Mercaptans titration

Hence the slope of the plot of log[S] against log[M] must equal Cs-The concentration [S] of unreacted mercaptan regulator may be determined by amperometric titration with silver nitrate,or the combined sulfur in the polymer may be determined radiochemically using mercaptan containing S, and the unreacted mercaptan concentration [S] calculated by difference. Transfer constants for carbon... 

The water (moisture) content can rapidly and accurately be determined in polymers such as PBT, PA6, PA4.6 and PC via coulometric titration, with detection limits of some 20 ppm. Water produced during heating of PET was determined by Karl Fischer titration [536]. The method can be used for determining very small quantities of water (10p,g-15mg). Certified water standards are available. Karl Fischer titrations are not universal. The method is not applicable in the presence of H2S, mercaptans, sulfides or appreciable amounts of hydroperoxides, and to any compound or mixture which partially reacts under the conditions of the test, to produce water [31]. Compounds that consume or release iodine under the analysis conditions interfere with the determination. 

Electrodes of the first kind have only limited application to titration in non-aqueous media a well-known example is the use of a silver electrode in the determination of sulphides and/or mercaptans in petroleum products by titration in methanol-benzene (1 1) with methanolic silver nitrate as titrant. As an indicator electrode of the second kind the antimony pH electrode (or antimony/antimony trioxide electrode) may be mentioned its standard potential value depends on proton solvation in the titration medium chosen cf., the equilibrium reaction on p. 46). 

Die Ubertragungskonstante kann also dadurch bestimmt werden, daC man den Verbrauch an Ubertrager und an Monomerem wahrend der Polymerisation verfolgt. Im Falle der Mercaptane kann die jeweils nodi in der Losung befindliche Konzentration an tlber-trager z. B. durch amperometrische Titration (mit Silbemitrat) bestimmt werden. 

Before being charged to an alkylation unit, olefin streams are normally treated for removal of sulfur compounds. The type of treating used depends for the most part on the amount and kind of sulfur compounds present. Quantities are normally reported in parts per million units. According to Industry sources, estimates of the amount of sulfuric acid diluted lie in a range of 15 to 60 pounds per pound of sulfur in the feed. A very commonly used factor is 20 pounds per pound. In the case of methyl mercaptan, if one assumes a mole for mole reaction occurs and that the reaction products are soluble in the acid and do not titrate, then the calculated amount of acid diluted is 53.7 pounds per pound of mercaptan sulfur (dilution range 98.5 to 90.0%). This is a very severe assumption however commercial tests indicate about 45 pounds per pound and this is probably more realistic. Somewhat lower dilution rates are normally assumed for H2S and COS, typically ranging between 15 and 18 pounds per pound of sulfur present. 

Many samples have redox potentials such fiiat fiiey can be oxidized by iodine. Therefore, file iodine in file titrant may be consumed by readily oxidizable samples fiiat will give a false high value for file water content. Some common substances fiiat can be oxidized by iodine are ascorbic acid, arsenite (As02 ), arsenate (As04 ), boric acid, tetraborate (3407 ), carbonate (COs ), disulfite (8205 ), iron(ll) salts, hydrazine derivatives, hydroxides (OH ), bicarbonates (HCOs"), copper(l) salts, mercaptans (RSH), nitrite (N02 ), some metal oxides, peroxides, selenite (SeOs "), silanols (RsSiOH), sulfite (SOs ), tellurite (TeOs ), fiiiosulfate (8203 ), and tin(ll) salts. For situations such as fiiese where file material under analysis reacts wifii iodine, an oven can be used to liberate fiie moisture from file sample, which is fiieii carried into file reaction vessel and titrated wifiiout interference. 

Anodically generated silver ions can be used for precipitation titrations of various substances. A cell constructed from a piece of heavy silver wire can be used. Substances precipitated include Cr, Br , D, and mercaptans. Similar applications using mercury(I)... 

V(OH).f, fatty acids, mercaptans Electroanalytical Direct titration with Ag ... 

Pilot test the apparatus consist of a fixed-bed reactor containing 100 g of 1 mm catalyst particles. Hydrocarbon feed (carbureactor containing 125 ppm RSH) and air (6 bar) are passed trough the reactor with a space velocity of 1.2 h at 313 K. The residual mercaptan content was also determined using the same silver nitrate potentiometric titration method [23]. 

As an addition to the test for mercaptan sulfur by potentiometric titration (ASTM D-3227, IP 342), a piece of mechanically cleaned copper is also used to determine the amount of corrosive sulfur in a sample (ASTM... 

Other test methods for the determination of corrosive sulfur (specifically hydrogen sulfide) are also available (ASTM D-6021, ASTM D-5705), which have been developed for residual fuel oil but can be applied to medicinal oil. Another test method (ASTM D-3227, IP 324) is used for the determination of mercaptan (R-SH) sulfur in petroleum products up to and including distillate fuels. However, there is no reason that the test could not be applied to mineral oil, with the use of any necessary modifications for solubility of the mineral oil in the alcoholic solution used for the titration. 

As mentioned previously, the conductometric method has few apphcations in food analysis. During the last year, CA Selects, Analytical Electrochemistry section, hsted only three papers on conductometric titrations, one of them on a PC-controUed automatic operation. In most cases, the apphcation involves the titration of acids or bases, although some apphcation applies to the determination of the halides, sulphides and mercaptans. 

Some typical important industrial applications of coulometry include the continuous monitoring of mercaptan concentration in the materials used in rubber manufacture. The sample continuously reacts with bromine, which is reduced to bromide. A third electrode measures the potential of B12 vs. Br and, based on the measurement, automatically regulates the coulometric generation of the bromine. Coulometry is used in commercial instruments for the continuous analysis and process control of the production of chlorinated hydrocarbons. The chlorinated hydrocarbons are passed through a hot furnace, which converts the organic chloride to HCl. The latter is dissolved in water and the Cl titrated with Ag" ". The Ag" " is generated by coulometry from a sUver electrode, Ag°. It is necessary for the sample flow rate to be constant at all times. Integration of the coulometric current needed to oxidize the silver to silver ion results in a measurement of the Cl concentration. 

The Karl Fisher method is recommended for general use in solvents to determine the water eontent. It is not suitable if mercaptans, peroxides, or appreciable quantities of aldehydes and amines are present. Water in halogenated solvents may cause corrosion, spotting, reduce shelf-life of aerosols, or inhibit chemical reactions, thus special method, also based on the Karl Fischer titration, was developed for halogenated solvents. ... 

Other example is the determination of fuel parameters including the diene value and mercaptan sulfur. First, the robot weighs the sample, refluxes it in the presence of maleic anhydride, and extracts the analytes into an aqueous phase, which is poured into the titration vessel, where the robot inserts the photometric or potentiometric probe. For the determination of mercaptan sulfur, the robot also weighs the sample and removes sulfide by precipitation with a CdS04 solution and liquid-liquid extraction. Once the fuel is sulfide free, which is checked by using a photometric probe, it is poured into the titration vessel by the robot arm, which also plunges an Ag electrode prior to addition of the titrant (an AgNOs solution). In both cases, the automated titrator acts as a module of the robotic station and is operated by the robotic arm. 

The potentiometric detection of the endpoint of precipitation titrations is very often used because not many visual indicators are available, in particular when mixtures of analytes are titrated. Halides, cyanide, sulfide, chromate, mercaptans, and thiols can be titrated with silver nitrate, using the silver sulfide-based ISE. Also complex mixtures, such as sulfide, thiocyanide, and chloride ions, or chloride, bromide, and iodide ions, can be titrated potentio-metrically with silver(I) ions. When the solubility of a compound formed during titration is too high, nonaqueous or mixed solvents are used, for example,... 

Traditional procedures for quantification of total cyclopropene acid content have been reviewed by Christie (1970) and include titration with hydro-bromic acid and GC of the methyl mercaptan derivatives, products from reaction with silver nitrate/methanol and from methanethiol addition. However, a colorimetric test involving reaction with sulphur/carbon disulphide (the Halphen test) was found to be one of the most reliable methods for determining the small levels of cyclopropene acids present in oils containing cottonseed oil (Coleman, 1970). Another approach is GC of the relatively stable cyclopropane acids after hydrogenation with hydrazine (Conway, Ratnayake and Ackman, 1985)... 

There have since been many similar instruments, largely dedicated to measurement of mercaptans and other sufur compounds by reaction with generated bromine. An early example was the Titrilog, manufactured by Consolidated Electrodynamics Corporation in the 1950s. Others, produced by European or Japanese firms, are intended for the determination of SO2 or CO2 from combusion analyzers (47). Stock (48) has summarized the development and present status of continuous "coulometric" titrators. 

With mercaptans and other compounds bearing isolated thiol groups, the situation is relatively simple practically any of the reagents named can be used, the most frequently used being silver nitrate. Accurate results can be obtained with pure solutions when both the titrated solution and the standard solution of the titrant are deaerated. In an ammoniacal medium, the determination is not influenced by chloride, sulphate, phosphate and small amounts of bromide but iodide, cyanide, heavy metals such as zinc. 

For the determination of primary and tertiary mercaptans two aliquot portions are titrated, one argentometrically and the other iodometrically. In the first titration the total amount of mercaptans present is determined, as all react in the ratio 1 1. When titrated with a standard iodine solution, primary mercaptans react with 0-5 mole, and tertiary with 1-0 mole of iodine. To prevent the formation of disulphides, an excess of a lead salt is added during the iodometric titration. 

The disulphides, in the determination of disulphides in the presence of mercaptans,< ) are reduced first to mercaptans with zinc amalgam. The titration before and after reduction gives the quantity of mercaptans and the sum of mercaptans and disulphides respectively. For the determination of traces of disulphides in mercaptans, the mercaptans are separated before reduction as silver mercaptans. The methods were used in polymer< > and petroleum< > chemistry. 

---