140
VOL. 11, NO. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
acid, and the cold titration of the acid liberated from a previously heated mixture of sodium citrate solution with the unknown aluminum solution. The ca,lculation of the ahminum content is based on substitution in an appropriate equation of the amount of sodium hydroxide used in titrating the liberated acid. The method is applicable to solutions which are free from appreciable quantities of iron Or other interfering ions.
Literature Cited (1) Congdon, L. (2)
*.,and
J., Chem.
128, 98 (1924).
craig, T.J. I,,J ,sot. Chern,In&30, 184 (1911).
(3) Pavlinova, A.
V., J. Applied Chern. (U. S. S.E.), 9, NO. 9,
1682-
9 (1936).
(4)
**
H*i
J*
Chern*
249
457 (lgo2).
R~~~~~~~September 10, 1938. Contribution NO. 52 from the Chemioal Laboratories of the University of LTtah.
Pycnornetric Determination of Lead as Sulfate W. WALKER RUSSELL AND J. H. A. HARLEY, JR. Brown University, Providence, R. I.
The new method in pycnometric analysis has been adapted to the determination of lead as sulfate in two nonferrous alloys with satisfactory results. The behavior of the lead sulfate precipitate is such that centrifugalization can be eliminated, thereby considerably simplifying the method.
I
N A RECENT preliminary report (3)upon a new method in pycnometric analysis, its underlying theory and probable accuracy were discussed, and pycnometric analyses were presented for the barium, iron, or silver in several simple substances, primarily salts. The present work deals with practical analyses of two nonferrous alloys of widely different lead contents. The lead was determined as sulfate because this is the compound most frequently used for its separation and gravimetric determination. The relatively high density of lead sulfate is an asset in the pycnometric method, while its solubility offered an opportunity to apply the pycnometric method to a precipitate of considerably greater solubility than any studied previously in this laboratory.
Density of Lead Sulfate Inasmuch as a pycnometric analysis actually determines the volume of a precipitate, the weight can be ascertained only if the density of the precipitate, or strictly speaking the apparent density characteristic of the experimental conditions existing during the analysis, is accurately known. Other things being equal, the more soluble the precipitate, the greater will be the difference between its absolute and apparent density. I n order to determine the apparent density of lead sulfate, a primary standard of accurately known lead content must be subjected to the pycnometric procedure which is to be used subsequently in the analysis. The principal primary standard used in the present work was the Bureau of Standards lead block, No. 49, melting point 327.3' C. The results of nine determinations, leading to an average value of 5.791, are given in Table I. This value is to be used in computations of analyses in which 3 per cent sulfuric acid is used as the standard washing medium in approximately the amount and manner specified below. The values for the density of lead sulfate given in the literature show a considerable variation. The numerous values listed in Mellor (2) vary from 5.97 to 6.393, while the value given in the International Critical Tables is 6.2. Furthermore, Krings (I) found that lead sulfate formed in
aqueous solution has a lower density (6.03), than that formed by fuming with sulfuric acid (6.272), and that ignited lead sulfate has a lower density than the freshly prepared precipitate. A comparison of the apparent with the real density of lead sulfate therefore required a determination of the latter value for the lead sulfate precipitate formed under the conditions of the analyses. The lead sulfate precipitates in the present work redissolved in the strong sulfuric acid during the fuming and were formed again upon dilution with water. I n six density determinations values of 6.309, 6.277, 6.290, 6.298, 6.338, and 6.292 were obtained, leading to a mean value of 6.301 for the real density of the lead sulfate precipitate. Thus, the real density of the lead sulfate precipitates in the present work is 6.301 while the apparent density when using 3 per cent sulfuric acid is 5.791. The densities of the sulfuric acid used in the present work varied between 1.02475 and 1.0351 a t 30' C.
Apparatus The apparatus employed was essentially the same as that already described (3). Most of the work has been carried out with 1-cc. pycnometers, although 3-co. pycnometers were occasionally used. In the latter part of the work the hand-made pycnometers were replaced by vessels of similar design provided with precision ground-glass joints of standard taper (made by the Scientific Glass Apparatus Company, - - . Bloomfield, N. J.) which have proved very saiiifactory. The evaooration of liauid from the orecioitation flasks can be efficienily conducted- by surrounding ihe vessel with a close-fitting conical asbestos pa er hood, which is readily molded from wet asbestos paper and folds its shape after baking. It is held in position around the flask by a separate, split collar of asbestos paper which is kept firmly in place around the narrow flask neck by a twist of wire. These hoods are readily slipped on and off the precipitation flasks as required.
Method The analytical procedure is in general similar to that already described (3). Since the alloys analyzed contain tin
TABLEI. APPARENT DENSITY OF LEADSULFATB Weight of Lead Block 49 GTam
0.20735 0.2079 0.20835 0.20905 0.2002 0.499s 0,50045 0.19945 0.19985
Apparent Density at 30' Grame/oc. 5.746 5.827 5.771 5.832 5.807 5.77s
5.785 5.778 5.791 Av. 5.791
MARCH 15, 1939
ANALYTICAL EDITION
and antimony, these elements are first removed in the customary manner. The weighed sample is dissolved in a mixture of 15 cc. of concentrated nitric acid and 10 cc. of water and evaporated in the course of an hour t o 5 t o 10 cc. on a steam bath. After adding 40 cc. of water the precipitate is digested on a steam bath for 15 to 20 minutes. The precipitate containing tin and antimony, well mixed with paper pulp, is filtered off and well washed with 5 per cent nitric acid. The combined filtrate and washings, having a volume of about 400 cc., are evaporated to about 10 cc. and then rinsed in a 125-cc. precipitation flask with 20 to 30 cc. of 5 per cent nitric acid. For evaporations the precipitation flask should not contain more than 30 to 40 cc. of liquid. An asbestos hood is now placed around the precipitation flask, and its contents are evaporated to about 10 cc. on a hot plate. With the asbestos hood in place the solution boils quietly without bumping. To the cold solution 20 cc. of concentrated sulfuric acid are now added and evaporation is continued on the hot plate to copious fumes of sulfur trioxide. By proper regulation of the hot-plate temperature this operation also occurs quietly without bumping. The precipitate of lead sulfate dissolves shortly before the end of the fuming operation. T o the cold clear concentrated acid solution, 80 cc. of water are added, introducing the first portions very cautiously with good agitation. The mixture is digested for at least an hour on the steam bath to ensure a uniformly coarse precipitate. After digestion the solution is cooled and the precipitate settled while the precipitation flask is left in an inclined position. If then the flask is carefully placed in the upright position the precipitate remains at the side in the bottom and the siphon may be inserted to the bottom of the flask without contacting any precipitate. After siphoning off the original clear liquid, five successive 25-cc. charges (measured sufficientlyaccurately with a graduate) of 3 per cent sulfuric acid of carefully predetermined density, are introduced with intervening siphonings. After theintroduction of each charge of acid, the fliask contents are gently agitated by rotating the flask, conveniently while it floats inclined in a beaker partly filled with water. Violent shaking is to be avoided, as it breaks up the precipitate particles, causing a slight amount of the precipitate to float on the liquid surface as scum. After siphoning away as much as possible of the fifth charge of acid, 75 cc. of the 3 per cent acid are added and agitated with the precipitate. After the recipitate has settled, a pycnometer filled with the 3 per cent sulfuric acid is attached to the flask neck, as explained elsewhere ( 3 ) , and the assembled apparatus is slowly inverted so that the precipitate of lead sulfate slides smoothly into the pycnometer, By giving the inverted apparatus a rotary motion a number of times, the precipitate is transferred to the pycnometer so completely that no centrifuging is required. If a suitable centrifuge is available a more rapid transfer is accomplishedby centrifuging a minute or two at about 1200 r. . m. A t this relatively low s eed danger of leakage is so sligft, if the pycnometer is tightyy attached to the precipitation flask, that the centrifuge cup need not be filled with water during centrifuging. When the transfer of the precipitate t o the pycnometer is complete, the pycnometer is removed, its top is inserted firmly, and it is placed in a water bath at 30” C. After 15 minutes in the bath the height of the liquid in the pycnometer capillary is carefully adjusted t o the scratch, by inserting through the capillary extremely narrow and slender slivers cut from filter paper. Then the pycnometer is withdrawn, the stopper and outer ground glass surface are carefully wiped dry with filter paper, and the dry ground glass ca is firmly put in place. The vessel may now be completely wipe: dry, or preferably wiped over its outer surfaces with a moist cloth or chamois, then weighed after 15 to 20 minutes. The calculations of the results of the analyses are extremely simple (3).
Results The results of a series of pycnometric analyses for lead on two Bureau of Standards nonferrous alloys are given in Table 11. The phosphor-bronze bearing metal, Standard Sample 63, contains nearly 10 per cent each of tin and lead, and about 78 per cent of copper. The lead-base bearing metal, Standard Sample 53, contains nearly 79 per cent of lead and about 10 per cent each of tin and antimony. Thus a copperrich alloy and a lead-rich alloy have been subjected to pycnometric analysis. In both cases, by following the procedure outlined, using a 3 per cent sulfuric acid standard washing
141
liquid and an apparent density for lead sulfate of 5.791, analyses for lead are obtained in close agreement with Bureau of Standards averages. However, in the last analysis the density of the sulfuric acid liquor in which the precipitate formed was separately determined to be 1.2086, and the real density of 6.301 for lead sulfate was used in calculating the result.
TABLE11. PYCNOMETRIC ANALYSES FOR LEADAS SULFATE Sample
Veight
Lead Found
Lead Present
arums
%
%
%
9.74 9.74 9.75 9.75 9.74
9.74
0.00 0.00 $0.10 $0.10 0.00
78.92 78.84 78.90 78.74 78.85 78.88 78.71
78.87
$0.06 -0.03 $0.04 -0.16 -0.03 +0.01 -0.20
Allnv ~~~~. 63“ 1.0004 2.0001 2.00065 1,50095 1.0004 Alloy 53 0.19970 0.200204 0.99960 0.10035 0.512604 0.5O75Oa 0.21135a,l,
Relative Error
Analysis was made without use of centrifuge. b Precipitate was not washed but density of supernatant liquid was separately determined.
Discussion The pycnometric method is readily adapted to the determination of lead as sulfate in nonferrous alloys. Employing 3 per cent sulfuric acid as standard washing medium, an apparent density considerably lower than the real density of the precipitate must be used, but the compensation for solubility losses of the precipitate thereby afforded, allows pycnometric results to be quickly and directly obtained in close agreement with gravimetric values obtained only after carrying out extra procedures for the recovery of small amounts of dissolved lead sulfate. Because of the dependence of the apparent density of lead sulfate upon the strength and amount of sulfuric acid washing medium, as well as upon the general mode of the analysis, it is recommended that, having adopted a given analytical procedure, pycnometric analyses be carried out on samples of known lead content, and the proper value for the apparent density experimentally ascertained. This is readily done (3). The precision with which this precipitate density value needs to be known for calculating pycnometric analyses is diminished somewhat by the fortuitous fact that such calculatiom depend less strongly upon the value of the precipitate density in the case of denser precipitates (3). Thus in the case of the pycnometric method for lead any error in the value of the apparent density of lead sulfate appears in the calculated analyses diminished to about one fifth its original value and as an error of opposite sign. A considerable simplification of the pycnometric method is offered in the case of lead sulfate because it is possible to dispense with the centrifuge. This should also be possible with other dense, compact precipitates which settle readily. There is also always the possibility of entirely omitting the washing of the precipitate and substituting for this operation a determination of the density of the liquid present along with the precipitate in the pycnometer.
Literature Cited (1) Krings, W., 2. anorg. allgem. Chem., 181,323-6 (1929).
(2) Mellor, “Comprehensive Treatise of Inorganic Chemistry,” Vol. VII, p. 806, New York, Longmans, Green and Co., 1927. (3) Russell, W.W., IND.ENQ.CHEM., Anal. Ed., 9, 692-7 (1937). Rlocarvmn August 26, 1938.
