Hydrochloric acid liquid junction

Molisch s test A general test for carbohydrates. The carbohydrate is dissolved in water, alcoholic 1-naphthol added, and concentrated sulphuric acid poured down the side of the tube. A deep violet ring is formed at the junction of the liquids. A modification, the rapid furfural test , is used to distinguish between glucose and fructose. A mixture of the sugar, 1-naphthol, and concentrated hydrochloric acid is boiled. With fructose and saccharides containing fructose a violet colour is produced immediately the solution boils. With glucose the appearance of the colour is slower. 

The two equations show that the nearer the cationic transport is to 0.5, the smaller is the liquid junction potential (other factors being unchanged). Among common electrolytes one of the highest numerical values of the factor (2 t+ - I) is given by hydrochloric acid, at 0.65. Hence a potential difference of about 39 mV develops at 25 °C across the junction between 0.001 N and 0.01 N hydrochloric acid. In the case of potassium chloride solution,... 

In addition, a solution of pure hydrochloric acid might be present between the two saturated solutions. In this particular case, the considerations outlined in previous sections would lead us to believe that the liquid-junction potential in such a cell is negligible. If, however, hydrogen and silver chloride were more soluble, the resulting liquid-junction potential might not be negligible. 

Consider the liquid junction of the concentration cell given above. There are two phases a and /3, which contain hydrochloric acid at different activities. The transport of ions at equilibrium is expressed by... 

For the purpose of exact calculation of the liquid junction potential according to the last mentioned equation the activity coefficient of the hydrogen or chloride ions would have to be known. As their value is unknown it is assumed that their ratio is equal to the ratio of the mean activity coefficients of hydrochloric acid, or in other words ( +)2/( +)i — (a+)2l(a )v Then the equation (VI-28) will be written in this form ... 

To establish a pH scale, Sorensen chose a dilute hydrochloric acid solution for a standard. He took the concentration of hydrogen ions in such a solution to be given by aC, where C is the concentration of hydrochloric acid and a is a degree of dissociation determined from conductance measurements. His procedure had drawbacks first, there is evidence that the extrapolation procedure does not actually reduce the liquid-junction potential of Cell (3-6) to zero second, the hydrochloric acid is completely dissociated (dissociation constant about 1.6 x 10 ), and therefore the concentration of H is C rather than a somewhat smaller quantity. 

Smith and Getz determined the formal electrode potentials in various concentrations of perchloric, nitric, and sulfuric acids, with the results given in Table 18-1. They postulated that Ce(IV) existed as anionic complexes Ce(C104)6 , Ce(N03)6 , Ce(S04)3, and CeClg in perchloric, nitric, sulfuric, and hydrochloric acids. Although Ce(rV) is not particularly stable in hydrochloric acid, a value of 1.28 V in 1 M was reported. Note that Smith and Getz measured formal potentials using a saturated calomel electrode, and the measured values, therefore, include liquid-junction potentials that cannot be estimated accurately. 

In this book the following conventions will be used in representing galvanic cells (a) a semicolon ( ) will be used to indicate a metal electrolyte boundary, such as Zn ZnSO (b) a liquid junction or electrolyte-electrolyte boundary will be shown by a colon ( ), thus, for instance, hydrochloric acid at the concentrations Ci and C2 may form the liquid junction... 

The most extensive study of cells "without liquid junction involving hydrochloric acid has been made by Harned and Ehlers.15 From a critical summary of their work they have obtained potentials at rounded molalities from which emf values of cells of the type... 

The Standard Potential of Chlorine. Measurements of the potentials of galvanic cells without liquid junctions from which the standard potential of chlorine may be deduced have been made by Lewis and Ruppert40 who used, as one electrode, platinum over which a mixture of chlorine and nitrogen was bubbled, and, as reference, a calomel electrode and hydrochloric acid as electrolyte. The arrangement may be represented by... 

The electrode is directly immersed in the electrolyte solution without liquid junction. In general hydrochloric acid of 0.01 or 0.001 mol kg molality is used as electrolyte [7]. The equilibrium potential depends on the hydrogen ion activity and therefore on the pH of the sample electrolyte. The electrode surface and therefore also the solution must be saturated with hydrogen. 

The hydrogen electrode is still the basis for all pH measurements to date. The pH of the standard buffers used to calibrate pH electrodes is traced back to primary buffer solutions [11]. The primary method for pH is based on the measurement of the potential of an electrochemical cell without liquid junction, involving a selected buffer solution, a platinum hydrogen electrode, and a silver/silver chloride reference electrode. The standard potential of the silver/silver chloride electrode in hydrochloric acid at a molality of 0.01 mol kg is determined simultaneously. 

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