The term naphtha collectively describes a family of volatile liquid hydrocarbon mixtures. Naphtha is a major component of gasoline and other fuel types; it is primarily produced from the distillation of crude oil, and it can also be produced from coal tar and extracted from wood. Naphtha is often used as a solvent for petrochemical feedstocks, as well as for the production of natural gas. Chlorides are frequently occurring contaminants in naphtha and crude oil, often exceeding concentrations of 3 ppm (mg/L). While some chlorides may be naturally found in crude oil, they can also be introduced during cleaning processes.
Chlorides can be present as organic chlorides, organic compounds with a carbon–chloride bond, or undesaltable chlorides, which are organic compounds that cannot be removed during a salt separation process. High chloride levels are often reported in refining equipment, and are undesirable as they can cause scaling and ultimately damage to the equipment. During the oil refining process, sulfur must be removed to reduce sulfur dioxide emissions, as the gas has harmful effects on humans and the environment. The process to remove sulfur, along with nitrogen, oxygen, and trace metals is called hydrotreatment.
Hydrotreatment of naphtha and other fuels involves a reaction between hydrogen and the sulfur present in hydrocarbons that effectively removes sulfur. However, during the hydrotreatment process, chlorides that are present in naphtha and crude oil can lead to the formation of hydrochloric acid, which can then corrode equipment. To minimize corrosion and scaling, chloride levels should be kept below 1 ppm. Chlorides can be detected and monitored in naphtha and crude oil through a reduction with sodium biphenyl and subsequent potentiometric titration.
An oil and natural gas supplier was looking to determine the concentration of chloride in naphtha. The supplier wanted to follow the Standard Test Method ASTM D4929-99 for the determination of chloride in crude oil; this method approves the use of a silver-silver chloride billet-type electrode. For this application, Hanna Instruments recommended the Automatic Potentiometric (pH/mV/ISE) Titration System - HI902C with the Silver ORP Half-Cell Electrode - HI5110B. The HI5110B has a silver ring as part of the indicating electrode. This silver ring is also known as a “silver-billet.”
Since the method uses organic solvents as part of the sample preparation, it is necessary to have chemically compatible materials including glass body electrodes. For the reference the double junction glass body HI5311 Reference Half-cell is recommended.
For the overhead stirrer it is recommended to use the PVDF Propellers for Titrator Overhead Stirrer - HI900303 in place of the supplied Propellers for Titrator Overhead Stirrer - HI900302. The titration is then performed with a known volume of a prepared naphtha sample that is titrated potentiometrically with a standard solution of silver nitrate. This titration results in a precipitation reaction of silver chloride from the silver nitrate titrant and the chloride present in the naphtha. The method also requires a blank titration to be performed, which is used to determine the exact volume of titrant that is consumed by the reagents, excepting the naphtha sample. Electrode preparation is a critical step in this method.
It is necessary to coat the silver billet before performing both the blank and sample titrations, as it is the sensing element for the reaction. Coating the silver billet prevents any imperfections in the silver-halide coating, which can lead to erratic ORP readings. Prior to analysis, the electrode is filled with HI7082 3.5M KCl electrolyte fill solution for double-junction electrodes. The silver billet electrode is first soaked in sodium hypochlorite, rinsed with deionized water, and polished with fine grit sandpaper. The electrode is then placed in a saturated potassium chloride solution and an alternating voltage is applied for intervals of several seconds. The reference electrode is then emptied of the aqueous KCl electrolyte and filled instead with aqueous HI7072 Electrolyte Fill Solution, 1M KNO₃. The customer was pleased with the support provided by Hanna Instruments in providing an affordable way to automate the process of determining the chloride concentration in naphtha. The customer appreciated that the titrator produced quick, accurate, and reliable results.
