Author Institution: Department of Chemistry, The Ohio State University, Columbus 1

Caley, Earle R.

Farrohha, Sabri M.

The Ohio Journal of Science

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A NEW TYPE OF COMPLEXOMETRIC TITRATION1EARLE R. CALEY AND SABRI M. FARROHHA2Department of Chemistry, The Ohio State University, Columbus 10This method of titration depends on the equilibria relations that may exist insolution between a metal ion and an anion, which form, according to their con-centrations, either an insoluble precipitate or a soluble complex ion of sufficientlylow stability to decompose rapidly and reversibly on dilution. The solution ofthe metal ion is prepared for titration by adding a measured large excess of theanion in the form of a soluble salt, considerably more being added than is sufficientmerely to redissolve the precipitate that is first formed. This prepared solution istitrated with distilled water up to the point where the soluble complex just beginsto decompose as is indicated by the appearance of a slight turbidity due to theincipient separation of the insoluble precipitate formed between the metal ion andthe anion. With other variables fixed, the volume of water required is a measureof the weight of metal ion present. This paper describes experiments with thesilver iodide-iodide system, as a typical example, to determine whether this typeof titration has any practical advantages.TITRATION APPARATUSThe effect of temperature on volume of titrant required is much greater thanin the usual type of titration. To avoid large corrections, titrations were performedat a standard temperature of 25 C. The titration vessel in the form of a liplessbeaker was half immersed in a constant temperature bath, and was provided witha rubber stopper through which could be passed an electrically operated stirrerand the tip of the buret. The buret, with a capacity of 50 ml, was provided withan elongated offset tip and with a glass water jacket 4 cm in diameter which en-veloped the buret from the stopcock to above the graduated portion. This jacketwas connected to the constant temperature bath with rubber tubing so that thewater of the bath could be constantly circulated through the jacket by an electricpump.EXPERIMENTSI. The first series of experiments was devised to estimate the minimum weightsof ammonium iodide required to produce entirely clear solutions when added tosilver nitrate solutions of various concentrations. A given weight of dried silvernitrate was weighed into a beaker and dissolved in a given volume of water deliveredfrom a pipet. The beaker was then half immersed in the constant temperaturebath maintained at 25 C. Dried ammonium iodide was added to the solution insuccessive weighed increments while the solution was being rapidly stirred. Thefirst additions were relatively large, but as soon as the precipitated silver iodideshowed signs of dissolving, the successive increments were sharply and progressivelyreduced, and the times of stirring were prolonged. Toward the end, the incrementswere in the range from 0.01 to 0.001 g. When the solution had lost its last traceof turbidity on thorough stirring, the weights of all the increments were addedtogether to determine the total weight of ammonium iodide required. Representa-tive results are shown in table 1. Since the results of duplicate trials often differedby as much as 0.01 g, these averaged results are given only through two decimalJPart of a paper presented at the Sixty-ninth Annual Meeting of The Ohio Academy ofScience, Antioch, Ohio, April 22, 1960.2Present address: Department of Chemistry, College of Science, Baghdad, Republic of Iraq.THE OHIO JOURNAL OF SCIENCE 63 (2): 92, March, 1963.No. 2 COMPLEXOMETRIC TITRATION METHOD 93places. No greater accuracy was needed because the data were used merely asa guide to insure addition of excess weights of ammonium iodide in subsequentexperiments.II. In this series ammonium iodide in excess over that needed to dissolve thesilver iodide was added to silver nitrate solutions of fixed concentration, and theresulting solutions were titrated with water to the point where silver iodide repre-cipitated. Dried silver nitrate was weighed into the titration vessel and dissolvedTABLE 1Minimum weight of ammonium iodide needed toform clear solution of iodide complex in silvernitrate solutions of different concentrations at 25 CAgpresent g1.0001.0001.0001.0001.0001.0000.2000.2000.2000.1000.1000.100Initial volumeof solution ml50.0025.0020.0015.0010.005.0015.0010.005.0015.0010.005.00NHJ neededav. weight g14.859.518.337.105.754.343.912.971.913.182.391.49TABLE 2Volume of water required for titration at 25 C of iodide complex formed by adding different weights ofammonium iodide to silver nitrate solutions containing 0.1000 g of silverNHJadded g4.0004.5005.0005.5006.0006.5007.000Volume ofAg solution ml10.0010.0010.0010.0010.0010.0010.00Waterrequired ml10.7414.4018.2322.2026.2830.5434.90Volume ofAg solution ml20.0020.0020.0020.0020.0020.0020.00Waterrequired ml0.584.258.0612.0316.1120.3724.72in a given volume of water delivered from a pipet. A given weight of dried am-monium iodide was then added and the solution was stirred until it reached thetemperature of the bath. The titration was performed by the slow addition ofwater to the stirred solution until the first faint permanent turbidity appeared.Representative results are shown in table 2.III. This series was similar to Series II except that parallel experiments weremade at four different temperatures. The results in table 3 illustrate the effect oftemperature. This effect is much larger than can be ascribed to changes in volumecaused by temperature, and must be due largely to a decrease in the stability ofthe silver iodide-iodide complex with increase in temperature. Though the94 EARLE R. CALEY AND SABRI M. FARROHHA Vol. 63figures in table 3 might seem to indicate that the changes in the volume of waterrequired follow a simple regular pattern, an analysis of these figures shows thatthe effect is a complex one that cannot be corrected by a single temperature co-efficient. It is better, therefore, to titrate at some standard temperature ratherthan to attempt to apply complex corrections. The results in table 3 show thatfor accurate results the temperature in titrations of this type should be held towithin 0.1 C of the selected standard temperature.IV. In this series the conditions simulated those for actual determinations ofsilver. In a given group of experiments a graded series of weights of dried silvernitrate were separately dissolved in identical volumes of water, and the same weightof dried ammonium iodide was added to each solution, which was brought to 25 Cand titrated. Representative results are shown in table 4. The unusual featureof these results is that, contrary to any other type of titration, the volume oftitrant increases with decrease in the weight of substance being titrated. ThisTABLE 3Volume of water required at different room temperatures for titration of iodide complex formed byadding different weights of ammonium iodide to 5.00 ml of silver nitratenitrate solution containing 0.1000 g of silverNHJ added g2.5003.0003.5004.0004.5005.0005.5006.00015Cml6.479.8213.3016.9320.7324.7228.8633.14Volume of water required at20Cml6.109.3212.7116.3020.0523.9527.9532.05given temperature25Cml5.808.9412.2615.7719.4123.2227.1931.2830Cml5.588.6711.9715.4519.0922.8526.7330.70means that the absolute error of measurement should decrease as this weightdecreases. Consequently, this type of titration should be especially advantageousfor the accurate determination of very small weights of silver. However, it mayalso be used for the determination of larger amounts since the volume of titrantmay be controlled by the proper selection of the volume of initial solution and theweight of ammonium iodide added. These results also show that the amount ofsilver to be determined should be known approximately beforehand. If entirelyunknown, it may be rapidly estimated by adding test weights of ammonium iodideto aliquots of the unknown silver solution and titrating. The following is a de-tailed procedure for a silver solution estimated to contain between 0.00200 g and0.01000 g of silver per ml.PROCEDUREBy means of a micropipet or microburet, deliver a 5-ml aliquot of the silversolution into a 100-ml titration vessel for use with the apparatus already described.Add 4.000 g of ammonium iodide previously dried at 110 C, and when this hasdissolved add 20.00 ml of water at 25 C from the jacketed buret. Stopper thevessel, half immerse it in the constant temperature bath maintained at 25 C, andstir slowly for about 30 minutes. Continue stirring and complete the titration bythe slow addition of water until the first permanent turbidity appears. Find theNo. 2 COMPLEXOMETRIC TITRATION METHOD 95weight of silver corresponding to the total buret reading from a titration curve ortable prepared by titrating by this same procedure a graded series of standardsilver solutions having a range of concentration which includes that of the unknown.Notes on Procedure.—If a buret or pipet of high precision is not available, it isbetter to weigh 5.000 g of the sample solution into the titrating vessel. The purposeof first adding a portion of the water required for the total titration is to providesufficient volume for efficient stirring. The solution is stirred for the recommendedTABLE 4Volume of water required for titration at 25 C of iodide complex formed by adding a fixed weight ofammonium iodide to a fixed volume of silver nitrate solution of varying concentrationNHJadded g8.0008.0008.0008.0008.0008.0004.0004.0004.0004.0004.0004.0001.0001.0001.0001.0001.0001.000Agtaken g0.050000.040000.030000.020000.01000Volume ofsolution ml10.0010.0010.0010.0010.0010.005.005.005.005.005.005.001.001.001.001.001.001.00Standardtitrationreading ml22.0724.3627.6733.7948.40Agcontent g1.00000.80000.60000.40000.20000.10000.10000.08000.06000.04000.02000.01000.01000.00800.00600.00400.00200.0010TABLE 5Test determinationsSampletitrationreading ml22.1324.4027.6333.7048.32Waterrequired ml8.9012.1516.0921.6831.7143.4315.7017.7620.3524.3833.7548.366.246.857.739.4815.7922.50Difference involume ml+0.06+0.04-0.04-0.09-0.08Ag per mlof water g0.11240.06580.03730.01850.006310.002300.006360.004500.002950.001650.000590.000210.001600.001170.000780.000420.000130.00004Absolute errorfor Ag g+0.00014+0.00007-0.00004-0.00005-0.00002period in order to bring it to the temperature of the bath. Cross illumination ofthe solution during titration is helpful in obtaining sharp and reproducible end-points. Concentrations of ions other than silver should be similar in sample andstandards. The solution should be approximately neutral but exact adjustmentis not necessary because the ammonium iodide buffers the solution to the properpH range. Metals that form insoluble iodides interfere.96 EARLE R. CALEY AND SABRI M. FARROHHA Vol. 63RESULTS AND DISCUSSIONResults of a test of this procedure are shown in table 5. As these resultsindicate, and as other experiments have clearly shown, the precision tends todecrease appreciably with increase in volume of water required for tit ration, sothat the relative error of measurement remains approximately the same for differentweights of silver. The endpoint error caused by the volume of water needed toproduce a noticeable turbidity increases with increase in volume of the finalsolution, but is normally less than 0.10 ml, and is in effect cancelled out whenstandards and unknown are titrated to identical turbidities. The volumetricerror involved in measuring out the solutions and in reading the buret probablyaccounts for some of the observed inaccuracy. However, the absolute accuracyin the determination of these small amounts by this method exceeds that of theusual titration methods for silver. Experiments on a few other systems indicatedthat the principle of this method of titration may be applied to the accuratedetermination of small amounts of various metals.

A New Type of Complexometric Titration

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A New Type of Complexometric Titration

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