T H E JOUR,VAL O F 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
910
let was plugged. As further proof, comparisons were made of t h e flash point of a n oil obtained in t h e usual way a n d of t h e flash obtained b y placing a q u a n t i t y of low flash oil in t h e overflow cup. T h e results are shown in Table \-. TABLE V
Sample
Nc . Test
1087
No. 1 . . . . . . . . . . . . . .
No. 3 . . . . _ . . . .
.
.,.
Average.. . . . . . . . . . . .
Sample No. 108i with S cc. of No. l08G (F. P. = 3 7 ’ ) in the overflow cup
45.4
45.4 45.4
45.8
45.4
-
-_
45.5
45.4
I n s t r u m e n t used
B. of hZ. modified Abel-Fensky
S U 11 31 A R Y
T h e i m p o r t a n t factors t o be considered in t h e construction a n d manipulation of a n i n s t r u m e n t t o determine t h e flash point of a n oil m a y be stated as follows : ( I ) T h e conditions under which t h e test vapors are generated should be as like as possible t o t h e conditions found in practice. ( 2 ) Corrections should be made for variations from t h e normal barometric pressure. (3) T h e size a n d dimensions of t h e cup. (4) T h e size, shape, d e p t h of immersion a n d exposed p a r t of t h e thermometer should be definite a n d in every case t h e thermometer should be calibrated. (j) T h e oil should never be exposed for a n y length of t i m e a t temperatures greatly higher or lower t h a n the normal temperature. (6) T h e r a t e of heating on testing should be constant a n d a t a r a t e of a b o u t I O C . for l a m p oils a n d 3 O - j O C. for high flash-point oils. ( 7 ) T h e oil should be uniformly stirred during t h e test. (8) T h e test flame should be of definite size, should be exposed a t a fixed distance above. t h e surface of t h e oil a n d exposed for a definite length of time. (9) A11 water should be removed from t h e oil prior t o testing. ( I O ) I n general testing, so far as possible, t h e effect of t h e personality of t h e operators should be eliminated a n d t h e manipulation of t h e tester made entirely mechanical a n d automatic. T h e most accurate tester a n d t h e tester t h a t most nearly reproduces actual working conditions should be adopted for official tests. Considering all of t h e above factors, it is believed t h a t these t w o testers, t h e Abel-Pensky a n d t h e P e n s k y - M a r t e n s , as modified b y t h e Bureau of htines, most nearly meet with t h e desired conditions. T h e Bureau of Mines flash testers have been officially adopted b y t h e Kational Fire Protection Association, t h e Independent Petroleum h,Iarketers’ Association of t h e United States a n d a r e now being considered for adoption b y committees on tests of t h e National Petroleum Association, t h e American Chemical Society, t h e American Society for Testing Materials a n d t h e International Petroleum Commission. B U R E A UOF M I N E S PITTSBURGH
Vol.
s 9 No.
11
THE TITRATION OF CALCIUM AND MAGNESIUM IN THE SAME SOLUTION By PAULJ. Fox’ Received September 9, 1913
Few determinations are more frequently made in chemical laboratories t h a n those of calcium a n d magnesium, a n d t h e writer was led into the following investigation with a view t o lessening the labor required for these determinations: T h e titration of calcium as oxalate with permanganate is, of course, a well known a n d s t a n d a r d process, a n d . t h e titration of magnesium as magnesium a m m o n i u m arsenate with sodium thiosulfate in acid solution after t h e addition of potassium iodide has been described b y Meade* a n d b y G. B. Frankforter a n d Lillian Cohen.3 It has occurred t o t h e writer t h a t it might be possible t o combine these processes in one operation b y first precipitating t h e calcium as oxalate, a n d t h e n in t h e same solution a n d without filtering, precipitating t h e magnesium as magnesium ammonium arsenate, filtering off a n d washing t h e combined calcium oxalate a n d ammonium magnesium arsenate, dissolving in acid, titrating t h e calcium oxalate with potassium permangante a n d t h e n t h e magnesium ammonium arsenate with sodium thiosulfate after t h e addition of potassium iodide. As will be shown in t h e sequel, this combined method is perfectly feasible, t h e t w o titrations not interfering with each other. T h e titration of calcium as oxalate calls for no rem a r k except t o observe t h a t t h e objection t h a t t h e calcium sulfate formed prevents complete decomposition of t h e calcium oxalate is not, in t h e writer’s experience, valid. With moderate skill in manipulation, i t is always possible t o obtain good results. I n t h e titration of t h e arsenate, however, certain irregularities were encountered. For example, following approximately t h e directions in t h e paper of F r a n k forter a n d Lillian Cohen, when I O cc. portions of a certain solution of a m m o n i u m arsenate were acidified, a n d made u p t o IOO cc. so t h a t I O O cc. contained a b o u t ~j cc. concentrated sulfuric acid, cooled, j grams potassium iodide added, a n d titrated after standing j minutes, 2 1 . 2 1 , 21.j2, a n d 2 0 . 7 8 cc. thiosulfate solution were required t o decolorize t h e solution. All t h e portions were, of course, measured out b y t h e same pipette a n d measured over t h e same portion of the burette. T h e figures given are selected from m a n y a n d are representative of t h e magnitude of t h e variations. T h e y were measured out a n d titrated a t different times, a series t i t r a t e d a t t h e same time showing less variation. T h e e n d point was always certain a n d definite. M a n y experiments were made t o find t h e cause a n d remedy for t h e irregularities, b u t t o cite t h e figures in detail would scarcely be profitable. T h e a d v a n tages of t h e improvements suggested are more or less self evident. T h e principal causes of irregularity -assuming of course t h a t t h e iodide used is free from iodate or other substance capable of setting free iodine 1 Scientist in Soil Investigations, Bureau of Soils, U. S. D e p t . of Agriculture. 2 R . K. Meade, Jour. A m . C k e m . SOL.,21, 146 (1899). 3 Jour. A m . Chem. Soc., 29, 1464 (1907).
NOV.,
I913
T H E JOCR,V=IL O F I N D G S T R I A L A X D ENGIAVEERING C H E M I S T R Y
911
in acid solution-are (I) t h e slowness with which straw color, a n d t h e flask allowed t o s t a n d in t h e d a r k pentavalent arsenic sets free iodine in acid solution; for a b o u t six minutes. T h e rule was observed t o allow eight minutes t o elapse between t h e adding of t h e ( 2 ) in strong solutions, t h e escape of iodine before titration: (3) especially t h e effect of light on setting potassium iodide a n d t h e completing of t h e titration. free iodine from hydriodic acid; a n d (4) t h e returning T h e n thiosulfate was slowly added t o decolorization, a n d t h e flask placed in t h e black paper cylinder for color a t t h e e n d point. T h e setting free of iodine from pentavalent arsenic one minute, a n d if a color returned, i t n-as discharged. Commonly there was no return of color, if t h e l a s t m a y be represented b y t h e equation drops of thiosulfate h a d been added slowly. I n t h r e e 212 Xs?Oj 4 H I = X S ~ O ~2H2O 2 0 . 8 j cc., 2 0 . 8 2 cc., a n d 2 0 . 8 0 cc. thioexperiments, This reaction proceeds from left t o right rather slowly. sulfate solution were consumed. I n general there was Fine s a n d was tried as a catalyzer b u t with no appreciable accelerating effect. Heating t o a b o u t 40' ac- no difficulty in getting check results within one or celerated t h e reaction, b u t t h e temperature m u s t be t w o drops of t e n t h normal thiosulfate on titrations so closely managed t o avoid a precipitation of yellow of a b o u t 2 0 cc. I n this case n o correction has been arsenic trisulfide, t h a t i t cannot be recommended: subtracted for light, as t h e procedure was merely t o o m a n y determinations are lost. Removing t h e being tested. T h e most convenient means of finding t h e correction reaction products on t h e right-hand side of t h e equafor iodine set free b y light or b y other means t h a n tion naturally suggested itself. It is n o t practicable t o use sulfuric acid of sufficient strength t o t a k e u p pentavalent arsenic is b y titrating with thiosulfate, t h e water, b u t it was found distinctly advantageous as described above, t w o portions of ammonium arsenate t o a d d t h e thiosulfate immediately after t h e potassium solution (of which t h e exact concentration need n o t iodide, t o a light straw color, a n d t h e n t o allow t o s t a n d , be known) one. say, of I O cc., a n d t h e other of double for j t o 6 minutes. before titrating t o t h e e n d point. t h e a m o u n t or 2 0 cc. If t h e I O cc. arsenate required This procedure also operates t o prevent t h e escape x cc. thiosulfate solution a n d t h e 20 cc. required 3' thiosulfate solution, t h e n t h e correction is ( 2 x - y ) of iodine before titration. The principal cause of irregularity, however, was cc. I t m a y be 0.2 t o 1.0cc. Of course t h e correction found t o be light. Of course. titrations were n o t made m a y be found b y titrating magnesium ammonium in sunlight, b u t even t h e diffused daylight of Washing- arsenate, b u t i t is longer a n d not necessary. It is not possible t o find t h e correction by making a blank t o n in summer is sufficient t o set free considerable in t h e ordinary manner, a correction found by such iodine. T h a t t h e light a n d not t h e presence of iodates means coming out too low. The following figures or other oxidizing bodies sets free t h e iodine was shown shorn t h e results of t h e corrected method of titration b y dissolving five grams of t h e potassium iodide used in recently boiled acidified water, displacing t h e air of I O a n d 2 0 cc. portions of a n ammonium arsenate in t h e flask b y carbon dioxide. a n d allowing t o s t a n d solution: ( T h e arsenate solution is not t h e one prefor some t i m e in t h e dark. N o color was developed. viously referred to.) .%RSENATE THIOSULFATE M€4X T h e returning color a t t h e e n d point appears cc. cc cc. t o be caused b y t h e incomplete setting free of t h e 10 20.98 20.94 iodine b y t h e arsenic a n d b y light. T h e reaction 10 20.91 20 4 0 . 8 3 I? zSa2S203 = z S a I Na2S406 seems t o have no 40.92 20 41.02 tendency, even in t h e light, t o go from right t o left. I t was found possible t o eliminate t h e effects of F r o m which t h e correction proves t o be 2 X 20.94 light, or rather t o eliminate t h e irregularities, b y plac- 40.92 = 0.96 cc. This correction is unusually large. ing t h e Erlenmeyer flask in which t h e titration was made I n general, i t will amount t o from 0.3 t o 0.7 cc. A solution of calcium chloride free from magnesium in t h e d a r k during t h e time i t was standing after having h a d t h e greater p a r t of t h e thiosulfate added, a n d a n d iron was prepared b y boiling a solution of calcium preparatory t o completing t h e titration. Likewise, chloride with calcium oxide a n d acidifying t h e filtrate after bringing t o a n e n d point, t h e flask is allowed t o barely t o acid reaction with hydrochloric acid. A s t a n d in t h e dark one minute. If there is n o r e t u r n solution of magnesium chloride free from iron a n d calof color t h e e n d point has been reached. T h e dark- cium was prepared b y boiling magnesium chloride ness was conveniently provided b y rolling a hollow solution with magnesium oxide, filtering, evaporating cylinder o u t of black paper a n d placing i t over t h e t h e filtrate with a little ammonium oxalate. taking u p with water a n d a little ammonia, fi tering a n d slightly flask in which t h e titration h a d been made. I n spite of these precautions, however, i t is necessary acidifying t h e filtrate. Portions of these were t a k e n t o make a correction for iodine set free otherwise for analysis. I n t h e case of t h e calcium chloride t h a n b y arsenate. T h e correction has t h e effect solution it was found gravimetrically, each experiment of making t h e variation more nearly uniform. T h e i m - being in triplicate with unusual care, t h a t 2 j cc. yielded provement in results brought a b o u t b y these precautions 0.1283 gram CaO, j o cc. o . n s 7 j gram, I O O CC. 0 . j 1 j 8 is shown b y t h e following experiments: T e n cc. of g r a m CaO. T h u s if we determine t h e CaO yield dia solution of a m m o n i u m arsenate was diluted t o I O O rectly on 2 j cc. CaC12 solution, we find t h a t i t a m o u n t s cc. with water containing I j cc. concentrated sulfuric t o 0.1283gram. B u t if we t a k e t h e difference, 5 0 cc. acid, a n d j grams of potassium iodide added. Sodium z j cc. = o . a j 7 j - 0.1283 gram = 0.1292 gram CaO. thiosulfate (about t e n t h normal) was added t o a light SO also '/3 (100- 2 j) cc. a n d '/z (100 - j o ) both
+
+
+
+
+
}
T H E JOL-RSAL OF IXDcSTRI.4L - 4 S D ENGINEERING CHEMISTRY
912
equal 0 . 1 2 9 2 g r a m after rounding off t h e t e n t h milligrams. T h u s 2 5 cc. of t h e CaC12 solution is equivalent t o 0 . 1 2 9 2 g r a m CaO, or I O cc. = o . o j 1 6 j CaO. I n all t h e experiments t h e conditions of bulk of solution. temperature, a n d wash water were kept exactly alike, a n d t h e 0.9 mg. difference is evidently owing t o t h e solubility of t h e calcium oxalate under the conditions of t h e experiment. This source of error is also mentioned b y Hillebrand.' Strictly speaking, i t is a n absolute error which should be applied t o each determination, b u t i t mill not be necessary t o apply i t if t h e permanganate is standardized b y weighing o u t pure Iceland spar, especially if t h e bulk of permanganate used in standardization does not differ v e r y greatly from t h a t consumed in t h e analysis. I n t h e case of t h e magnesium chloride solution, no such solubility error was found. By precipitation with ammonium phosphate. each determination being i n duplicate : I O C C . = 0.0391 l l g o 2 0 cc. = O . O i 8 0 N g O 3 0 CC. = 0 . 1 1 7 3 h l g o Average for I O cc. = 0.0390; SlgO N o volumetric method for magnesium, so far as t h e a u t h o r knows, can hope t o yield so exact results. I t m u s t be remembered, however, t h a t in t h e present case, t h e magnesium ammonium phosphate was precipitated under ideal conditions, there being present n o other salts, a n d only slightly more t h a n t h e calculated a m o u n t of ammonium phosphate necessary t o precipitate t h e magnesium. Under working conditions this determination is well known t o be subject t o a serious plus error.2 Under working conditions. probably t h e volumetric method Rill give as good results as t h e gravimetric unless special precautions a n d care are t a k e n with t h e latter. F o r t h e volumetric solutions t h e mean of all titrations showed each cc. of permanganate = 0.002816 CaO. F o r t h e thiosulfate each cc. = 0 . 0 0 2 0 j 4 SIgO. 31 ET H 0 D F 0 R C 0 MB I N E D D E T E R 31 IS A T IO N
T h e combined determination is made in t h e following manner: Precipitate t h e iron a n d alumina with a m monia or basic acetate in t h e usual manner.3 There m u s t always be sufficient ammonium salts present t o prevent a precipitation of magnesia when t h e solut i o n is made slightly alkaline. I n t h e filtrate, precipitate t h e calcium hot with excess of oxalic acid4 a n d dilute ammonia added slowly, t o slight alkaline reaction, a drop of methyl orange having been added t o the solution. T h e n a d d sufficient ammonium arsenate t o precipitate t h e magnesia. a n d slowly a d d ammonia with constant stirring t o t h e hol solution until t h e magnesium a m Bull. 422, C . S . G. S . , page 119. Hillebrand, loc. (it., p. 128. F r o m a qualitative experiment, i t appears t o be practicable t o precipitate t h e calcium. a d d tartaric acid, make alkaline. a n d precipitate t h e magnesia with arsenate, t h e iron a n d alumina remaining in solution in t h e well-known combination with alkaline tartrate. Some other hydroxy organic solvents for iron oxide were tried, e . g . , sucrose a n d mannite, h u t did not work so well. Avo quantitative experiments were made in this direction. This plan would be convenient where iron a n d alumina are not wanted, or are determined on separate portions of t h e material. 4 T h e precipitation of calcium in acid solution yields larger crystals. as m a n y authors have mentioned, a n d free from material contamination b y occluded magnesia. 1
2
T'ol.
s 3 NO. I I
monium arsenate comes down, or if so much calcium oxalate is present t h a t this cannot be observed. a d d about I O cc. strong ammonia water. Let cool a n d a d d I O t o I j C C . more strong ammonia. i l d d t h e ammonium arsenate t o t h e hot solution within a few minutes after t h e solution has been made slightly alkaline in t h e precipitation of calcium oxalate, as magnesium oxalate soon begins t o come down, which is. of course, one of t h e sources of error in t h e standard gravimetric procedure. After t h e arsenate has been added, i t is immaterial how long t h e determination stands before filtering. Let s t a n d over night,' filter a n d mash thoroughly with dilute ammonia water. T h e precipitate is t h e n transferred t o a 300 t o j o o cc. Erlenmeyer flask. If a paper filter has been used, t h e quantitative transfer is most conveniently made by spreading t h e paper out on t h e inner surface of a 4-inch funnel, of which t h e stem has been bent t o rest against t h e inner wall of t h e flask into which i t has been inserted. T h e precipitate is rinsed off with hot water, t h e n with dilute acid.* Add I O cc. sulfuric acid ( I acid t o I water), make up t o about 7 j t o 80 cc. a n d t i t r a t e hot with permanganate solution. Let cool a n d a d d 2 j CC. more of t h e acid. It is well not t o have t h e bulk of t h e solution much over I 2 j cc. when about t o t i t r a t e t h e magnesia; still, if much lime is present, t h e 7j-80 cc. mentioned above m a y not be sufficient, in which case, a large'r bulk a n d more acid can be used. These proportions of acid are necessary in order t o give a satisfactory e n d point. Add j grams of potassium iodideS a n d immediately t i t r a t e t o a straw color with sodium thiosulfate. Stopper t h e flask, a n d cover with black paper, or set in t h e dark for about j minutes. Eight minutes should elapse between t h e putting in of t h e iodide a n d t h e last p a r t of t h e titration. Complete t h e titration drop b y drop. Read t h e end point a n d place in t h e d a r k for one minute. If there is a return of color, discharge it. Csually there will be none if the last drops of thiosulfate have been added slowly. T h e writer strongly concurs in what Frankforter a n d Lillian Cohen4 say about t h e use of starch, namely, t h a t i t is very much better to do without it. The writer tried four samples of starch, a n d was unable t o obtain a n y satisfactory end point a t all. Methylene blue is also useless as a n indicator in t h e strong acid solution. There is no indefiniteness in t h e e n d point when simply t h e fading out of t h e brown color of iodine is depended on. The amount of acid prescribed will be f o u n d sufficient when amounts up t o 4 0 t o j o cc. t e n t h normal thiosulfate solution are required. If more magnesia is present, more acid must be added, which can even be done during t h e titration if done carefully. I t is much better, however, t o use a stronger solution of 1 One determination gave good results after being s h a k e n twenty min Utes after cooling, a n d immediately filtered. 9 If too strong acid is used, t h e filter paper is acted on, giving presumably a cellulose hydrolysis product, which produces a color with iodine. If 5 0 cc. strong sulfuric acid (sp. gr. 1.84) are mixed with 500 cc. water. a n d t h e mixture applied a t water b a t h temperature, all t h e precipitate will be promptly dissolved a n d t h e filter paper d o t attacked. 3 If much magnesia is present, it is well t o a d d t h e potassium iodide slo~vly,as otherwise a precipitate sometimes falls o u t which does not readily redissolve. I t s composition was not studied. 4 I.0;. r i i .
NOV.,
T H E J O ~ R Z S . 1 LO F I S D l - S T R I . I L d S D E - Y G I S E E R I S G C H E M I S T R Y
1913
91 3
be noticed t h a t the n-riter's results n-ould be improved b y standardizing a t t w o different concentrations. T h e irregularities seem t o be due t o a lack of absolute uniformity in t h e composition of t h e magnesium ammonium arsenate. There seems t o he no limit in reason t o the amount of magnesium n-hich m a y be titrated. if the concentration of the acid is kept sufficient. b u t the strength of t h e thiosulfate solution should be so adjusted -and t h e principle is. of course. x-alid in all volumetric work-that not more t h a n 30 t o j o cc. are used in a titration. If it is not so adjusted, the addition of t h e considerable volume of thiosulfate solution used in t h e titration so reduces the acid concentration t h a t a good end point cannot be obtained. The permanganate was standardized with Sorensen's sodium oxalate as supplied b y the Bureau of Standards and vias found t o be 0.0996 normal. which n-ould amount t o 0 . 0 0 2 7 9 4 CaO per cc. For t h e hIgO ThioMgO Error in CaO CaO 340.j 2 cc. of permanganate used in 'all the titrations, Prmgt. found taken hlgO sulfate found taken Grams Grams cc. cc. Grams M g . Grams me should have 1 . 2 3 0 9 grams CaO. instead of I.~.+O.+, 18.70 0 , 0 5 2 7 0 . 0 3 9 1 1 8 . 3 2 0.0380 - 1 , l I . . . . . . 0.0517 which would correspond t o a loss liy solubility of 0 . 7 9 18.40 0 0518 0.0781 3 7 . 3 1 0.Oii4 - 0 . i 2 . . . . 0.05li mg. per determination. T h e conditions of each ex16.38 0 . 0 5 1 8 0 . 1 1 7 2 5 6 . 6 3 0 1174 - 0 . 2 3 . . . . . 0.0517 45.97 0.1295 0.0331 1 8 . 4 9 0.0383 -0.8 4 . . . . , . 0.1292 periment n'ere kept uniform. T h e thiosulfate solus . . . . . . 0 , 1 2 9 2 4 5 . 9 4 0.1294 O.Oi81 37.38 O . O i i 5 -0.6 tion w a s standardized b y twice sublimed iodine a n d 5 6 17 0 . 1 1 6 5 -0.7 45 8 6 0 . 1 7 9 1 0 . 1 1 7 2 6 . . . . . . 0.1292 b y permanganate, both according t o t h e directions in 4 5 . 9 0 0 1292 0.097i 47 36 0 0982 f 0 . 5 0.1292 I . . . . . . 4 5 . 9 2 0 . 1 2 9 3 0.0977 46.48 0 0064 - 1 . 3 8 . . . . . 0.1292 Treadwell a n d Hall.' I t w a s found t o be 0 . 1 0 1 1 nor4 5 . 6 1 0 . 1 2 8 5 0.0977 46 42 0 . 0 9 6 2 - 1 . 5 9 . , . . 0.1292 mal, corresponding t o 0 . 0 0 2 0 3 8 1 I g O per cc. or a total 4 5 . 7 2 0 . 1 ? 8 i 0.0977 48.00 0.0995 C1.8 10.. . . . . 0.1202 4 5 . 6 7 0.1286 0 . 0 9 7 7 of 1 . 1 3 2 8 grams l l g 0 instead of 1.1j26. This would 47.93 0.0994 + I . : 11 . . , . . 0 . 1 2 9 2 1 8 . 4 2 0 . 0 5 1 9 0.1953 95.36 0.1977 + 2 . 4 12 . . . . . O.05li correspond t o an average loss of 1.65 mg. AIgO per determination. Whether this loss is caused b y solu1 ,2404 4 4 0 . 5 2 1.2405 1.1526 5 5 5 . 8 5 1 , 1 5 2 5 -6.7 f6.6 bility or b y t h e magnesium ammonium arsenate as T h e results of t h e titration of calcium call for no precipitated not corresponding t o i t s formula on t h e special remark. K i t h regard t o t h e magnesium, average, t h e writer does n o t know. These figures i t m a y be s t a t e d a t once t h a t t h e writer was unable are given t o emphasize t h e necessity of standardizing t o obtain t h e very exact results reported b y Meade. t h e volumetric solutions with t h e same substances hleade, however, worked under ideal conditions as a n d under t h e same conditions as are used in practice. This method of determining calcium a n d magnesium t o t h e composition of his solutions, vhile t h e results is obviously not adapted t o use in making c o m p l e t e here set forth were obtained under t h e conditions exrock analysis, nor is a n y volumetric method.? isting in actual analysis. The writer's results are. Outside of technical uses, there are m a n y cases in however, comparable with those of Frankforter a n d Lillian Cohen. I n cases of this kind we do better t o soil a n d rock work v h e r e only partial analyses are consider t h e absolute rather t h a n t h e relative error. T h e desired. For example. in a very interesting article results of t h e l a t t e r are reported in p a r t s per million by E . S. Bastin.3 it is shown how chemical composition of AXg, having been obtained from mater analyses serves as a criterion for identifying metamorphosed i n which j o o cc. m i t e r were used. T h u s t h e results sediments. Substantially t h e only elements considered m u s t have been multiplied b y tn-0. In the writer's are iron a n d alumina. a n d t h e ratios 1 I g O CaO and results, as previously mentioned, t h e s t a n d a r d for t h e K n O S a p o . I t is believed t h a t t h e present method \-oliimetric solutions is t a k e n from t h e mean of all n.oulcl be useful in this sort of work. especiall5- where t i t r a t i on s , w hi1e t h e x.01 u metric result s of F rank f or t er i t is desired t o make a large number of determinations. a n d Lillian Cohen are in t h e mean 3.3 per cent higher I t is also to be noted t h a t sodium and potassium m a y t h a n their gravimetric results. If \ve make t h e be determined in t h e filtrate from t h e precipitation results of Frankforter a n d Lillian Cohen comparable of calcium a n d magnesium. the arsenic being volatilized with those of t h e writer b y dividing b y two. coni-erting b y means of hydrobromic acid as shown b y Browning AIg i n t o 1 I g O a n d adding 3.3 per cent t o each gral-i- a n d Drushel.A BUREAUO F SOILS metric result. we find t h a t t h e average error a m o u n t s v. s. DEP.ARTXEST OF X C R I C C L T C R E t o 1.7 mg. AIgO in 8 4 . 1 mg. I I g O , a n d t h a t t h e maxiTYASHISGTOK m u m error a m o u n t s t o 6.j mg. 5IgO in 1 3 1 mg. V g O . Pnge 602ff. Vol. 11. T h e standardization b y permanganate agrees I n t h e writer's results t h e a\-erage error a m o u n t s t o well with t h a t b y iodine, a n d is much m o r r convenient. even if the p e r mangnnate itself has t o h e standardized with sodium oxalate. 1.1 mg. R i g 0 in 9 6 . 0 mg. l l g 0 , a n d the maximum : Hillebrand, ioc. c i f . , p . 1 2 1 . error t o 2.3 mg. 1IgO in 1 9 j . 3 mg. 11gO. I t will 3 Joiwnal 0.f G e o l o g y . 17, 445 (1909). 1 Hillehrand, loc i r l * A m . J . Sei., 23, 293 (19071.
thiosulfate. say fifth normal. One cubic centimeter t e n t h normal thiosulfate solution equals theoretically only 0 . 0 0 2 0 1 6 magnesia ( L I g O ) . so t h a t a fifth normal solution would not be unduly strong. ;ipply a correction for iodine set free b y t h e light. e t c . , b y titrating as described above I O cc. a n d 2 0 cc. of a solution of ammonium arsenate (of which t h e exact concentra20 grams t o liter is tion need n o t be known-about convenient). If I O cc. required s cc. thiosulfate a n d zo cc. required y cc.. t h e n the correction is ( Z Y cc. I t is very i m p o r t a n t , as the experiments cited show, t o standardizc t h e volumetric solutions b y weighing out a n d titrating some suicable compound for t h e permanganate. pure Iceland spar.' a n d for t h e arsenate: specially prepared magnesium oxide. or some samples of magnesium ribbon. I-ery considerable errors n-ill be made b y assuming t h e theoretical values for the solutions. T h e following results were obtained : >I)
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