Spectrophotometric microdetermination of sulfate

Abstract

A spectrophotometric method has been developed for the selective determination of sulfate in the 0-10 and 0-100 microgram (ppm in water) ranges. The aqueous samples are treated with a mixture of hydriodic acid, acetic anhydride, and sodium hypophosphite and heated in a modified countercurrent reaction apparatus to evolve hydrogen sulfide. This gas is swept by nitrogen into a buffered solution of ferric ion and 1,lO-phenanthroline, where it reduces ferric ion to ferrous ion. The bright orange tris(1,lO-phenanthroline)iron(ll) complex is formed and is measured spectrophotometrically at 510 nm. Of over 20 common ions, only nitrite and those ions capable of yielding hydrogen sulfide under the same conditions interfered. Sulfonated surfactants did not interfere. SULFATE REDUCTION to hydrogen sulfide is a very attractive basis for a simple, selective, and sensitive method for the determination of this extremely important anion. Use of this one step eliminates the shortcomings of barium sulfate and related precipitation methods. Hydrogen sulfide is a gas, so its quantitative transfer presents no problems. It is highly reactive, so its measurement is also easy. Determination of small amounts of sulfate thus need be no more difficult than the determination of small amounts of nitrogen by the Kjel-dah1 method.Sulfate reduction, however, is a problem because the reaction is highly irreversible. Thermodynamically, sulfate should be reduced in a Jones reductor. In reality, few reagents are known which can satisfactorily accomplish the task, and reduction has best been carried out by the use of hydriodic acid under drastic conditions. It has been found in this work that a mixture of hydriodic acid and acetic anhydride containing a small amount of sodium hypophos-

The Analysis of Airborne Sulfate

“…Wet chemical reduction of all sulfur compounds in a sample and determination of the resultant H2S was shown by Davis and Lindstrom 13 to be a sensitive analytical method of potential application to aerosol samples. Sulfate and other sulfur compounds were reduced by a mixture of hypophosphorus acid and hydriodic acid, the evolved H2S collected in ferric solution and determined spectrophotometrically as the ferrous-1,10-phenanthroline complex.…”

A simple and reliable method reducing sulfate to sulfide for multiple sulfur isotope analysis

“…In the reducing solution of Thode et al, high concentrations of HI seem to be the most important component of the reducing agent for complete sulfate reduction, and the presence of H 3 PO 2 or NaH 2 PO 2 increases the reduction speed by maintaining a high hydroiodic acid to iodine ratio which is one of the factors favoring the reduction . HCl is only of secondary importance and its presence is suggested to increase the acidity and volume, and reduce the use of relatively expensive HI .…”

“…HCl is only of secondary importance and its presence is suggested to increase the acidity and volume, and reduce the use of relatively expensive HI . However, Gustafsson found the presence of water to be detrimental for the reduction because water tends to dilute and thus lower the concentration of HI, and at lower HI concentration, side products ( viz , SO 2 and elemental S) will be formed . In this regard, mixing 50% H 3 PO 2 and concentrated HCl with the concentrated HI may have drawbacks for the reduction efficiency, because both H 3 PO 2 (50%) and concentrated HCl (37%) contain more than 50% water by weight.…”

“…To avoid additional water in the reducing solution, the H 3 PO 2 can be replaced with dry NaH 2 PO 2 salt, and HCl can be omitted. Gustafsson and Davis and Lindstrom have used a reducing solution containing only HI (57%) and NaH 2 PO 2 salt, and found a good reduction yield. In particular, Davis and Lindstrom found that the optimum composition of the reducing solution for complete and fast sulfate reduction is 0.13 g NaH 2 PO 2 in 1 mL HI (57%).…”

Development and Validation of a Spectrophotometric Method to Measure Sulfate Concentrations in Mine Water without Interference

“…Many direct and indirect methods are used to measure sulfate ions in mine water, including ion chromatography (IC) (Kaksonen et al 2003;Morales et al 2000), inductively coupled plasma absorption spectrophotometry (ICP-AES) (Christensen et al 1996), turbidimetric (Elenkova et al 1980;Raghavan and Raha 1991), colorimetric (Bertolacini and Barney 1957), titrimetric (Budesinsky and KrumLova 1967), spectrophotometric (Davis and Lindstrom 1972), and gravimetric methods (APHA 1985).…”