Silver/ions determination

An example of an anion buffer that may be suitable for standardizing an ion-selective electrode is based on the greater insolubility of silver iodide than silver chloride (Havas et al., 1971). This iodide ion buffer comprises a solution containing a high concentration of free chloride ion to which is added a small amount of silver ion and a smaller amount of iodide ion. The chloride ion activity determines the activity of silver ion (because of the solubility product of AgCl) and, in turn, the activity of silver ion determines the activity of the iodide ion in the buffer solution. For chloride ion concentrations of IM, O.lM, and O.OlM, pi is approximately 5.85, 6.85 and 7.85, respectively. 

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

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. 

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. 

Instmmental methods are useful for the determination of the total silver ia a sample, but such methods do not differentiate the various species of silver that may be present. A silver ion-selective electrode measures the activity of the silver ions present ia a solution. These activity values can be related to the concentration of the free silver ion ia the solution. Commercially available silver ion-selective electrodes measure Ag+ down to 10 flg/L, and special silver ion electrodes can measure free silver ion at 1 ng/L (27) (see Electro analytical techniques). 

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). 

Brewis et al. used TOF-SIMS to determine the surface composition of hydrocarbon polymers after electrochemical pretreatment with nitric acid alone or in the presence of silver ions [58J. AgNO was generated by electrolysis of a 0.1 M solution of silver nitrate in 3.25 M nitric acid in the anode compartment of a... 

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. 

Other useful solid-state electrodes are based on silver compounds (particularly silver sulfide). Silver sulfide is an ionic conductor, in which silver ions are the mobile ions. Mixed pellets containing Ag2S-AgX (where X = Cl, Br, I, SCN) have been successfiilly used for the determination of one of these particular anions. The behavior of these electrodes is determined primarily by the solubility products involved. The relative solubility products of various ions with Ag+ thus dictate the selectivity (i.e., kt] = KSp(Agf)/KSP(Aw)). Consequently, the iodide electrode (membrane of Ag2S/AgI) displays high selectivity over Br- and Cl-. In contrast, die chloride electrode suffers from severe interference from Br- and I-. Similarly, mixtures of silver sulfide with CdS, CuS, or PbS provide membranes that are responsive to Cd2+, Cu2+, or Pb2+, respectively. A limitation of these mixed-salt electrodes is tiiat the solubility of die second salt must be much larger than that of silver sulfide. A silver sulfide membrane by itself responds to either S2- or Ag+ ions, down to die 10-8M level. 

A color film makes use of sensitizer molecules that absorb photons and then reduce silver ions. A color film contains three emulsions overlaid on one another, each emulsion containing a different sensitizer. One sensitizer selectively absorbs red light, one selectively absorbs blue light, and the third selectively absorbs green light. In each layer, absorption of photons of the selected color results in clumps of neutral silver atoms. These clumps occur in different places in each layer, as determined by where photons of each color were absorbed. 

Tu, A.T. and Reinosa, J.A., The interaction of silver ion with guanosine, gua-nosine monophosphate and related compounds Determination of possible sites of complexing, Biochemistry, 5,3375,1966. 

Schildkraut, D.E., P.T. Dao, J.P. Twist, A.T. Davis, and K.A. Robillard. 1998. Determination of silver ions at submicrogram-per-liter levels using anodic square-wave stripping voltammetry. Environ. Toxicol. Chem. 17 642-649. 

Helmerson 2881, and more recently Bartels 2891, determined chloride in serum by adding excess silver and then measuring the excess silver ion in the filtrate. Ezell 29°) used a similar procedure to determine chloride in plant liquors and Gutsche etal.29 determined chlorine in milk by measuring the excess silver by flame photometry. 

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. 

As an example of the determination of electrochemical reaction orders, we consider the deposition of silver from an aqueous solution containing cyanide. The latter forms various complexes with silver ions, such as AgCN, Ag(CN) ", Ag(CN)3. Therefore, in the bulk of the solution reactions of the type ... 

And finally, the same procedure was followed as above, but instead of obtaining an unknown concentration of silver ion, the student prepared a solution containing 2 drops of 1.00 M Cu(N03)2 in 10. mL of 6.00 M NH3. The student determined that it took 20. drops to equal 1 mL. This solution was then added to the cell containing the copper electrode. The voltage was read as 0.56 V. The cell can be represented as ... 

The determination of chloride using an instrument known as a chloride meter is probably the most common application of coulometry in biochemistry. The instrument is designed to generate silver ions electrolytically from a silver anode. These ions are removed from the solution as undissociated silver chloride, which is either deposited on the anode or precipitated in the solution. A low concentration of carrier electrolyte (nitrate ions) permits a small current... 

Oxidative microcoulometry has become a widely accepted technique for the determination of low concentrations of sulfur in petroleum and petroleum products (ASTM D3120). The method involves combustion of the sample in an oxygen-rich atmosphere followed by microcoulometric generation of a triiodide ion to consume the resulting sulfur dioxide. It is intended to distinguish the technique from reductive microcoulometry, which converts sulfur in the sample to hydrogen sulflde that is titrated with coulometrically generated silver ion. 

Step 1 Determine the concentrations of silver ions and chloride ions in the reaction mixture. 

Silver ions are being exchanged for ammonium ions on a ion-exchange column and traces of silver ions in the eluent from the column are determined every minute when an aliquot of eluent is taken. In order to measure a(Ag ), a detector of silver wire is immersed in the liquid to determine the electrode potential Ag+.Ag-Assume that the silver wire only sees the silver ions, i.e. that no other ions interfere. 

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