T H E JOL-R.YA4L OF I L Y D U . 5 T R I A L A.YD E S G I S E E R I S G C H E i I I I S T R Y .
198
be represented as directly proportional t o t h e area integrated and inversely proportional t o the time or t o some power of the time. Protective power may be represented as the inverse of this. Hence, P = C t" / A = K t * / Q , where K is a proportionality constant, t the time u p t o the maximum current, Q coulombs passed through t h e circuit up t o the time t , a n d 12 an arbitrarily chosen constant. Relative values have been obtained by calling K unity and b y assigning to the exponent n the values ( D ) I a n d ( E ) 2 . This method may be used only in case of the one-coat tests. The quantitative results of the above methods are, for the sake of brevity, summarized below in a qualitative manner. The capital letters refer to the preceding methods, and the fikures t o the paint mixtures as follows: ( I ) zinc oxide, ( 2 ) zinclead chromate, (3) American vermilion, (4) red lead, (j) white lead, (6) iron oxide, ( 7 ) barytes, ( 8 ) silica, (9) carbon black, ( I O ) graphite. The higher the position of a number in any column, the greater the excluding power, and as a rule, the less the depolarizing power of the paint film. One-coat tests.
TKO-coat tests. A,B.
C.
1
-
2 10 9 4 3 5
7 8 6
-
A.
B.
D.
E.
1
1
2
2 9
2 1 10
2 1 9 10 5 4
10 9 4 3 5 7
9 10 3
8 6
5
4 8 6 7
10 3 8
9
4
3 4 8
6 7 5
6 5 i
3 6 8 I
WORK NOT REPORTED
Although the method uses current values, a large number of electromotive forces were measured, taken after the current readings and after breaking the circuit. Data relative t o the change in resistance of the film were thus obtained. Direct measurement of resistance is greatly disturbed b y the condenser-like action of the paint films. All this will probably be considered more fully later. An attempt was also made t o determine the cause for certain inexplicable deviations among the average values of the current readings t h a t sometimes occurred during the course of a test. An examination of Plots I a n d I1 shows a few widely displaced points, especially on the third curve for red lead. All conditions were guarded as carefully as possible : the electrolyte was kept a t constant level, thus preventing change in conductance due to change in concentration. Temperature changes were watched. The resistance of the electrolyte between bare iron cathodes a n d anodes was only 18.6 ohms, which formed a p a r t of the resistance of the total circuit of about 119 ohms. This being the case and, further, the temperature changes in the tanks being small, no corrections were made in the current readings. These corrections would have accounted for b u t a few per cent. of the wide deviations referred t o above. Except for a few days when the temperature varied 18 5 4 O , i t varied only 19 5 2 " . The change in solubility of
Mar., 1 9 1 2
oxygen was thus very small, and could not account for the deviations anyway, as they were frequently in the wrong direction. c o x c L us I O h-
The work done thus far indicates t h a t the method is sound in principle, reasonably practicable, and reliable, in t h a t under the same conditions of testing, results can be duplicated. I t determines definitely, a n d under artificial conditions made as nearly natural as possible, the excluding power of a n y paint film against water and air. As regards the testing of paints for use on iron and steel, the method should develop into one of especial value, as the nature of the corrosion through the painted surfaces (according t o the electrolytic theory of corrosion) is entirely analogous to the mechanism of the reactions in the paint cell. I t has further been shown t h a t , a t least under the conditions maintained in the test-and it is probably true to varying degrees under conditions of actual service-films prepared from certain of the pigments experimented with possess a marked depolarizing action. This may be in part the saturation of unsaturated bonds in the linoxyn b y addition of nascent hydrogen, as i t has been shown t h a t linoxyn will depolarize hydrogen under the conditions maintained; and i t may be due in part to depolarizing properties possessed b y the pigments themselves ; b u t i t appears t h a t the rate of depolarization is determined largely b y the character of the whole filmt h a t is, pigment and vehicle combined. Increasing the porosity or the thinness of a film makes it possible for the depolarizing action t o appear, b u t it apparently does not necessarily cause i t t o appear. The exact nature of the depolarizing action, and its true relation to the thickness of the film will be investigated more fully in the future. REFEREXCES. 1 Gardner and Cushman. "The Corrosion and Preservation of Iron and Steel," pp. 241-243. 2 Gill and Foster, Tech. Quarterly. 17, 145 (1904). 3 Thompson, Trans. A m SOC. T e s t i n g iMalerzals, 7, 493 (1907). Benson and Pollork, THISJOURNAL, 3, 9 (1911). 5 IValker and Lewis. I b i d . , 1, 11 (1909).
RESEARCHLABORATORY O F h P P L l E V CHEMISTRY, > f A S S . IXST. O F TECHNOLOGY. BOSTON.
A SHORT METHOD FOR THE DETERMINATION OF SOLUBLE ARSENIC I N COMMERCIAL LEAD ARSENATES. BY B. E. CURRYAND T. 0. SMITH. Received October 23. 1911.
As the result of a request for an immediate report on the amount of soluble free arsenic oxid in a sample of commercial lead arsenate, the writers began a series of experiments t o develop a method of analysis which would give satisfactory results without incurring the tedious shaking of two liter bottles, eight times daily for ten days as in the procedure outlined in the standard A. 0. A. C. method for this determination. Since i t was desired mainly to reduce the time,
the method which naturally suggested itself was while the dry sample required several days. Beone of determining the amount of continuous stirring cause of this difference in the rate of solubility i t is at room temperature, t h a t would be equivalent to better to determine the water and then weigh out t h e eighty shakings a t intervals during ten days. an amount of the moist sample equivalent to two grams of dry lead arsenate. All the following deAccordingly the samples were placed in a thermostat at z o o C. and stirred continuously b y means of a hot terminations were made on moist samples. To overcome the inconvenience of washing the moist air engine. In order t h a t the results might be directly compar- sample from a weighing tube or watch glass, a small able with the results obtained b y the A. 0. A. C. piece of oiled-paper may be balanced on the pan. method i t mas necessary to use two liter bottles or The sample is weighed on this paper and the paper less than two gram samples of lead arsenate. The then transferred t o the jar. I t is necessary t o stir thermostat available would not conveniently carry the sample well into suspension with a glass rod bebottles with a capacity of more than 500 cc. and i t fore placing the j a r in the thermostat. did not seem advisable to use less than two gram samSeveral experiments were made to determine the time required t o bring the lead arsenate and water ples. A few preliminary determinations demonstrated into equilibrium. It was found t h a t with the temperat h a t more than eighteen hours of continuous stirring ture a t z o o C. the maximum time was for all practical did not appreciably increase the amount of arsenic purposes not more than 18 hours. Satisfactory reoxid in solution. ,41so these results showed t h a t sults were had where stirring had been continued no obvious direct relation existed between the A. 0. only twelve hours. This makes i t easily possible A. C. results, and those obtained b y continually stirring to complete the determination the second day. By tm-o gram samples in the thermostat with 500 cc. of the A. 0. A. C. method the report is delayed until water. The solubility of the lead arsenate itself the eleventh day. When 500 cc. jars are used as in these experiments, entered in and caused the difference between results obtained by the two methods. The difference in the a zoo cc portion of the solution is a satisfactory volumes of r a t e r used in the two methods amounted amount for analysis. Most lead arsenate solutions to Ijoo cc. This quantity of water dissolves a con- do not subside rapidly. In order to obtain a solution free from solid lead arsenate it is usually necessary siderable amount of lead arsenate. This consideration involved a side problem. It t o pour the contents of the jar on a filter and take has been pointed out by others, t h a t arsenate of the sample for analysis from the filtrate. In a few lead may mean the triplumbic arsenate Pb,(AsO,), samples of commercial lead arsenate, small amounts o r plumbic hydrogen arsenate PbHAsO,. Most com- of arsenious oxid were found. The exact amount mercial samples may consist of a mixture of these may readily be determined b y simply adding t o this two, the amount of each depending upon the method z o o cc. portion, the starch and bi-carbonate and t i and the conditions under which the precipitation was trating with a standard iodine solution. made. I t is evident, also, t h a t in some cases the byAs,O, 41 zH,O = As,O, 4HI. products formed in the process of manufacture have n o t been washed out. In some samples the odor of The addition of bi-carbonate, starch and iodine acetic acid, which is formed when lead arsenate is solution does not interfere with the subsequent demade from lead acetate and di-sodium arsenate, termination of arsenic oxide. In the reduction would indicate t h a t this step in the manufacture the bicarbonate is neutralized, the starch is conhad probably gone no farther than the filter press. verted into sugar and in the acid solution the iodine Based on the theoretical percentage of As,O, in lead becomes a reducing agent. Since arsenious oxid arsenates consisting of mixtures of Pb,(AsO,), and is not always present, it is desirable t o test for i t qualiPbHAsO, and also upon the analysis of a number tatively in a part of the solution remaining after the of samples from some of the leading manufacturers, two samples of z o o cc. each have been taken. The reduction of the arsenic oxide to arsenious i t is evident t h a t most commercial grades carry from 2 5 per cent. t o 33 per cent. of As,O, in the mois- oxide in the zoo cc. sample is readily accomplished ture-free product. As the solubility of these lead b y the modified method of Mohr. In this laboratory, arsenates is lorn the variation within these limits j cc of sulphuric acid and one gram of potassium is slight and a solubility factor may be determined iodide are added to the sample in a 400 cc. beaker which introduces no appreciable error within this and the solution boiled until the volume is reduced t o 40 or j o cc. The solution is then cooled and diluted range. o I n most cases the iodine will not Lead arsenate is generally p u t on the market in t o about ~ j cc. air-tight containers in order to keep i t in a moist all have been removed and the solution will retain condition until ready for use. After i t has once been a yellon- color This excess of iodine is removed by dried i t is much more difficult to keep in suspension. the addition of twentieth normal sodium thiosulphate This fact suggested the relative rate of solubilities solution added carefully from a burette or' the dry arsenate and the commercial form which 21 zSa,S,O, = Sa,S,O, zNaI. usually contains about 40 per cent. to 60 per cent. moisture. Investigation showed t h a t the moist samIf the flask or beaker is of colorless glass and set ple reached its maximum solubility in about 18 hours, on a white plate carefully screened from reflected '
+
+
+
+
+
2 00
T H E JOC'RA\-AL OF I-\L9CSTRIAL
colors, i t is not necessary to add starch t o determine the end point. The addition of the starch here is undesirable as will be noted later, and under these conditions the disappearance of the yellow due to the presence of iodine gives a satisfactory end point. After the excess of iodine is removed the clear solution is made slightly alkaline. A sodium hydroxid solution is more convenient and rapid for this purpose than sodium carbonate and gives satisfactory results. The solution is again made acid with a few drops of dilute sulphuric acid and stirred well to make certain t h a t all alkali is neutralized ; the solution is made alkaline again with sodium bicarbonate and is ready for titration. A considerable excess of bicarbonate, as is sometimes recommended, has not been found necessary in these experiments. The solution is titrated with a standard iodine solution according t o the equation given for the oxidation of free arsenious oxid in the original solution Various comments have been made on the use of starch as an indicator in this reaction and many methods proposed for eliminating its objectionable features. A number of these methods were tried in these experiments. The digestion of the starch in dilute hydrochloric acid for twenty-four hours and subsequent drying a t 100' C. for three hours is not entirely without merit, b u t was not found necessary when a good starch was procured and well boiled. The addition of three drops of a ten per cent. solution of potassium iodide increases the delicacy of the end point when only a part of a cubic centimeter of iodine solution is used. When a larger amount is used the potassium iodide in the iodine solution itself serves the purpose and makes further addition unnecessary. In a few samples the lead was removed as sulphate before beginning the analysis for arsenic ; b u t , with the amount of lead present in these samples, its presence was not found objectionable. The number of cases in which this was investigated, however, and the small amounts of lead present in all the samples preclude any generalization as t o how far this statement will hold true. The results thus obtained include all arsenic oxid in solution and are lower than those obtained b y the A. 0. A. C. method, This is because the solubility of lead arsenate is left out of consideration and the volumes of water are unequal In order t o fix the relation between the results obtained b y the two methods i t was necessary first to determine the solubility of lead arsenate. Accordingly, a number of samples were prepared in the laboratory and a number of commercial samples were selected and washed as free as possible from by-products with distilled water. The solubility of the lead arsenate in these samples was determined by stirring a t 20' in the thermostat and also b y the A. 0. A. C. method. I t was found t h a t the amount of soluble arsenic determined by the A 0. A. C method mas exactly four times as much as t h a t found b y the short method, the amounts being directly proportional to the volumes of water used in the different methods. The average of the results obtained by the A. 0. A. C.
AlVD ENGIIYEERISG C H E M I S T R Y .
Mar., 1912
method calculated to percentage was 0.605 per cent. while the average of the results obtained b y the short method was 0.151 per cent. These results include the blank for the end point in the titration. The results obtained b y both methods agree very closely. In order to obtain the amount of soluble arsenic not in the form of lead arsenate the above percentage should be subtracted from the total amount of soluble arsenic. Manufacturers supplied us with 17 samples of lead arsenate which carried various amounts of soluble arsenic oxid. These samples were analyzed for soluble arsenic by the short method outlined above a n d also by the A. 0 . A4.C. method. The results of these determinations are given in Table I. In this table, column z gives the results obtained by the short method; column 3 shows the results obtained b y the A. 0. A. C. method. N o corrections are made in these columns for the solubility of lead arsenate. These results give the total percentage of soluble A4s,0,. These samples contained no soluble arsenious TABLEI. Per cent. Per cent. soluble arsenic. soluble arsenic. A . 0 . A . C. Short method method Der cent. of Per cent. corrected for corrected for solubility solubility soluble arsenic soluble arsenic of lead of lead Lab. no. by short by A. 0. A. C. arsenate. arsenate. of sample. method. method. 1 2 3 4
5 6 7 8 9 10 11 12 13 14 15 16 17
0.26 0.37 0.37 0.44 0.40 0.40 0.41 0.44 0.46 0.66 0.77 0.96 1.22 1.42 1.99 2.61 3.24
0.70 0.81 0.85 0.83 0.85 0.91 0.92 0.92 0.96 1.12 1.26 1.50 1.79 1.98 2.50 3.03 3.78
0.11 0.22 0.22 0.29 0.25 0.25 0.26 0.29 0.31 0.35 0.62 0.81 1.07 1.27 1 .84 2.46 3.09
0:10 0.21 0.25 0.23 0.25 0.31 0.32 0.32 0.36 0.52 0.66 0.90 1.19 1.38 1.90 2.43 3.18
oxid. Column 4 shows the results obtained by the short method after subtracting the factor 0 . I j I per cent. for the solubility of the arsenate of lead. Column j shows the results by the A. 0. A. C. method after making the correction of 0.605 per cent. The results given in columns 4 and 5 are practically identical. Referring still to the table i t is a t once obvious t h a t by correcting for the solubility of lead arsenate t h a t the results determined b y either method may be quickly changed t o terms of the other. The corrected results obtained by the short method give only the soluble arsenic oxid uncombined x i t h lead oxid. On this account it more truly tells the nature and quality of an]- commercial lead arsenate. The A. 0. A. C. method gives results uniformly 0 . 6 0 j per cent, higher than the corrected results given b y the shorter method. Stated otherixTise, when 0.605 per cent, is added t o the results obtained b y this method the results become the same as those obtained by the A. 0. A. C. method and with the expense of much less time. The result of this method
which, however, is limited, varying with the amount of base metals present; should they be present in considerable quantities the residue must be weighed, after drying at 1 1 0 ’ C., t o avoid an error caused b y their oxidation. Ordinarily. hon-ever, this oxidation is so small t h a t i t need not be taken into account. I n the separation of carbonates and ignition of residue, the loss will also include. small amounts of water, volatile and organic matter. The residue left insoluN r a H.v.iPsrx I R E ble in acetic acid contains all the silica originally presAGRICULTURAL E X P E R I 3 I E N T S T A T I O S , ent in the ore. The silica is now determined b y DCRHAX.NE\\. I I A > l P S € i I R E . volatilizing the same as silicon fluoride. I h e presence of sulfides of iron, zinc and lead must be considered, and for this reason I have found i t best t o VALUATION OF FLUORSPAR. oxidize t h e sulfides in the residue b y heating with merB y E. BIDTEL. curic oxide before evaporating with hydrofluoric Received January 2 , 1912. acid. In this operation lead sulfide is entirely oxitlI n the commercial analysis of fluorspar the deter- ized t o lead sulfate, b u t the ferruginous zinc sulfide minations usually required are calcium fluoride, is b u t slightly acted upon and remains unaffected b y silica and calcium Carbonate; in some particular cases hydrofluoric acid, in so far as final results are conlead, iron, zinc and sulfur. We have received several cerned, as shown b y analysis of synthetical samples calls for percentage of barium sulfate b u t t h e most of known percentages. careful qualitative tests have failed t o shox- the 111 regard t o t h e determination of calcium fluoride, presence of this mineral. as \Till be shown later, in I avoid decomposing t h e sample and eliminate the either n-hat is known as Rosiclare or Fairview fluor- calcium fluoride b y treating the residue obtained from spar. the silica determination with hydrofluoric acid t o There is no practical method for the separation transform the iron oxide into iron fluoride and e s of fluorine from silica and of determining fluorine tracting the same together with the lead and zinc direct in its ore, which answers the commercial re- b y solution of ammonium acetate containing ammoquirements of a mine laboratory. The usual prac- nium citrate. The method as used at present follows: tice follon-ed b y most chemists is to determine the Weigh into a small Erlenmeyer flask one gram of total calcium oxide and t o calculate from this the t h e finely powdered sample, add I O cc. of ten per amount of calcium fluoride, after deducting the cent. acetic acid, cover with a short-stemmed glass amount equal t o the calcium carbonate present, this funnel and heat on a water bath for one hour, agitalatter being determined from a separate sample in ting from time t o time. Filter through a j cm. ashthe usual wa>- b y absorbing the liberated carbonic less filter, mash nTith warm v a t e r four times and burn acid in soda-lime tubes. This method would be correct off the filter paper in a weighed platinum crucible if all the calcium present were combined with either at a temperature as low as possible. The loss in weight carbonic acid or fluorine and all t h e carbonic acid minus O . O O I ~gram (the amount of calcium fluoride present combined with calcium. The first assumption soluble in acetic acid under t h e conditions named) is probably correct as all our experiments go t o prove is calcium carbonate. this, b u t the latter we know is wrong. for lead carbonAdd t o the residue in the platinum crucible about ate, zinc carbonate and iron carbonate are frequently one gram of yellow mercuric oxide in t h e form of an Con- emulsion in water; break up any hard lumps t h a t present, especially in the gravel fluorspar. sequently, the calculated amount for calcium carbonate m a y have formed; evaporate t o dryness and heat n-ill be too high and the calcium fluoride in consequence t o a dull red h e a t ; cool and weigh. Add about 2 cc. will be too l o n . of hydrofluoric acid and evaporate to dryness, repeatI n T-iv7.v of t h e above error. I have found it better ing this operation twice, using I cc. of hydrofluoric t o diss,.jl\.e all carbonates from the sample b y acetic acid in each of the last two operations. Add a few acid. lea\-ing the calcium fluoride and silica as a resi- drops of hydrofluoric acid and some macerated fildue (UII the filter read!* for the determination of both ter paper, as recommended b y Dittrich, then a few silica ant1 calcium fluoride. I t must he remembered drops of ammonium hydroxide to precipitate the iron t h a t calcium fluoride is slightly soluble i n acetic acid. and evaporate t o dryness. Heat t o a dull red heat, .I have reduced this solubilitJ- t o a factor which I . cool and weigh; the loss in v-eight is reported as silica. use in m?- calculatians. Generallh- the amount of KOK add 2 cc. uf hydrofluoric acid and a few drops of calcium carbonate present is greatlj- in excess oi the nitric acid, cover the crucible with its lid and place other c n i - b ( n i a t e s . so t h a t frequently i t will be sufi- on a moderately warm water bath thirty minutes ; cientl!- accurate fur commercial purposes t o report remove the lid and evaporate t o dryness. If the conthe amount soluble in acetic acid after deducting t h e tents of the crucible are not now perfectly white, amount of calcium fluoride soluble in this acid as evaporate again with hydrofluoric acid, add a few calcium carbonate. I t is to be remembered t h a t a t drops of hydrofluoric acid anti I O cc. of the solution this point there is oliportunit>- for another small error of ammonium acetate (this ammonium acetate is as given in column 4 gives t h e soluble arsenic oxid not combined with lead oxid. The method outlined in this paper gives a rapid and accurate procedure for determining soluble arsenic oxid in commercial lead arsenate, It is pointed out t h a t the solubilit-\- of lead arsenate affects the total amount of soluble arsenic oxid and a correction is worked out which gives the amount of soluble arsenic oxid not combined with lead oxid.
