial chemical compounds.Enormous amount of CS 2 is produced annually for synthetic �bres, rubber-making chemical compounds, or pesticides.CS 2 is commonly present in oil industry.Signi�cant part of CS 2 is released into the working environment and contaminates it.Determination of CS 2 in the working environment of chemical companies and plants is therefore one of the basic conditions for the health protection of workers and population.
Recently series of strongly powerful instrumental techniques have been applied in analytics using chromatographic separation methods [2][3][4].Additionally traditional methods based on coloured reactions keep their relevancy.e oldest colorimetric and spectrophotometric methods are based on the reaction of CS 2 with alcohols transformed on yellow copper xanthates [5].Coloured dithiocarbamates have the most important relevance, which results from the reaction of CS 2 with the prime and secondary amines and usually with ion of copper and nickel [6][7][8][9].Other methods were described as well.One of the selection methods is based on the reaction of CS 2 , phloroglucinol, and nickel with production of the red salt of 2,4,6-trihydroxydithiobenzoic acid [10].Reactions with thiols were used for spectrophotometry determination of CS 2 , for instance, benzyl thiol, which in the presence of potassium hydroxide reacts on yellow potassium benzyl-trithiocarbonate [11].Spectrophotometric kinetic iodide-azide method was also used for the sensitive determination of carbon disulphide [12].Indirect determination of CS 2 with reagent of bis[4(4 � -nitrophenyl)azo-2nitrophenyl] disulphide is also known [13].
Determination methods of CS 2 in the air are based mostly on its consumption of solid sorbent [14] or amine solutions, for instance, triethanolamine [15].Based on previous experiments [16], a determination method of CS 2 in the air was proposed and based on absorption of CS 2 in aprotic solvent and its transformation to sulphides.Determination of sulphides was performed using 5,5-dithiobis(2-nitrobenzoic) acid or blue tetrazolium chloride, which are reduced to typical colouring products.
Two analytical reagents (A and B) were prepared.
Analytical reagent A was prepared in the 100 mL volumetric �ask by weighing 0.01 g of DTNB, then 0.2 mL ammonium hydroxide (28%) was added using pipette, and DMFA was added up to the mark.
Analytical reagent B was prepared by solving 0.1 BTC in the mixture which contained 2 mL of ammonium hydroxide (28%) and 2 mL anhydrous ethanol and DMFA was added to the volume of 100 mL.
Reagents must be fresh (8 hours).Under alkaline conditions, DTNB or BTC is already transformed into the colour product without any reactant.
Measurements of absorbance were done on UV-VIS spectrophotometer Helios Alpha (ermo Electron).e pump AirCheck 2000 (S�C) with �ow meter was used for the air sampling.

Construction of Calibration Graph.
Working solution was prepared by dissolving 0.1 mL CS 2 in DMFA to a total volume of 100 mL.Cleanliness of CS 2 was controlled by iodine-metric titration [17].4 mL of analytic reagent was measured off into a set of test tubes; using pipette 0 up to 100 L base solution of CS 2 was added and levelled with DMFA up to 20 mL.Absorbance values were measured against pure DMFA aer 30 minutes of reaction time.

Determination of Carbon
Disulphide in the Air.Steam-air mixture with CS 2 prepared in testing chamber was absorbed into 15 mL of DMFA in the washing apparatus.Speed of air �ow was 200 mL⋅min −1 .Aer sampling, 4 mL of analytical reagent (A or B) was added to the absorbed substance and levelled with DMFA to 20 mL.Absorbance values were measured against pure DMFA aer 30 minutes of reaction time.
Concentration of CS 2 in the air ( air ) was calculated according the following formula: where  abs is the concentration of CS 2 in the absorbed substance (g⋅mL −1 ),  abs is the volume of absorbed substance (mL), and  air is the volume of air sample (L).

Results and Discussion
3.1.Reaction Mechanism.In the presence of ammonium hydroxide, CS 2 reacts with ammonium dithiocarbamate, which with excess ammonium hydroxide is transformed into ammonium sulphide and ammonium thiocyanate.Sulphides reduced DTNB or BTC to coloured products suitable for spectrophotometric measurements.e course of chemical reaction for the case of DTNB reagent is displayed in Figure 1.Chemical structure of coloured product of reduction of BTC is presented in Figure 2. Value of  max 500 nm, respectively 520 nm, was read from the course of dependence of absorbance on wavelength (Figure 3).Aprotic solvent in�uences intensi�cation of colouring of thiol (reaction product of sulphides with DTNB) from yellow (ethanol,  max = 412 nm) into red.Shi of  max was not observed with violet-coloured product during reaction with BTC.

Reaction Conditions.
Ammonium hydroxide was used for transformation of CS 2 to sulphides, but other compounds with ammonium ions (e.g., ammonium acetate) are suitable also.Reaction is progressing in the signi�cant surplus of ammonium ions.Dependence of absorbance on concentration of ammonium ions, described as ratio of molar concentration of NH 4 + and reagent (DTNB, BTC), is shown in Figure 4.
Transformation of CS 2 to sulphides is progressing very quickly and with a good yield in aprotic solvents.is report describes application of DMFA, but it is also possible to use dimethyl sulfoxide (DMSO).Since DMSO solidi�es at the temperature of 18 ∘ C, in the �eld and unfavourable climate conditions it is advantageous to use the mixture of DMFA/DMSO (1 : 1).
Stability of reaction balance is achieved for both methods within 30 minutes.e start of colouring is quicker with the DTNB method.One-half of the maximal absorbance at wavelength  max was achieved aer 2 minutes and with the BTC method aer 9 minutes.
One of the limiting factors of CS 2 determination in the air is the effectiveness of its absorption in aprotic solvent.Effectiveness of absorption is mainly dependent on the speed of air �ow during sampling.It is valid that the lower the speed of air �ow, the higher the effectiveness of absorption.In practice the optimal time of the air sampling must also be taken into account, as not to exceed the acceptable limit.Air sampling with the speed of air �ow 200 mL⋅min −1 for 50 minutes seems to be optimal.It corresponds with air sampling with the total volume of 10 L.

Analytical Characteristics.
Basic parameters of determination of CS 2 in the absorbed substance (molar absorptivity, detection limit, determination limit, Sandell's sensitivity, and parameters of linear regression) are presented in Table 1.Sandell's sensitivity was calculated for  = .Detection limit was calculated from the formula 3 and determination limit from the ratio 1, where  is a standard deviation of blind attempt and  is the slope of calibration line.
Calibration line (Figure 5) was constructed on the basis of absorbance dependence on CS 2 concentration in the air.Detection limit of CS 2 in the air by the proposed procedure is 0.2, respectively 0.4 mg⋅m −3 (calculated according to the  formula  air ).In practice, with this procedure it is possible to determine CS 2 (without dissolving the absorption solution) up to the concentration of 4.8 mg⋅m −3 .It is fully acceptable for the requirements of hygienic control of the air in the working environment (e.g., in the Czech Republic PEL = 10 mg⋅m −3 , NPK-P = 20 mg⋅m −3 ).Correctness of determination was veri�ed by the method �inserted-found�, (see Table 2).Determination error is not higher than 20%.

Effect of Foreign Components.
Reducing components present in the controlled air disturb determination of CS 2 by the proposed method.Hydrogen sulphide reacts mostly with DTNB or BTC, which can originate for instance like a byproduct of synthesis of CS 2 with methane and sulphur.
To eliminate hydrogen sulphide, we can use �ltration insert produced by impregnation of �ltration paper or �ltration materials from glass �bres by the solution of salts from certain heavy metals, for instance, lead, copper, or bismuth.is �ltration insert is placed in the form of tube in front of the container with absorptive liquid.Sulphur dioxide (a result of pyrolysis of CS 2 ) and volatile aliphatic triplets have signi�cant disturbing in�uence.�verview of the selected the disturbing in�uences, which with DTNB and BTC produce the coloured reduction products, is in Table 3.

Conclusion
e proposed method allows spectrophotometric determination of CS 2 in the workplace air of chemical companies and plants.is method is based on transfer of CS 2 with the help of ammonium ions to sulphides, which react further with DTNB or BTC to the characteristically coloured reaction products.Reactions are in progress in the medium of aprotic solvent, with advantage in DMFA.e method is simple and easy to instrumentation and technique.e sensitivity method, detection limits, and the range of use are comparable with other spectrophotometric methods described in the literature [9,14,15].e advantage of the proposed analytical process (based on transfer of CS 2 to sulphides) is in the opening of new development possibilities of colorimetric and spectrophotometric methods for CS 2 determination.Interconnection of CS 2 and sulphides analysis allows using and modifying a T 3: Concentration limit of foreign components which with reagents produce coloured products.

Component
Concentration limit, mg⋅m
Concentration of CS 2 in air (mg⋅m −3 ) F 5: Calibration curves obtained for the determination of CS 2 in the air by the DTNB method (1) and BTC method (2).T 1: Base parameters of CS 2 determination (in the absorbed substance).