Direct titration with silver nitrate

Argentometric titrations may be divided into two broad categories, namely (/ ) Direct titration with silver-nitrate, and... 

Pharmaceutical substances essentially containing halides may be estimated by direct titration with silver nitrate solution as a titrant. 

The pharmaceutical substances in Table 8.1, can be assayed by direct titration with silver nitrate using a suitable indicator. 

Table 8.1 Substances Assayed by Direct Titration with Silver Nitrate...

First, we recall that several direct titrations with silver nitrate have been recommended in formularies and pharmacopeias older than the present European pharmacopeia. These include the titrations of... 

When the interferents cannot be easily masked, or eliminated, then bromide can be removed from the sample as BrCN by distillation and quantitatively absorbed in sodium hydroxide solution. BrCN can be decomposed by sulfuric acid and the bromide content measured by titration with silver nitrate reagent, or by direct potentiometry. Chloride or iodide content of the sample does not interfere with the determination. 

For direct titration a number of alternative methods of end-point detection are possible. The oldest procedure, and still in use to a considerable extent for fairly pure samples, is Mohr s method. In this method potassium chromate is added to the chloride solution which is then titrated with silver nitrate. Because it is so much more insoluble than silver chromate, silver chloride first precipitates in a colloidal form which tends to coagulate as the end-point is reached immediately all the chloride is titrated silver chromate is formed as a brownish-red precipitate which gives a good indication of the end-point. It is important that the quantity of chromate added should not be too excessive or the red precipitate may form before all chloride is precipitated and so give a premature end-point. In very dilute or hot solution the end-point indication is poor, due to solubility of silver chromate. Mohr s method may be used for chlorides or bromides, but is unsatisfactory for iodides because (i) a mixed precipitate of iodide and chromate may be obtained and (ii) the colour of silver iodide masks that of the precipitating chromate. The following general procedure may be used ... 

Give a comprehensive account of the direct titration method with silver nitrate with reference to the following ... 

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). 

Puschel and Stefanac ° use alkaline hydrogen peroxide in the oxygen flask method to oxidize arsenic to arsenate. The arsenate is titrated directly with standard lead nitrate solution with 4-(2-pyridylazo) resorcinol or 8-hydroxy-7-(4-sulpho-l-naphthylazo) quino-line-5-sulphonic acid as indicator. Phosphorus interferes in this method. The precision at the 99% confidence limit is within 0.67% for a 3-mg sample. In another variation, Stefanac used sodium acetate as the absorbing liquid, and arsenite and arsenate are precipitated with silver nitrate. The precipitate is dissolved in potassium nickel cyanide (K2Ni(CN)4) solution and the displaced nickel is titrated with EDTA solution, with murexide as indicator. The average error is within + 0.19% for a 3-mg sample. Halogens and phosphate interfere in the procedure. 

An alternative method of end-point detection in the direct titration of halides with silver nitrate is by the use of adsorption indicators. First described by Fajans, these methods depend upon the fact that the colloidal precipitate of silver halide adsorbs ions from solution before the end-point is reached a layer of negatively charged chloride ions is adsorbed but as soon as silver ions are present in excess these are adsorbed on to the precipitate and attract the negatively charged anion of indicators of the fluores-... 

The purity of the crystallized product, determined volu-metrically by Volhard s method, exceeds 98%. In this procedure, 10 ml. of a 1% solution of methylisourea hydrochloride is acidified with a few drops of nitric acid and treated with 20 ml. of 0.1 N silver nitrate. After removal of the silver chloride by filtration, the excess of the silver nitrate is estimated with 0.1 TV thiocyanate solution, using ferric alum as indicator. Alternatively, 10-ml. portions of 0.1 N silver nitrate, acidified with nitric acid, may be titrated directly with the 1% methylisourea hydrochloride solution in the presence of tartrazine. 

Chloride Content (as Cl) Transfer about 1 g of sample, previously dried in vacuum for 1 h and accurately weighed, into a 250-mL beaker, and add sufficient water to make 100 mL. Equip a pH meter with glass and silver electrodes, and set it on the + millivolt scale. Insert the electrodes and a motor-driven, glass stirring rod into the sample beaker. Add 1 to 2 drops of methyl orange TS. Stir, and add, drop wise, 10% nitric acid until a pink color appears, then add 10 mL in excess. Titrate the solution with 0.1 A silver nitrate to a reading of +1.0 millivolt on the pH meter. Each milliliter of 0.1 A silver nitrate is equivalent to 3.545 mg of chloride. Lead Determine as directed in the APDC Extraction Method under Lead Limit Test, Appendix IIIB. 

Assay Dissolve about 0.4 g of sample, accurately weighed, in 20 mL of 16 100 sulfuric acid, add 5 mL of 85% phosphoric acid, dilute with approximately 50 mL of water, and immediately titrate with 0.1 N ceric sulfate, using orthophenanthroline TS as the indicator. Perform a blank determination (see General Provisions), and make any necessary correction. Each milliliter of 0.1 N ceric sulfate is equivalent to 5.585 mg of Fe. Chloride Heat 100 mg of sample, accurately weighed, with 25 mL of water and 2 mL of nitric acid until the sample dissolves. Cool, dilute to 100 mL with water, and mix. Take 10 mL of this solution, and dilute to 30 to 40 mL with water. Proceed as directed in the Chloride Limit Test under Chloride and Sulfate Limit Tests, Appendix IIIB, beginning with add 1 mL of silver nitrate TS... . Any turbidity produced does not exceed that shown in a control containing 20 pig of chloride (Cl). 

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. 

A similar approach can be used with other electrodes. In the following example, the titration of a chloride sample with standard silver nitrate, the potential of a silver electrode in combination with a saturated calomel reference electrode was used to follow the course of the titration. The potential of the electrode pair is a direct measure of the free chloride ion concentration as the chloride ion concentration decreases, the potential increases. The titration results are shown in Figure 20-3. 

Table 11 shows some results obtained in titrating gasoline and No. 2 fuel oil samples for mercaptan sulfur using 0.001 N silver nitrate. These results were reduced directly to ppm of sulfur and standard deviations of 0.05 ppm were obtained with both the gasoline and fuel oil. 

Huber and Van der Wielen determine the volatile oil (thiocyanate) in mustard seeds as follows. Their experiments were directed towards determining to what extent the time of maceration of the crushed seeds in water influences the result. Five gram samples of the mustard were macerated for 1, 2, 4, 18, and 20 hours respectively, with 100 c.c. of water, after which were added 20 c.c. of alcohol and 2 c.c. of olive oil. Of the mixture about 50 c.c. were distilled into a 100 c.c. measure containing 10 c.c. of ammonia, taking care that the delivery tube was immersed in the ammonia. After adding 20 c.c. of deci-normal silver nitrate solution the whole was heated over a water-bath until the silver sulphide had aggregated and the liquid was clear. The liquid was then cooled and made up with water to 100 c.c., the excess of silver nitrate was determined by titration with deci-normal ammonium thiocyanate, usW iron alum as an indicator. 

Mohr s method is a direct titration. It consists of making a silver nitrate solution to react with the solution containing the halide to be determined in neutral or weakly... 

The number and variety of ion-specific electrodes is rapidly increasing with no end in sight. At the present writing, it is possible to use such electrodes to determine, either by direct or indirect measurement, ionic concentrations of the following species ammonia, bromide, cadmium, calcium, chloride, cupric, cyanide, fluoride, fluoroborate, iodide, lead, nitrate, perchlorate, potassium, sulfide, sodium, sulfur dioxide, and thiocyanate, all by direct measurement, and by titration methods aluminum, boron, chromium, cobalt, magnesium, mercury, nickel, phosphate, silver, sulfate, and zinc. 

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