170
ANALYTICAL CHEMISTRY Table I. Calculation of Concentration Seriesc1 - ( l / a l ) loglorn
loglom
(l/d)logiom
1 log10 1 = 0 . 0 0 0
0.0000 0.0015
wa
2 loglo 2 9 log10 3 4 loglo 4 5 logio 5 6 login 6
= 0.301 = 0.477 = 0.602 = 0.699 = 0.778
0.0024 0.0030 0.0035 0,0039
CI
-
Transmittance Readings PhotoPhoto(l/ul)loglom cell 2 cell 4
0.0040 0.0025 0.0016 0.0010 0.0005
0.0001
0.506
0.655
0.770 0.850 0,920
0,980
0.270 0.435 0.575 0.705 0.840 0.960
where
Now or1 has a fixed but unknown value. Then since c1 - log,o?n a1 is a real concentration it cannot be negative; a value for m (number of check points required) which will fix a minimum value for c1 and therefore z and T can be chosen. Alternatively c1 can be arbitrarily fixed as equal to the highest concentration used in a test, thereby fixing z and T , and limiting m. The latter choice is used for illustration using the data in Figure 1. Since a1 is unknown the most accurate value to select will be the highest ever recorded by the spectrophotometer under test for a series of photocells. This assumes that any nonlinear photocells will have 7~ less than unity (see Equation I), which implies that a t
higher light intensities they will exhibit a saturation effect. This is the most probable form of nonlinearity which may develop and is very common in barrier layer photocells such a s those used in the instrument described. To simplify the arithmetic in this illustration a1 is taken as 200. This value is slightly higher than the highest ever recorded for the instrument. c1 is set = 0.004%; thus the ntth concentration is
- I%. Trial shons that the highest possible value for m is 6. Thus the six concentrations can be calculated. The transmittance readings for the two photocells a t these points are given in Table I and are plotted against m in Figure 4. The nonlinearity of the defective photocell (No. 2) is very apparent in the graph. The limitation of this method of checking linearity is the uncertainty in al, so that a t best it can only give an approximate check on the absolute response of a photocell. As a method of comparing the relative response curves of photocells, as illustrated above, it is very useful. Furthermore the data obtained for an analytical ralihration curve-e.g., Figure 1-can be used, and no additional clperimental work is required.
0.004
4CKNOWLEDGSIENT
~
Thanks are due to British Xylon Spinners, Ltd., for permission to publish this work. LITERATURE CITED
(1) Green, M., J . Optical SOC. Am., 41, 867 (1951). (2) Philpotts, 4 . R., Thain, William, and Smith, P. G., CHEM.,23,268 (1951). RECEIVED for review July 26, 1952.
h A L .
Accepted September 24, 1952.
Precipitation of Basic Stannic Sulfate from Homogeneous Solution HOBART H. WILLARD AND LOUIS GORDON' University of Michigan, Ann Arbor, Mich.
ANY names, formulas, and other differences have been attributed to the oxides or basic salts formed when ammonium hydroxide (or an alkali) is added to a solution containing tin(1V) or when nitric acid reacts with tin metal. Weiser (g) rites x-ray evidence to show that the differences in properties of these compounds are the resuh of particle size because they furnish identical diffraction patterns. The many contradictory statements about the character of these precipitates appear to have been the result of investigators unknowingly working JT ith mixtures, since the oxide produced by ammonia precipitation readily changes, especially when heated, to the form produced by the action of nitric acid on metal. The metal oxide precipitates obtained from homogeneous solution by the urea process ( 3 ) have been generally referred to as basic salts. The stannic compound similarly precipitated by urea contains sulfate and will thus be referred to as basic stannic sulfate; one analysis of such a precipitate, washed fifteen times with hot 2% ammonium nitrate solution, indicated a molar ratio of tin to sulfate of approximately 44. In outward appearance this very dense precipitate produced with urea bears little resemblance to the oxides of tin produced by other methods.
the precipitate was washed with hot 27, ammonium nitrate eolution (pH adjusted to 1.5 with nitric acid). COLLOID FORMATION
In much of the initial work it was difficult to obtain a precipitate which was not colloidal or did not readily becorn-e colloidal &-hen filtration was attempted. l l a n y variables were studied, such as the effect of ( a ) approximately twenty anions, ( b ) continued boiling and long standing, ( c ) initial pH, ( d ) quantities of urea, ammonium sulfate, and sulfuric acid, and ( e ) other hydrolytic agents such as acetamide and formamide. The conditions described in the method of precipitation were those which finally led in every case to the production. within a reasonable time ( 2 . 5 hours), of an extremely dense ant1 easily filtered and wYnshrt1 pre-
E
14
o
13
p
12
METHOD OF PRECIPITATION
In 400 ml. the solution contained 0.25 to 0.30 gram of tin(1V) aa chloride, 50 grams of urea, 15 ml. of sulfuric acid, 2 grams of ammonium sulfate, and sufficient hydrochloric acid to furnish an initial p H value of 0.50. The solution was rapidly heated to boiling and then transferred to an air bath heater designed to heat most of the liquid portion of the beaker. A stirring rod with a 1-mm. carpet tack indentation was inserted into the beaker to minimize bumping. The solution was then boiled for 2.5 hours, a t which time the pH was usually about 1.5. Distilled water mas added a t intervals to maintain liquid level. After filtration 1 Present addreas, Department of Chemistry, Syracuse University Syracuse, N. Y .
Sn ADDED (mg.)
Figure 1. Relation between Metal &precipitated and Tin Added Manganese present, 100 mg.; nickel present, 200 mg.
V O L U M E 25, NO. 1, J A N U A R Y 1 9 5 3 cipitate. Of the anions tested, only the presence of sulfate or selenate resulted in the formation of a dense precipitate. The large quantity of urea, 50 grams, is required to attain the proper pH within 2.5 hours. Either an initial lack of ammonium ions or the presence, initially, of a large quantity actually resulted in the occasional formation of a colloidal precipitate, whereas the addition of 2 grams of ammonium sulfate completely inhibited colloid formation in every case.
171 tate will peel off in transparent sheets which exhibit highly colored interference patterns. Some of the films had an area of as much as 1 square inch. The precipitates (total tin present = 0.5 gram) and films from the surfaces of six 600-ml. beakers contained 8.8 mg. of silica. The average weight of the films from a beaker is about 20 mg. Basic stannic sulfate adhered just as tenaciously to platinum and silver dishes.
INITIAL AND FINAL pH VALUES O F PRECIPITATION
I t has been demonstrated, in the case of other basic salts precipitated by the urea method, that there is an optimum pH value above which the hydrolysis reaction must not be started else an initially gelatinous precipitate will be formed. This pH value is
Table I.
Distribution of Manganesea in Washings and in Adherent Filmsb Erpt. I 0.49 1.64
Initial pH Final pH
Expt. 2 0.48 1,52 Mg.
hln in filtrate hln in first two washings hfn in next 13 washings Mn in each of subsequent washings h l n in precipitate Total hIn found &In taken originally
0.23
29 4 0 58 0 13
3.48 33.5 31.0
3.43 33.6 34.0
29.0 0.76 c
c
a 273 mg. of tin(1V) present. In four separate experiments in which were present 273 mg. of tin(1V) and 100 nig. of manganese (II), manganese in adherent films amounted to 0.04,0.1,6,0.15,and 0.11 mg. Estimated to he no greater than 0.0001 mg. in 16th, 17th, or 18th washing.
*
2.6 2.4 22 -
3.0 28
-
2.0
M E T A L ADDED (mg)
Figure 2. Relation between Metal Added and Its Analytical Coprecipitation 273 nig. of tin present
0.50 in the case of basic stannic sulfate. All pH measurement.s were made at, room temperature with a glass electrode using a Leeds and Sorthrup universal pH indicator, adjusted to p H 4.01 with 0.05 JI potassium acid phthalate. The rate of change of p H is shown in Figure 3 of ( 1 ) . Quantitative precipitation of tin(IV) is attained at pH 1.3, altliough in moat cases a final pH value of 1.5 was used. PROPERTIES O F T H E PRECIPITATE
The iirecipitate is initially observed as an opalescence which appears in 5 to 10 minut,es after the start of boiling. The solution will remain turbid, although some precipitate settles out, until a pH of 1.2 tjo 1.3 is reached. I n this pH range the precipitate will settle, even though there is a vigorous state of agitation due to escaping carbon dioxide. The precipitate is pure white and very dense and appears grainy and crystalline although microscopic examination shows no crystals. Sulfuric, hydrochloric, and nitric acids, even when hot, do not, appreciably dissolve the basic salt, although it is dissolved, with difficulty, by boiling phosphoric acid. Protract.ed heating v i t h strong sodium hydroxide solution results in solution of the precipitate. Just as wit,h the basic salts precipitated in other urea processes, this precipitate exhibits a strong tendency to adhere to beaker surfaces. Howevcarlit x i s not possible to remove this precipitate with hot dilute hytirochloric acid as is normally done. The adherent precipitate ran be removed in two different ways. 1. With dilut'e hydrofluoric acid, and even then it dissolves very slowly, tending rather to peel off in sheets. 2. If the beaker is filled with a solution containing 50 grams of ammonium sulfate per liter, adjusted t'o pH 8.5 to 9.5 with sodium hydroxide, and then gently warmed for 30 minutes, the precipi-
1.a
:3
a
X
3
IS!-
:'i I4
Fe
12-
1 01.0
Mn 00 -02
-
-
-0.4 -04-
