Chloride precipitation titration

The indicator electrode employed in a potentiometric titration will, of course, be dependent upon the type of reaction which is under investigation. Thus, for an acid-base titration, the indicator electrode is usually a glass electrode (Section 15.6) for a precipitation titration (halide with silver nitrate, or silver with chloride) a silver electrode will be used, and for a redox titration [e.g. iron(II) with dichromate] a plain platinum wire is used as the redox electrode. 

A second method which is now probably the most widely used method in the Pediatric Laboratory is to use amperometric titration. In this connection, a constant current flows through the solution. The silver dissolves and reacts stolchlometrlcally with chloride, precipitating silver chloride. When all of the chloride has reacted, there is a sharp increase in conductivity which is read as an end point. This instrument, therefore, measures the amount of time a current flows. Instruments are now available for which 5 microliters can be used routinely, rapidly, titration being of the order of about 20 seconds. 

Precipitation titrations are typified by the titration of chloride with silver or vice versa. In this case, interferences with the precipitation reaction may occur because of components in the soil, and the soil itself may interfere with detection of the end point. Thus, complexation reactions are rarely applied directly to soil however, they can be applied to soil extracts. Common environmental titration methods described in the United States Environmental Protection Agency (USEPA) methods are summarized in Table 10.1 [1,2],... 

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). 

Chlorides, bromides, and iodides can be quantitatively determined by treatment with silver nitrate, and, with suitable precautions, the precipitated halide is washed, dried, and weighed. Chlorides in neutral soln. can be determined by F. Mohr s volumetric process 27 by titration with a standard soln. of silver nitrate with a little potassium chromate or sodium phosphate as indicator. When all the chloride has reacted with the silver nitrate, any further addition of this salt gives a yellow coloration with the phosphate, and a red coloration with the chromate. In J. Volhard s volumetric process, the chloride is treated with an excess of an acidified soln. of silver nitrate of known concentration. The excess of silver nitrate is filtered from the precipitated chloride, and titrated with a standard soln. of ammonium thiocyanate, NH4CN8—a little ferric alum is used as indicator. When the silver nitrate is all converted into thiocyanate AgN03-fNH4CNS=AgCNS +NH4NOS, the blood-red coloration of ferric thiocyanate appears. 

Example of (3).—0 99 gm. of Chrysophenine G (C30H26N4O8SaNa2) is dissolved in a litre of water. 100 c.cs. of this solution are withdrawn and boiled, with a current of carbon dioxide passing through 10 c.cs. of cone, hydrochloric acid and 50 c.cs. of titanous chloride are then added, and the mixture boiled until the precipitate dissolves and the solution turns a slight violet colour. After cooling, the excess of titanous chloride is titrated with ferric alum. 

The bronzes are decomposed in a stream of oxygen and hydrogen chloride at 600° C. The residue of sodium chloride is titrated with standard silver nitrate solution, giving the sodium content of the sample. Tungsten is determined by precipitation of tungstic acid and ignition to tungsten (VI) oxide in the residue from the above treatment. Before analysis all of the samples analyzed were leached successively with concentrated ammonia and hydrochloric and hydrofluoric acids. 

This expression shows that under equilibrium conditions the concentration of chromate ions in the solution is always much greater than that of the chloride ions. If therefore to a mixture of chloride and chromate ions, silver ions are added, these will combine with chloride ions, forming silver chloride precipitate until the concentration of chloride ions in the solution decreases to such an extent, that the ratio expressed in equation (iii) is achieved. From then onwards the two precipitates will be formed simultaneously. If a 01m solution of sodium chloride is titrated with silver nitrate in the presence of 0 002m potassium chromate, the concentration of chloride ions at which silver chromate starts to precipitate can be expressed from equation (iii) ... 

Small quantities of molybdenum may be determined by reduction with excess of titanous chloride, and titrating back ith ferric chloride solution, potassium thiocyanate being used as indicator or the reduction may be followed electrometrieaUy. If tungsten is present it must be eliminated by precipitation before the addition of the reducing agent,... 

Precipitation titrations can be extended to mixtures that form precipitates of different solubilities. The titration curve in Fig. 2 for a chloride/iodide mixture is a composite of the individual curves for the two anionic species. Because silver iodide has a much lower solubility than does silver chloride, the initial additions of the reagent result exclusively in formation of iodide. Thus, two equivalence points are evident. 

Salts are the products of the acid-base neutralisation reaction. The salts used most in textile wet processes are common salt (NaCl, sodium chloride) and Glauber s salt (Na SO, sodium sulphate). The content analysis of salts is usually conducted by using a precipitation titration method which may be followed by fdtering and weighing procedures to obtain the final results. 

Industrial grade NaCl has a content of 92-98%. The precipitation titration can be conducted using 0.1 N AgNO, as the titrant and 5% K,Ci<) as the indicator (the Mohr method). The sample chloride solution should be buffered with calcium carbonate to a pH between 6.3 and 7.2 in order to avoid any interference from other... 

Precipitation. In the case of precipitation, the titrant forms an insoluble product with the analyte. An example is the titration of chloride ion with silver nitrate solution to form silver chloride precipitate. Again, indicators can be used to detect the end point, or the potential of the solution can be monitored electrically. 

Indicators Reacting with the Titrant. There are several examples of an indicator forming a colored compound with a titrant. The Mohr method for determining chloride serves as an example. The chloride is titrated with standard silver nitrate solution. A soluble chromate salt is added as the indicator. This produces a yellow solution. When the precipitation of the chloride is complete, the first excess of Ag reacts with the indicator to precipitate red silver chromate ... 

The indicating electrode in precipitation titrations is used to follow the change in pM or pA, where M is the cation of the precipitate and A is the anion. In the titration of chloride ion with silver ion, for example, either Equation 13.3 or 13.10 will hold. In the former equation, the term log (1/uAg ) is equal to pAg and in the latter equation, the term log Oq- is equal to —pCl. Therefore, the potential of the silver electrode will vary in direct proportion to pAg or pCl, changing 2.30 RT/F V (ca. 59 mV) for each 10-fold change in or flci-- A plot of the potential versus volume of titrant will give a curve identical in shape to that in Figure 11.1. (Note that since aAg+<3a- = constant, Uci- is proportional to and pCl is proportional to — pAg, so the same shape curve results if we plot or measure either pCl or pAg.)... 

During heat treatments, zinc and calcium stearates react with hydrogen chloride that has evolved from the polymer or with labile chlorine atoms to liberate zinc and calcium chlorides. These compounds are titrated by a coulometric method in tetrahydrofuran (7) using a generator of Ag+ ions that are precipitated in the presence of chloride ions. When zinc and calcium stearates are used together, both chlorides are titrated at the same time, and they are not differentiated. 

In this experiment you will determine the molar solubility of PbCl2 as a function of temperature by means of a precipitation titration to determine the molar concentration of chloride ion at each temperature. From this data you will calculate Kgp at each temperature, plot In Kgp versus 1/T, determine AH° and AS° from the slope and intercept, and then calculate AG<. ... 

As with the other reaction types, the precipitation titration curve can be described by a single equation. If of a chloride solution is titrated with C gM AgN03, then at any point after the addition of mL, balance equations can be written for [Cl ] and [Ag ]. The total Ag content is distributed between dissolved Ag and precipitated AgCl. Similarly, total Cl is the sum of dissolved Cl" and precipitate. 

In precipitation titrations the titration reaction produces an insoluble precipitate. Despite the many known precipitation reactions, very few of them have the necessary requirements to be the basis of a volumetric method. Determining chloride using silver ion precipitation, using several methodologies, is the most appropriate application of this group of titrations. 

Table 1 shows some data for commercially available electrodes and a selection of interesting analytical uses of solid ISEs. The majority of applications has been the determination of fluoride in many kinds of samples. Chloride determinations are next in order of importance. The precision of direct determinations is usually limited to 1-10% rsd. The ruggedness of solid ISEs makes them suitable for online monitoring applications. If monitoring is based on direct measurement frequent recalibrations may be necessary and the temperature compensation may not always be straightforward. Monitors can also be based on titration reactions in this case less problem may be encountered but more sophisticated hardware will be necessary. Solid ISEs can also be used in different solvents. This may be useful, e.g., in precipitation titrations. 

The potentiometric detection of the endpoint of precipitation titrations is very often used because not many visual indicators are available, in particular when mixtures of analytes are titrated. Halides, cyanide, sulfide, chromate, mercaptans, and thiols can be titrated with silver nitrate, using the silver sulfide-based ISE. Also complex mixtures, such as sulfide, thiocyanide, and chloride ions, or chloride, bromide, and iodide ions, can be titrated potentio-metrically with silver(I) ions. When the solubility of a compound formed during titration is too high, nonaqueous or mixed solvents are used, for example,... 

The scientist who was responsible for the introduction of titrimetry into mainstream chemistry was Gay-Lussac. He further developed the apparatus used in volumetric analysis and was the first to use the word burette (Figure 14.3). In 1832 he introduced precipitation titrations for the estimation of silver. Sodium chloride solution was added to a solution of a silver salt until no further precipitation occurred. This was a time-consuming procedure as near the endpoint the precipitate had to be allowed to settle before each new portion of sodium chloride was added. 

After the work of Nemst on electrode potentials in 1889 (Chapter 13), it became apparent that EMF measurements could be used in analytical work. The first potentiometric titration was performed in 1893 by Robert Behrend (1856-1926). He titrated a solution of mercury(i) nitrate with potassium chloride, thus precipitating mercury(i) chloride. The titration vessel contained a mercury electrode, and was separated by a porous membrane from another container of mercury(l) nitrate with a mercury electrode. The EMF of the resulting cell depended on the concentration of the mercury ions in the titration vessel, and showed a pronounced change in the region of the end-point. 

Precipitation titrations, e.g. determination of halides by titration with silver nitrate, using a silver-silver chloride electrode. 

Actual Chloride, either titrated with N/10 silver nitrate solution, using neutral potassium chromate solution as indicator, or, preferably, estimated gravimetrically as silver chloride by precipitation with silver nitrate solution, the precipitate transferred to a tared filter paper, washed, dried and weighed. 

More methods are available for chlorine. As free chlorine, the element may be analyzed by reduction to chloride using iodide, arsenite, alkaline hydrogen peroxide, sulfur dioxide, or sodium thiosulfate (Na2S203), or determined colorimetrically by treatment with ort/to-toluidine in hydrochloric acid. Chloride ion may be precipitated as silver chloride or titrated with silver nitrate in the presence of potassium chromate (K2Cr04). 

A common type of precipitation titration uses silver nitrate to determine the concentration of chloride ions. Silver nitrate solution is added to a chloride solution in the presence of potassium chromate(vi), which acts as an indicator . 

Of the five anions checked, only iodide and bromide form soluble complexes the thiocyanate and cyanide complexes are insoluble, but are indicated by potential changes chlorides precipitate immediately at the beginning of the titration. A consideration of the mid-complex potentials (mcp) indicates that in acetone the cyanide 2 1 complex is the most stable and the thiocyanate the least stable. The halides fall between these extremes mcp values for the insoluble compound are not, of course, equilibrium values. 

In order to determine chloride ions, it is necessary to remove the silver chloride precipitate, for example, by filtration or by centrifugation, before carrying out the back titration with thiocyanate ions. The reason is the following after the addition of an excess of silver ions, the precipitation of chloride ions as silver chloride may be considered complete because of the common ion effect. During the second stage of the determination, the excess of silver ions disappears with the addition of thiocyanate ions. Close to the equivalence point of the back titration, when both precipitates coexist, the following solubility products are satisfied simultaneously ... 

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 ... 

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