STUDIES ON AGING AND COPRECIPITATION.
XXVII
THECOPRECIPITATION OF NITRATEWITH LEADSULFATE AND THE DECOMPOSITION OF COPRECIPITATED NITRATEON HEATING’ I. M. KOLTHOFF
AND
ROY A. HALVERSEN
School of Chemistry, Institute of Technology, University of Minnesota, Minneapolis, Minnesota Received November 19, 1988
Several studies (2) have been carried out in this laboratory on the degree of perfection and the aging of lead sulfate obtained by precipitation of lead nitrate with potassium sulfate or by reverse precipitation. In the present study the effect of various factors, such as the concentratidh of the reactants, the method of precipitation, the temperature during the precipitation, and the conditions of aging, upon the amount of coprecipitated nitrate has been determined. Moreover, the effect of the heating of the air-dried precipitate upon the nitrate content of the precipitate has been determined. EXPERIMENTAL
Determination of coprecipitated nitrate The nitrate was reduced in alkaline medium with Devarda’s alloy, and the ammonia was transferred to a wash bottle partly filled with dilute sulfuric acid. The reduction chamber consisted of a 75-ml. Pyrex distillation flask provided with an inlet tube for air. The sample was transferred to the reduction flask, 8 ml. of a saturated sodium hydroxide solution was added and enough water to make a volume of 20 ml. Four-tenths of a gram of Devarda’s alloy was then added, and a rubber stopper immediately inserted in the neck of the flask. Air washed through dilute acid and water was passed through the flask, and the ammonia was collected in a cylinder containing 40 ml. of 0.5 N sulfuric acid. The contents of the reduction chamber were heated gently for a few minutes a t the beginning of the experiment. After 50 min. heat was again applied for a 10-min. interval to drive out the last traces of ammonia. The contents of the 1 This article is based upon a thesis submitted by Roy A. Halversen to the Faculty of the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the degree of Master of Arts, June, 1938.
805
606
1.
M. KOLTHOFF A N D ROY A. HALVERBEN
flask with 0.5 N sulfuric acid were transferred to a 250-ml. Erlenmeyer flask, the excess of acid was nearly neutralized with sodium hydroxide, and the ammonia was determined by the hypochlorite method of Kolthoff and Stenger (3). A blank determination was made under similar conditions, and the amount of ammonia found was subtracted from the amount TABLE 1 Effect of concentration of reactante, method of precipitation, temperature, and conditions of aging upon wpren'pitation of nitrate tcrith lead sulfate m m . NO
1 2 3
4
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
:ONDENTEASON OF LEA1 AND OF
OWLFATS
FIIMFEMTURD OF 'RUXPPCATlON
M
*C.
0.4 0.4 0.4 0.1 0.1 0.1 0.1 0.1 0.1(*) O.l(*) 0.1 0.1 0.1(.) 0.1(.) 0.1(0) 0.1(0) 0.1(0)
25 25 25 25 25 25 95-100 95-100 25 25 25 25 25 25 25 25 25 0.025 25 0.025 25 25 0.025 25 0.025(0 0.025 25 0.025 25
PIYPSBA. TURD DVBINQ AGING
HITBATE I N PBCCIFITATB AFTER AQINO FOR
YSTEOD OF PBDXPITATTION
I
1 min. FCau
Direct Direct Reverse Direct Direct Reverse Direct Direct Direct Direct Direct Direct Direct Direct Reverse Direct Direct Direct Direct Reverse Direct Direct Direct
-
-
0.61 0.41 0.64 0.22 0.33
0.39
0.28
1 br.
pa euni
0.19 0.53
DQ
cdtli
0.46 0.33 0.35 0.36 -0.35 0.42 0.29 0.40 0.16 0.16 0.31 0.27 0.28 0.37 0.29 0.18 0.08 0.38 0.25 0.12 0.04 0.31 0.32 0.34
24 hr. DQ
ant
0.27 0.22
0.20 0.22 0.26 0.24 0.19 0.01 0.16 0.01 0.30 0.31
0.26
0.30 0.55
6 hr.
0.50 0.43 0.16
0.44
0.20 0.05 0.44 0.32
0.14 0.62 0.22
0.14 0.09 (a) Lead nitrate solution waa 0.01 N in nitric acid; (b) supernatant liquid waa made 0.01 N in nitric acid; (0) lead nitrate and potassium sulfate in 15 per cent ethanol; (d) precipitate was filtered and aged in 0.1 N nitric acid; (e) lead nitrate solution was 0.5 N in potassium nitrate; (f) precipitate was filtered and aged in 0.6 N nitric acid.
found in the unknown. Working with 1 mg. of nitrate and subtracting the blank yielded results varying between 0.995 and 1.00 mg. in ten experiments. In the determination of coprecipitated nitrate in lead sulfate it was not found necessary to decompose the precipitate first by boiling with sodium
607
COPRECIPITATION OF NITRATE WITH LEAD SULFATE
carbonate solution; the same results were found by the above procedure as after boiling with the carbonate solution.
Methods of precipitation Direct method (sulfate added to lead): (1) 0.4 M solutions: 2.447 ml. of 0.4 M potassium sulfate added to 2.567 ml. of 0.4 M lead nitrate. (2) 0.1 M solutions: 9.79 ml. of 0.1 M potassium sulfate added to 10.27 ml. of 0.1 M lead nitrate. (3)0.025 M solutions: 39.16 ml. of 0.025 M potassium sulfate added to 41.08 ml. of 0.025 M lead nitrate. The time of precipitation in all cases was 20 to 30 sec. The precipitations were carried out a t room temperature in most cases, but a t boiling temperature in some cases (see table 1). Several experiments have been carried out TABLE 2 Direct precipitation
Ten ml. of 0.1 M potassium sulfate was added to 10 ml. of 0.15, 0.3, or 0.5 M lead nitrate CONCENTBATII OI LEAD NITBATE
M 0.15 0.15 0.30 0.30 0.30 0.50 0.50 0.50
NITRATTIP I N PRECIPITATE AFTEB AQINQ FOB
PEYPEBATUBI OF PBDCIPITATION
1 min.
1 hr.
*e.
*C.
P B cent
PCI Csnl
25 95 25 95 25 25 95 25
25 25 25 25
0.43 0.32 0.34 0.27
0.40 0.25 0.30 0.26
_-
90' 25 25 25'
0.36 0.32
0.34 0.29
0.37 0.22 0.30 0.24
0.36 0.21 0.29
0.23 0.09 0.34
* Aged in 0.01 N nitric acid. in which 0.1 M potassium sulfate was added to a large excess of a stronger lead nitrate solution (table 2). Method of reverse precipitation: The lead nitrate solution was added to the potassium sulfate solution. The ratio of lead to sulfate was the same as in the direct precipitations. For the determination of nitrate in the fresh precipitatas the latter were filtered immediately and washed with six successive 5-ml. portions of conductivity water. After drying, weighed portions were transferred to the reduction flask. (The time required for filtration and washing was 5 to 6 min.) When the precipitates were aged they were placed with the supernatant liquid on a shaker (400 rotations per minute) a t room temperature or digested a t 90°C. Unless stated otherwise, the aging occurred in the
608
I . M. KOLTHOFF AND ROY A. HALVERSEN
supernatant liquids. After aging the precipitates were washed, etc., as described above. The results are expressed in per cent of nitrate in the precipitates.
Experimental results A condensed summary of a great number of experiments is given in table 1. In table 2 a few of the results are reported in which the 0.1 molar sulfate solution was added to a large excess of lead nitrate. All the figures reported are the average of a t least three independent experiments. It was of interest to know whether all of the coprecipitated nitrate was occluded or if part was adsorbed on the active surface of the precipitate. It is to be expected that adsorbed lead nitrate would be easily removed upon treatment with a slight excess of sulfate. Upon addition of 9.79 ml. of 0.1 M potassium sulfate to 10 ml. of 0.1 M lead nitrate a t room temperature the fresh precipitate contained 0.64 per cent of coprecipitated nitrate. When 10.27 ml. of 0.1 M sulfate was added to the nitrate solution instead of 9.79 ml. the fresh precipitate contained 0.40 per cent of coprecipitated nitrate. This amount of coprecipitated nitrate decreased very slowly upon aging a t room temperature in the supernatant liquid (0.34 per cent after 24 hr.). These experiments would indicate that, in the presence of a slight excess of lead nitrate in the supernatant liquid, 0.64 - 0.40 = 0.24 per cent of the coprecipitated nitrate, or about 35 per cent of the total amount, is adsorbed on the active surface. This conclusion was substantiated by the following experiments: Equivalent amounts of 0.1 M sulfate and lead nitrate were mixed (direct precipitation); the fresh precipitate contained 0.59 per cent of coprecipitated nitrate. When 0.5-1 ml. of 0.1 M potassium sulfate was added 15 sec. after the precipitation the amount of coprecipitated nitrate was 0.46 per cent, while this amount was equal to 0.64 per cent when a slight excess of lead was added after the precipitation. These experiments would indicate that about 30 per cent of the coprecipitated nitrate is adsorbed on the active surface. Thermal aging an the dry state The progress of perfection and the change of the specific surface of fresh air-dried lead sulfate prepared from 0.1 M solutions has been studied by Kolthoff and Rosenblum (2, e). In the following experiments 30 g. of a similar product was prepared; it was immediately washed four times with 100-ml. portions of water, and air-dried. Samples of 300 mg. were placed in a furnace and heated for 24 hr. at looo, 200°, 300°, 400", and 5OO0C., respectively. After cooling, the samples were washed six times with 5-ml. portions of water, The nitrate contents of the collected washings and of the washed precipitate were then determined. The original air-dried product contained 0.71 per cent of coprecipitated nitrate; after washing
609
COPRECIPITATION OF NITRATE WITH LEAD SULFATE
six times with the 5-ml. portions of water 0.50 per cent of nitrate was found in the precipitate and 0.21 per cent (referred to the original precipitate) in the combined washings. -4second series of washings yielded only 0.03 per cent of the nitrate in the combined filtrates (30 ml.); a third washing 0.01 per cent. Apparently, in the first six washings mainly adsorbed nitrate is removed; upon further washing a trace of occluded nitrate enters the liquid as a result of recrystallization. The results of the heating experiments are given in table 3. DISCUS8ION O F RESULTS AND CONCLUSIONS
From table 1 it is evident that, when equimolecular solutions of lead nitrate and potassium sulfate with concentrations varying between 0.4 and 0.025 M are mixed a t room temperature (direct precipitation), the amount of coprecipitated nitrate is of the same order of magnitude (0.5 to 0.6 per cent). When the sulfate solution (0.1 M ) is added to a large excess TABLE 3 Nitrate content of precipitate and $rst set of washings after heating f o r 24 hr. at 1oO-600"C. KITRATE IN WAOHED PRECIPITATE
NITRATE IN WABHINGB (PER CENT O F PRICIPITATE)
TOTAL NITRATE I N HEATED PRECIPITATE
'C.
per cent
per cenl
per cent
100 200 300 400 500
0.50 0.50 0.50 0.31 0.01
0.21 0.21 0.17 0.04 0.0
0.71 0.71 0.67 0.35 0.01
TEYPERATURI O F HEATING
_____-
610
I. M. KOLTHOFF AND ROY A. HALVERSEN
I n agreement with the rules of coprecipitation (1) it is found that by reverse precipitation less nitrate is obtained in the precipitate than by direct precipitation. Upon aging of the precipitate the amount of coprecipitated nitrate decreases. The speed of purification of the precipitate upon aging depends upon the degree of perfection of the fresh precipitate and upon the solubility of the precipitate in the aging medium. The precipitates obtained from 0.025M solutions (table 1) are more perfect than those obtained from more concentrated solutions; consequently the amount of coprecipitated nitrate in the former decreases much less upon aging a t room temperature and a t 90°C.than in the latter (compare, e.g., experiments 18 and 19 with experiments 1, 2, 4, and 5 in table 1). When the solubility of the lead sulfate in the aging medium is very small, as is the case in the experiments in table 2 when the aging medium contained a large excess of lead, very little purification (recrystallization) occurs on aging. The same is true for the precipitates prepared and aged in 15 per cent ethanol (experiments 13 to 17,table 1). On the other hand, when the solubility of the precipitate in the aging medium is increased by the addition of nitric acid, a drastic perfection and purification occurs, particularly a t 90°C.,as is evidenced from experiments 9,10,11, 12,16,17,22,and 23 in table 1 and the last experiment in table 2. Even the fairly perfect precipitate obtained from 0.025 M solutions loses the greatest part of the coprecipitated nitrate upon aging in 0.5 N nitric acid. From an analytical viewpoint it is of great interest that an impure precipitate obtained from 0.1 M solutions becomes free of nitrate when aged for 24 hr. a t 90OC.in 0.01 N nitric acid. The precipitate formed by direct precipitation of 0.1 M solutions a t room temperature contains about 30 per cent of the coprecipitated nitrate adsorbed on the active surface when the supernatant liquid contains an excess of lead. During the aging and perfection the active surface decreases rapidly; the initial decrease of the amount of coprecipitated nitrate on aging is, therefore, mainly due to a decrease of the active surface. Lead nitrate when exposed to the air decomposes a t 233°C. It is evident from table 3 that occluded nitrate in lead sulfate does not decompose, and the adsorbed nitrate decomposes only very slightly a t 300°C. After heating for 24 hr. a t 400°C.practically all of the adsorbed nitrate is decomposed and only a small fraction of the occluded nitrate. After heating for 24 hr. a t 500°C. all of the coprecipitated nitrate is decomposed. SUMMARY
1. The effect of conditions of precipitation, of concentration of the reactants (lead nitrate and potassium sulfate), and of aging under varying conditions upon the coprecipitation of nitrate with lead sulfate has been determined.
COPRECIPITATION OF NITRATE WITH LEAD SULFATE
61 1
2. Of analytical interest is the fact that after aging for 24 hr. in dilute nitric acid a t 90°C. practically all of the coprecipitated nitrate is eliminated from the precipitate. 3. Evidence is given that about one-third of the nitrate coprecipitated with lead sulfate formed a t room temperature by direct precipitation of 0.1 M lead nitrate with 0.1 M potassium sulfate is adsorbed on the active surface. This adsorbed nitrate decomposes when the precipitate is hcated for 24 hr. a t 400"C., while only a small fraction of the occluded nitrate decomposes a t this temperature. At 500°C. all of the occluded nitrate decomposes after 24 hr. of heating. REFERENCES (1) KOLTHOFF, I. M.:J. Phys. Chem. 36, 860 (1932). (2) KOLTHOFF, I. M.,AND ROSENBLUM, CH.:(a) J. Am. Chem. SOC. 66, 1264 (1934); (b) J. Am. Chem. SOC.66,1658 (1934);( c ) J. Am. Chem. SOC.67,597 (1935); (d)J. Am. Chem. SOC.67,607 (1935);(e) J. Am. Chem. SOC.67,2573 (1935); (f) J. Am. Chem. SOC.67,2577 (1935);( 9 ) J. Am. Chem. SOC.66,116 (1936); (h) J. Am. Chem. SOC.68,121 (1936). (3) KOLTHOFF, I. M., AND STENGER, V. A.: Ind. Eng. Chem., Anal. Ed. 7, 79 (1935).
