G. G. Long and F. C. Hentz, Jr. ~ o r t hCarolina State University Raleigh, 27607
Mercurimetric Determination of Chloride in Lite salt@ A freshman chemistry experiment
Binary compounds of halogens with metals are usually considered as essentially ionic compounds which behave as strong electrolytes in ionizing solvents such as water. This description is rather a good one when the metal is in a low oxidation state and the halogen is fluorine or chlorine (Fajans' Rules); however, like most generalizations there are exceptions, e.g., compounds of mercury(I1) with halogens other than fluorine. Thus, when a solution containing mercury(I1) ions (such as aqueous Hg(N03)z or Hg(C104)d is added to a weakly acidic 2-propanol-water solution containing C1-, Br-, or I-, the net result is the stoichiometric formation of neutral HgX2 molecules1
In the case of iodide, the endpoint is indicated by the formation of red crystals of HgI2; for Cl- or Br- the indicator diphenylcarbazone can be used to form an easily discernible pink-violet complex of m e r c ~ r y ( I 1 )when ~ the first slight excess of titrant has been added. This titration can be used to determine CI-. . Br-. . or I- in a wide variety of routine situations or to determine total halide in a mixture such as sea water. A oarticularlv interesting student unknown is Lite Salta which, except for traces of additives, consists of sodium and potassium chlorides. Indeed, the composition is indicated on the label by the statement that "100 g of Morton's Lite Salta contains 19,480 mg of sodium and 25,975 mg of potassium." Thus, the student can use the chloride determination to verify the above information from the label on the box. This in itself is an instructive stoichiometric calculation for the freshman chemistry student. Experimental Preparation and Standardization of 0 . 0 5M~Mercuryp) ~~ Nitrate Solution
About 36 g of Hg(NOdrH20 is dissolved in a mixture of 5 ml of concd HN03 and 15 ml of deionized water which is then made up to 1 I. Reagent grade KC1 is dried at llO°C and 0.1700-0.2100 g samples are weighed into 250-ml Erlenmeyer flasks. Each sample is dissolved in 50 ml of deionized water and a maenetic stirrine bar is nrldrd to the flask. Three drops of hrumphenol blue (0.1% in 2. propnnolj are added and the p H adjusted by adding 0.1 A1 HNO? until the indicawr changes from blue to yellow. The solutiun is di-
714 / Journal of ChemicalEd~~dtbn
luted to 100 ml with 2-propanol, 10 drops of diphenylcarbazone (0.1%in 2-propsnol) indicator are added, and the solution is titrated with the mercury(I1) solution to the first permanent pink color.
Determination of Chloride in Lite S a p Lite Saltm (2.5xxx g, previously dried at llO°C) is weighed out;' as much of the solid as possible is dissolved in deionized water and diluted to 250.0 ml in a volumetric flask. The resulting solution will have a slight turbidit9 (which may be ignored) due to traces of insoluble additives. Aliquots (20.00 ml) are pipetted into 250-ml Erlenmeyer flasks, diluted to 50 ml with deionized water, and the procedure continued in the same manner as given for the standardization of the mercury(I1) solution. The students are asked to calculate the total percent chloride from the product label, the average percent chloride found, and the difference between %round and %,M. We have used this mercurimetric titration for some time in our freshman chemistry classes and found i t to work well due to the ease with which the endpoint is detected. The experiment can serve as a n introduction to volumetric analysis since the student needs to use a volumetric flask, a pipet, and a buret. We frequently have our chemistry majors standardize the mercury(I1) solutions, but with the other classes the student is provided with standardized mercury(I1) titrant. Based on the label, theoretical %C1 in "Lite Salt" is 53.6%. At about midterm a recent class of first semester freshmen carried out the standardization, sample-solution preparation, titrations, and calculations, with the result that over half of the students reported %Clin the range 53.6 z t 0.5%. Clarke, F. E., Anal. Chem., 22,553 (1950). This endpoint is much easier for the inexperienced student to detect as compared to endpoints for the Mohr or Fajans' titration of halide with silver nitrate. 3 Lite Salt@is the registered trademark of the Morton Salt Company, Chicago Illinois, a division of Morton-Norwich Products, Inc. It is preferable to weigh out s relatively large quantity of Lite Salta and take aliquots of the solution rather than weighing out small individual samples since the Lite Salta is not truly homogeneous. 5 If the small amount of precipitate which causes the turbidity of the solution has started to settle, the volumetric flask containing the salt solution should be shaken to homogenize the solids before pipetting out aliquots. 2
