Development of a Suitable Technique of Estimating Soluble Sulphur Using Less Health Hazardous Chemical

An analytical method is a process or pathway to estimate,
determine, or solve any type of measurand or problem...


Introduction
An analytical method is a process or pathway to estimate, determine, or solve any type of measurand or problem or product [1,2]. It needs some instruments, tools, glassware, mathematical theorems to solve the problem [3]. Various national and international bodies, e.g. International Organization for Standardization (ISO), European Committee for Standardization (CEN), American Society for Testing and Materials (ASTM), Indian Standard (IS), National Bureau of Standards (NBS) are involved in this work to create standard analytical methods with respect to quality and reliability [4,5]. In rubber industry various standard methods are used for testing purpose such as characterization of incoming raw materials as well as in process materials, and finished products [6]. Such a method is ASTM D4578: "Standard Test Method for Rubber Chemicals -Determination of Percent Sulfur by Extraction [7]." At rubber product manufacturing units, Sulfur is used as curing or vulcanizing or cross-linking agent [8,9].
Two allotropes of Sulfur are used: soluble Sulfur (S 8 crown) and insoluble Sulfur (polymeric) [10]. For better performance of rubber product, insoluble Sulfur is preferred during rubber compounding [11,12]. Use of soluble Sulfur in rubber matrix leads to blooming phenomena and also makes the rubber product more prone to oxygen and ozone attacks [11,13].
It is essential to quantify the fraction of soluble Sulfur present in the total Sulfur to be used for curing purpose [14]. The S 8 allotrope of Sulfur is soluble in Carbon disulfide (CS 2 ) and Toluene (C 6 H 5 CH 3 ) [15]. In the ASTM D4578 method, one of these two solvents is used for extraction purpose, leaving behind the insoluble material.
Total amount of solvent required for extraction is 100 to 150 ml over a minimum extraction period of 8 minutes [7]. CS 2 is a colourless, highly volatile, obnoxiously odorous, flammable, toxic liquid solvent having both acute and chronic toxicity. Exposure of CS 2 having concentration of 10 ppm or more in atmosphere may lead to numbness, blurred vision, cramps, muscle fatigue, neurophysiological impairment, psychosis, keratitis, priapism, erectile dysfunction, and death by respiratory failure [16]. On the other hand for acute (short-term) and chronic (long-term) exposure of toluene toxicity, affect Central Nervous System (CNS) primarily in both humans and animals. Acutely exposed to elevated airborne levels of toluene; symptoms include fatigue, sleepiness, headaches, and nausea [17].

We have developed liquid chromatographic [High Performance
Liquid Chromatography (HPLC)] technique as a successful analytical method for the estimation of soluble sulphur content in total sulfur. Here also we have used toluene but very less quantity (5 ml) is sufficient to get repeatable and reliable result. Necessary study has been done to establish as a successful validated method

Repeatability Measurement
It is the experiment performed under a set of similar condition to evaluate how repeatable the results are. Ten replicate tests of each calibration standard samples have been analysed to study the repeatability of developed technique and results are tabulated in Table 2

Limit of Detection (LOD) and Limit of Quantification (LOQ)
Limit of detection is defined as the lowest concentration of an analyte in a sample that can be detected, though not necessarily  (Table 2) x i : i th test result of lowest calibration standard sample µ: Mean of ten replicates data of lowest calibration standard sample n: Number of replicate test performed (n = 10)

Estimation of Working Range
Working range of a method defines the interval over which the uncertainty will be acceptable. To estimate the working range, linearity of response obtained from different calibration standards tested and achieved correlation coefficient as 0.9998. The linearity study has been shown in Figure 3.

Conclusion
While estimating soluble sulphur content using HPLC as a tools, it was found that regression coefficient factor obtained during calibration is 0.9998 which signifies that the developed method is exhibit linear relationship during analysis of standard sample having concentration from 0.096 mg/ml to 1.257 mg/ml. Trueness study of standard samples indicates both the positive and negative bias obtained during the analysis and the maximum is 0.376% and -0.315% respectively. Recovery in different concentration has been analysed and found it is lying between 98.91%-101.00%.
The experiment of estimation of limit of detection and limit of quantification confirms that, through this technique, soluble sulphur having concentration 0.002 mg/ml can be detected in the sample and can be quantified above 0.006 mg/ml. Thus the above technique can be adopted as an analytical method as far as environment point of view with respect health hazardous effect and less quantity chemical use.