Potassium thiocyanate standardization

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. 

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. 

Bromides can also be determined by the Volhard method, but as silver bromide is less soluble than silver thiocyanate it is not necessary to filter off the silver bromide (compare chloride). The bromide solution is acidified with dilute nitric acid, an excess of standard 0.1M silver nitrate added, the mixture thoroughly shaken, and the residual silver nitrate determined with standard 0.1 M ammonium or potassium thiocyanate, using ammonium iron(III) sulphate as indicator. 

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. 

After the addition of the potassium iodide solution, run in standard 0.1M sodium thiosulphate until the brown colour of the iodine fades, then add 2 mL of starch solution, and continue the addition of the thiosulphate solution until the blue colour commences to fade. Then add about 1 g of potassium thiocyanate or ammonium thiocyanate, preferably as a 10 per cent aqueous solution the blue colour will instantly become more intense. Complete the titration as quickly as possible. The precipitate possesses a pale pink colour, and a distinct permanent end point is readily obtained. 

To use KBr discs for quantitative measurements it is best to employ an internal standard procedure in which a substance possessing a prominent isolated infrared absorption band is mixed with the potassium bromide. The substance most commonly used is potassium thiocyanate, KSCN, which is intimately mixed and ground to give a uniform concentration, usually 0.1-0.2 per cent, in the potassium bromide. A KBr/KSCN disc will give a characteristic absorption band at 2125 cm 1. Before quantitative measurements can be carried out it is necessary to prepare a calibration curve from a series of standards made using different amounts of the pure organic compound with the KBr/KSCN. A practical application of this is given in Section 19.9. 

These and similar results can be explained if the simultaneous reduction of hydrogen peroxide is due to an induced reaction. To show the characteristic features of this reaction some results are presented in Table 19 and Table 20. The procedure for these measurements was as follows. The solution of peroxy compounds given in columns 1 and 2 was made up to 20 ml and the pH was adjusted to the given value. Then potassium thiocyanate solution was added and, after the reaction time noted, the process was quenched by adding potassium iodide solution (0.3 g KI). After 5 sec the solution was acidified with 1 ml 2 iV sulphuric acid then using, molybdate catalyst solution, the iodine liberated was titrated with standard thiosulphate. 

Potassium thiocyanate (KSCN) is considered to be the choicest internal standard. In usual practice, it must be preground, dried and subsequently reground, and used at a concentration of 0.2% (w/w) along with the dried spectroscopic grade KBr. The mixture of KBr-KSCN is stored over P205. 

Foster et al. have developed a method for determining technetium in dissolved nuclear fuel solutions. Tetrapropylammonium pertechnetate is doubly extracted from a basic medium into chloroform and the colored technetium (V) thiocyanate complex is formed in the chloroform phase by the addition of sulfuric acid, potassium thiocyanate and tetrapropylammonium hydroxide. The colored complex absorbs at 513 nm, has a molar extinction coefficient of 46,000 and is stable for several hours. Of more than 50 metals studied, none impairs measurements at ratios less than 100 to 1 mol with respect to technetium. Most anions do not disturb the determination of technetiiun. The standard deviation for a single determination is 0.09 fig over the range of 1 to 20 fig of technetium. 

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. 

This solution is then diluted and titrated with standard potassium thiocyanate solution, using ammonium iron alum as indicator.1... 

The AgCl is filtered and washed, and excess Ag+ in the combined filtrate is titrated with standard KSCN (potassium thiocyanate) in the presence of Fe3+. 

Chloride is analyzed by some form of reaction with silver to form insoluble silver chloride. Direct titration of milk with silver nitrate yields erroneously high and variable results, and pre-ashing cannot be used because chloride is lost by volatilization. Satisfactory procedures involve adding an excess of standardized AgN03 directly to milk and back titrating with potassium thiocyanate (KSCN), using a soluble ferric salt as the indicator (Sanders 1939). 

Often, greater accuracy may be obtained, as in Volhard type titration, by performing a back titration of the excess silver ions. In such a case, a measured amount of standard silver nitrate solution is added in excess to a measured amount of sample. The excess Ag+ that remains after it reacts with the analyte is then measured by back titration with standard potassium thiocyanate (KSCN). If the silver salt of the analyte ion is more soluble than silver thiocyanate (AgSCN), the former should be filtered off from the solution. Otherwise, a low value error can occur due to overconsumption of thiocyanate ion. Thus, for the determination of ions (such as cyanide, carbonate, chromate, chloride, oxalate, phosphate, and sulfide, the silver salts of which are all more soluble than AgSCN), remove the silver salts before the back titration of excess Ag.+ On the other hand, such removal of silver salt is not necesary in the Volhard titration for ions such as bromide, iodide, cyanate, thiocyanate, and arsenate, because the silver salts of these ions are less soluble than AgSCN, and will not cause ary error. In the determination of chloride by Volhard titration, the solution should be made strongly acidic to prevent interference from carbonate, oxalate, and arsenate, while for bromide and iodide analysis titration is carried out in neutral media. 

A standard solution was prepared by dissolving 6.7035 g AgN03 in water to a volume of 1 L. What volume of potassium thiocyanate of strength 0.0957 M would be needed to titrate 50 mL of this solution ... 

Thiocyanate standard solns. 1.673 g potassium thiocyanate, KSCN in distilled water, diluted to 1 L. This stock soln. = 1000 mg SCN7L or 1 mL = 1 mg SCN". Prepare a secondary standard from the stock solution by diluting 1 mL stock solution to 100 mL with distilled water. Secondary std = 10 mg SCN-/L or 1 mL = 0.01 mg SCN". Prepare a series of calibration standards by diluting the secondary standard with distilled water as follows ... 

A 0.5-g. sample is dissolved in 50 ml. of 3 M aqueous ammonia, and the solution is made slightly acidic to litmus paper with 6 M nitric acid. Fifty milliliters of standard 0.1 M silver nitrate, 3 ml. of nitrobenzene, and 10 drops of ferric alum indicator are added, and the excess Ag+ is titrated with 0.1 M potassium thiocyanate to a brownish-red end point. 

For compounds containing chlorine or bromine, a weighed sample is dissolved in 10 ml. of methanol 10 ml. of 20% sodium hydroxide, 2 g. of zinc, and 0.5 g. of Raney nickel are added, and the mixture is heated with reflux for 1 hour over a water bath. It is then cooled and decanted. After acidification with nitric acid, 20 ml. of standard silver nitrate solution and 5 ml. of a ferric alum solution are added, and the solution is titrated with standard potassium thiocyanate. The procedure is modi-... 

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. 

Niobium pentachloride decomposes readily in the presence of moisture thus it is very important in this synthesis that the acetonitrile be rigorously dried. The method described by Coetzee et al.1 is suitable for the purification of this solvent. The niobium pentachloride starting material should also be freed of any oxy species. This can be effected at about 100° by sublimation of the pentachloride away from the less volatile oxychlorides using a standard vacuum sublimer with a water-cooled probe. Appearance of commercial samples is a poor guide to their purity, and this purification step should not be omitted. The potassium thiocyanate should be purified by recrystallization and then thoroughly dried. 

To standardize the solution with iron, an iron solution containing exactly 2 grams of iron per liter is used. This solution should contain 5 per cent hydrochloric acid or sulfuric acid. As an example, it is foimd that 6.85 cc. of a titanium trichloride solution corresponds to 25 cc. of the iron chloride solution, using potassium thiocyanate as the indicator (spot test). Since the iron solution contains 2 grams of iron per liter, 1 cc. TiCls solution corresponds to 0.05/6.85 or 0.007299 grams of iron. 

Standard Thiocyanate Solution. Dissolve 9.718 g of potassium thiocyanate in 100 ml of water and dilute with sufficient water to produce 1000ml. This solution contains 0.1 mmol of CNS in 1 ml. Its exact strength should be ascertained by tih ation widi silver nitrate. Dilute 0.5 ml of this solution to 100 ml with water. 

Excess silver ions are then titrated with standard potassium thiocyanate solution in the presence of an iron (III) salt ... 

The excess silver nitrate is then back-titrated with a standard solution of potassium thiocyanate ... 

Table 13-3 lists some typical applications of precipitation titrations in which silver nitrate is the standard solution. In most of these methods, the analyte is precipitated with a measured excess of silver nitrate, and the excess is determined by a Volhard titration with standard potassium thiocyanate. 

Both silver nitrate and potassium thiocyanate are obtainable in primary-standard quality. The latter is, however, somewhat hygroscopic, and thiocyanate solutions are ordinarily standardized against silver nitrate. Both silver nitrate and potassium thiocyanate solutions are stable indefinitely. 

Titration. After cooling, 10 ml water and 1 ml potassium thiocyanate (20%) are added. The titration with titanous chloride is carried out in the digestion tube in a COa atmosphere. The titanous chloride solution (about 0.15%) is frequently standardized against the ferric ammonium sulfate stock solution (mentioned in Section 2.1.2). 

A second example of this type of indicator is illustrated in the Volhard titration. This is an indirect titration procedure for determining anions that precipitate with silver (Cl , Br , SCN"), and it is performed in acid (HNO3) solution. In this procedure, we add a measured excess of AgNOa to precipitate the anion and then determine the excess Ag" " by back-titration with standard potassium thiocyanate solution ... 

Standard Ag" was prepared by mixing 25.00 mL of 0.102 6 M AgN03, 5 mL of 6 M HNO3, and 1 mL of Fe indicator solution (40 wt% aqueous (NH4)Fe(S04)2 with a few drops of 6 M HNO3). To standardize a solution of potassium thiocyanate, KSCN is delivered to the standard Ag solution from a buret. The initial precipitate is white and later it is reddish brown. The color disappears on shaking. At the end point, one drop of KSCN solution produces a faint brown color that does not disappear on shaking. A volume of 24.22 mL was required to reach the end point and the blank correction was 0.02 mL. 

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