Ammonium thiocyanate-silver nitrate titrations

Ammonium thiocyanate-silver nitrate titrations (Volhard s Method). 

AMMONIUM THIOCYANATE-SILVER NITRATE TITRATIONS (VOLHARD S METHOD)... 

In practice this determination is carried out by adding to a weighed quantity of the chloropicrin in a small flask fitted with a condenser, an excess of an aqueous-alcoholic solution of sodium sulphite, prepared by dissolving 10 gm. sodium sulphite in 250 ml. water and diluting with an equal volume of ethyl alcohol. The liquid in the flask is then carefully heated so as to distil off all but about 10 ml. This is then diluted with water to 100 ml. and 10 ml. of nitric acid and an excess of a standardised solution of silver nitrate are added. The solution is then warmed to drive off the nitrous gases and to coagulate the silver chloride, and then cooled and the excess silver nitrate titrated with a solution of ammonium thiocyanate (ferric alum indicator). 

Argentometric titration method has been applied to the determination of sulfonamide mixtures. The sulfonamides are quantitatively precipitated by the addition of excess standard silver nitrate solution, the precipitated silver salts removed by filtration, and the excess silver nitrate titrated with standard ammonium thiocyanate using ferric alum as indicator (32). 

Chloride may be determined by the Volhard method. The sample is acidified with concentrated nitric acid and silver nitrate is added, which reacts with the chloride, and then the mixture is boiled. After cooling, the pale yellow solution is diluted and the excess silver nitrate titrated with potassium thiocyanate solution using ammonium ferric sulfate as indicator. The ash may also be used for this determination. 

IQ. To determine the concentration of chloride ion, - a 5-mL aliquot of the methyl lithium solution is cautiously added to 25 ml of water and the resulting solution is acidified with concentrated sulfuric acid and then treated with 2-3 ml of ferric ammonium sulfate [Fe(NH4)( 04)2 12 H2O] indicator solution and 2-3 ml of benzyl alcohol. The resulting mixture is treated with 10.0 mL of standard aqueous 0.100 M silver nitrate solution and then titrated with standard aqueous 0.100 H potassium thiocyanate solution to a brownish-red endpoint. 

Both ammonium and potassium thiocyanates are usually available as deliquescent solids the analytical-grade products are, however, free from chlorides and other interfering substances. An approximately 0.1M solution is, therefore, first prepared, and this is standardised by titration against standard 0.1 JVf silver nitrate. 

Pipette 25 mL of the standard 0.1 M silver nitrate into a 250 mL conical flask, add 5mL of 6M nitric acid and 1 mL of the iron(III) indicator solution. Run in the potassium or ammonium thiocyanate solution from a burette. At first a white precipitate is produced, rendering the liquid of a milky appearance, and as each drop of thiocyanate falls in, it produces a reddish-brown cloud, which quickly disappears on shaking. As the end point approaches, the precipitate becomes flocculent and settles easily finally one drop of the thiocyanate solution produces a faint brown colour, which no longer disappears upon shaking. This is the end point. The indicator blank amounts to 0.01 mL ofO.lM silver nitrate. It is essential to shake vigorously during the titration in order to obtain correct results. ... 

Procedure C. Pipette 25 mL of the diluted solution into a 250 mL conical flask containing 5mL of 6M nitric acid, add a slight excess of 0.1M silver nitrate (30-35 mL) from a burette, and four drops of tartrazine indicator (0.5 per cent aqueous solution). Shake the suspension for about a minute in order to ensure that the indicator is adsorbed on the precipitate as far as possible. Titrate the residual silver nitrate with standard 0.1 M ammonium or potassium thiocyanate with swirling of the suspension until the very pale yellow supernatant liquid (viewed with the eye at the level of the liquid) assumes a rich lemon-yellow colour. 

Iodides can also be determined by this method, and in this case too there is no need to filter off the silver halide, since silver iodide is very much less soluble than silver thiocyanate. In this determination the iodide solution must be very dilute in order to reduce adsorption effects. The dilute iodide solution (ca 300 mL), acidified with dilute nitric acid, is treated very slowly and with vigorous stirring or shaking with standard 0.1 M silver nitrate until the yellow precipitate coagulates and the supernatant liquid appears colourless. Silver nitrate is then present in excess. One millilitre of iron(III) indicator solution is added, and the residual silver nitrate is titrated with standard 0.1M ammonium or potassium thiocyanate. 

Soliman and Belal investigated argentimetric (67,68) and mercurimetric (69) methods. Hydralazine precipitates silver from ammoniacal silver nitrate solution. The silver is dissolved with hot nitric acid and titrated with ammonium thiocyanate solution. Alternatively, mercury is precipitated from alkaline potassium mercuric iodide solution. The precipitated mercury is dissolved by adding excess standard iodine solution. The excess iodine is back-titrated with sodium thiosulfate solution after acidifying with acetic acid. 

Procedure Weigh accurately about 0.2 g of chlorobutol in a flask and dissolve in 5 ml of alcohol. Add to it 5 ml of sodium hydroxide solution, and boil under a reflux condenser for 15 minutes. Cool, dilute with 20 ml of DW, add 5 ml of nitric acid, 1 ml of nitrobenzene and 50 ml of 0.1 N silver nitrate solution. Shake the contents vigorously for 1 minute, add 4 ml of ferric ammonium sulphate solution and titrate the excess of silver nitrate with 0.1 N ammonium thiocyanate solution. Each ml of 0.1N silver nitrate is equivalent to 0.005917 g of C4H7C130. 

Elemental composition K 40.23%, S 33.00%, C 12.36%, N 14.41%. Potassium can be measured in an aqueous solution by flame photometry, AA, or ICP/AES (see Potassium). Thiocyanate anion can be measured by Volhard titration against a standard solution of silver nitrate in the presence of ferric ammonium sulfate. The color of the solution turns red at the end point. 

Chlorides, bromides, and iodides can be quantitatively determined by treatment with silver nitrate, and, with suitable precautions, the precipitated halide is washed, dried, and weighed. Chlorides in neutral soln. can be determined by F. Mohr s volumetric process 27 by titration with a standard soln. of silver nitrate with a little potassium chromate or sodium phosphate as indicator. When all the chloride has reacted with the silver nitrate, any further addition of this salt gives a yellow coloration with the phosphate, and a red coloration with the chromate. In J. Volhard s volumetric process, the chloride is treated with an excess of an acidified soln. of silver nitrate of known concentration. The excess of silver nitrate is filtered from the precipitated chloride, and titrated with a standard soln. of ammonium thiocyanate, NH4CN8—a little ferric alum is used as indicator. When the silver nitrate is all converted into thiocyanate AgN03-fNH4CNS=AgCNS +NH4NOS, the blood-red coloration of ferric thiocyanate appears. 

Yolumetrically, thiocyanate is estimated by Volhard s method, which involves titration with standard silver nitrate solution containing nitric acid, ferric alum being used as indicator. Cuprous thiocyanate dissolved in ammonium hydroxide solution and acidified with dilute sulphuric acid may be titrated with permanganate.6 An iodometrie method has also been described.7... 

The above methods cannot be used if the spirit contains chlorides, as may happen if it has been broken down with water containing these salts. In this case the total hydrocyanic acid may be determined by distilling 100 c.c. of the spirit and collecting at least three-quarters (which will contain all the hydrocyanic acid present) in a dilute solution of silver nitrate of known titre. The liquid is then made up to a definite volume and filtered, the excess of silver in an aliquot part of the filtrate being titrated with thiocyanate as already described. The free hydrocyanic acid, in presence of chlorides, should be determined colorimetrically as follows a solution of about 0-05 gram of potassium cyanide per litre is prepared and its exact content of HCN determined by titration with silver nitrate and ammonium thiocyanate. In a series of test-tubes are placed such quantities of this... 

Visual titration was also reported, the excess of silver being titrated with potassium thiocyanate in the presence of ferric ammonium sulfate indicator [51]. Mercuric nitrate titrations were also reported [62]. 

Weigh accurately about 400 mg of acetylcholine chloride, previously dried at 105°C for 3 h, and dissolve in 50 mL of water in a glass stoppered 125mL flask. Add with agitation 30.0 mL of 0.1 N silver nitrate VS, then add 5mL of nitric acid and 5mL of nitrobenzene and shake well. Thereafter, add 2mL of ferric ammonium sulfate TS, and titrate the excess silver nitrate with 0.1 ammonium thiocyanate VS. Each milliliter of 0.1 N silver nitrate is equivalent to 3.545mg of Cl. Between 19.3 and 19.8% of Cl is found. 

Determination of Orthophosphates.—(1) With Silver Nitrate.— This depends upon the precipitation of silver orthophosphate in solutions of low and controlled acidity. In the assay of commercial 85 per cent, phosphoric acid of density 1-710 the syrup is diluted to a convenient volume and an aliquot part is taken which contains about 0-1 gram of H3P04. It is neutralised to phenolphthalein with approximately decinormal alkali (free from chloride). 50 c.c. of decinormal silver nitrate are then added while the solution is kept neutral to litmus by stirring in zinc oxide or a suspension of the hydroxide. The whole or a measured part of the filtered solution is acidified with nitric acid and, after the addition of ferric alum, the unused silver nitrate is titrated with standard decinormal ammonium thiocyanate in the usual manner. Alkali phosphates may also be determined in this way. 

This reaction is also used in the method of Holleman 2 as modified by Wilkie.3 A phosphate solution containing phenolphthalein is reddened by the addition of alkali, then just decolorised with nitric acid. An excess of standard silver nitrate is then added and decinormal sodium acetate and alkali to slight pink colour, followed by 2 c.c. of decinormal H2S04. The solution is diluted and filtered and the excess of silver determined by titration with decinormal ammonium thiocyanate. 

Sodium Chloride Content Accurately weigh about 20 mg of sample, and dissolve it in 50 mL of water contained in a glass-stoppered flask. Add, while agitating, 3 mL of nitric acid, 5 mL of nitrobenzene, 50.0 mL of 0.1 N silver nitrate, and 2 mL of ferric ammonium sulfate TS. Shake well, and titrate the excess silver nitrate with 0.2 N ammonium thiocyanate. The titration endpoint is indicated by the appearance of a red color. Perform a blank determination (see General Provisions). Calculate the content of sodium chloride in the sample by the formula... 

Total Chlorine Dissolve 1 g of sample in 100 mL of water, add enough 6% sulfurous acid to give the solution a distinct odor of sulfur dioxide, boil gently until the odor of the sulfur dioxide is no longer evident, and adjust the volume to 100 mL with water. Add 1.0 mL of 0.1 A silver nitrate followed by 3 mL of nitric acid and 3 mL of nitrobenzene, and shake vigorously. Add ferric ammonium sulfate TS, and titrate the excess silver nitrate with 0.1 A ammonium thiocyanate. No more than 0.6 mL of the 0.1 A silver nitrate is consumed. 

Sodium Chloride Dissolve about 10 g of sample, accurately weighed, in 50 mL of water in a 250-mL beaker. Add sufficient nitric acid to make the solution slightly acid, then add 1 mL of ferric ammonium sulfate TS and 1.00 mL of 0.05 A ammonium thiocyanate, and titrate with 0.05 A silver nitrate, stirring constantly, until the red color completely disappears. Finally, back titrate with 0.05 A ammonium thiocyanate until a faint red color appears. Subtract the total volume of 0.05 A ammonium thiocyanate added from the volume of 0.05 A silver nitrate required. Each milliliter of 0.05 A silver nitrate is equivalent to 2.922 mg of sodium chloride (NaCl). Calculate the percentage of sodium chloride in the sample taken. Water Calculate the percentage of water by subtracting from 100 the sum of the percentages of Sodium Bicarbonate, Sodium Carbonate, and Sodium Chloride found in the sample. 

Transfer 50 mL of filtrate to a 250-mL flask. Add 2 mL of 6 N nitric acid, 5 mL of nitrobenzene, and 10 mL of standardized 0.1 N silver nitrate solution. Shake the flask until the silver chloride coagulates. Prepare a saturated solution of ferric ammonium sulfate, and add just enough concentrated nitric acid to discharge the red color add 1 mL of this solution to the 250-mL flask to serve as the indicator. Titrate with 0.1 N ammonium thiocyanate solution that has been standardized against the silver nitrate solution until the color persists after shaking for 1 min. Calculate the weight percent of sodium chloride, P, by the equation... 

Ammonium Thiocyanate, 0.1 N (7.612 g NH4SCN per 1000 mL) Dissolve about 8 g of ammonium thiocyanate (NH4SCN) in 1000 mL of water, and standardize by titrating the solution against 0.1 N Silver Nitrate as follows Transfer about 30 mL of 0.1 N Silver Nitrate, accurately measured, into a glass-stoppered flask. Dilute with 50 mL of water, then add 2 mL of Ferric Ammonium Sulfate TS and 2 mL of nitric acid, and titrate with the ammonium thiocyanate solution to the first appearance of a red-brown color. Calculate the normality, and, if desired, adjust the solution to exactly 0.1 A. If desired, 0.1 N Ammonium Thiocyanate may be replaced by 0.1 A potassium thiocyanate where the former is directed in various tests and assays. 

Determination of Benzyl Bromide. The determination of this substance may be carried out by the method already described for benzyl chloride. However, according to Van der Laan, it is sometimes more convenient to decompose the substance directly with a measured volume of standardised alcoholic silver nitrate solution and to titrate the excess of the latter with ammonium thiocyanate solution by the Volhard method. 

The azide value may be determined 1 shaking with dilute nitric acid until dissolved and precipitating as silver azide with excess of standard stiver nitrate. A little sodium acetate is added to decrease the solubility of the silver azide. The excess of silver nitrate is titrated with thiocyanate. The hydrazoic acid may also be distilled off after addition of acetic acid or ammonium nitrate and estimated as silver azide. 

Precipitation of silver azide from acetate solution and gravimetric determination of the silver as silver azide or silver chloride, or volumetrically by addition of an excess of standard silver nitrate solution and back-titration with standard ammonium thiocyanate using ferric alum as internal indicator. 

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