200
4
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
minutes, I cc. is withdrawn from t h e first t u b e of disinfected bacteria a n d added to a bottle containing 99 cc. of sterile water; one-half minute later t h e second minute intervals tube is so treated a n d t h e others a t in the same way. T o t h e tube of bacteria in which no disinfectant was placed, I O cc. sterile water are added a n d I cc. is transferred t o a bottle containing 99 cc. sterile water. These bottles are well shaken, a n d duplicate Petri dishes are poured from each one, I cc. solution being first added t o the dish followed b y enough nutrient agar t o make a satisfactory culture plate. When t h e dishes set they are inverted a n d allowed t o incubate a t 3 7 . j o C. for 48 hours. At t h e end of this time t h e plates are counted a n d the ratio of t h e number of surviving bacteria t o t h e number of colonies on t h e non-disinfected plate is determined. When plotted against t h e dilution t w o curves are obtained, types of which are shown on the accompanying plate. If a number of disinfectants are examined a t t h e same time, it is of course not necessary t o repeat t h e phenol series a n d t h e bacteria blank with each. T h e coefficient of total killing for Disinfectant A is seqn t o be 2.88, while for B it is 4.76. 4 study of a large number of these plates shows t h a t t h e curves for t h e so-called emulsion disinfectants are almost straight lines; a t most there is b u t a slight curvature. T h u s a small number of points will locate a line with a fair degree of accuracy a n d t h e necessity of using small increments of dilution as obtains in t h e older method is not here present. As is clearly shown in this plot, though desirable, i t is not necessary, as in t h e other methods, t h a t t h e most concentrated solution of t h e disinfectgnt used should produce a sterile tube or plate, t h a t is, show total killing. If a t least three points have been found t o lie fairly well upon a smooth curve, t h e line may safely be interpolated until i t cuts t h e axis representing complete killing a n d t h e coefficient calculated from this intersection. T h e results obtained may be duplicated with sufficient precision t o warrant confidence in them. This is true not only of a single operation carrying on duplicate determinations, b u t b y two analysts working separately. 1 ........................ 2 ........................ 3........................ 4 ........................
5 ........................ 6 ........................
I
I1
4.82 5.17 6.95 9.64 4.28 2.04
4.58 5.05 6.64 9.30 4.08 2.24
T h e above duplicate results obtained by t h e same analyst will give a n idea of the degree of accuracy which may be readily attained. RESEARCH LABORATORY OF APPLIEDCHEMISTRY MASSACHUSETTS INSTITUTE OF TECHNOLOGY
BOSTON
users of this pigment. Both among rubber manufacturers a n d producers of paints, i t is being found essential t h a t t h e contents of lead oxide a n d lead sulfate be known, so t h a t advantage m a y be fully t a k e n of its characteristic properties. This control necessitates a n analysis of t h e compound in t h e laboratory. I n analyzing sublimed white lead b y t h e usual method, it is found t h a t t h e percentage composition can be determined only b y a n analysis entailing lengthy manipulation, in which t h e content of lead oxide is directly dependent upon t h e accuracy of t h e other determinations, owing t o t h e necessity of estimating its percentage b y a calculation based upon t h e percentage of t h e other constituents present. T h e steps in t h e procedure must therefore be closely watched for slight inaccuracies a t all times. As is well known, t h e average composition of sublimed white lead is given as follows: Lead sulfate.. . . . . . . . . 78.5 Lead o x i d e . . . . . . . . . . . 1 6 . 0 Zinc oxide., , , , , , , , , , , 5 . 5
T h a t its composition varies only slightly from t h e above analysis during a long period of time, is shown by its comparison with a n average of the entire o u t p u t of t h e Picher Lead Company extending over, five months time, a n average embracing 2 7 0 total analyses. This average shows t h e composition t o be: Lead sulfate.. . . . . . . . . 7 6 . 6 8 Lead oxide.. . . . . . . . . . 17.23 Zinc oxide.. . . . . . . . . . . 5 . 7 9 99.70
A slightly higher lead oxide a n d zinc oxide content a n d a correspondingly lower lead sulfate content is found, t h a n in t h e usually stated formula. It shows, however, only slight variation. T h e average total percentage, consisting of lead sulfate, lead oxide, and zinc oxide, was found t o be 99.70 per cent. T h e remaining 0.3 of a per cent is only rarely determined, a n d when actually sought is found t o consist of moisture, occluded gas a n d ash. A definite ratio exists between t h e total lead content a n d the lead sulfate a n d lead oxide contents, a n d advantage m a y be t a k e n of this relation for a rapid a n d accurate determination of t h e lead constituents in sublimed white lead. I n order t o arrive a t t h e short method for t h e analysis which is based upon a direct calculation of the lead a n d zinc contents, i t is necessary t h a t the usual method of analysis be considered. USCAL
THE LEAD CONTENTS IN SUBLIMED WHITE LEAD-A CALCULATION B y JOHN A. SCHAEFFER Received December 1 1 , 1913
T h e composition of sublimed white lead, the basic sulfate of lead, has become a most important factor to
Vol. 6, No. 3
METHOD ADOPTED
FOR
THE A N A L Y S I S O F SUB-
LIMED WHITE LEAD DETERMINATION O F TOTAL
of t h e sample with beaker. Treat the boil gently for ten hours. Dilute t h e
SULFATE-Mix
0.5
gram
3 grams of sodium carbonate in a mixture with 30 cc. of water a n d
minutes. Allow t o stand for four contents of t h e beaker with hot
Mar., 1913
T H E J O l ' R S d L O F I , V D r S T R I d L d,VD E,VGI-ITEERISG C H E M I S T R Y
water; filter off t h e residue a n d wash until t h e filtrate is about zoo cc. in volume. Reject t h e residue. By this reaction all t h e lead sulfate is changed t o carbonate, t h e sulfate being transposed into sodium sulfate, which is found in t h e filtrate. Xcidulate t h e filtrate with hydrochloric acid a n d a d d a n excess of about z cc. of t h e acid. Boil, a n d a d d a slight excess of barium chloride solution ( 1 2 cc. of a n 8 per cent solution). When t h e precipitate has well settled, filter o n a n ashless filter, wash, ignite a n d weigh as BaSOI. Calculate t h e B a s 0 4 t o PbS04 b y using t h e factor 2 . 6 when a half gram is used. Weight of B a S 0 4 X 1.3 = weight PbS04. On 0 . j gram sample t h e factor B a s 0 4 t o PbSOd = 2 . 6 . DETERRIIXATIOS O F LEAD.
MOLYBDATE
METHOD' -
Dissolve I gram of t h e sample in IOO cc. of an acid ammonia acetate solution made u p as follows: Eighty per cent acetic a c i d . . . . . . . . . . . . . . . . 125 cc. Concentrated ammonia hydroxide.. . . . . . . 95 cc. LVater.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 cc.
Add this solution hot a n d dilute with about jo C C . of water. Boil until dissolved. Dilute t o 2 0 0 cc. a n d titrate in t h e usual manner with standard ammonium molybdate solution, spotting out on a freshly prepared solution of tannic acid. Ammonium molybdate is a slightly variable salt, b u t a solution containing 8.6; grams per liter usually gives a standard solution: I cc. = 0.01gram P b Standardize against pure P b O , pure PbSO4 or clean lead foil. Deduct t h e lead found as lead sulfate from t h e total lead a n d calculate t h e residual lead t o PbO. D E T E R MI K A T I 0N
0F Z I N C
.
F E R R 0 CY A XI D E
MET H 0 D
'
I gram of t h e sample i n a beaker with t h e fol-Boil lowing solution:
important factors; namely, t h e accurate estimation of the total lead content, t h e zinc content a n d t h e complete transposition of t h e lead sulfate present t o lead carbonate with t h e a t t e n d a n t formation of sodium sulfate from which t h e sulfate is determined as barium sulfate. The percentage of lead oxide depends wholly upon t h e accuracy of these determinations, a s i t is never directly determined. The follov,-ing method for ascertaining t h e lead sulfate a n d lead oxide contents is based upon a calculation depending upon t h e percentage of total lead found. T h e complete analysis can be easily carried out in a half hour. T h e essential requisites are t h e volumetric determination of t h e zinc present with i t s subsequent calculation t o zinc oxide, a n d t h e volumetric determination of t h e total lead. regardless of t h e proportion of lead sulfate a n d lead oxide present. These volumetric determinations are carried out according t o t h e method abo7.e outlined. Using t h e percentages of zinc oxide a n d total lead, together with t h e average total, 99.70 per cent, determined from t h e large number of analyses, t h e contents of lead oxide a n d lead sulfate are readily estimated b y t h e following calculation: T o t a l percentage of lead compounds present = total percentage found of ZnO, P b O a n d PhSOI - percentage of Z n O T o t a l percentage of lead compounds present = 99.70 per cent (average total1 - percentage ZuO. Atomic weight l e a d . . . . . . . . . . . . . . . . . . . . . . . . 207.1 Molecular weight lead oxide., . . . . . . . . . . . . . . 223. 1 Molecular weight lead sulfate. . . . . . . . . . . . . . . 303.1 As a hypothetical case, we can assume t h e presence of a 4.70 per cent ZnO a n d 69.00 per cent metallic lead. Determining t h e percentage of lead oxide a n d lead s u l f a t e present b y t h e above formulas we find:
303.1
"Technical Methods of Ore Analysis," 1 Modification of Low's method. Low. p . 149. 2 "Technical M e t h o d s of Ore Analysis," Low, p . 284.
X 69.00) - 95.00
--
207.1
Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ammonium chloride.. . . . . . . . . . . . . . . . . . . . . 4 grams Concentrated hydrochloric acid. . . . . . . . . . . . 6 cc.
If t h e sample is not quite dissolved t h e result is not affected, as t h e residue is lead sulfate or precipitated lead chloride. Dilute t o 2 0 0 cc. with hot water, a d d z cc. of a saturated sodium hyposulfite sol u t ion a n d titrate \vi t h a standard solution of potassium ferrocyanide, spotting out on a j per cent solution of uranium nitrate. Calculate t h e zinc t o zinc oxide. S U L F U R DIOXIDEDigest z grams of t h e sample with frequent stirring i n j per cent sulfuric acid for t e n minutes in t.he cold. Add starch indicator a n d iodine solution. titrate with N,/IOO -1 more accurate method is t o a d d a n excess of standard iodine solution t o t h e sample before t h e addition of t h e acid a n d t h e n t o titrate t h e excess of iodine with N,'roo sodium thiosulfate solution. It will be seen from t h e above method of analysis t h a t the concordance of results depends upon several
201
_
303.1 - 223.1 223.1 ~~
~
(2g x moo) XC-1 L,IVI.
"
_+
nL
= per ~ cent PhC) = 16.68
- 95.00
223.1 - 303.1 303. I
{--Mol wt. PhSOI \ .%t. wt. Ph
_
~~~~
= per cent PhSOi = 78.32
'' PbSOd
-
mol. wt. P h O
=
yc PbO present
(hfol. wt. P b O ~
- $5 Pb constituents -4t. wt. P b = Mol. wt. P b O - Mol. w t . PhSOa Mol. wt. PhSOi
.~
~
~
yc PhSOr present
.I comparison of the actual results obtained b y the complete analysis of sublimed white lead a n d its calculated composition shows t h a t t h e values obtained are concordant. Indeed t h e only essential factors for t h e short method are accurate determinations of t h e lead a n d zinc contents. The removal of several steps in t h e analysis leads t o greater accuracy coupled with a considerable curtailment of time. A table of comparisons shows t h e following concordance of results:
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.. . . . .
......... 77.97
3
5
6
8
10 11
Calculated.. Complete. ....
Complete . . . . . . . . . . . . 78.00 Calculated, . . . . . . . . . . 7 7 . 9 7 Complete . . . . . . . . . . . . 7 7 . 8 4 Calculated,. . . . . . . . . . 77.69 Complete . . . . . . . . . . . . 74.10 Calculated., ., , , ., . , . 77.41 C o m p l e t e . , . . . . . . . . . . 76.05 Calculated.. . . . . . . . . . 76.04 Complete. . . . . . . . . . . . 7 6 . 9 8 Calculated,. . . . . . . . . . 76.85
Lead oxide
Zinc oxide
Total lead
Total
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
5.23
68.30
99.72
5.11
68.70
99.66
5.73
68.40
99.77
4.86
68.40
...
99.72
.... 5.11
68.70
99.71
....
.... ....
...
...
...
...
...
...
....
...
...
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
THE 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 a n d 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, a n d 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, a n d 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; a n d 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. T h e possibility, or even the probability, therefore, of either or both normal lead carbonate a n d some form of lead hydroxide being present in stack process white lead must be admitted. T h e 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 a n d definite compound, zPbC03.Pb(OH)z, admixed with chance amounts of normal lead carbonate a n d 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 a n d 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 a n d 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 a n d precipitate, when t h e apparatus used is of sufficient size (say, 1000 pounds product per hour), a n d when proper tests are known, close control of t h e precipitation process is attainable; and in many thous a n d 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 a b o u t 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 a n 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 a n d analyses 8.6 per cent COZ or slightly higher, corresponding t o t h e formula P b C 0 3 . P b ( O H ) 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 a n d 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 a n d 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 3 P b C 0 3 . P b ( O H ) 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 a n d 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 a n y fixed a n d 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
.
