Carbonate precipitation titration

Industrial grade NaCl has a content of 92-98%. The precipitation titration can be conducted using 0.1 N AgNO, as the titrant and 5% K,Ci<) as the indicator (the Mohr method). The sample chloride solution should be buffered with calcium carbonate to a pH between 6.3 and 7.2 in order to avoid any interference from other... 

Applications of precipitation titrations are listed in Table 6-1. The Volhard method is an argentometric titration, but the Fajans method has wider application. Because the Volhard titration is carried out in 1 M acid, it avoids some interference that affects other titrations. Silver salts of anions such as C03 (carbonate), C204 (oxalate), and AsO (arsenate) are soluble in acidic solution, so these anions do not interfere with the analysis. 

Carbonate is measured by evolution of carbon dioxide on treating the sample with sulfuric acid. The gas train should iaclude a silver acetate absorber to remove hydrogen sulfide, a magnesium perchlorate drying unit, and a CO2-absorption bulb. Sulfide is determined by distilling hydrogen sulfide from an acidified slurry of the sample iato an ammoniacal cadmium chloride solution, and titrating the precipitated cadmium sulfide iodimetrically. 

In an oversimplified way, it may be stated that acids of the volcanoes have reacted with the bases of the rocks the compositions of the ocean (which is at the fkst end pokit (pH = 8) of the titration of a strong acid with a carbonate) and the atmosphere (which with its 2 = 10 atm atm is nearly ki equdibrium with the ocean) reflect the proton balance of reaction 1. Oxidation and reduction are accompanied by proton release and proton consumption, respectively. In order to maintain charge balance, the production of electrons, e, must eventually be balanced by the production of. The redox potential of the steady-state system is given by the partial pressure of oxygen (0.2 atm). Furthermore, the dissolution of rocks and the precipitation of minerals are accompanied by consumption and release, respectively. 

This method yields only approximate results because of the precipitation of basic barium carbonate in the presence of hydroxide. More accurate results are obtained by considering the above titration as a preliminary one in order to ascertain the approximate hydroxide content, and then carrying out another titration as follows. Treat 25-50 mL of the solution with sufficient standard hydrochloric acid to neutralise most of the hydroxide, then heat and precipitate as before. Under these conditions, practically pure barium carbonate is precipitated. 

A slight excess of 10 per cent barium chloride solution is added to the hot solution to precipitate the carbonate as barium carbonate, and the excess of sodium hydroxide solution immediately determined, without filtering off the precipitate, by titration with the same standard acid phenolphthalein or thymol blue is used as indicator. If the volume of excess of sodium hydroxide solution added corresponds to timL of 1M sodium hydroxide and u mL 1M acid corresponds to the excess of the latter, then v — v = hydrogencarbonate, and V— v — v ) = carbonate. 

The method may be applied to commercial boric acid, but as this material may contain ammonium salts it is necessary to add a slight excess of sodium carbonate solution and then to boil down to half-bulk to expel ammonia. Any precipitate which separates is filtered off and washed thoroughly, then the filtrate is neutralised to methyl red, and after boiling, mannitol is added, and the solution titrated with standard 0.1M sodium hydroxide solution ... 

Note. A better blank is obtained by adding about 0.5 g of calcium carbonate before determining the correction. This gives an inert white precipitate similar to that obtained in the titration of chlorides and materially assists in matching the colour tints. 

Notes. (1) If in a similar determination, free mineral acid is present, a few drops of dilute sodium carbonate solution must be added until a faint permanent precipitate remains, and this is removed by means of a drop or two of acetic acid. The potassium iodide is then added and the titration continued. For accurate results, the solution should have a pH of 4-5.5. 

Procedure. Weigh out accurately about 2.5 g of finely powdered mercury(II) chloride, and dissolve it in 100 mL of water in a graduated flask. Shake well. Transfer 25.0 mL of the solution to a conical flask, add 25 mL water, 2mL 1M hydrochloric acid, and excess of 50 per cent phosphorous(III) acid solution. Stir thoroughly and allow to stand for 12 hours or more. Filter the precipitated mercury(I) chloride through a quantitative filter paper and wash the precipitate moderately with cold water. Transfer the precipitate with the filter paper quantitatively to a 250 mL reagent bottle, add 30 mL concentrated hydrochloric acid, 20 mL water, and 5 mL carbon tetrachloride or chloroform. Titrate the mixture with standard 0.025M potassium iodate in the usual manner (Section 11.127). 

Carbon dioxide is not a common oxidation product in periodate work, but it does appear in the oxidation of ketoses,49 a-keto acids,14,39 and a-hydroxy acids,14 39 and it is often a product23 141 of overoxidation. Carbon dioxide analyses have been carried out using the Plantefol apparatus,49 the Warburg apparatus,14 23 and the Van Slyke-Neill mano-metric apparatus,39 and by absorption in standard sodium hydroxide141 followed by back-titration with acid. A most convenient method is the very old, barium hydroxide absorption scheme.16 The carbon dioxide is swept from the reaction mixture into a saturated, filtered barium hydroxide solution by means of a stream of pure nitrogen. The precipitated barium carbonate is filtered, dried, and weighed. This method is essentially a terminal assay. The manometric methods permit kinetic measurements, but involve use of much more complicated apparatus. 

Singhal et al. [78, 79] have described a titrimetric method for the determination of low levels of Oxamyl residues in soils. Their investigations revealed that after hydrolysis Oxamyl gave a brown precipitate with carbon disulphide and an alkaline solution of copper(II) sulphate. This reaction suggested a procedure for the determination of Oxamyl by titration with ethylene diamine tetracetic acid of the copper remaining unreacted to the 1-(2 pyridylazo)-2-naphthol end-point indicator). The following stoichiometric reaction appeared to occur between Oxamyl and the reagents ... 

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. 

Titrate 0.1 M Fe" sulphate or chloride solution with M NaOH in a closed system and under N2 to pH 6.5-7 and then, while maintaining the pH by addition of M NaOH, slowly oxidize with a stream of C02-free air. Upon formation of a green precipitate (partial oxidation), separate this precipitate, still keeping it under N2 and freeze dry it (Schwertmann and Fechter, 1994 Lewis, 1997 Refait et ak, 1999). The preparation of a carbonate green rust is described by Taylor (1982) and Genin et ak (1998). 

Occasionally it is desired to determine sulphurous acid by a gravimetric method it is then usual to expel the sulphur dioxide from the solution under examination by distilling in an atmosphere of carbon dioxide and oxidise the gas to sulphuric acid by absorption in bromine water or iodine solution, subsequently adding barium chloride and weighing the precipitated barium sulphate. This method also gives accurate results volumetrieally if steps are taken to prevent loss of iodine by volatilisation in the current of carbon dioxide. The excess iodine is titrated with a solution of sodium thiosulphate.8... 

The precipitation of mercuric acetylide resulted in alkali consumption so that titrating the excess hydroxide gave a direct measure of the amount of acetylide and hence the concentration of acetylene in the gas mixture. The more usual method, precipitating silver acetylide from ammoniacal silver nitrate, was unsatisfactory in this case since the carbon monoxide in the product gases reduced the silver nitrate to silver. 

Example of (3).—0 99 gm. of Chrysophenine G (C30H26N4O8SaNa2) is dissolved in a litre of water. 100 c.cs. of this solution are withdrawn and boiled, with a current of carbon dioxide passing through 10 c.cs. of cone, hydrochloric acid and 50 c.cs. of titanous chloride are then added, and the mixture boiled until the precipitate dissolves and the solution turns a slight violet colour. After cooling, the excess of titanous chloride is titrated with ferric alum. 

For the determination, about 0-3 gram of the finely-powdered substance is treated in a beaker with 25 c.c. of water, concentrated hydrochloric add being then added drop by drop until the pigment is dissolved as a rule not more than 10 drops are required. If free arsenious anhydride is present in appreciable amount, it remains undissolved under these conditions and may be filtered off and washed. The filtrate is treated with sodium carbonate to indpient precipitation and then with a solution of 2-3 grams of sodium potassium tartrate it is next diluted to about 200 c.c., mixed with about 5 grams of solid sodium bicarbonate and titrated with iodine solution in presence of starch paste in this way the combined arsenious anhydride is determined. 

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