T B E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
202
Lead
No. Analysis sulfate 1 Complete ............ 79.20 Calculated..
3
5
6
8
10 11
....
Calculated.. ......... 77.97 Complete. ....
Complete . . . . . . . . . . . . 78.00 Calculated, . . . . . . . . . . 77.97 Complete . . . . . . . . . . . . 7 7 . 8 4 Calculated,. . . . . . . . . . 77.69 Complete . . . . . . . . . . . . 74.10 Calculated., . , , , . , . , . 77.41 Complete., . . . . . . . . . . 76.05 Calculated.. . . . . . . . . . 7 6 . 0 4 Complete. . . . . . . . . . . . 7 6 . 9 8 Calculated,. . . . . . . . . . 7 6 . 8 5
Lead oxide
Zinc oxide
Total
5.23
Total lead 68.30
15.28 15.30 16.81 16.62 16.95 17.12 14.66 14.69 16.60 16.62 17.10 17.15 16.20 16.06 20.21 20.22 15.92 16,06 17.93 17.93 17.78 17.87
5.11
68.70
99.66
5.73
68.40
99.77
4.86
68.40
...
99.72
.... 5.11
68.70
99.71
....
.... ....
99.72
...
...
...
...
...
...
....
...
...
4.86
69,OO
99.80
...
...
6.23
67.80
99.63
...
...
5.4s
69.40
99.79
...
...
6.23
67.80
99.78
...
...
5.73
68.60
99.71
4.98
69.10
....
.... ....
.... ....
....
...
...
...
’
99.74
...
T h a t this method will prove of value will be readily appreciated b y all chemists who have t o determine t h e percentage composition of a n y basic sulfate of lead, either for t h e purpose of meeting specifications or for accurate control of finished products. CHEMICAL LABORATORY, PICHERLEADCo. JOPLIN, MISSOURI
T H E CONSTITUTION OF WHITE LEAD By
EDWINEUSTON
Received December 1. 1913
T h e usual view is t h a t t h e widely used Dutch, or tan-bark stack, process of corrosion of metallic lead to form white lead results in a product containing principally a fixed and definite compound, zPbC03.Pb(0H)Z. This view is based on t h e physical appearance of the crust formed under t h e best conditions, and assumes t h a t t h e higher percentage of combined carbon dioxide almost always found on analysis is due t o admixed normal lead carbonate, and t h a t t h e lower percentage of carbon dioxide occasionally found results from admixed lead hydroxide. I t is certain t h a t a considerable percentage of normal lead carbonate in t h e crystalline form (“sandy. lead”) is usually present in t h e product of the stack process, even t o t h e extent of j per cent or more; and under abnormal conditions of corrosion de-hydrated lead hydroxide is found t o be present in quantity sufficient even to injure the color of t h e white lead. The possibility, or even the probability, therefore, of either or both normal lead carbonate and some form of lead hydroxide being present in stack process white lead must be admitted. The purpose of t h e writer is t o consider whether true white lead (the so-called basic carbonate of lead) consists substantially of a fixed and definite compound, zPbC03.Pb(OH)z, admixed with chance amounts of normal lead carbonate and lead hydroxide, or whether some other form or forms of combination of the elements involved may not better accord with t h e facts. During t h e long time required in t h e stack process t o effect t h e commercial degree of corrosion, usually IOO t o 1 2 0 days, no important control can be exercised over t h e process, and even in t h e separate tiers of the same stack, wide variations of the governing conditions
Vol. 6 , No. 3
of heat, ventilation, moisture and vaporization of t h e acetic acid occur, rendering t h e stack process entirely unsuitable as a means of investigating experimentally t h e steps in t h e formation of white lead and the nature of t h e product. Precipitation b y carbon dioxide gas from basic lead acetate solution, as first suggested by Thenard as a means of forming white lead, has been found t o be subject t o so many difficulties of control t h a t heretofore no results of value have been derived from this method. Yet when due care is exercised t o ensure uniformity of treatment of the entire mass of solution and precipitate, when t h e apparatus used is of sufficient size (say, 1000 pounds product per hour), and when proper tests are known, close control of t h e precipitation process is attainable; and in many thousand repetitions on this manufacturing scale the writer has found t h a t very definite results may be obtained, throwing new light on the series of changes occurring when basic lead acetate solution is subjected t o t h e action of carbon dioxide. For practical reasons a n approximately di-basic solution of lead acetate formed from about 4 per cent acetic acid is preferable, uniformly circulated (without permitting separation of t h e resulting precipitate from t h e liquid) b y spraying a number of times through an atmosphere of moderately or strongly concentrated carbon dioxide gas, a t room temperature. When t h e precipitation is slowly performed under standard conditions, the precipitate first appearing is colloidal in character and analyses 8.6 per cent COZ or slightly higher, corresponding t o t h e formula PbC03.Pb(OH)2. When more quickly performed, t h e precipitate contains 9.0 t o 10.0per cent COZ,therefore also less t h a n t h e 11.3 per cent COz corresponding t o t h e formula 2PbCO3.Pb(OH)2 constituting the usually accepted view of the principal compound in white lead. On continuing t h e treatment, t h e percentage of combined COS increases gradually until it approximates 16.6 per cent. corresponding t o the formula of t h e normal carbonate P b C 0 3 , after which no further precipitation and no further change in t h e composition of the already formed precipitate occurs. Throughout this treatment the increase in t h e COa percentage in the precipitate is progressive and gradual, with no evidence of t h e formation of hypothetical intermediate compounds such as t h e supposed 2PbC03.Pb(OH)*, or such as 3PbC03.Pb(OH)z. T h e change in the solution as t h e available lead is precipitated is gradual, from strong alkalinity t o slight acidity. Similarly, t h e properties of t h e precipitate indicate only gradual change as t h e process progresses, except t h a t when t h e solution passes t h e point of exact neutrality t h e ‘(apparent density” of the precipitate suddenly and sharply increases, b u t thereafter resumes the more gradual rate of increase which is also characteristic in the earlier stage of t h e precipitation. This sudden increase in ‘(apparent density” is doubtless due t o coagulation occurring when t h e solution becomes acid. Analyses do not indicate any fixed and definite composition of the precipitate when this sudden increase in ((apparent density” occurs, as t h e determinations may range variously from 1 1 . 0 per cent t o even 13.5
.
Mar., 1914
T H E J O r R S A L O F I - V D L 7 S T R I d L A N D E,VGI,TEERIXG
per cent COS. The foregoing, therefore, affords no evidence of the formation of t h e usually assumed compound z P b C 0 3 . P b (O H ) z , b u t on t h e contrary shows t h e initial formation of a precipitate of t h e composition P b C 0 3 . P b ( O H ) 2 and the gradual change t o t h e normal carbonate. Lead hydroxide is soluble in a solution of cane sugar in water. At no stage in t h e above described process is t h e separated and washed precipitate appreciably soluble in cane-sugar solution. Therefore, t h e precipitates described are not simple mixtures of normal carbonate and of lead hydroxide, b u t hold t h e lead hydroxide in some form of combination with all or part of t h e carbonate. This fact, in connection with the analyses of t h e first formed precipitate, shows t h a t t h e first product obtained is a basic carbonate of the formula P b C 0 3 . ( 0 H ) 2 . Inasmuch as t h e final product obtained by continuing t h e process t o t h e extreme degree is P b C 0 3 , t h e question then becomes whether these two compounds, PbC03.Pb(OH)? and P b C 0 3 , when mixed in various proportions, constitute white lead, or whether other compounds must also be postulated. X s mixtures of P b C 0 3 . P b ( O H ) ? and PbC03 in proper proportions give all the intermediate percentages of combined carbon dioxide between 8 . 6 per cent and 16.6 per cent, no other compounds t h a n these two need t o be assumed from t h e point of view of composition. A mixture of one equivalent of PbCO3.Pb( O H ) , with one equivalent of PbC03 will give t h e average 11.3 per cent COS of the heretofore assumed zPbCO3.Pb(OH)s, and t h e physical properties of a sample of white lead analyzing 11 3 per cent COS and directly prepared are identical with t h e physical properties of a sample of the same average composition obtained b y mechanically mixing together separately prepared equivalent quantities of PbCO3.Pb(OH), and of P b C 0 3 , e . g., t h e space occupied by 20 grams of the pulverized sample settling freely under water in a zoo cc. cylinder, t h e amount of linseed oil necessary t o form a paste, t h e ease of miscibility with linseed oil, also t h e opacity and spreading power when mixed as a paint. This same result is true for all other variations of composition between 8.6 per cent and 16.6 per cent COS, i. e . , the entire range. The molecular volume affords further evidence t h a t no other compounds t h a n PbC03.P b (OH):! and P b C 0 3 are present in white lead properly prepared:
(a) (b)
(c)
(4
Molecular volume Ratio 1.0 PbCO3. . . . . , . . , . . . , , . . 5 7 . 0 PbCOa.Pb(OH)z.. . . . . . 9 7 . 8 1.71 2PbCO3.Pb(OH)z.. . . . . . 154.8 2.71 JPbCOs.Pb(OH)a . . . . . . . 211.8 3.71
Percentage linseed oil required t o form paste 7 12 19 26
(actual) (actual) (indicated) (indicated)
Ratio 1.0 1.7 2.7 3.7
The close correspondence between the ratios for calculated molecular volume and for actual oil requirement in the case of ( a ) and ( b ) is very striking. The indicated oil requirements for the hypothetical compounds (c) and ( d ) , v i z . , 19 per cent and 2 6 per cent, are entirely too high t o accord with the known oil requirements of white lead of the corresponding
CHEMISTRY
203
analyses, and would imply t h a t as the percentage of carbon dioxide in white lead increases, the oil requirement would rapidly increase until. when the normal carbonate should be reached, a sudden great drop in oil requirement would result. Certainly this is not the case. for actually the oil requirement, as t h e percentage of carbon dioxide increases, shows gradual reduction from 1 2 per cent t o 7 per cent for all intermediate stages. Instead of the indicated oil requirement of 19 per cent €or ( c ) , therefore, this figure becomes ( 7 X 266.5) ~
~
+
(12
X ~
773.5
jo;) =
10.27
per cent,
which accords with the facts. provided allowance is made for gain in compactness due t o the intermeshing of two sizes of particles, resulting in a n actual requirement of about y per cent oil. Inasmuch as the Dutch or stack process also involves the use of basic lead acetate solution and carbon dioxide gas, there is no reason t o consider t h a t the course of reactions in the stack process (in which the reactions cannot be watched) differs from t h e now known course of reactions in the precipitation process (in vhich the reactions can be watched); and, except in its comparative coarseness and granularity and its less degree of whiteness due to tan-bark and other stains. stack process lead has no characteristics different from those of a precipitated white lead known t o contain the two compounds P b C 0 3 . P b ( O H ) 2 and P b C 0 8 . The conclusion is clear, therefore, t h a t white lead does not contain a compound of the heretofore assumed formula zPbC03.Pb(OH)2, b u t t h a t on the contrary white lead consists of a mixture of the two amorphous compounds PbC03.Pb(OH)Z and PbC03, complicated in t h e case of stack white lead by the presence of some lead carbonate in crystalline form and occasionally some form of lead hydroxide due to abnormal conditions in the corroding stack. EUSTOXWHITE L E A D CO S T LOUIS
ON THE DETERMINATION OF TITANIUM AS PHOSPHATE B y GEORGES J ~ M I E S O S A K D R I C H A R D WRESSH~LL Received October 29, 1913
Several years ago Eric John Ericsonl described a method for t h e determination of titanium in ferrotitanium and in ores. The method was based upon the precipiLation of titanium phosphate in acid solution by t h e addition of ammonium phosphate and boiling, after reducing the iron t o the ferrous state by means of ammonium bisulfite or sulfur dioxide. He gave the factor 0.336 for calculating the titanium from t h e weight of t h e phosphate, assuming t h a t the latter has t h e formula Ti2P208. On account of the simplicity and rapidity of this method its accuracy and application have been studied in this laboratory. Some modifications of the original method have been found desirable and its application t o t h e separation of titanic acid from alumina has been worked out. A series of eighteen experiments 1 Iron
Age, Aug. 27, 1903, p. 4.
.
