The apparatus determines from 10 to 600 grams/l. hydroxide and from 3 to 120 grams/l. alumina in Bayer Process streams. Essential components include a constant rate buret, a glass enclosed thermistor, and a specially designed operational amplifier circuit which by generating the first derivative' of the thermistor signal, detects the end point and stops the titration. The sample is diluted with sodium tartrate and titrated with 1 . 3 N HCI in a thermos jar. The first titration volume is equivalent to hydroxide; after the addition of potassium fluoride, the second titration volume is equivalent to alumina. The complete titration takes 4 minutes. Suspended solids do not interfere, neither do other constituents in Bayer liquors. The relative standard deviation is 0.32% for hydroxide at the 180 grams/l. level, and 0.17% for alumina at the 120 grams/l. level.Good analytical values for hydroxide and alumina are needed to efficiently operate the Bayer Process, in which hot sodium hydroxide is used to dissolve the alumina from bauxite to give a concentrated solution of alumina. Impurities such as iron oxide, titania, and sand are left behind as insolubles, commonly referred to as red mud. After filtering off the red mud, the dissolved alumina is recovered from the filtrate by allowing it to cool, and then seeding it with aluminum hydroxide. About one half of the alumina precipitates out by this procedure. The Bayer Process control strategy involves constant measurement of the concentrations of hydroxide, which in North America is traditionally ( I ) expressed as grams/l. NaZC03, and of alumina, which is expressed as grams/l. A1203. In some cases the ratio of the alumina to hydroxide concentrations is used for process control. This ratio is related to the degree of saturation of the solution; it has values of less than 0.5 for saturated solutions, and values up to 0.7 for supersaturated solutions, which are fed to the precipitators.Thermometric titrations have been used to determine caustic ( 2 ) and alumina (3) in 2-or 3-component solutions. These methods are not well suited for routine, process control analyses, since the titration must be continued beyond the equivalence point, and a strip chart recording of the titration curve must be inspected to locate the end point. Zenchelsky and Segatto ( 4 ) were the first to point out that thermometric titration curves can be differentiated. They demonstrated this by mechanical amplification of the bridge signal followed by differentiation of the amplified signal in a resistance-capacitance network. Later, a much simpler operational amplifier circuit ( 5 ) was used to generate the first and second derivatives 1 Present address, Aluminum Company of Canada, Limited, Kitimat, British Columbia, Canada.
We have found that a useful polarographic method for the determination of zirconium can be based on the findings that m-nitrobenzoic acid is polarographically reducible (1,4) and that this compound is a highly selective precipitant for zirconium (3,5). In principle, the method consists of precipitating zirconium with m-nitrobenzoic acid, dissolving the filtered and washed precipitate in hydrochloric acid, and measuring the height of the polarographic wave corresponding to the reduction of the m-nitrobenzoic acid (to the hydroxylamine stage) in a suitable supporting electrolyte. The method is accordingly an indirect one for zirconium, and depends on the specificity of m-nitrobenzoic acid as a precipitant. We have applied the method successfully to the determination of zirconi~~m in magnesium-base alloys. PROCEDURETo a 5-gm. sample of the alloy, about 25 ml. of water is added followed by sufficient 16 M nitric acid (about 35 ml., in small portions) to cause ready dissolution, except for the relatively small residue which contains the so-called "insoluble" zirconium. The mixture is filtered through paper (Whatman No. 42) and the residue washed with 2.0 M nitric acid. The filtrate and washings are caught in a 250 ml. volumetric flask, from which, after making up to the mark with 2.0 M nitric acid, one or more 25 ml. aliquot samples are taken. T o each of these is added aqueous ammonia (1 :I) to the Congo red end point, with care being taken to avoid precipitation of zirconium. There are then added 20 ml. of 2.0 M hydrochloric acid, 10 gm. of ammonium nitrate, water to give a volume of about 100 ml., and then, a t the boiling point with stirring, about 100 ml. of a hot 1% aqueous solution of m-nitrobenzoic acid (5). The mixture is boiled gently for five minutes and then filtered hot through a Gooch crucible. The precipitate is washed with hot water and then dissolved in 25 ml. of 6.0 M hydrochloric acid by placing the crucible and acid in the original beaker and warming the mixture. The crucible, after it is washed down with water, is re~noved from the beaker and the contents of the beaker are neutralized to theCongo red end point wit11 aqueous ammonia (1:l) and washed with water into a 250 ml. volumetric flask. One or more 25 ml. aliquot samples are then pipetted into 100 ml. volumetric flasks, and, to each, 50 ml. of a 0.20 M solution of potassium chloride, 10 ml. of 0.20 M hydrochloric acid (by pipette), and 1 ml. of 0.75% gelatin solution (as a ~naxinlum suppressor) are added. The solution is made up to volume with water, a portion is decanted into the polarographic cell, bubbled with purified nitrogen for 15 min., and then polarographed.The polarographic wave for the first reduction step of nz-nitrobenzoic acid For personal use only.
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