Industrial and Engineering Chemistry : analytical edition, Vol. 9, No. 5

Analytical Edition Vol. 9, No. 5

I N D U S T R I A L

andENCINEERmG

C H E M I S T R Y

Vol. 29, Consecutive No. 19

Published by th e Am erican Chemical Society H arrison E. Howe, E ditor

May 15, 1937

Publication Office: Easton, Pa. . Editorial Office: Room 706, Mills Building, Washington, D . C. . Telephone: National 0848 Cable: Jiechem (Washington) . Advertising Departm ent: 332 West 42nd Street, New York, N. Y. . Telephone: B ryant 9-4430

C O N T E N T S

19,300 Copies of This Issue Printed

Determination of Silicon in Aluminum and Aluminum Alloys . II. V. Churchill, R. W. Bridges, and M . F. U c 201 Determining the Aniline Point of D ark Petroleum Prod­

ucts ... Leon Donn 202 Refractive Index of Strontium N i t r a t e ...

... M . L. Yakowitz and P. S. Jorgensen 204 A Comparison of Quantitative Methods for the Determi­

nation of Copper . . . II. W. Foote and John E. Vance 205 Determination of Rotenone in Derris and Cube . . . .

... Howard A. Jones 206 Measurement of Particle-Size Distribution by Optical

M e t h o d s ... II. E. Schweyer 211 A Simplified Sealed S tirre r...Lionel Joseph 212 Determination of T ar Acids and Bases by Extraction

M e t h o d s ... C. H. Fisher and Abner Eisner 213 Determination of the Moisture Content of Tobacco . .

... Richard M . Cone and Loyal II. Davis 219 Separation of Beryllium in the Presence of Complex

T a r t r a t e s ... Hoke S. Miller 221 Interference of Phosphorus in the Determination of Fluo­

rine . II. V. Churchill, R. W. Bridges, and R. J . Rowley 222 Direct Determination of Oxygen in Organic Compounds

by Hydrogenation . Paul Goodloe and J . C. W. Frazer 223 An Inexpensive Metal Chimney for Fusions...

... .... Iyjuis J . Curtinan and Leo Lehrman 225

Determining the Gelatinization Temperature of Starches.

...D. II. Cook and Joseph II. Axtrnayer 226 Determination of Sugars in Plants by Oxidation with

Ferricyanide and Ceric Sulfate Titration . W. Z. Ilassid 228 Turbidity in Sugar Products. V. Color and Turbidity

of H ard Refined Sugars . F. IK. Zerban and Louis Saltier 229 Estimation of Potassium in Silicates and Soils...

... J . E . Gieseking and II. J . Snider 232 Pretreatm ent of Wood with H ot Dilute Acid...

...Wilby E. Cohen and Elwin E. Harris 234 Determination of the Common and Rare Alkalies. Note

on Wells and Stevens’ Method . . . John C. Hillyer 236 The Analysis of P o llu c ite ...

... Roger C. Wells and Rollin E. Stevens 236 Borax as an Acidiinetric Standard. I I ...

... Frank H. Hurley, Jr. 237 The Calibration of Weights . . . Frank H. Hurley, Jr. 239 An Apparatus for Determining Odor in W a t e r ...

W. B. Hart 243 Twenty-Plate la b o ra to ry Bubble-Cap Still for Low-

Boiling M a t e r i a l s ...

... Johannes H. Bruun and Scott D. West 247 A Variable-Voltage Autotransformer . . . D. II. Cook 248 Siphon-Starting D e v ic e ...T. W. Chandler 249 Apparatus for the Extraction of Liquids with Immiscible

Solvents of Greater Density . . . Lindsay H. Briggs 250 Calcium Chlorite as a Volumetric Oxidizing Agent (Corre­

spondence)... 250

T h e A m erican C hcm ical Society assum es no resp o n sib ility for th e sta te m e n ts a n d opinions ad v an ced by co n trib u to rs to its p u b lic a tio n s.

P u b lish ed b y th e A m erican C hem ical Society, P u b lica tio n Office, 20 th <fc N o rth a m p to n S ts.. E a sto n , P a. E n te re d as second-class m a tte r a t th e P o st Office a t E a s to n , P a ., u n d e r th e A ct of M arch 3, 1879, as 48 tim es a year.

In d u s tria l E d itio n m o n th ly on th e 1st: A n aly tical E d itio n m o n th ly on the 15thj N ew s E d itio n on th e 10th an d 20 th . A cceptance for m ailing a t special ra te of p ostage pro v id ed fo r in Section 1103, A ct of O ctober 3, 1917, a u th o r­

ized Ju ly 13. 1918.

A n n u al su b scrip tio n ra ti« : (o) In d u s t r i a l Ed i t i o n$5.00; (?;) An a l y t i­ c a l Ed i t i o n $2.00; ( e ) Ne w s Ed i t i o n $1.50; ( o ) a n d (i>) to g e th e r, $6.00;

(a), (b), a n d (c) com plete, $7.50. Fo reig n p ostage to co u n tries n o t in th e P a n A m erican U nion, (a) $1.20; £6) $0.30; (c) $0.60; to C an a d a o n e-th ird these ra te s. Single copies: (a) $0.75; (&) $0.50; (c) $0.10. Special ra te s to m em bers.

C laim s fo r copies lo st in m ails to be honored m u st be received w ith in 60 d a y s of d a te of issue a n d based o n reasons o th e r th a n “ m issing from files."

T e n d a y s' ad v an ce n o tice of change of address is re q u ire d . A ddress C harles L. P arso n s, B usiness M an ag er, M ills B uilding, W ash in g to n , D . C ., U . S. A.

4 INDUSTRIAL AND EN G IN E ER IN G CHEM ISTRY VOL. 9, NO. 5

A. O. SMITH CORPORATION

RESEARCH AND ENGINEERING BUILDING

In th e se laboratories the finest oi p recision equipm ent is u se d to d e v elo p a n d control the production standards of Smith products. H evi Duty Furnaces are u se d for m etal a n d ceram ic in vestigation s.

H E V I D U T Y E L E C T R I C C O M P A N Y

H E A T T R E A T IN G F U R N A C E S E L E C T R IC EXCLUSIVELY

M I L W A U K E E , W I S C O N S I N

MAY 15, 1937 ANALYTICAL ED ITIO N 5

KIMBLE G L A S S C O M P A N Y

v i n e l a n d

, N . J .

N E W Y O R K - - C H I C A G O • • P H I L A D E L P H I A • • D E T R O I T • • B O S T O N

A WORLD ON WHEELS

. . . CONTROLLED BY GLASS!

A long every highw ay and b y -w a y from coast to co ast roll th e p ro d u c ts o f th e au to m o tiv e ind u stry — approxim ately 24,000,000 pleasure cars, 3,800,000 trucks, consum ing upw ards o f 17 billion gallons o f gasoline a year! T ru ly a w orld on wheels, k e p t spinning by m ore th an 6,000,000 w orkers and o v er $2,000,000,000 in sales.

T his industrial giant is th e largest single user o f steel, plate glass, ru b b e r, nickel, lead, m ohair, gas an d oil.

A nd Kimble Blue Line Exax G lassw are helps to guard p ro d u c ­ tion and quality in th e great laboratories controlling this w orld on wheels.

Kimble Blue Line Exax w are assures unerring accuracy, speed of determ ination, and reliability in test, control, analysis and research

— w h erev er it stands guard in chemical, clinical, educational and research cen ters o f th e nation. Its brilliant BLUE L IN E calibrations are fused-in. It is re te ste d and annealed (stra in -fre e). It is th e choice o f industry, science and m edicine — fo r A S S U R A N C E !

Stocked by leading L aboratory Supply Houses throughout the U n ited States a n d C anada

P i <ÊxÂx> Ü B L U E m m

• ___• ____ •

The V i s i b l e G u a r a n t e e o f I n v i s i b l e Q u a l i t y * *

^•

6 INDUSTRIAL AND E N G IN E ER IN G CHEM ISTRY VOL. 9, NO. 5

Reliable ^and Accurate pH ^Control

with the Rugged C O L E M A N pH

E L E C T R O M E T E R

M O D E L 3D with Automatic Temperature Compensator

. . . ^Combines P R A C T I C A L pH ^and R E L I A B L E R E D O X

The New M odel 3 Coleman Electrometer combines over four years pioneering with the glass electrode and is meeting with an enthusiastic reception by its consistent and reliable operation. It incorporates the latest electronic developments, reduces to practice details of design and technique that insure maximum accuracy and sim­

plicity. A single switch instantly shifts the instrument from pH to R E D O X . The New Coleman Glass Electrode is small, and adaptable to measuring the pH of nearly any material that is capable of wetting the surface of the bulb. Glass Electrode Is completely insulated from G rid Current the original True Impulse A m plifier.

Features that make the C O LEM A N Superior

^ Rugged and Simple— the liquid junction does not drift— cannot

"freeze."

J Flexible— wide V ariety of Sealed Electrode Assemblies available.

J Assymetry correctors on all models.

Automatic Temperature Corrector for special conditions.

^ Buffer standardization insures con­

sistent accuracy because the instru­

ment is corrected to A C T U A L operating conditions.

5

S E A L E D G L A S S E L E C T R O D E — A lw a ys Ready for use— no prepara­

tion necessary.

ï P O R T A B L E — Compact and Self contained.

M A IL E D O N R E Q U E S T

"M easu rin g p H with the G lass E le c ­ tro d e " an 1 8 page original work b y one o f A m e rica 's leading authorities.

Covers a discussion of the theo ry of the glass electrode and factors that must b e considered in its practical ap ­ p licatio n . Includes a b ib lio grap h y of 1 3 0 scientific articles bearing on the subiect.

ADDRESS A L L INQUIRIES TO THESE RELIABLE DEALERS

Canadian Laboratory Su p p lias, Ltd . 32 G re n v ille Street

Toronto, O ntario H o w e A French In c.

9 9 Broad Street Boston, Massachusetts Em il G re in e r C o . 55 Vandam Street N e w Y o rk , N e w Yo rk

N e w Je rsey Laboratory Su pp ly C o . 2 3 5 Plane Street

N ew ark, N e w Jersey

R . W . Bergen 3 2 8 Chestnut Street P h ilad e lp h ia, Pennsylvania Burrell Technical S u p p ly C o . 1 9 3 6 Fifth A v e n u e Pittsburgh, Pennsylvania W . A . T aylo r & C o ., In c.

87 2 Lin d e n A v e n u e Baltim ore, M aryland P hip p t & B ird , In c 9 1 5 East Cary Street Richm ond, V irg in ia

Cincinnati Scien tific C o . 2 2 4 M ain Street C in cin n ati, O h io Eberbsch & Son C o . 2 0 3 East L ib e rty Street A n n A rb o r , M ichigan W ilk en s-A n d e rso n C o . 111 North Canal Street C h icag o , Illin o is G e o . T . W alk e r A C o . 3 2 4 Fifth A v e n u e , South M in n e a p o lis, M in n .

G ree n e Brothers, In c.

1 8 1 2 G riffin Street D a lla s, Texas

The D enver Fire C la y C o . D enver, Colorado Redman Scien tific Com pany 58 5 Ho w ard Street San Francisco , C alifornia

C O L E M A N ELECTRIC COMPANY, INC.

308 M ADISON ST., M A Y W O O D , ILLINOIS

MAY 15, 1937 ANALYTICAL E D IT IO N 9

R U N T H I S T E S T ON PETRI DISHES

Corning, New York

L A B O R A T O R Y G L A S S W A R E

J

t e r h a p s

, in your culture work, you have noticed but little visible difference in Petri Dishes. You m ay have even thought they were all alike. Sooner or later, however, the difference becomes apparent, frequently at a time when it is m ost annoying and inconvenient.

Here is a test you can run in your own laboratory, if you wish. It will prove conclusively that while there is small difference in cost there is a vast difference in the quality o f Petri Dishes.

Take several Petri Dishes— one of them " P Y R E X ” Brand Ware.

Subject them to from 15 pounds to 50 pounds steam pressure in your sterilizer for 120 hours or longer. At the end o f the test, remove and inspect them , and determine the loss in weight. <

The " P Y R E X ” dish will have lost less weight. It will be clear, transparent, unclouded, permitting unhampered examination o f the growing culture.

The com paratively small loss in weight o f " P Y R E X ” dishes, even under these accelerated test conditions, indicates definitely that the pH o f the culture will not be appreciably changed by glass solubility.

This test will demonstrate to you, once and for all, the ability o f a

" P Y R E X ” Petri D ish to withstand repeated sterilization, frequent washings—hard usage. Its exclusive beaded edge gives it unparalleled mechanical strength. You already know o f its superior heat-resistant qualities.

Run this destructive, y et con­

structive test. Convince yourself that it pays to specify " P Y R E X ” Brand Petri Dishes.

Only 38c per pair; substantially lower prices in quantity purchases.

“ P Y R E X " is a registered trade-m ark a n d indicates m anufacture by C O R N I N G G L A S S W O R K S

10 INDUSTRIAL AND EN G IN E ER IN G CHEM ISTRY VOL. 9, NO. 5

THE FRONT LINE OF DEFENSE

O I n e v er y w e ll c o n t r o lle d p la n t , lik e t h e K e ls e y - H a y e s W h e e l C o r p ., t h e la b o r a to r y is t h e fr o n t lin e o f d e f e n s e , in b e h a lf o f q u a lit y . S t a lw a r t s in t h a t lin e a re H o s k in s E le c ­ t r ic F u r n a c e s , w h o s e C h r o m e l u n i t s a re n o t e d fo r t h e ir a b i l ­ i t y t o “ t a k e i t . ” N o t e d a ls o , fo r t h e e a se w i t h w h ic h t h e y m a y b e r e n e w e d . . . F o r y o u r o w n n e e d s , w e or y o u r d e a le r w ill g la d ly g iv e y o u f u ll in f o r m a t io n o n H o s k in s E le c tr ic F u r n a c e s , d e s c r ib e d in C a t a l o g - 5 5 Y . . . . H o s k in s M a n u ­ f a c t u r in g C o ., D e t r o it , M ic h ig a n . 0

H O S K I N S

Electric FURNACES

MAY 15, 1937 ANALYTICAL E D ITIO N 11

CEN CO-DU N OUY

INTERFACIAL TENSIOMETER

For Precision M easurem en ts of Ten sio n s at L iq u id - L iq u id and V a p o r-L iq u id Interfaces

r J ''H E phenomena of absorption, cataphoresis, condensation, émulsification, evaporation, m iscibility, osmotic pressure, solubility, etc., are present in m any industrial procedures or are involved in the use of some industrial products. Through a clear knowledge of the surface or interfacial tensions of the m aterials employed, modifications m ay be m ade to establish optim um values for the most rapid or effi­

cient prom otion of a process or the improved u tility of the product.

T he rapidity with which m easurem ents m ay be m ade, the small q u an tity of liquid required for the m easurem ent, and the lack of necessity for arduous m athem atical calculations to determ ine the results, make the ring m ethod a preferred m ethod for all kinds of work involving the m easurem ent for the sur­

face tensions of liquids. For colloidal solutions where the surface tensions change rapidly, only the ring m ethod can be employed with success.

70540 IN T E R F A C IA L T E N S IO M E T E R , Cenco-du N o uy, complete with torsion wire, 6 cm platinum-lridium ring, olive green enameled metal carrying case and directions for use...Each 1175.00

C H IC A G O 1700 Irvins Pk. Blvd.

Lakeview Station

S C I E N T I F I C I N S T R U M E N T S

m

New York • Boston • C H I C A G O • Toronto • Los Angeles L A B O R A T O R Y A P P A R A T U S

B O S T O N 79 Amherst St.

Cambridge A Station

12 IND U STR IA L AND E N G IN E E R IN G CHEM ISTRY VOL. 9, NO. 5

A .H .T. CO. S P E C IFIC A T IO N

B U R E T T E S

Stopcocks. The stopcocks are attached directly to the cylinder of the burette without the slender connecting spindle formerly used. The plugs are all of 12 mm diameter a t the central bore.

Stopcock plugs are all finished with a deep ground corrugation and supplied with a rubber washer—a most im portant practical improvement.

All stopcocks of these Burettes are properly lubricated and fastened in place. I t is necessary only to wash and dry before use. The stopcock plug need not be removed.

These Burettes are tested for leaks by immersing the lower portion, including the stopcock, in a beaker of water and applying air under pressure—15 inches of mercury—to the barrel.

“All-round” graduations. Thin, shar-ply defined, while lines extend over half the circumference of the cylinder, with every fifth division extending beyond the single division, and every tenth graduation continued entirely around the cylinder, this method being commonly designated as “ all-round” graduation, which feature, in combination with the thin white line used by us, greatly facilitates reading of the meniscus particularly with No.

2501 Burette Reader (described in pam phlet EE-84, copy sent upon request).

Tapered tips. The internal diameter of the delivery tips on stopcock burettes is a gradual taper, extending from 20 to 30 mm from the end, to avoid the inaccuracy caused by suddenlv constricted tips. The wall a t the end of the tip is approximately l l/< mm thick and carefully fire finished. The tips all extend for 80 mm below the stopcock to avoid splashing in use.

Delivery time. The delivery time of stopcock burettes is controlled between approxi­

mately 40 seconds for 25 ml burettes and 65 seconds for 100 ml burettes.

Uniformity. Tubing is selected of uniform bore so that, within reasonable tolerances, the linear graduated distance on all burettes of the same capacity is identical.

Accuracy. All A.H.T. Co. Specification Burettes are guaranteed to be within the following tolerances for total or partial capacity, these having been established as reason­

able for routine work:

Capacity less than, and including:

10 ml 25 ml 50 ml 100 ml

Limit of Error:

=*=0.04 ml

=*=0.06 ml

±0.10 ml

±0.20 ml

Percentage Error:

0.4%

0.24%

0.2%0.2%

2 5 J jk

-

5

2436

2404. Burettes, A.H.T. Co. Specification, as above described, with straight glass stopcock.

Capacity, m l... 10 25 Graduation interval, m l... 1/20 1/10 Each... L65 1.80 20% discount in case containing... 48 48 Code W ord ... Berow Berrq

10% discount in carton containing 12; 25% discount in case containing 144.

50

1/10 ¹⁰⁰1/5 2.00 2.70

36 24

Beryc Besfo

SCHELLBACH BURETTES, A.H.T. Co Specification, with dark blue enameled stripe on white background for con­

venient and accurate reading of meniscus. For obvious reasons Schellbach Burettes are not furnished with the “all-round”

graduation. They are otherwise in exact accordance with the requirements for A.H.T. Co. Specification Burettes as above described.

2436. Burettes, Schellbach, with straight glass stopcock.

Capacity, m l... 25 50 100 Graduation interval, m l... 1/10 1/10 1/5 Each... ~ O o 3^75 5^5 20% discount in case containing... 48 36 24 Code W ord...Bhdah Bhdoe Bhdyk

10% discount in carton containing 12; 25% discount in case containing 144.

For c o m p le te a s s o r tm e n t o f A.H .T. Co. Specification a n d o th e r B u re tte s, including Certified B u rettes, see pages 114 to 122, incl., o f our c u rren t catalogue.

ARTHUR H. THOMAS COMPANY

R E T A IL — W H O LE SA LE— E X P O R T

LA B O R A TO R Y APPARATU S AND R EA G EN T S

W E S T W A S H IN G T O N S Q U A R E P H IL A D E L P H IA , U .S .A .

Cable Address, “ B alance,” Philadelphia

I N D U S T R I A L

andENGINEERINti

C H E M I S T R Y

H a r r iso n E . H o w e , E d ito r A N A L Y T IC A L E D IT IO N

D eterm in ation o f Silicon in A lum inum and A lu m inu m A lloys

H . V. C H U RC HILL, R. W. B R ID G ES, AND ¡VI. F. LEE, A lu m in u m R esearch L ab o rato rie s, New K en sin g to n , P a.

A

R E V IE W of th e literatu re shows th a t analytical chem­

ists have been m ore or less dissatisfied w ith the sta tu s of procedures for th e determ ination of silicon in alum inum and alum inum alloys. The m ain basis of uncer­

ta in ty lies in th e fact th a t conventional m ethods of acid decomposition of th e sample often yield a residue after de­

hydration which contains both silicon and silica. This situa­

tion is satisfactorily m et by an alkaline fusion of the residue which oxidizes the silicon so th a t all the silicon before vola­

tilization is present as silica. C allendar (2), however, pointed o u t th e possibility and th e actu ality of errors in the determ ination of silicon b y m eans of acid solution of the sample. T he error was shown to be caused by a volatiliza­

tion loss of silicon during solution of th e sample. The possibility of such error has been tacitly recognized in m ost silicon m ethods by the use of oxidizing acids. T hus the inclusion of nitric acid in m ost acid m ixtures recommended has silicon oxidation as one of its purposes.

A nalytical work on various alum inum alloys prepared under careful control seems to indicate th a t acid decom­

position yields satisfactory results in m ost cases, although in others low results seem to be produced.

I t seems obvious th a t silicon losses would be affected by the exact kind of acid used, th e physical condition of the silicon as controlled by th e therm al history of the m etal being analyzed, and possibly by the compounds in which the silicon occurred. T ypical analytical d ata covering the above points are shown in T able I. In the second column 0, II, IF, and T have th e following m eaning: 0 designates m etal which is fully annealed. H designates m etal in a hard tem per produced by cold working. IF m eans tem per of m etal produced by solution h eat trea tm e n t w ithout any subsequent precipitation h e a t trea tm e n t. In the case of 17S, T refers to th e tem per of m etal produced by solution heat tre a tm e n t followed by aging a t room tem perature.

In th e case of 5 IS, T m eans th e tem per produced by solu­

tion h eat trea tm e n t followed by a precipitation h eat tre a t­

m ent. T he th ird column refers to the use of an acid mix­

ture m ade up of 485 cc. of w ater, 115 cc. of sulfuric acid, 200 cc. of hydrochloric acid, and 200 cc. of nitric acid. These proportions are n o t fortuitous b u t are th e resu ltan t of evo­

lution based upon necessity. Smaller proportions of nitric acid result in low silicon recoveries.

The d a ta in T able I are representative of a larger am ount of d a ta which cannot be presented within allowable space lim itations. D a ta indicate satisfactory agreem ent between

tri-acid and sodium hydroxide results except in specific cases discussed below, and indicate a tren d tow ard low re­

sults in th e case of perchloric acid.

Ta b l e I. Si l i c o n Co n t e n t o p Al u m i n u m a n d Al u m i n u m Al l o y s

(As determ in ed a fte r th re e m ethods of decom position) M etal D ecom posed b y : T ri- P erchloric Sodium

Alloy T em per or T h erm al H isto ry acid acid hy droxide

% % %

51S O 0 .9 3 0 .9 2 0 .9 4

0 .9 3 0 .9 3 0 .9 3

51S W 0 .8 5 0 .7 5 0 .9 4

0 .8 0 0 .7 3 0 .9 3

51S T 0 .8 9 0 .8 4 0 .9 3

0 .8 8 0 .8 4 0 .9 4

17S H 0 .5 0 0 .5 1 0 .5 1

0 .5 1 0 .5 1 0 .5 1

17S O 0 .4 8 0 .4 9 0 .4 9

0 .4 8 0 .4 9 0 .4 8

17S T 0 .4 8 0 .4 8 0 .4 9

0 .4 8 0 .4 7 0 .4 9

2S II 0 .1 9 0 .1 7 0 .1 9

0 .1 9 0 .1 7 0 .1 9

2S 24 hours a t 148.89° C. (300° F.) 0 .2 0 0 .1 6 0 .2 0

0 .1 9 0 .1 6 0 .2 0

2S 24 hours a t 260° C. (500° F .) 0 .2 0 0 .1 7 0 .2 1

0 .1 9 0 .1 7 0 .2 1

3S II 0 .2 5 0 .2 5 0 .2 5

0 .2 5 0 .2 4 0 .2 5

3S 24 hours a t 300° C. 0 .2 6 0 .2 4 0 .2 5

0 .2 5 0 .2 5 0 .2 7

3S 24 hours a t 500° C. 0 .2 5 0 .2 6 0 .2 6

0 .2 5 0 .2 5 0 .2 6

43S II 5 .0 6 4 .9 6 5 .0 5

5 .0 5 4 .9 7 5 .0 5

43S 24 hours a t 148.89° C. (300° F.) 5 .0 6 4 .9 7 5 .0 3

5 .0 5 4 .9 8 5 .0 6

4 .9 9 4 .9 1 5 .0 0

43 S 24 hours a t 260° C. (500° F.) 5 .0 0 4 .9 1 5.0 3

M arked differences between the tri-acid and th e sodium hydroxide results are shown in th e case of 5 IS-IF and 51S-7\

Satisfactory' agreem ent is shown between the two m ethods when 51S-0 is analyzed for silicon. T able I I shows the norm al alloying constituents or im purities present in the alloys.

Ta b l e II. No m in a l Co m p o s i t i o n o r Me t a l s An a l y z e d

Silicon Iro n M anganese C opper M agnesium

1 I in d icates elem en t p re sen t only as im p u rity .

2S 3S 43S 17S 51S

% % % %

I« I 5 .0 0 I 1 .0 0

I I I I I

1 .2 5 0 .5 0

Ï I Ï 4 .0 0 Ï

0 .5 0 0 .6 0

201

202 INDUSTRIAL AND E N G IN E ER IN G CHEM ISTRY VOL. 9, NO. 5 I t is ap p aren t th a t significantly discrepant results occur

only in th e case of alum inum alloyed w ith m agnesium sili- cide and then only when th e m etal is in a heat-treated con­

dition.

In laboratories wherein alum inum and alum inum alloys are analyzed on a routine basis, th e filtrate from the silicon determ ination is used for the determ ination of other ele­

m ents. W hen the tri-acid m ethod is used for silicon, this filtrate is a sulfate solution which is convenient for further work. T he filtrate, when th e sodium hydroxide m ethod is used, m ay consist of either sulfates or perchlorates according to which acid is used for dehydrating silica. However, the solution contains large am ounts of sodium salts which are som ew hat undesirable.

T he tendency tow ard low results found when perchloric acid is used as the decomposition reagent is som ew hat disap­

pointing; th e use of this acid is usually desirable in determ in­

ing silica, since dehydration of silica is satisfactory and the subsequent re-solution of salts is more easily effected than when sulfuric acid is used.

T he d a ta shown are representative of m an y other data, all of which indicate th a t while th e tri-acid m ethod is satisfactory in m ost cases th e sodium hydroxide m ethod should be used when alum inum -m agnesium silicide alloys are analyzed for silicon.

T he following m ethods give satisfactory service in deter­

mining silicon in alum inum and alum inum alloys.

T r i-A c id S o lu t io n M e th o d

Place 1 gram of sample in a 250-ec. beaker. Keeping the beaker covered as much as possible, cautiously add 35 cc. of acid mixture No. 1 (485 cc. of water, 115 cc. of sulfuric acid, 200 cc.

of hydrochloric acid, and 200 cc. of nitric acid). When no fur­

ther action can be seen, evaporate till heavy fumes of sulfuric acid are evolved for 15 minutes, cool, add 10 cc. of 1 to 3 sulfuric acid and 100 cc. of hot water, and boil until salts are dissolved. Stir in some paper pulp, filter through a close-textured paper, and wash well with not water. Evaporate the filtrate to fumes, cool, dissolve in water, add pulp, filter, and wash as before. Ignite the residues in a platinum thimble. After cooling, mix with 1 to 8 grams (depending on amount of residue) of sodium carbonate.

Fuse cautiously until nearly quiet, then finish with a strong heat.

Run the melt up the sides of the crucible, cool, and place in a beaker with 50 cc. of 1 to 3 sulfuric acid.

When the melt has dissolved, remove the crucible, washing it out into the beaker. Evaporate, continue heating until heavy fuming has taken place for a t least 15 minutes, and remove from heat. While still moderately warm, add a little cold water, fol­

lowed by 100 cc. of hot water. H eat to complete solution of the

soluble salts, but avoid too long treatm ent, as the silica tends to redissolve. Filter, after stirring in some paper pulp, and wash thoroughly with hot water. Evaporate the filtrate and heat to fuming again to separate silica, which may have escaped the former dehydration, and combine with the first residue obtained.

D ry the filters with contents, then ignite in a platinum crucible a t 500° C. until free from carbon, finish a t 1000° C., cool, and weigh. Moisten with a few drops of diluted sulfuric acid and add several cubic centimeters of hydrofluoric acid. Evaporate dry, ignite, cool, and weigh again. The loss in weight represents silica.

Deduct a determined blank. Silicon = silica X 0.4672.

S o d iu m H y d r o x id e S o lu t io n M e th o d

Dissolve 0.5 to 1.0 gram of sample in a covered Monel metal beaker, using 15 cc. of 30 per cent sodium hydroxide solution.

When violent action ceases, place on a hot plate and heat gently until the volume of solution is reduced to about 5 cc. If the solution is still dark, add 2 or 3 cc. of 3 per cent hydrogen peroxide to hasten oxidation and again reduce the volume to about 5 cc.

Transfer the concentrated sodium hydroxide solution to a Pyrex beaker containing 80 cc. of 1 to 1 sulfuric acid. Thoroughly police the Monel metal beaker and, using a few cubic centimeters of dilute sulfuric acid, wash any adhering material into the Pyrex beaker. Add 2 cc. of concentrated nitric acid. Evaporate to copious fumes and finish by the usual silica volatilization proce­

dure. (This method with double dehydration was used to obtain results shown in last column of Table I.)

Al t e r n a t i v e Me t h o d. (This procedure is now preferred to the one given above because after dehydration salts are more easily dissolved.) Transfer the solution to a Pyrex beaker con­

taining 65 cc. of 1 to 1 sulfuric acid and 20 cc. of 60 per cent per­

chloric acid. Thoroughly police the Monel beaker and cover and, using a few cubic centimeters of dilute sulfuric acid, wash any ad­

hering material into the Pyrex beaker. Make double dehydra­

tion by evaporation to copious fumes and finish by usual silica volatilization procedure.

Another alternative procedure, substantially as published by the Aluminum Research Institute (1), is: Neutralize the con­

centrated sodium hydroxide solution with 1 to 1 hydrochloric acid and transfer it to a Pyrex beaker. Add 20 cc. of 60 per cent perchloric acid. Evaporate to copious fumes, cool, add 50 cc.

of hot water, bring to a boil, filter a t once using aD ashless paper pulp, and wash with warm 1 per cent hydrochloric acid. Add 10 cc. of perchloric acid to filtrate, fume, and filter as before. Dry the filters with contents, then ignite in a platinum crucible at 1000° C. Add a few drops of sulfuric acid and ignite to constant weight. Cool and weigh. Finish by the usual silica volatiliza­

tion procedure.

L ite r a tu r e C ited

(1) A lum inum Research In stitu te , S tan d ard M ethods for Sam pling and A nalyzing of A lum inum and C ertain Aluminum Alloys, 1932.

(2) C allendar, L. H ., Analyst, 58, SI (1933).

Re c e i v e dF e b ru a ry 6, 1937.

D eterm ining the A n ilin e P o in t o f D ark P etroleu m Products

LEON DONN, T h e Texas C o m p an y , B eacon, N. Y.

T

H E aniline point (£) is th e solution tem perature of an oil w ith an equal p a rt by volum e of aniline. T he In sti­

tution of Petroleum Technologists (1) describes a m ethod for the determ ination of th e aniline point of an oil, and rem arks, regarding opaque oils (3), th a t their aniline points can usually be determ ined w ith suitable illum ination or by observation of the thin film of the m ixture which is splashed up on th e sides of th e tube during stirring.

T his m ethod works well w ith both tran sp aren t and n o t too d ark oils. In th e case of really opaque m aterials, however, it is practically impossible to obtain b e tte r th a n a rough ap ­

proxim ation of the correct result. In view of this, a m ethod has been developed particularly for dark products an d has been found to give results th a t are in excellent agreem ent w ith those obtained by th e I. P. T . m ethod.

A consideration of th e viscosity of a slowly cooling solution of tw o liquids in th e region of their solution tem perature re­

veals th e following facts:

1. Above this temperature where true solution holds, the viscosity of the solution increases uniformly with decreasing tem­

perature.

2. When, on cooling, the solution temperature is reached

MAY 15, 1937 ANALYTICAL E D IT IO N 203

Fi g u r e 1. Cu r v e f o r Sa m p l e 10

and the solution begins to separate into finely divided particles of its components, the emulsion-like mass thus momentarily formed causes a sharp increase in viscosity. This sharp peak corresponds very closely to the aniline point obtained by visual methods.

3. As the temperature continues to fall below the solution tem perature, the particles of components grow rapidly in size, so th a t the emulsion no longer holds. During this period of cooling the viscosity is found to decrease rather than increase with cooling. This serves to accentuate the high viscosity peak.

A pparently, a stu d y of the viscosity of a solution in the neigh­

borhood of its solution tem perature reveals its aniline point.

Procedure

Equal parts by volume of dry distilled aniline and sample are introduced into an Ostwald viscometer and the viscosity tube is heated by means of a suitable bath to a temperature sufficiently high to cause complete solution. During the heating, the mix­

ture of sample ana aniline is agitated by applying very gentle suction to the wide end of the viscometer, thereby causing a slow bubbling of air through the mixture which stirs it thoroughly.

Solution having been effected, the bath is permitted to cool very slowly. At the same time relative viscosity readings are begun at this temperature above the aniline point, and continued as rapidly as possible during the cooling period. The time of out­

flow steadin' increases with falling temperature until a maximum time of outflow is obtained. Thereafter, through a comparatively long period, the readings continually decrease with falling tem­

perature. I t has been found th a t the temperature corresponding to this maximum reading corresponds very closely with the ani­

line point obtained by the I. P. T. method.

The aniline used was of c. ^p. grade, and was dried by contact with anhydrous potassium carbonate for several days, and then distilled. Only the middle portion of the distillate was retained for use.

The Ostwald pipet corresponded to Eimer and Amend Catalog No. 32804 and had a time of outflow of 15 seconds a t room tem­

perature for a charge of 5 ml. of water.

T he approxim ate tem perature of com plete solution can be determ ined in several ways, th e sim plest being by noting the appearance of th e m ixture in th e capillary of th e viscometer after a sh o rt stirring period. F or very d ark samples, whose solutions were even too opaque for observation through th e

finest viscom eter capillary, a m icrom ethod was employed, which consisted in rapidly transferring a drop of h o t solution to a h o t b u t gradually cooling microscope warming stage equipped w ith a therm om eter, and prom ptly covering the drop w ith a cover glass. O bservation through th e micro­

scope of th e th in film produced by th e drop between th e two glasses showed the aniline point by the change in appearance from a clear, transparent, though often highly colored film, to th e form ation of a grainy film, the grains of which grew con­

tinually larger until small droplets were definitely formed.

Fi g u r e 2. Cu r y e f o r Sa m p l e 25

T he first indication of graininess in the film was taken as the approxim ate aniline point, and th e heating b ath for th e vis­

cosity m ethod was started a t a tem perature a few degrees higher th a n this. The behavior of th e sample and aniline in th e viscom eter serves also to show w hether th e solution is complete. Successive viscosity readings of a com plete solu­

tion w ith agitation of th e solution between each reading should agree consistently w ith each other, while readings of an in­

complete solution taken in th e same way will be erratic and

Ta b l e I. So l u t i o n Te m p e r a t u r e I. P . T .

C apillary-

T ube M icro- N arro w -T u b e

M eth o d V iscosity

M eth o d m ple M eth o d M ethod m ethod R u n 1 R u n 2 R u n 1 R u n 2

° C. 0 C. ° C. ° C. ° C. 0 C. 0 C.

1 71 .2 7 1 .8 7 1 .9 5

2 O ver 80 8 8 .0 8 8 .1

3 O ver 90 9 4 .1 9 3 .9

4 8 9 .8 8 9 .9

5 6 3 .6 63 ! 7

6 5Q.5 57'.5 5 7 .5 5 7 .7

7 6 5 .0 6 6 .0 6 5 .8 6 5 .7

8 48 49 4 8 .7 4 8 .8

9 4Î* 4 1 .8 4 1 .6

10 2 6 .5 2 6 .8 2 6 .8 5

11 35 3 7 .3

12 7 7 .5 7 6 .8 7Q.9

13 7 2 .5 7 5 .6 7 5 .3

14 23 2 2 .8 2 2 .7

15 69 6 7 .3 6 7 .3

16 60 5 7 .5 5 8 .0

17 54 4 9 .0 4 9 .0

18 60 5 8 .0 5 9 .0

19 69 64

20 7 3 .5 73 7 3 .3 7 3 .5

21 68 » » • 68 68 .1

22 4 3 .8 45

23 4 6 .9 49

24 4 8 .4 49

25 6 3 .5 64*

26 A pprox. 98 9 9 .6 9 9 .5

27 O ver 98 104 .2 104.2

28 O ver 95 101.4 101 .5

29 91 9 0 .5

204 INDUSTRIAL AND EN G IN E ER IN G CHEM ISTRY VOL. 9, NO. 5

/04.0 /03.S /030 /ozs /oe.o /o/.s

T E M P E R A T U R E D E C R E E S C E N T /G R A D E Fig u r e 3. Cu r v e f o r S a m p l e 28

n o t agree, because of varying am ounts of each ingredient ru n ­ ning through the capillary.

Slow cooling of the b a th is accomplished b y having the immersion h eater wired in series w ith a heavy rheostat. By adding or taking aw ay resistance from th e heater, th e cooling can be cu t down to a very slow rate. A sm ooth ra te of cool­

ing of ab o u t 1 ° C. in 5 to 10 m inutes was found to be satisfac­

tory. In case a sample has a solution tem perature below room tem perature, an imm ersed copper cooling coil is a con­

venience for further lowering th e b a th tem perature.

T able I compares solution tem peratures of different samples by various m ethods. T he narrow -tube m ethod is a modifica­

tion of the I. P. T . m ethod in which the te st tube em ployed is a very narrow one and th e q uantities of sample and aniline are reduced to 1 ml. each.

Ta b l e II. Re a d i n g s o n Th r e e Sa m p l e s Sam ple 10

T im e of Sam ple 25

T im e of Sam ple 28

T im e of

T em p. outflow T em p. outflow T em p. outflow

° C. Sec. ° C. Sec. ° C. Sec.

2 8 .0 3 6 .2 6 8 .0 20 .1 104.2 3 7 .4

2 7 .9 3 6 .6 6 7 .5 2 0 .3 103.8 3 8 .0

2 7 .8 3 7 .1 6 5 .7 21 .1 103.6' 3 8 .2

2 7 .7 3 7 .4 6 4 .8 2 1 .8 103.1 3 9 .0

2 7 .5 3 8 .0 6 4 .5 22 .7 102.8 3 9 .4

27 A 3 8 .7 6 4 .0 2 3 .8 102.25 4 0 .2

2 7 .3 3 9 .2 6 3 .6 2 3 .4 101.8 4 1 .4

2 7 .2 3 9 .8 6 3 .2 2 3 .2 101.6 4 2 .6

2 7 .1 4 0 .5 6 2 .4 2 2 .9 101.3 4 2 .3

2 7 .0 4 1 .4 6 2 .0 2 2 .8 101.2 4 1 .6

26 .9 5 4 2 .4 5 6 .2 2 4 .4 101.0 3 8 .0

2 6 .9 4 4 .0 5 5 .7 2 4 .7

2 6 .8 4 4 .9 5 4 .9 2 5 .5

2 6 .7 4 4 .8 2 6 .6 4 4 .0 2 6 .5 4 3 .6 2 6 .4 4 3 .0

Table I I and Figures 1, 2, and 3 illustrate the rise of the viscosity to a peak and its subsequent fall, th e peak represent­

ing th e aniline point. Readings on ru n of sam ple 25 were con­

tinued in order to show th e second upw ard tu rn of viscosity after the peak in viscosity had been reached.

S u m m a r y

A new m ethod is described for th e determ ination of aniline poin t or solution tem perature, applicable p articularly to d ark- colored samples.

L ite r a tu r e C ite d

(1) In st. Petroleum T ech., “ S tan d ard M ethods of T esting Petroleum and I ts P ro d u cts,” 3rd ed., p. 20, 1935. I. P. T. Serial Desig­

nations F. 0 . 23.

(2) T izard, H. T., and M arshall, A. G., J . Soc. Chem. In d ., 40, 20T (1921).

(3) V an W yk, W. R ., J . In st. Petroleum Tech., 22, 754 (1936).

R e c e i v e d F e b ru a ry 6 , 1937.

R efra ctiv e I n d e x o f S tro n tiu m N itrate

M . L . Y A K O W IT Z AND P . S . J O R G E N S E N

F o o d a n d D r u g A d m i n i s t r a t i o n , U . S . D e p a r t m e n t o f A g r i c u l t u r e , S a n F r a n c i s c o , C a lif .

T

H E use of imm ersion m ethods for determ ining the refractive indices of solids requires a series of immersion liquids whose refractive indices are known w ith a n accuracy of abo u t 0.001. T he refractive indices of the immersion liquids m ay be determ ined in a refractom eter or by use of a series of isotropic crystals whose refractive indices are known.

C ham ot and M ason (2) list such a series of isotropic crystals to be used in determ ining th e refractive indices of liquids by imm ersion m ethods. In this list, th e refractive index of strontium n itra te [Sr(NOj)2] is given as 1.567.

W hile checking th e refractive indices of a set of immersion oils, it was found th a t th e value given by C ham ot and M ason is incorrect. T he refractive index of strontium n itra te deter­

m ined b y th e immersion m ethod, using w hite light, is 1.586.

T he sam ple of strontium n itra te used was analyzed and found to be pure S r(N 0 3); w ith less th a n 0.1 per cent water.

A check of th e literatu re showed th a t th e refractive index of strontium n itra te is given as 1.5667 by International Critical Tables (7), F ry (5), and Landolt, Bornstein, and R o th (5). T he compilers of these tables apparently took th e value of 1.5667 from th e sta n d ard w ork of G ro th (6),

who depended upon th e determ inations of Fock (4) and Craw (3). T h e correct value is given by B ehr (1) who found stro n tiu m n itra te to have a refractive index of 1.5878, using sodium light and th e more accurate crystal refractom eter.

K eenan (8) sta te s th a t he has depended on B ehr’s d eter­

m ination, which appears to be m ore nearly correct.

L ite r a tu r e C ited (1) Behr, Neues Jahrb. Mineral. Geol., 1, 138 (1903).

(2) C ham ot, E . M., and M ason, C. W ., "H an d b o o k of Chemical M icroscopy,” Vol. I, p. 3S7, New Y ork, Jo h n W iley & Sons, 1931.

(3) Craw, Z. phys. Chem., 19,277 (1890).

(4) Fock, Z. Kryst. Mineral. Petrog., 4,5 8 5 (1880).

(5) F ry, W . H ., U. S. D ept. Agr., Bull. 1108 (1922).

(6) G roth, P., “ Chemische K rystallographie,” Vol. I I , p. 104, Leipzig, W . Engelm ann, 1908.

(7) In tern . C ritical T ables, Vol. I, p. 165, G eneral Index N o. 2458 (1926).

(8) K eenan, G. L., p riv ate com m unication.

(9) L andolt, B ornstein, and R o th , “ Physikalisch-chem ischo Tabel- len,” p. 983, T able 212-D (1912).

Re c e i v e d M arch 19, 1937.

A C om parison o f Q uantitative M ethods for the D eterm ination o f Copper

H . W . F O O T E AN D J O I I N E . V A N C E , Y a le U n iv e r s ity , N e w H a v e n , C o n n .

T

H E three m o st widely used m ethods for th e q u a n tita ­ tive determ ination of copper are th e electrolytic, the gravim etric, based upon th e precipitation of cuprous thio- cyanate (or a com bination of th is w ith an oxidation titra tio n ), and th e volum etric, which depends upon th e fact th a t bi­

valent copper will liberate a n equivalent am ount of iodine from a soluble iodide. E ach m ethod has certain advan­

tages and disadvantages.

T he electrolytic m ethod is m ost susceptible to error in th e presence of o th e r ions. T he gravim etric thiocyanate m ethod is applicable in th e presence of ions o ther th a n those of lead, m ercury, tellurium , selenium, and th e precious m etals. T he iodom etric m ethod is also widely applicable, th e interfering substances being arsenic, antim ony, iron, hexavalent molyb­

denum , selenium, and th e oxides of nitrogen. In th e m ajority of cases m e t in practice, th e interfering elem ents in the iodom etric m ethod can be controlled b y th e use of a buffer solution and by th e addition of suitable reagents (6, 6).

R ecently it has been shown (£) th a t th e volum etric m ethod is capable of giving m ore exact results w hen th e usual pro­

cedure is modified slightly, and th e present work has been carried o u t w ith th e purpose of com paring th is modified iodo­

m etric m ethod w ith th e stan d ard m ethods. T his modifica­

tion consists sim ply of th e ad dition of 1 to 2 gram s of a soluble thiocyanate to the solution afte r alm ost all of th e iodine liberated has been rem oved by th e thiosulfate solution. W ith this m odification th e m ethod is so nearly q u an tita tiv e th a t th e iodine sta n d ard of th e thiosulfate m ay be calculated in term s of copper w ith o u t th e need for an em pirical sta n d ­ ardization w ith copper. I n th e presence of th e interfering elem ents already noted, th e arsenic, antim ony, an d iron are oxidized and a buffer solution is prepared, which will prevent th e p en tav a len t arsenic and antim ony from liberating iodine from th e potassium iodide. T he ferric iron is converted into a complex fluoride (<?). P a rk (6) recom m ended th e use of a p h th a la te buffer, for th e reason th a t th e reaction liberat­

ing iodine proceeds too slowly in th e presence of th e more usual ac etate buffer solution. However, w ith th e modifica­

tion m entioned (2), an a c e ta te buffer m ay be used. Sodium fluoride or am m onium bifluoride m ay be used to form the complex iron fluoride. D uring th e progress of this work an article appeared (1) pointing o u t th a t th e addition of am­

m onium bifluoride to an acid solution produces a buffer solu­

tion of th e correct pH to prev en t th e interference of arsenic and antim ony, as well as th e interference of iron. T his pro­

cedure was also included in th e m ethods studied.

P r o c e d u r e s

T he electrolytic m ethod (a) was th e usual procedure as described by H illebrand and Lundell (3).

T he gravim etric thiocy an ate m ethod (&) was according to Hillebrand and Lundell (8) or th a t recom m ended by Kolthoff and v. d. M eene (4).

T he iodom etric m ethod (c) using an a c etate buffer was carried o u t in th e following m a n n e r:

The samples were dissolved in aqua regia with subsequent addition of potassium chlorate. The equivalent of 10 ml. of 6 N sulfuric acid was added and the nitric acid was removed com­

pletely, often with an additional evaporation with hydrochloric acid. The residue was taken up in 20 to 40 ml. of water and the equivalent of 25 ml. of 6 V acetic acid and 12 ml. of 6 Ar ammonium

hydroxide was added. The resulting buffer solution has a pH of approximately 3.7. Interfering elements were, in general, added before the solution of the copper ore. Following the preparation of the buffer, sodium fluoride was added directly before titra ­ tion, 1 gram being used for every 0.1 gram of iron.

T he iodom etric m ethod (d) using am m onium bifluoride was identical w ith (c) except th a t no acetic acid nor am m onia was added, th e bifluoride being added directly to th e sulfuric acid solution.

T a b l e I. A n a l y s i s o p O r e a n d C o p p e r S u l f i d e »

M axim um M ax im u m

M eth o d C halcocite Ore D ev iatio n C opper Sulfide D ev iatio n C uC N S 7 4 .4 7 0 .0 5 (7)«» + 0 .1 1 64 .0 1 0 .0 2 (4) - 0 . 0 6 Io d o a c etate 7 4 .3 6 0 .0 3 (7) + 0 . 0 8 6 3 .9 8 0 .0 3 (5) + 0 .0 9 E lectro ly tic 7 4 .4 4 0 .0 4 (3) + 0 . 0 7 6 4 .0 5 0 .0 2 (3) + 0 . 0 3 Iodofluoride ... . . 6 3 .9 8 0 .0 3 (6) + 0 .1 1

° D ev iatio n in d ic a ted is th e m ean deviation.

& N um bers in p aren th eses refer to th e n u m b er of ex p erim en ts carried o u t using th e p a rtic u la r procedure.

To th e solutions resulting from procedures (c) or (d), potassium iodide was added and th e liberated iodine titra te d w ith sodium thiosulfate w ith th e addition of am m onium thiocyanate near th e end point. T he sodium thiosulfate was standardized against nearly pure copper of known copper content.

The source of th e copper used in th e determ inations was either a specially prepared sample of cuprie sulfide or a chalco­

cite ore, kindly provided by Professor B atem an of th e D ep a rt­

m ent of Geology of this university. T he results showed no essential differences, whichever of the two sam ples was used.

Arsenic an d iron were added to gether in th e form of arseno- pyrite, or separately as dihydrogen potassium arsenate and a solution of ferric n itra te . T he antim ony was in th e form of potassium antim onate.

T he sam ples of copper sulfide and chalcocite were very finely ground and as a consequence either oxidized or ad­

sorbed m oisture very slightly over th e period of tim e during which th e experim ents were perform ed. In order always to be able to com pare th e results w ith th e known copper con­

te n t of th e pure samples, th e two sources of copper were analyzed frequently, in m ost cases a sam ple of th e pure ore being ru n sim ultaneously w ith th e analysis of th e same ore to which interfering elem ents had been added.

T able I shows th e results of analyses of th e synth etic cupric sulfide or chalcocite. T he term “iodoacetate” refers to th e modified iodom etric m ethod in an a c etate buffer, while

“ iodofluoride” refers to th e same m ethod using a fluoride- bifluoride buffer solution alone.

I n te r fe r in g E le m e n ts

In th e presence of th e interfering elem ents norm ally found in copper ores or alloys, th e electrolytic m ethod is n o t appli­

cable, so it was used only in analyses of th e p ure sulfides. I ts use here, however, serves to show th e high accuracy of the volum etric m ethod; in general, th e volum etric m ethod agrees approxim ately w ithin one p a rt in a thousand w ith th e two gravim etric m ethods, though it is definitely low by th a t am ount. T he possibility th a t there were traces of interfering elem ents in th e chalcocite ore was considered; in fact, 0.02 per cent of silver was found and th e results are corrected by th a t am ount. All other elem ents which m ight interfere wrere found to be absent.

205