Nitric acid, titration

Pipette 25.0 cm of standard 0.1 M silver nitrate solution into a 250 cm flask and add 1 cm of 40% w/v solution of purest ammonium iron(lll) sulphate acidified with nitric acid. Titrate with the thiocyanate solution until the appearance of a faint brown colour in the flocculated suspension of white silver thiocyanate. Repeat and calculate from the average titre [SCN j in mol dm. ... 

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. 

Add a known volume ofo oaM.AgNOj solution (in excess) and boil the solution until the silver chloride has coagulated. Filter through a conical 5 cm. funnel, ensuring that the filter-paper does not protrude above the r m of the funnel. Wash the silver chloride and the filter-paper several times with a fine jet of distilled water. To the united filtrate and washings add i ml. of saturated ferric alum solution. The solution should be almost colourless if it is more than faintly coloured, add a few drops of concentrated nitric acid. Then titrate with 0 02M-ammonium thiocyanate solution until the permanent colour of ferric thiocyanate is just perceptible. (Alternatively the chloride may be determined potentiometrically.)... 

In a back titration, a slight excess of the metal salt solution must sometimes be added to yield the color of the metal-indicator complex. Where metal ions are easily hydrolyzed, the complexing agent is best added at a suitable, low pH and only when the metal is fully complexed is the pH adjusted upward to the value required for the back titration. In back titrations, solutions of the following metal ions are commonly employed Cu(II), Mg, Mn(II), Pb(II), Th(IV), and Zn. These solutions are usually prepared in the approximate strength desired from their nitrate salts (or the solution of the metal or its oxide or carbonate in nitric acid), and a minimum amount of acid is added to repress hydrolysis of the metal ion. The solutions are then standardized against an EDTA solution (or other chelon solution) of known strength. 

Titrations conducted with microliter or picoliter sample volumes require a smaller absolute amount of analyte. For example, diffusional titrations have been successfully conducted on as little as 29 femtomoles (10 mol) of nitric acid. Nevertheless, the analyte must still be present in the sample at a major or minor level for the titration to be performed accurately and precisely. 

Acetylene can be deterrnined volumetricaHy by absorption in Aiming sulfuric acid (or more conveniently in sulfuric acid activated with silver sulfate) or by reaction with silver nitrate in solution and titration of the nitric acid formed ... 

Quantitative Analysis. The total acidity of nitric acid solution may be deterrnined by conventional titration using phenolphthalein as the indicator. 

Ferrous Sulfdte Titration. For deterrnination of nitric acid in mixed acid or for nitrates that are free from interferences, ferrous sulfate titration, the nitrometer method, and Devarda s method give excellent results. The deterrnination of nitric acid and nitrates in mixed acid is based on the oxidation of ferrous sulfate [7720-78-7] by nitric acid and may be subject to interference by other materials that reduce nitric acid or oxidize ferrous sulfate. Small amounts of sodium chloride, potassium bromide, or potassium iodide may be tolerated without serious interference, as can nitrous acid up to 50% of the total amount of nitric acid present. Strong oxidizing agents, eg, chlorates, iodates, and bromates, interfere by oxidizing the standardized ferrous sulfate. 

Possible interferences and variation of results from modified techniques can be avoided by titrating the sample in exacdy the same way and by employing approximately the same amounts of materials as in the initial standardization of the ferrous sulfate against a known quantity of nitric acid. The ferrous sulfate solution is added in a thin stream until the initially yellowish solution turns brown. The titration is complete when the faint brownish-tinged end point is reached. 

Devarda s Method. Nitrogen in nitrates or nitric acid also may be deterrnined by the Kjeldahl method or by Devarda s method. The latter is both convenient and accurate when no organic nitrogen is present. The nitrate is reduced by Devarda s alloy to ammonia in an alkaline solution. The ammonia is distilled and titrated with standard acid. 

Chemical analysis methods maybe used for assay of silver alloys containing no interfering base metals. Nitric acid dissolution of the silver and precipitation as AgCl, or the Gay-Lussac-VoUiard titration methods are used iaterchangeably for the higher concentrations of silver. These procedures have been described (4). 

Determination. The most accurate (68) method for the deterrnination of copper in its compounds is by electrogravimetry from a sulfuric and nitric acid solution (45). Pure copper compounds can be readily titrated using ethylene diamine tetracetic acid (EDTA) to a SNAZOXS or Murexide endpoint. lodometric titration using sodium thiosulfate to a starch—iodide endpoint is one of the most common methods used industrially. This latter titration is quicker than electrolysis, almost as accurate, and much more tolerant of impurities than is the titration with EDTA. Gravimetry as the thiocyanate has also been used (68). 

A 0.4000 M solution of nitric acid is used to titrate 50.00 mL of0.237 M barium hydroxide. (Assume that volumes are additive.)... 

Procedures have also been devised for the determination of metallic constituents. Thus, mercury is absorbed in nitric acid and titrated with sodium diethyldithiocarbamate, whilst zinc is absorbed in hydrochloric acid and determined by an EDTA titration (see Section 10.65). 

Hydrochloric acid and sulphuric acid are widely employed in the preparation of standard solutions of acids. Both of these are commercially available as concentrated solutions concentrated hydrochloric acid is about 10.5- 12M, and concentrated sulphuric acid is about 18M. By suitable dilution, solutions of any desired approximate concentration may be readily prepared. Hydrochloric acid is generally preferred, since most chlorides are soluble in water. Sulphuric acid forms insoluble salts with calcium and barium hydroxides for titration of hot liquids or for determinations which require boiling for some time with excess of acid, standard sulphuric acid is, however, preferable. Nitric acid is rarely employed, because it almost invariably contains a little nitrous acid, which has a destructive action upon many indicators. 

Discussion. When a solution of an orthophosphate is treated with a large excess of ammonium molybdate solution in the presence of nitric acid at a temperature of 20-45 °C, a precipitate is obtained, which after washing is converted into ammonium molybdophosphate with the composition (NH4)3[P04,12Mo03]. This may be titrated with standard sodium hydroxide solution using phenolph-thalein as indicator, but the end point is rather poor due to the liberation of ammonia. If, however, the ammonium molybdate is replaced by a reagent containing sodium molybdate and quinoline, then quinoline molybdophosphate is precipitated which can be isolated and titrated with standard sodium hydroxide ... 

Fluoride may be determined by precipitation as lead chlorofluoride, the precipitate being dissolved in dilute nitric acid and, after adjusting the pH to 5-6, the lead is titrated with EDTA using xylenol orange indicator.10... 

Pipette 25 mL of the bismuth solution (approx. 0.01 M) into a 500 mL conical flask and dilute with de-ionised water to about 150 mL. If necessary, adjust the pH to about 1 by the cautious addition of dilute aqueous ammonia or of dilute nitric acid use a pH meter. Add 30 mg of the xylenol orange/potassium nitrate mixture (see Section 10.50) and then titrate with standard 0.01 M EDTA solution until the red colour starts to fade. From this point add the titrant slowly until the end point is reached and the indicator changes to yellow. 

Procedure. Pipette 25 mL of the test solution (which may contain both calcium and lead at concentrations of up to 0.01 M) into a 250 mL conical flask and dilute to 100 mL with de-ionised water. Add about 50 mg of methylthymol blue/potassium nitrate mixture followed by dilute nitric acid until the solution is yellow, and then add powdered hexamine until the solution has an intense blue colour (pH ca 6). Titrate with standard (0.01 M) EDTA solution until the colour turns to yellow this gives the titration value for lead. 

To the resulting solution now add a further 20 mL portion of the standard EDTA solution, add nitric acid (1M) to adjust the pH to 1-2, and then boil the solution for 15 minutes. Cool, dilute to 400 mL by the addition of de-ionised water, add hexamine to bring the pH to 5-6, add more of the indicator solution, and titrate the excess EDTA with the standard lead nitrate solution. 

Procedure. Dissolve a weighed amount of ferro-manganese (about 0.40 g) in concentrated nitric acid and then add concentrated hydrochloric acid (or use a mixture of the two concentrated acids) prolonged boiling may be necessary. Evaporate to a small volume on a water bath. Dilute with water and filter directly into a 100 mL graduated flask, wash with distilled water and finally dilute to the mark. Pipette 25.0 mL of the solution into a 500 mL conical flask, add 5 mL of 10 per cent aqueous hydroxylammonium chloride solution, 10 mL of 20 per cent aqueous triethanolamine solution, 10-35 mL of concentrated ammonia solution, about 100 mL of water, and 6 drops of thymolphthalexone indicator solution. Titrate with standard 0.05M EDTA until the colour changes from blue to colourless (or a very pale pink). 

The analysis of low-melting alloys such as Wood s metal is greatly simplified by complexometric titration, and tedious gravimetric separations are avoided. The alloy is treated with concentrated nitric acid, evaporated to a small volume, and after dilution the precipitated tin(IV) oxide is filtered off heavy metals adsorbed by the precipitate are removed by washing with a known volume of standard EDTA solution previously made slightly alkaline with aqueous... 

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. 

Similar remarks apply to the determination of bromides the Mohr titration can be used, and the most suitable adsorption indicator is eosin which can be used in dilute solutions and even in the presence of 0.1 M nitric acid, but in general, acetic (ethanoic) acid solutions are preferred. Fluorescein may be used but is subject to the same limitations as experienced with chlorides [Section 10.77(b)], With eosin indicator, the silver bromide flocculates approximately 1 per cent before the equivalence point and the local development of a red colour becomes more and more pronounced with the addition of silver nitrate solution at the end point the precipitate assumes a magenta colour. 

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. 

The iron(III) indicator solution consists of a cold, saturated solution of ammonium iron(III) sulphate in water (about 40 per cent) to which a few drops of 6M nitric acid have been added. One millilitre of this solution is employed for each titration. 

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. ... 

A commercial silver alloy in the form of wire or foil is suitable for this determination. Clean the alloy with emery cloth and weigh it accurately. Place it in a 250 mL conical flask, add 5 mL water and 10 mL concentrated nitric acid place a funnel in the mouth of the flask to avoid mechanical loss. Warm the flask gently until the alloy has dissolved. Add a little water and boil for 5 minutes in order to expel oxides of nitrogen. Transfer the cold solution quantitatively to a 100 mL graduated flask and make up to the mark with distilled water. Titrate 25 mL portions of the solution with standard 0.1 M thiocyanate. 

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. 

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. 

Standardisation. Pipette 10.0 mL of the sodium tetraphenylborate solution into a 250 mL beaker and add 90 mL water, 2.5 mL 0.1 M nitric acid, 1.0 mL iron(III) nitrate solution, and 10.0 mL sodium thiocyanate solution. Without delay stir the solution mechanically, then slowly add from a burette 10 drops of mercury(II) nitrate solution. Continue the titration by adding the mercury(II) nitrate solution at a rate of 1-2 drops per second until the colour of the indicator is temporarily discharged. Continue the titration more slowly, but maintain the rapid state of stirring. The end point is arbitrarily defined as the point when the indicator colour is discharged and fails to reappear for 1 minute. Perform at least three titrations, and calculate the mean volume of mercury(II) nitrate solution equivalent to 10.0 mL of the sodium tetraphenylborate solution. 

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