Sulfate precipitation titration

Sulfate in soil on Mars, A barium sulfate precipitation titration described at the opening of this chapter is shown in the figure. The initial concentration of Cl" before adding BaCl2 was 0.000 19 M in 25 mL of aqueous extract of Martian soil. At the end point, when there is a sudden rise in Ba, [cr] = 0.009 6 M. 

Barium sulfate precipitation titration from Phoenix Mars Lander. [Data from Reference 1, courtesy S. Kounaves, Tufts University.)... 

An alloy containing only cadmium and zinc is claimed by the manufacturer to contain 63-percent cadmium by weight. A 356-mg sample of the alloy is dissolved in nitric acid, the solution buffered, and both ZnS and CdS are precipitated by treatment with H2S. The mixed precipitate is dissolved in a solution containing 5 millimoles of ferric sulfate, precipitating sulfur. The filtrate is acidified and the divalent iron is titrated with standard KMn04, 1.60 millimoles of the latter being consumed. How nearly correct is the manufacturer s estimate ... 

Sulfate can be determined by titrating with barium ion, to precipitate BaS04. As in the gravimetric determination of sulfate by precipitation of barium sulfate, this titration is subject to errors by coprecipitation. Cations such as K", Na", and NHU" " (especially the first) coprecipitate as sulfates ... 

The j8-form, unlike the a-form, is only sUghtly soluble in sulfuric acid solution. The mixture containing the dense white europium(II) sulfate is cooled and filtered. The precipitate is washed with dilute hydrochloric acid and then with a few nuUiliters of methyl alcohol containing hydrogen chloride. An atmosphere of carbon dioxide is not required during the filtration. The final product is dried in air at 75°. Yield 4.4 g. (90 per cent). Anal. 99.7 per cent purity (acid iron(III) sulfate-permanganate titration). 

Professor Sam Kounaves and his students at Ihfts University were filled with excitement as their Wet Chemistry Laboratory experiment aboard the Phoenix Mars Lander returned a stream of information about the composition of Martian soil scooped up hy a robotic arm. The arm delivered 1 gram of soil through a sieve into a lieakei fitted with a suite of electrochemical sensors described in Chapter 15. Aqueous solution added to the beaker leached soluble salts from the soil while sensors measured ions appearing in the liquid. Unlike other ions, sulfate was measured hy a precipitation titration with Ba ... 

The study of nonaqueous titrations covers a very broad field. It can include acid-base titrations, oxidation-reduction titrations, precipitation-titrations, and titrations involving complex formation. In order to limit things somewhat, I would like to discuss only nonaqueous acid-base titrations. However, I do not mean to imply that other types of titrations carried out in nonaqueous media are of lesser importance. For example, some excellent oxidation-reduction titrations have been carried out by Dr. Stone of Michigan State and as an example of a precipitation titration, we developed a method several years ago for titrating sulfate with barium that is carried out at least partly in nonaqueous solution. Other examples could be given but to mention all of them would lead to an excessively prolonged discussion. 

Indicators for silver-halide precipitation titrations are of two types. The first react specifically when an excess of titrant becomes present immediately after the end point - for example, if a small amount of potassium chromate is added, it will react with excess silver ions to produce deep red silver chromate in neutral solutions (Mohi s method). In acid solutions, the silver is titrated with potassium thiocyanate (KCNS) solution (Volhard s method). Iron (III) ammonium sulfate solution is added and reacts with an excesss of thiocyanate to produce a deep red iron thiocyanate species. 

Note 13—It is helpful to match end point colors with solutions saved from prior standardization titrations performed within the last IS min and well stirred to prevent drop-out of the colored barium sulfate precipitate. People having a low r -green color sensitivity find that using the blue light of Method D 1S6, sharpens the end point very considerably. 

It is convenient to titrate the suspended barium salt with 18 N sulfuric acid (approx. 12-14 ml.) to a pH. of 1.5 using a pH meter. After removal of the barium sulfate the slight excess of sulfate ion may be precipitated using barium chloride solution. The end point is taken when several drops of filtrate show no turbidity either upon addition of sulfuric acid or barium chloride solution. 

Lebel [224] has described an automated chelometric method for the determination of sulfate in seawater. This method utilises the potentiometric end-point method for back titration of excess barium against EDTA following precipitation of sulfate as barium sulfate. An amalgamated silver electrode was used in conjunction with a calomel reference electrode in an automatic titration assembly consisting of a 2.5 ml autoburette and a pH meter coupled to a recorder. Recovery of added sulfate was between 99 and 101%, and standard deviations of successive analyses were less than 0.5 of the mean. 

EDTA titrations are routinely used to determine water hardness in a laboratory. Raw well water samples can have a significant quantity of dissolved minerals that contribute to a variety of problems associated with the use of such water. These minerals consist chiefly of calcium and magnesium carbonates, sulfates, etc. The problems that arise are mostly a result of heating or boiling the water over a period of time such that the water is evaporated, and the calcium and magnesium salts become concentrated and precipitate in the form of a scale on the walls of the container, hence the term hardness. This kind of problem is evident in boilers, domestic and commercial water heaters, humidifiers, tea kettles, and the like. 

The normality or molarity of the acid can be determined by titration with a standard solution of sodium hydroxide using a color indicator, or by potentio-metric titration using a pH meter or a millivoltmeter. The sulfate anion in dilute acid can be measured by precipitation with barium chloride or by ion chromatography. 

An acidic solution of tellurium (IV) or tellurium (VI) is treated with sulfur dioxide and hydrazine hydrochloride. Tellurium precipitated from solution can be estimated by gravimetry. Selenium interferes with this test. A volumetric test involves converting tellurium to tellurous acid and oxidizing the acid with excess ceric sulfate in hot sulfuric acid in the presence of Cr3+ ion as catalyst. The excess ceric sulfate is measured by titration with a standard solution of ferrous ammonium sulfate. 

In their applications of A.C. oscillopolarographic titration for pharmaceutical analysis, Huang et al. reported a method for the titration of procaine hydrochloride with sodium tetraphenylborate [62]. Procaine hydrochloride was mixed with sodium tetraphenylborate in acetate buffer (pH 4.6). The precipitate was filtered off, and the unconsumed tetraphenylborate titrated with thallium sulfate by A.C. oscillo-polarography. The recovery was found to be 99.9 to 100.0%, and the coefficient of variation (n = 10) was 0.19%. The method could also be used to identify outdated samples of procaine hydrochloride injection solution, as its loss of water solubility is indicated by an incision in the titration curve. 

Nikolic et al. reported the preparation and coulometric determination of quaternary ammonium iodides of procaine and of other local anesthetics [63]. After extraction from 0.33 M NaOH, the quaternary iodide salts were prepared by precipitation with methyl iodide in ethyl ether. The quaternary iodides were then coulometrically determined with the use of a Radiometer titrator. The method used a silver cathode and anode (in electrolytes of 2 M and 0.4 M H2SO4, respectively), and a reference mercurous sulfate electrode. For drug determinations in the range of 0.12 to 0. 96 mg, the standard deviations were typically found to be 4 to 8 pg. 

Anions that precipitate with certain metal ions can be analyzed with EDTA by indirect titration. For example, sulfate can be analyzed by precipitation with excess Ba2+ at pH 1. 

The precipitation as barium sulfate requires boiling in the absence of air to remove sulfur dioxide. (The residue from the distillation procedure for sulfur dioxide may be used.) Some skill in handling the precipitate is required for accurate results. Complexometric titration after precipitation as lead sulfate (4) is possible. By using three solutions containing different amounts of barium chloride, it is possible to rapidly determine from the clouding whether a wine contains more or less than 0.7, 1.0, or 2.0 grams/liter of potassium sulfate. 

Other typical reagents generated for coulometric titrations are hydrogen and hydroxyl ions, redox reagents such as ceric, cuprous, ferrous, chromate, ferric, manganic, stannous, and titanous ions, precipitation reagents such as silver, mercurous, mercuric, and sulfate ions, and complex-formation reagents such as cyanide ion and EDTA [8-10]. 

A solution suspected of containing lead(II) nitrate is titrated with 0.035 M Na2S04. How much Na2S04 solution is required to produce a precipitate of PbS04 from 1 L of a saturated lead(II) sulfate solution The solution is saturated at 1.3 x 10-4 M PbS04 ... 

Issa et al. [9] used various metal ions for the volumetric determination of mefenamic acid. Mefenamic acid was precipitated from its neutral alcoholic solution by a standard solution of either silver nitrate, mercurous acetate, or potassium aluminum sulfate. In the argentimetric procedure, residual Ag(I) was titrated with standard NH4SCN. With Hg(OAc)2 or potash alum, the residual metal was determined by adding EDTA and conducting back titration of excess of EDTA with standard Pb(N03)2 using xylenol orange indicator. The applied methods were used for the determination in bulk drug substance, and in its formulations. 

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