Spectrophotometric determination of small amounts of cerium in stainless steel

Abstract

The determination of 0.005 to 0.1% of cerium in stainless steel with a spectrophotometric method was described.
Absorption spectrum of cerium(IV) (600 μg/50 ml) in sulfuric acid solution was shown in Fig. 1. The absorbance of cerium(IV) was maximum at 320 nm. At this wave length, iron, manganese, chromium, molybdenum, vanadium, and niobium interfered with the measurement of cerium(IV), but the absorbance of nickel, cobalt tungsten, titanium, and calcium could be neglected.
The conditions for the oxidation cerium(III) to cerium (IV) were studied. The larger and smaller amounts of sulfuric acid decreased the absorbance of cerium (I V) (Fig. 4). The effects of silver nitrate and ammonium persulfate in the cerium (I V)-sulfuric acid solution were shown in Fig. 5 and Fig. 6. Fig. 5 showed that the absorbance of cerium (IV) increased slightly with the amounts of silver nitrate.
In order to remove the interfering elements and to obtain a higher recovery of cerium(III), co-precipitation technique was employed in the present method. Cerium fluoride was co-precipitated with a large amounts of calcium fluoride from perchloric acid solution. The efficiency of the co-precipitation was shown in Table I. Calcium fluoride increased the recovery of cerium(III) from 6080% to 95100%. However, the correct values of cerium were not obtained in the presence of the interfering elements (Table II), and it was necessary to remove large amounts of iron, chromium, manganese, nickel, vanadium, and niobium. For this purpose the solution was treated with hydrogen peroxide and then electrolyzed with mercury cathode.
The proposed procedure was as follows. Two grams of the sample is decomposed with 20 ml of aqua regia, 20 ml of perchloric acid is added to the solution, then the mixture is evaporated until the white fume evolves. The residue is dissolved in water, and a small amount of hydrogen peroxide is added to reduce iron etc. After filtration, the solution is electrolyzed with mercury cathode for about 1 hour. The electrolyzed solution is evaporated to about 50 ml, then 15 ml of perchloric acid, 20 ml of hydrofluoric acid(47%), 4 ml of calcium chloride solution (0.28%), and filter paper pulp are added to the solution, and it is diluted to 100 ml with water. The precipitate of cerium fluoride and calcium fluoride are digested on a water bath for about 20 minutes. The precipitate is collected and is ignited in a platinum crucible. The residue is dissolved in 3 ml of sulfuric acid (18 N) and 1 ml of hydrogen peroxide (15%). Cerium(III) is oxidized to cerium(IV) by adding 1 ml of silver nitrate solution (0.12%) and 0.2 g of ammonium persulfate. Cerium(IV) is determined by measuring the absorbance at 320 nm.
The time required for the analysis is about 4 hours/ sample.
The elements commonly contained in stainless steel, except large amounts of niobium, did not interfer.
Good accuracy and reproducibility were obtained as shown in Table III and Table IV.