Silver nitrate standard solution

In Nebraska, state regulations require that the chemical makeup of animal feed sold in the state be accurately reflected on the labels found on the feed bags. The Nebraska State Agriculture Laboratory is charged with the task of performing the analytical laboratory work required. An example is salt (sodium chloride) content. The method used to analyze the feed for sodium chloride involves a potentio-metric titration. A chloride ion-selective electrode in combination with a saturated calomel reference electrode is used. After dissolving the feed sample, the chloride is titrated with a silver nitrate standard solution. The reaction involves the formation of the insoluble precipitate silver chloride. The electrode monitors the decrease in the chloride concentration as the titration proceeds, ultimately detecting the end point (when the chloride ion concentration is zero). 

S04 see Conservative Elements). The analytical results reported by W. Dittmar in 1884 for waters collected during the British RMS Challenger Expedition (1872-1876) from the world s oceans were almost the same as today s values. The constancy of major chemical composition has led oceanographers to define salinity as a fundamental property together with temperature to calculate the density of sea water. It was routine for classic physical oceanographers to titrate sea water for chloride (plus bromide) ion with silver nitrate standard solution, until the mid 1960s when salinity could be determined more practically by measurement of conductivity. 

If the chloride content of the water sample is unknown this should first be determined by pre-titration with silver nitrate standard solution of appropriate normality. Take 100 ml of the filtered water sample with a chloride content of 1 - 15 mg, or a smaller volume diluted to 100 ml with distilled water. The pH should lie between 7 and 9 (acid- or alkaline-reacting waters should first be neutralized with chloride-free reagents). Add 1 ml of potassium chromate solution. 

N silver nitrate. The solution is stored in the dark. This is the standard. 

Silver Nitrate, Standard Alcohtdic Solution (0.1 mol/ L)—Dissolve 17 g of AgNOj in 1(X) mL of water in a l-L volumetric flask and dilute to 1 L with 2-propanol (99 %) (Note 4). Store in a dark bottle and standardize at intervals frequent enough to detect a change of 0.0005 or greater in molarity. 

Silver Nitrate, Standard Alcoholic Solution (0,010 mol/L)—Prepare daily by dilution of the 0.1 N standard. Pipet 100 mL of the 0.1 mol/L standard into a 1-L volumetric flask and dilute to volume with 2-propanol. Calculate the exact molarity. 

Addition of silver nitrate to a solution of a chloride in dilute nitric acid gives a white precipitate of silver chloride, AgCl, soluble in ammonia solution. This test may be used for gravimetric or volumetric estimation of chloride the silver chloride can be filtered off, dried and weighed, or the chloride titrated with standard silver nitrate using potassium chromate(VI) or fluorescein as indicator. 

The cyanide content may be determined by titration with standard silver nitrate solution. ... 

Silver compounds, available from commercial suppHers, are expensive. Reagent grades of sHver(I) carbonate, cyanide, diethjldithiocarbamate, iodate, nitrate, oxide, phosphate, and sulfate are available. Standardized solutions of silver nitrate are also available for analytical uses. Purified grades of sHver(I) acetate, bromide, cyanide, and iodide can be purchased silver nitrate is also made as a USP XX grade for medicinal uses (6). 

Analysis. The abiUty of silver ion to form sparingly soluble precipitates with many anions has been appHed to their quantitative deterrnination. Bromide, chloride, iodide, thiocyanate, and borate are determined by the titration of solutions containing these anions using standardized silver nitrate solutions in the presence of a suitable indicator. These titrations use fluorescein, tartrazine, rhodamine 6-G, and phenosafranine as indicators (50). 

Assay of hydrogen cyanide can be done by specific gravity or silver nitrate titration. Sulfur dioxide in hydrogen cyanide can be deterrnined by infrared analysis or by reaction of excess standard iodine solution and titration, using standard sodium thiosulfate or by measurement of total acidity by... 

The sodium carbonate content may be deterrnined on the same sample after a slight excess of silver nitrate has been added. An excess of barium chloride solution is added and, after the barium carbonate has setded, it is filtered, washed, and decomposed by boiling with an excess of standard hydrochloric acid. The excess of acid is then titrated with standard sodium hydroxide solution, using methyl red as indicator, and the sodium carbonate content is calculated. 

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. 

Reagents. Silver nitrate solution, 10 4M. Prepare by dilution of a standard 0.1 M silver nitrate solutioa... 

Anion exchange resin. Proceed as in the previous experiment using 1.0 g, accurately weighed, of the air-dried strongly basic anion exchanger (e.g. Duolite A113, chloride form). Fill the 250 mL separatory funnel with ca 0.25M sodium nitrate solution, and allow this solution to drop into the column at the rate of about 2 mL per minute. Collect the effluent in a 500 mL conical flask, and titrate with standard 0.1M silver nitrate using potassium chromate as indicator. 

Theory. The anion exchange resin, originally in the chloride form, is converted into the nitrate form by washing with sodium nitrate solution. A concentrated solution of the chloride and bromide mixture is introduced at the top of the column. The halide ions exchange rapidly with the nitrate ions in the resin, forming a band at the top of the column. Chloride ion is more rapidly eluted from this band than bromide ion by sodium nitrate solution, so that a separation is possible. The progress of elution of the halides is followed by titrating fractions of the effluents with standard silver nitrate solution. 

Weigh out accurately about 0.10 g of analytical grade sodium chloride and about 0.20 g of potassium bromide, dissolve the mixture in about 2.0 mL of water and transfer quantitatively to the top of the column with the aid of 0.3 M sodium nitrate. Pass 0.3 M sodium nitrate through the column at a flow rate of about 1 mL per minute and collect the effluent in 10 mL fractions. Transfer each fraction in turn to a conical flask, dilute with an equal volume of water, add 2 drops of 0.2M potassium chromate solution and titrate with standard 0.02M silver nitrate. 

Before commencing the elution titrate 10.0 mL of the 0.3 M sodium nitrate with the standard silver nitrate solution, and retain the product of this blank titration for comparing with the colour in the titrations of the eluates. When the titre of the eluate falls almost to zero (i.e. nearly equal to the blank titration) — ca 150 mL of effluent — elute the column with 0.6M sodium nitrate. Titrate as before until no more bromide is detected (titre almost zero). A new blank titration must be made with 10.0 mL of the 0.6M sodium nitrate. 

The method may also be applied to the analysis of silver halides by dissolution in excess of cyanide solution and back-titration with standard silver nitrate. It can also be utilised indirectly for the determination of several metals, notably nickel, cobalt, and zinc, which form stable stoichiometric complexes with cyanide ion. Thus if a Ni(II) salt in ammoniacal solution is heated with excess of cyanide ion, the [Ni(CN)4]2 ion is formed quantitatively since it is more stable than the [Ag(CN)2] ion, the excess of cyanide may be determined by the Liebig-Deniges method. The metal ion determinations are, however, more conveniently made by titration with EDTA see the following sections. 

Pipette 25.0 mL of the bromide ion solution (0.01-0.02M) into a 400 mL beaker, add excess of dilute silver nitrate solution, filter off the precipitated silver bromide on a sintered glass filtering crucible, and wash it with cold water. Dissolve the precipitate in a warm solution prepared from 15 mL of concentrated ammonia solution, 15 mL of 1M ammonium chloride, and 0.3 g of potassium tetracyanonickelate. Dilute to 100-200 mL, add three drops of murexide indicator, and titrate with standard EDTA (0.01 M) (slowly near the end point) until the colour changes from yellow to violet. 

The titration error will increase with increasing dilution of the solution being titrated and is quite appreciable (ca 0.4 per cent) in dilute, say 0.01 M, solutions when the chromate concentration is of the order 0.003-0.005M. This is most simply allowed for by means of an indicator blank determination, e.g. by measuring the volume of standard silver nitrate solution required to give a perceptible coloration when added to distilled water containing the same quantity of indicator as is employed in the titration. This volume is subtracted from the volume of standard solution used. 

Other dyestuffs have been recommended as adsorption indicators for the titration of halides and other ions. Thus cyanide ion may be titrated with standard silver nitrate solution using diphenylcarbazide as adsorption indicator (see Section 10.44) the precipitate is pale violet at the end point. A selection of adsorption indicators, their properties and uses, is given in Table 10.8. 

The following sections are concerned with the use of standard solutions of reagents such as silver nitrate, sodium chloride, potassium (or ammonium) thiocyanate, and potassium cyanide. Some of the determinations which will be considered strictly involve complex formation rather than precipitation reactions, but it is convenient to group them here as reactions involving the use of standard silver nitrate solutions. Before commencing the experimental work, the theoretical Sections 10.74 and 10.75 should be studied. 

Discussion. Very pure silver can be obtained commercially, and a standard solution can be prepared by dissolving a known weight (say, 10.787 g) in nitric acid in a conical flask having a funnel in the neck to prevent mechanical loss, and making up to a known volume (say, 1 L for a 0.1 M solution). The presence of acid must, however, be avoided in determinations with potassium chromate as indicator or in determinations employing adsorption indicators. It is therefore preferable to employ a neutral solution prepared by dissolving silver nitrate (relative molecular mass, 169.87) in water. 

Analytical grade silver nitrate has a purity of at least 99.9 per cent, so that a standard solution can be prepared by direct weighing. If, however, commercial... 

Procedure. Pipette 25 mL of the standard OAM sodium chloride into a 250 mL conical flask. Add 10 drops of either fluorescein or dichlorofluorescein indicator, and titrate with the silver nitrate solution in a diffuse light, while rotating the flask constantly. As the end point is approached, the silver chloride coagulates appreciably, and the local development of a pink colour upon the addition of a drop of the silver nitrate solution becomes more and more pronounced. Continue the addition of the silver nitrate solution until the precipitate suddenly assumes a pronounced pink or red colour. Repeat the titration with two other 25 mL portions of the chloride solution. Individual titrations should agree within 0.1 mL. 

Either the Mohr titration or the adsorption indicator method may be used for the determination of chlorides in neutral solution by titration with standard 0.1M silver nitrate. If the solution is acid, neutralisation may be effected with chloride-free calcium carbonate, sodium tetraborate, or sodium hydrogencarbonate. Mineral acid may also be removed by neutralising most ofthe acid with ammonia solution and then adding an excess of ammonium acetate. Titration of the neutral solution, prepared with calcium carbonate, by the adsorption indicator method is rendered easier by the addition of 5 mL of 2 per cent dextrin solution this offsets the coagulating effect of the calcium ion. If the solution is basic, it may be neutralised with chloride-free nitric acid, using phenolphthalein as indicator. 

The method may be applied to those anions (e.g. chloride, bromide, and iodide) which are completely precipitated by silver and are sparingly soluble in dilute nitric acid. Excess of standard silver nitrate solution is added to the solution containing free nitric acid, and the residual silver nitrate solution is titrated with standard thiocyanate solution. This is sometimes termed the residual process. Anions whose silver salts are slightly soluble in water, but which are soluble in nitric acid, such as phosphate, arsenate, chromate, sulphide, and oxalate, may be precipitated in neutral solution with an excess of standard silver nitrate solution. The precipitate is filtered off, thoroughly washed, dissolved in dilute nitric acid, and the silver titrated with thiocyanate solution. Alternatively, the residual silver nitrate in the filtrate from the precipitation may be determined with thiocyanate solution after acidification with dilute nitric acid. 

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. ... 

Discussion. The chloride solution is treated with excess of standard silver nitrate solution, and the residual silver nitrate determined by titration with standard thiocyanate solution. Now silver chloride is more soluble than silver thiocyanate, and would react with the thiocyanate thus ... 

Procedure B. Pipette 25 mL of the diluted solution into a 250 mL conical flask containing 5mL 6 M nitric acid. Add a slight excess of standard 0.1M silver nitrate (about 30 mL in all) from a burette. Then add 2-3 mL pure nitrobenzene and 1 mL of the iron(III) indicator, and shake vigorously to coagulate the precipitate. Titrate the residual silver nitrate with standard 0.1M thiocyanate until a permanent faint reddish-brown coloration appears. 

From the volume of silver nitrate solution added, subtract the volume of silver nitrate solution that is equivalent to the volume of standard thiocyanate required. Then calculate the percentage of HCI in the sample. 

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. 

Discussion. Arsenates in solution are precipitated as silver arsenate, Ag3 As04, by the addition of neutral silver nitrate solution the solution must be neutral, or if slightly acid, an excess of sodium acetate must be present to reduce the acidity if strongly acid, most of the acid should be neutralised by aqueous sodium hydroxide. The silver arsenate is dissolved in dilute nitric acid, and the silver titrated with standard thiocyanate solution. The silver arsenate has nearly six times the weight of the arsenic, hence quite small amounts of arsenic may be determined by this procedure. 

Method A. Weigh out accurately about 3.5 g of potassium cyanide from a glass-stoppered weighing bottle, dissolve it in water and make up to 250 mL in a graduated flask. Shake well. Transfer 25.0 mL of this solution by means of a burette and NOT a pipette to a 250 mL conical flask, add 75 mL water, 5-6 mL 6M ammonia solution, and 2 mL 10 per cent potassium iodide solution. Place the flask on a sheet of black paper, and titrate with standard 0.1 M silver nitrate. Add the silver nitrate solution dropwise as soon as the yellow colour of silver iodide shows any signs of persisting. When one drop produces a permanent turbidity, the end-point has been reached. 

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