Titration of Sodium Carbonate

Primary standard sodium carbonate may also be used to standardize acid solutions. Sodium carbonate also possesses all the qualities of a good primary standard, like KHP and THAM. When titrating sodium carbonate, carbonic acid, H2C03, is one of the products and must be decomposed with heat to push the equilibria below to completion to the right  

Heat also will eliminate C02 from the solution, which further aids in this completion push. 

The H2C03 C02 equilibrium (Equation (5.13)) can be a problem in all base solutions because C02 

FIGURE 5.11 Titration curve for Na2C03 titrated with 0.10 N HCI. 

Equation (5.13), resulting in the formation of carbonate since carbonic acid is a weak acid  

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However, in a cold solution, with phenolphthalein as an indicator, the end-point of titration of sodium carbonate with 1 N sulphuric acid is exhibited when the sodium carbonate is fully transformed into sodium carbonate, thus ... 

Figure 15-5 illustrates that two end points are observed in the titration of sodium carbonate, the second being appreciably sharper than the first. It is apparent that the individual components in mixtures of sodium carbonate and sodium hydrogen carbonate can be determined by neutralization methods. 

Explain why boiling the solution near the end point in the titration of sodium carbonate increases the sharpness of the end point. 

The phosphorodichloridate was hydrolyzed by adding to a stirred solution of sodium carbonate (253 grams) in water (2.9 liters). After 1 hour the solution was cooled and acidified with a solution of concentrated sulfuric acid (30 ml) in water (150 ml) and then extracted with a mixture of tetrahydrofuran and chloroform (2.3/1 3 x 1 liter). The tetrahydro-furan/chloroform liquors were bulked and evaporated to dryness to give a light brown oil. This was dissolved in water (1 liter) and titrated with 2N sodium hydroxide solution to a pH of 4.05 (volume required 930 ml). The aqueous solution was clarified by filtration through kieselguhr and then evaporated under reduced pressure to a syrup (737 grams). 

Titration of carbonate ion with a strong acid. A solution of sodium carbonate may be titrated to the hydrogencarbonate stage (i.e. with one mole of hydrogen ions), when the net reaction is ... 

Notes. (1) For elementary students, an approximately 0.05 M solution of sodium carbonate may be prepared by weighing out accurately about 1.3 g of pure sodium carbonate in a weighing bottle or in a small beaker, transferring it to a 250 mL graduated flask, dissolving it in water (Section 3.28), and making up to the mark. The flask is well shaken, then 25.00 mL portions are withdrawn with a pipette and titrated with the acid as described above. Individual titrations should not differ by more than 0.1 mL. Record the results as in Section 10.30. 

Discussion. The hydroxides of sodium, potassium, and barium are generally employed for the preparation of solutions of standard alkalis they are water-soluble strong bases. Solutions made from aqueous ammonia are undesirable, because they tend to lose ammonia, especially if the concentration exceeds 0.5M moreover, it is a weak base, and difficulties arise in titrations with weak acids (compare Section 10.15). Sodium hydroxide is most commonly used because of its cheapness. None of these solid hydroxides can be obtained pure, so that a standard solution cannot be prepared by dissolving a known weight in a definite volume of water. Both sodium hydroxide and potassium hydroxide are extremely hygroscopic a certain amount of alkali carbonate and water are always present. Exact results cannot be obtained in the presence of carbonate with some indicators, and it is therefore necessary to discuss methods for the preparation of carbonate-free alkali solutions. For many purposes sodium hydroxide (which contains 1-2 per cent of sodium carbonate) is sufficiently pure. 

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 ... 

Consider as an example the standardization of a solution of hydrochloric acid by titration against a weighed amount of sodium carbonate. The strength of the hydrochloric acid will be computed from... 

Figure 5.11 represents the titration curve of sodium carbonate titrated with a strong acid. Notice that there are two inflection points. This is because sodium carbonate is a dibasic base—there are two hydrogen ions accepted by the carbonate. On the way to the first inflection point, hydrogen ions are accepted by the carbonate to form bicarbonate ... 

Why does the titration curve of sodium carbonate have two inflection points Why does this titration require that the solution be boiled as you approach the second equivalence point Why can bromcresol green be used as the indicator and not phenolphthalein ... 

Elemental composition Na 43.39%, C 11.33%, O 45.29%. Aqueous solution of sodium carbonate is strongly alkahne and its normahty can be measured by acid-base titration. Sodium content can be measured by AA, ICP, and other instrumental analyses. Carbonate anion can be measured by ion chromatography or from carbon dioxide hberated when the salt is treated with dilute... 

Some acids or bases can donate or accept more than one proton, i.e. 1 mole of analyte is equivalent to more than 1 mole of titrant. If the pA"a values of any acidic or basic groups differ by more than ca 4, then the compound will have more than one inflection in its titration curve. Sodium carbonate is a salt of carbonic acid and it can accept two protons. The pKa values of carbonate and bicarbonate are sufficiently different (p/fa 10.32 and 6.38) for there to be two inflections in the titration curve. The two stages in the titration are ... 

Thiosulfate solutions should be stored in the dark. Addition of 0.1 g of sodium carbonate per liter maintains the pH in an optimum range for stability of the solution. Three drops of chloroform should also be added to each bottle of thiosulfate solution to help prevent bacterial growth. An acidic solution of thiosulfate is unstable, but the reagent can be used to titrate If in acidic solution because the reaction with triiodide is faster than Reaction 16-20. 

Quantitative Determination. — Dissolve 3 gm. of sodium carbonate in 50 cc. of water, and titrate with normal hydrochloric acid, using methyl orange as the indicator. 

Quantitative Determination and Determination of Sodium Carbonate Content — Titrate the solution of 1 gnr. of sodium hydroxide in 100 cc. of water with normal hydrochloric acid in the cold, using phenolphthalein as indicator. At least 24 cc. of normal acid should be required to destroy the red color. Now add 1 drop of methyl orange, and titrate further until the color changes to red. In the second titration, at most 0.3 cc. of acid should be used (3.18 per cent of Na2C03). ... 

Permanent hardness can also be estimated by the alkalimetric method of Wartha and Pfeifer. A measured volume (200 e.e.) of the water is boiled with 50 c.e. of a mixture of decmormal solutions of sodium carbonate and hydroxide in equal amounts after restoring to the original volume and allowing the solution to settle, the residual alkali is determined by titration with standard acid. As the bicarbonates do not cause any consumption of alkali, there is a direct proportionality between the quantity of alkali which disappears and the total amount of sulphates and chlorides of calcium and magnesium. Sodium carbonate alone does not efficiently precipitate magnesium salts from solution, but precipitation as the hydroxide is complete if excess of sodium hydroxide is present it is for this reasoii that a mixture of sodium carbonate and hydroxide is applied 3 (see also p. 211). 

Determination of Sodium Carbonate Add 2 drops of methyl orange TS to the solution retained from Determination of Alkalinity (above), and continue the titration with 1 A hydrochloric acid to a permanent pink color. Calculate the percentage of Na2C03 by the formula... 

Sodium Thiosulfate, 0.1 N (15.81 g Na2S203 per 1000 mL) Dissolve about 26 g of sodium thiosulfate (Na2S203-5H20) and 200 mg of sodium carbonate (Na2C03) in 1000 mL of recently boiled and cooled water. Standardize the solution as follows Weigh accurately about 210 mg of primary standard potassium dichromate, previously pulverized and dried at 120° for 4 h, and dissolve in 100 mL of water in a 500-mL glass-stoppered flask. Swirl to dissolve the sample, remove the stopper, and quickly add 2 g of sodium bicarbonate, 3 g of potassium iodide, and 5 mL of hydrochloric acid. Stopper the flask, swirl to mix, and let stand in the dark for 10 min. Rinse the stopper and inner walls of the flask with water, and titrate the liberated iodine with the sodium thiosulfate solution until... 

Consider an analytical method involving the titration of hydrochloric acid with anhydrous sodium carbonate to determine the concentration of the acid. The measurements made are mass (weighing out a chemical to make up a solution of known concentration) and volume (dispensing liquids with pipettes and burettes). The reaction between the two chemicals is based on amount of substance - one mole of sodium carbonate reacts with two moles of hydrochloric acid - and the mass of a mole is known (e.g. the formula weight in grams of one mole of sodium carbonate is 105.99). All the measurements are based on either length or mass and are traceable to SI units, so the method is a primary method. 

Determination of hydroxyl ion in the presence of carbonate Owing to the small second ionization constant of carbonic acid K2 = 5 x 10" ), the titration of hydroxyl ion alone is not feasible in the presence of an appreciable concentration of carbonate ion. For example, a 0.01 M solution of sodium hydroxide has a pH of about 12, and a 0.01 M solution of sodium carbonate a pH of about 11.2. Thus the pH change in a titration of 0.01 M sodium hydroxide in the presence of 0.01 M sodium carbonate would be only about 0.8 pH unit. 

The recovery of iodine from waste liquids.—E. Beilsteini2 recovered iodine from laboratory residues by evaporation to dryness with an excess of sodium carbonate and calcination until the organic matter is all oxidized. The mass is dissolved in sulphuric acid and treated with the nitrous fumes, obtained by treating starch with nitric acid, until all the iodine is precipitated. The iodine is washed in cold water, dried over sulphuric acid, and sublimed. Other oxidizing agents less unpleasant than the nitrous fumes employed by F. Beil stein—e.g. hydrogen peroxide—-were recommended by G. Torossian for the residues obtained in copper titrations. F. Beilstein s process is applicable to soluble but not to insoluble, oxidized forms of ioffine. F. D. Chattaway... 

The contents of the flask are then boiled, being taken almost to dryness and taken up with water several times (generally twice) so as to remove the hydrogen peroxide completely. The residue is diluted once more with 50 ml. water and a solution of sodium carbonate added until a slight precipitate is formed, this being then redissolved with a few drops of dilute acetic acid. Excess potassium iodide is added and the liberated iodine is titrated with a decinormal solution of sodium thiosulphate. 

An aqueous solution of sodium carbonate, Na2C03, is titrated with strong acid to a point at which two H ions have reacted with each carbonate ion. (a) If 20.0 mL of the carbonate solution reacts with just 40.0 mL of 0.50 M acid, what is the molarity of the carbonate solution (b) If the solution contains 5.0 percent by mass sodium carbonate, what is the density of the solution (c) Suppose that you wanted to prepare a liter of an identical solution by starting with crystalline sodium carbonate decahydrate, Na2CO3 10H2O, rather than with solid Na2C03 itself. How much of this substance would you need ... 

In llraidy s modification, which is stated to give more consistent results, 2 5 g. of very finely divided air-dry cotton is treated with a mixture of 5 c.c. of 10 per cent, copper sulphate (cryst.) and 95 c.c. of an almost saturatcd solution of sodium carbonate and bicarbonate g crystals and 50 g. bicarbonate made up to 1 litre). The cotton is immersed by means of a rod and the air bubbles are allowed to escape the flask is then surrounded with boiling water for exactly three hours. The contents arc filtered off on an asbestos filter and washed first with dilute sodium carbonate solution and then with water. Then the residual cuprous oxide is dissolved by treatment with a solution containing 100 g. of iron alum and 140 g. of concentrated sulphuric acid per litre. Two such treatments usually suffice. The fitter is tben washed with 2 Af-sulphuric acid the combined filtrate and washings arc titrated with A /25 potassium permanganate solution. According to Brissaud the test is affected by air. 

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