Silver titrations

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. 

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. 

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

Potassium iodide prepared as described has been used in a very careful study of the absolute accuracy of the poten-tiometric iodide-silver titration,2 by comparing it directly against pure silver. The ratio KI Ag found in this way agreed to within 0.02 per cent with the theoretical ratio. This small deviation is to be attributed to a slight absorption of iodide ions by the silver iodide at the potentiometric end point and not to an impurity in the potassium iodide. 

An example of the use of a reference material as an intermediate point in this traceability mechanism is the reference material DMR-160a (CENAM identification), sodium chloride, with its purity value assigned. This reference material is intended to be applied by field laboratories in chloride measurement, silver titration, and in all those analytical methods which require NaCl with a specific purity value. In Fig. 2 a complete trace-ability chain is shown by the use of reference materials to the SI units. 

Finally, the electroanalytical methods described in chapter 6 may be used to scan the solution for metal ions. Table S.I2 includes some examples of substances determined by silver titrations and table S.I3 some miscellaneous precipitation methods. Other examples have already been mentioned under cornplexometric titrations. 

A current of 0.050 amp. was passed through a silver titration coulometer, and at the conclusion 23 8 cc. of 0.1 n sodium chloride solution were required to titrate the silver dissolved from the anode. How long was the current flowing ... 

Tris(l,10-phenanthroline)iron( n) salts were prepared according to the usual manner. Chloride of [Co(phen)3]3+ was synthesized by refluxing a water-ethanol mixture containing pentaamminechlorocobalt(ni) chloride and 1,10-phenanthroline and by precipitating the salt with an ethanol-acetone mixture the perchlorate was precipitated with sodium perchlorate from the chloride solution. The salts of [Co(bpy>3]3+ were prepared in a similar manner. All the salts were recrystallized and then air-dried their purities were confirmed by the silver titration for the chloride ions, by the visible-ultraviolet spectrum measurement, and by the analysis for the number of water of crystallization,3 of which values were 7.0, 4.7, and 4.0 for the chlorides of [Fe(phen)3]3+, [Co(phen)3]3+, and [Co(bpy)3]3+, respectively, and 0.6, 1.5, and 3.3 for the corresponding perchlorates. 

Analytical chemistry began in the late eighteenth century with the work of French chemist Antoine-Laurent Lavoisier and others the discipline was further developed in the nineteenth century by Carl Fresenius and Karl Friedrich Mohr. As a pharmacist s apprentice in Frankfurt, Germany, Fresenius developed an extensive qualitative analysis scheme that, when it was later published, served as the first textbook of analytical chemistry. He built a laboratory at his house that opened in 1848. Here he trained students in gravimetric techniques that he had developed. Mohr developed laboratory devices such as the pinch clamp burette and the volumetric pipette. He also devised a colorimetric endpoint for silver titrations. It was his 1855 book on titrimetry, Lehrhuch der Chemisch-Analytischen Titromethode, that generated widespread interest in the technique. 

Jacob Volhard (Darmstadt, 4 June 1834-Halle, 14 January 1910) was assistant to Liebig in Munich, Hofmann in London, and Kolbe in Marburg. He was assistant (1863), and associate professor (1869), in Munich, professor at Erlangen (1882) and Halle. He synthesised sarcosine, and creatinine by a method suggested by Strecker (1861), devised the thiocyanate silver titration, and (with H. Erdmann) synthesised thiophene by heating a mixture of sodium succinate and phosphorus trisulphide. ... 

Silver ion titrations are nice because AgN03 is a primary standard. After drying at 110°C for 1 h to remove moisture, the solid has the exact composition AgN03. Methods for finding the end point in silver titrations are described in Section 6-6. Silver compounds and solutions should be stored in the dark to prevent photodecomposition and should never be exposed to direct sunlight. 

Very recently—so recently that I must state the results only tentatively —Mr. Ellis has measured the protein SH in the course of mitosis, again using the amperometric silver titration. He does find the mirror image relation (Fig. 9). He does not, incidentally, confirm the hypothesis originating with Rapkine that protein SH increases just before division. Actually, it is decreasing, more or less parallel with the increase in soluble SH. 

In summary, it appears that the formation of silver complexes is a function of proton affinity and ultimately of 2a values for both phosphorus and nitrogen compounds. However, the stability of the complex depends on the extent of the substitution. The utility of silver ion for titration of compounds of the phosphine class is potentially much more useful than for the amines. It is probable that silver titrations can be used to differentiate P and N atoms either in mixtures or in a single molecule. 

Thus, when titrating iodide with silver nitrate, coagulation occurs as soon as a slight excess of silver ion has been added (so that a point of zero charge has been surpassed). 

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.)... 

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

The nickel ion freed may then be determined by an EDTA titration. Note that two moles of silver are equivalent to one mole of nickel and thus to one mole of EDTA. 

The equivalent amount of cadmium ion exchanged for the silver ion can readily be determined by EDTA titration procedures. 

For Volhard methods identified by an asterisk ( ) the precipitated silver salt must be removed before carrying out the back titration. 

The %w/w K in a 0.6712-g sample was determined by a Volhard titration. After adding 50.00 mb of 0.05619 M AgNOa and allowing the precipitate to form, the remaining silver was back titrated with 0.05322 M KSCN, requiring 35.14 mb to reach the end point. Report the %w/w K in the sample. 

Carbonate is measured by evolution of carbon dioxide on treating the sample with sulfuric acid. The gas train should iaclude a silver acetate absorber to remove hydrogen sulfide, a magnesium perchlorate drying unit, and a CO2-absorption bulb. Sulfide is determined by distilling hydrogen sulfide from an acidified slurry of the sample iato an ammoniacal cadmium chloride solution, and titrating the precipitated cadmium sulfide iodimetrically. 

An acidimetric quantitative determination is based on treatment of the hydantoia with silver nitrate and pyridine ia aqueous solution. Complexation of the silver ion at N-3 Hberates a proton, and the pyridinium ions thus formed are titrated usiag phenolphthaleia as an iadicator. In a different approach, the acidity of N-3—H is direcdy determined by neutralization with tetrabutylammonium hydroxide or sodium methoxide ia dimethylformarnide. 

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 ... 

The precipitated acetyHde must be decomposed with hydrochloric acid after the titration as a safety measure. Concentrated solutions of silver nitrate or silver perchlorate form soluble complexes of silver acetyHde (89). Ammonia and hydrogen sulfide interfere with the silver nitrate method which is less... 

In addition to modem spectroscopic methods ( H nmr spectroscopy, ftir spectroscopy) and chromatographic methods (gc, hplc), HBr titration (29) is suitable for the quantitative analysis of ethyleneimine samples which contain relatively large amounts of ethyleneimine. In this titration, the ethyleneimine ring is opened with excess HBr in glacial acetic acid, and unconsumed HBr is back-titrated against silver nitrate. 

The potentiometric micro detection of all aminophenol isomers can be done by titration in two-phase chloroform-water medium (100), or by reaction with iodates or periodates, and the back-titration of excess unreacted compound using a silver amalgam and SCE electrode combination (101). Microamounts of 2-aminophenol can be detected by potentiometric titration with cupric ions using a copper-ion-selective electrode the 3- and... 

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). 

Quantitative. Classically, silver concentration ia solution has been determined by titration with a standard solution of thiocyanate. Ferric ion is the iadicator. The deep red ferric thiocyanate color appears only when the silver is completely titrated. GravimetricaHy, silver is determined by precipitation with chloride, sulfide, or 1,2,3-benzotriazole. Silver can be precipitated as the metal by electro deposition or chemical reduciag agents. A colored silver diethjldithiocarbamate complex, extractable by organic solvents, is used for the spectrophotometric determination of silver complexes. 

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). 

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