Titrimetric Determination of Nitrate with Diphenylthallium(III) Sulfate Joseph S. DiGregorio’ and Michael D. Morris2 Department of Chemistry, The Pennsylvania State Unicersity, University Park, Pa. 16802 Nitrate may be titrated directly with diphenylthallium (111) sulfate. The titration is monitored either potentiometrically with a nitrate-selective electrode or amperometrically with a dropping mercury electrode. The amperometric titration is carried out at pH 1-2 and the potentiometric titration at pH 2-4, using sulfuric acid for pH adjustment, if necessary. Fluoride, sulfate, and phosphate do not interfere. Equimolar amounts of perchlorate do not:interfere with the amperometric titration. However, perchlorate interferes with the operation of the nitrate-selective electrode. Nitrite ion disproportionates, giving nitrate ion under the titration conditions, and must be removed by reduction. Halides interfere and must be removed by precipitation. The potentiometric titration can be carried out on samples 0.01M to 0.1M with a precision and accuracy of 10.6%,. The titration i s suitable for autotitrators. Interference removal procedures introduce sufficiently large quantities of other anions, chiefly sulfate, to impair the operation of the nitrate electrode and introduce systematic positive errors of 1-2% in some cases. Amperometric titration can be carried out on samples 0.005M to 0.02M. Interference removal does not affect either accuracy or precision, which are il%.
THE present commonly employed methods for nitrate determination are unsatisfactory in many respects. Gravimetric determinations-such as the familiar nitron procedure (I, 2)-are time consuming, restricted to relatively large samples, and are not very accurate. The nitron method in particular suffers from the fact that the precipitate is fairly soluble, difficult to free from excess reagent, and not always of the theoretical composition. Furthermore, several other ions are precipitated by this reagent. Perchlorate is the most serious interference and there is no satisfactory procedure for removing this ion before attempting precipitation of the nitrate. The nitrate precipitating properties of some N-benzyl- and substituted N-benzylamines have been investigated recently (3, 4 ) . A few of these offer several advantages over nitron. These include lower solubility of the nitrate salt and a smaller number of interferences. However, perchlorate remains an interference which cannot be removed readily. The commonly used nitrate titrations (1, 2) involve the reduction of nitrate to ammonia, nitric oxide, nitrogen dioxide, or nitrite ion by one of several different reducing agents. Subsequent titration of a product formed or of the excess reagent added is utilized to determine the amount of nitrate Present address, Shell Pipe Line Corp., Houston, Texas 77001 Present address, Department of Chemistry, University of Michigan, Ann Arbor, Mich. 48104 1 2
(1) A. J. Clear and M. Roth in “Treatise on Analytical Chemistry,” Part 11, Vol. 5, I. M. Kolthoff and P. J. Elving, Eds., Interscience Publishers, Inc., New York, 1961, p 217. (2) M. Williams, Iiidustrid Chemisf, 1954, 94. (3) R. C. Hutton, S. A. Salam, and W. I. Stephen, J . Clzem. SOC., 1966, 1573. (4) R. C. Hutton and W. I. Stephen, ibid.,1967, 1426. 94
originally present. The reduction method is limited by several factors. The selectivity and sensitivity of the reduction methods are low. The presence of any reducible nitrogen species other than nitrate is a source of large errors. I n 1925 Krause and von Grosse suggested the use of diphenylthallium(II1) salts as nitrate reagents (5). They noted that the precipitation of nitrate with diphenylthallium(II1) fluoride is rapid and sensitive and that strong solutions of perchlorate give no precipitate. The solubility product of (CGH&T1NO3is 1.0 X loF7a t 25 “C (6). In 1953 Hartmann and Bathge (7) reported that dicyclohexylthallium(II1) salts can be used in both gravimetric and indirect titrimetric nitrate determinations. Dicyclohexylthallium(iI1) nitrate is less than either nitron nitrate or soluble (KSp = 1 x diphenylthallium(II1) nitrate, but dicyclohexylthallium(II1) salts are difficult to prepare. Diphenylthallium(II1) salts, on the other hand, are readily prepared and commercially available. In a recent communication (6) we proposed the use of diphenylthallium(II1) sulfate as a nitrate titrant and showed that amperometric titrations of nitrate could be carried out over the concentration range 5-20 X M nitrate. I n the present paper we report the effect of several possible interferences on this titration and described the use of the reagent for a potentiometric titration of nitrate, using a nitrateselective electrode as the indicator. EXPERIMENTAL Reagents. Diphenylthallium(II1) oxide was prepared from diphenylthallium(II1) chloride (Alfa Inorganics, Beverly, Mass.) by refluxing with ethanolic potassium hydroxide, as previously described (6). The oxide was also prepared from diphenylthallium(II1) iodide (Strem Chemicals, Danvers, Mass.) or from diphenylthallium(II1) nitrate, recovered after titration, by the same method. The oxide was converted t o the sulfate by dissolution in 0.1M sulfuric acid (6) to give a solution about 0.1M in diphenylthallium(II1). The yield of diphenylthallium(II1) is about 65 of the amount of chloride, iodide, or nitrate taken. The chemical properties of the sulfate solutions d o not depend upon the source of diphenylthallium(II1). The reagent may be stored in the open for up to four weeks without visible decomposition or change of titer. Potassium nitrate (ACS reagent grade), dried at 115 O C for two hours, was used as the nitrate source. Titron X-100 (Hartmon-Leddon Co., Phila., Pa.) was used as received. All other materials were of ACS reagent grade and were used without further purification. Apparatus and Procedure. The amperometric titration apparatus was conventional and has been described previously (6). Potentiometric titrations were carried out with a Leeds and Northrup p H meter (Model 7401) whose feedback loop was modified t o increase the full scale sensitivity t o 280 (5) E. Krause and A. von Grosse, Chem. Ber., 58, 272 (1925). (6) J. S. DiGregorio and M. D. Morris, Aizal. Letters, 1,811 (1968). (7) H. Hartmann and G. Bathge, Angew. Chem., 65, 197 (1953).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
I14 6 O0
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Figure 2. Amperometric titration of 0.5515 mmole NO3with 0.1126M(C2H&TI+(as SO4*-)
Figure 1. Polarogram of 1.7 X 10-3M (C8H5)2TI+in 0.1M K*SOI,O.O~MH?SOI, pH 1.5 0.001% Triton X-100 as maximum supressor
mV (8). A nitrate-selective electrode (Orion Researches, Cambridge, Model 92-07) was used as the indicator and a saturated mercurous sulfate electrode (Radiometer Type K 601, The London Co., Westlake, Ohio) was used as the reference. The meter was adjusted to read zero in a solution of 0.1M potassium nitrate. The amperometric titration procedure was generally similar to that reported previously (6). An aliquot of a nitrate sample was diluted to 25 or 50 ml with distilled water and stock solutions of sulfuric acid and potassium sulfate to give a final composition 0.1M potassium sulfate, 0.05M sulfuric acid (pH 1.4-1.6). Triton X-100 was added (0.005 to 0.05%) and the solution was deaerated with nitrogen, and titrated with diphenylthallium(II1) sulfate. After each addition of titrant the solution was stirred with nitrogen and a magnetic stirrer and a current reading was taken at -1.0 V us. SCE ( - 1.4 V us. SMSE). Three of four readings were taken before the end point and three or four afterwards. All titrations were carried out at room temperature. Potentiometric titrations were carried out in sulfuric acidsulfate buffers of pH 2-4 using sample volumes of 5-50 ml. The solution was stirred during the addition of titrant and then allowed to become quiet before potential readings were taken. The end point was taken to be the mid-point of the potential break in the titration. It is convenient to use a mercurous sulfate reference electrode rather than the more common saturated calomel electrode. Diphenylthallium(II1) chloride is insoluble, and most calomel electrodes introduce enough chloride into the titration cell to cause serious positive errors. A calomel electrode could be used if a double junction were employed to prevent chloride leakage into the solution. Chloride, bromide, and iodide interfere and were removed by addition of a slight excess of silver sulfate. It was not necessary to remove the precipitate from the solution before beginning a titration. The excess silver ion causes relatively high background currents in amperometric titrations, but otherwise is without effect. Nitrite ion itself is not precipitated by diphenylthallium(II1). However, in the acidic solutions employed for the titration, nitrite disproportionates to give nitrate and nitric oxide. Therefore, nitrite was destroyed before a titration by adjusting (8) Leeds and Northrup Co., Philadelphia, Bulletin 177166 (1963).
the pH of the solution to about 4 and adding hydrazine sulfate. If carbonate was present in the sample, it was found necessary to remove the COa from the acidified (pH 4) sample by boiling it briefly before beginning a potentiometric titration, inasmuch as bicarbonate is precipitated by diphenylthallium(II1). RESULTS AND DISCUSSION
Amperometric Titration. The polarographic behavior of the diphenylthallium(II1) cation has been described in detail (9). In acidic sulfate supporting electrolytes the three waves coalesce into a single slowly rising wave (Figure 1). The diffusion current plateau is about 250 mV wide. Current readings for amperometric titrations were taken at the center of this plateau, but a potential as much as 100 mV positive or negative to -1.0 V us. SCE could be used equally well. A typical amperometric titration curve is shown in Figure 2. Before the end point only a slight background current is obtained. After the end point the current rises steeply, because of the reduction of electroactive diphenylthallium(II1). The current is relatively high at the end point because of the high (3.2 x lOP4M) solubility of diphenylthallium(II1) nitrate. The results of titrations of solutions of potassium nitrate have been presented earlier (6). The relative error is il or less over the concentration range 5 x lOFM t o 2.2 X 10-2M nitrate. The concentration range of the titration is limited by the slowness of reaction of solutions more dilute than 5 x lO-3M and by the clogging of the dropping mercury electrode by the precipitate formed when the solution is more concentrated than about 2.2 X 10e3M. Similar limitations are observed in amperometric precipitation titrations using tetraphenylarsonium (IO) and tetraphenylantimony (11) salts. Table I shows the effect of several foreign anions on the amperometric titration. Fluoride is without effect, as is carbonate, which exists as a CO2-H2CO3mixture at the pH of the titration. Sulfate is innocuous. Each sample contained (9) J. S. DiGregorio and M. D. Morris, ANAL. CHEM., 40, 1286 (1968). (10) H. E. Affsprung and V. S.Archer, ibid., 35,976 (1963). (1 1) M. D. Morris, ibid., 37,977 (1965).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
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Table I. Effect of Foreign Ions on the Amperometric Titration of Nitrate with Diphenylthallium(II1) Sulfate Foreign ion NO3- founde Error (mmoles) (mmoles) (mmoles) (mmoles) Error 0.5150 1.o COs-Zb 0.5150 f 0.0001 O.oo00 0.0 0.5150 0 . 1 C104-b 0.5192 f 0.0010 +o I0042 $0.8 0.5150 0.3 0.5200 f 0.0015 +O. 0050 $1 .o 0.5150 0 . 5 C104-b 0.5180 f 0.0063 +O. 0030 +0.6 0.5150 0.6 C104-b 0.5375 f 0.0025 +O. 0225 +4.3 0.5150 1 .o ClOa-b 0.5506 f 0.0030 +0.0356 +6.9 0.5150 1.o F-b 0.5108 f 0.0087 -0.0042 -0.8 0.5150 1 .o c1-c 0.5196 1. 0.0001 $0.0046 $0.9 0.5150 1.O B r c 0.5132 =k 0.0018 -0.0018 -0.3 0.5150 1.o I-c 0.5183 f 0.0009 +0.0033 +0.6 0.5150 1.O 0.5244 f 0.0010 +0.0094 $1.8 a 50-ml sample. No removal. c Removal by precipitation with silver sulfate. No coagulation or filtration of AgX. d Removal by reduction with hydrazine sulfate at pH 4 to 6. e The average of at least two titrations. NOs- taken4
I
I 260
Table 11. Potentiometric Titration of Nitrate with Diphenylthallium(II1) Sulfate
220
NO3-
taken (mmoles) 0.5130" 0.1026* 0. 2052b
0. 307Sb 0 . 4104b 0.5130b
NOt- foundc
(mmoles) 0.5098 j= O.OOO4 0.1021 + 0.0011 0.2062 It O.OOO9 0.3085 0.0006 0.4093 f 0.0001 0.5130 f O.OOO1
Error (mmoles) -0.0032 -0.0005
-
+0.0010
Error -0.6 -0.5 $0.5
+0.0007 +0.0011
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d
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140
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about 0.15M sulfate as a buffer. Equimolar quantities of perchlorate can be tolerated. If the perchlorate-nitrate ratio is increased above unity, increasingly large positive errors are obtained. Chloride, bromide, and iodide would be precipitated by the reagent, but at least two-fold excess can be tolerated if these ions are first removed by precipitation with silver sulfate. The perchlorate error is quite unexpected. We have found that diphenylthallium(II1) perchlorate itself is soluble to at least 0.1M , so that no precipitation would be expected from the 0.01M solutions used to obtain the data of Table I. In fact, Hartmann and Bathge recommend dicyclohexylthallium (111) perchlorate as precipitant (7) for gravimetric nitrate determination. Apparently perchlorate is coprecipitated with nitrate because it can fit readily into the crystal lattice of diphenylthallium (111) nitrate. Attempts to remove perchlorate by addition of tetraphenylarsonium sulfate led to low nitrate results, suggesting that tetraphenylarsonium nitrate, which is quite soluble, can be included in the crystal lattice of sparingly soluble tetraphenylarsonium perchlorate. Perchlorate may not be coprecipitated with nitrate from dicyclohexylthallium(III) solutions. However, it is also possible that the digestion procedure recommended by Hartmann and Bathge removes any perchlorate which might have precipitated initially. Potentiometric Titration. A typical potentiometric titration curve is shown as Figure 3. The theoretical curve (dashed line) was calculated assuming Nerstian electrode 96
> E
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Figure 3. Potentiometric titration of 0.4104 mmole NOawith 0.1263M(Cd-I&Tlt (as S042-) Solid line-experimental theoretical curve
potential differences.
Dashed h e -
response to nitrate alone to indefinitely low concentrations, Potentiometric titrations were carried out in solutions of p H 2-4. The nitrate electrode performs poorly in solutions more acidic than p H 2 (12, 13). Above p H 4, precipitation of diphenylthallium(II1) hydroxide begins and high results are obtained. Within the limitations set by these phenomena strict p H control is unnecessary. The results of potentiometric titrations 0.01M to 0.1M of nitrate solutions are shown in Table 11. The relative error for manual titrations is +0,6zor less. The effect of several possible interferences is shown in Table 111. There is a definite positive bias to the results and the errors are generally of larger magnitude than those encountered with the corresponding amperometric titration. No attempt was made to test the potentiometric titration in the presence of perchlorate. The selectivity coefficient of the electrode for perchlorate is 1 x l o 3 (12) and the electrode response would be governed by the perchlorate activity only. In order to allow the electrode t o respond t o nitrate activities as low as lO-4M, the perchlorate concentrations would have to be lowered at least to lO-'M. 2) Orion Research,
Instruction Manual for Nitrate Electrode, 1968. (13) S . S. Potterton and W. D. Shults, Anal. Letters, 1(2), 11 (1967).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
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Table 111. Effect of Foreign Ions on the Potentiometric Titration of Nitrate with Diphenylthallium(II1) Sulfate NO8- found' Error NOa- taken" Foreign ion (mmoles) (mmoles) (mmoles) (mmoles) Error 0.4104 0.4 CO,-* 0.4097 f 0.0006 -0.0007 -0.2 0.4104 0.4 F-c 0.4157 i 0.0010 +O. 0053 1-1.3 0.4104 0.4 C1-d 0.4142 f 0.0012 $0.0038 $0.9 0.4104 0.4 BrVd +0.0069 +1.7 0.4173 f 0.0007 0.4104 0.4 I-d +2.0 0.4185 f 0.0006 f0.0081 0.4104 0.5 0.4141 i 0.0025 +O. 0037 +0.9 10-ml sample. * Removal by heating to boiling. No removal. d Removal by precipitation with silver sulfate. No filtration or coagulation of AgX. e Removal by reduction with hydrazine sulfate at pH 4 to 6. f The average of at least two titrations.
The high positive errors encountered with the potentiometric nitrate titration after interferences have been removed are associated with the properties of the presently available nitrate electrode. The electrode response is described (12,13) by Equation 1.
E = Ea - 0.056 log (uxor
+ K~CZ~''~)
(1)
The slope of the response curve at 25 "C is 56 mV per decade instead of the theoretical 59.2 mV (13). The slightly less than theoretical sensitivity is of no consequence in titrimetry. The 56-mV slope is observed to below 10-4M nitrate and a usable response is observed to about 10-5M. The limiting factor in the use of the nitrate electrode is its response to other anions. This response is described by the second term in the argument of the logarithm in Equation 1. For sulfate, the selectivity coefficient is 6 X Consequently, it is expected that in the 0.1Mand more concentrated sulfate solutions employed in this titration, the electrode response will be determined by sulfate alone when the nitrate concentration falls much below 2-3 X 10-4M. When the nitrate concentration is below about lO+M, the titration curves will be severly distorted by the electrode's sulfate response. These effects are quite pronounced in Figure 2. Removal of those ions, other than nitrate, which precipitate the titrant, necessitates raising the sulfate content of the solution. Under these conditions, further distortion of the titration curve and a decrease of 15-20 mV in the final sulfatelimited potential are observed. Because of these effects the equivalence point of the titration then occurs somewhat before the midpoint of the potential break. It is possible to improve the appearance, but not the reality, of these results by making an ad hoc choice of the equivalence point potential at a point before the midpoint of the break. We have titrated potassium nitrate solutions with a homemade recording potentiometric titrator. The lowest titration time employed by us was about 3 minutes, for which a precision of + O S % and an accuracy of -0.7% was obtained. These results indicate that potentiometric titration of nitrate with commercial recording titrators or autotitrators is quite feasible.
CONCLUSIONS
The present nitrate titration has certain advantages over previously existing methods. It is rapid, direct, and amenable to automation. Its major drawbacks are the interference from the halides and the relatively high nitrate concentrations for which it is applicable. The reagent is easily prepared from commercially available materials. Diphenylthallium chloride and iodide are currently rather expensive, but the reagent can be recovered with little difficulty. Other organothallium salts which may ultimately prove superior to diphenylthallium salts as nitrate precipitants have been reported. Dicyclohexylthallium nitrate (7) has a much lower solubility than diphenylthallium nitrate. However, we were unable to repeat the literature preparation and a reagent house specializing in organometallic compounds was unable t o prepare it for us. Consequently, this reagent can not be considered practical at the present time. Bis-pentafluorophenylthallium salts have been prepared (14). The nitrate has been reported to be insoluble (14), but no quantitative data are available. It is to be expected that the properties of this reagent will parallel those of diphenylthallium nitrate. It is possible that the perfluoro compound or some other derivative may be usable in more concentrated perchlorate solutions or for more dilute nitrate solutions than is diphenylthallium sulfate. We are currently examining some organothallium compounds as possible nitrate reagents. It is unlikely, however, that any organothallium reagent will be usable without prior removal of halides from the sample.
RECEIVED for review June 16, 1969. Accepted November 3, 1969. Work supported in part by National Science Foundation Grant GP-8679. Presented in part, 17th Detroit Anachem Conference, September 1969.
(14) G. B. Deacon, J. H. S. Green, and R. S. Nyholm, J. Chem. Soc., 1965, 3411.
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