T itrim etric E xtraction , U sing D ith izo n e

A. G . A S S A F a n d W . C. IIO L LIB A U G H M a s s a c h u s e tts I n s t it u t e o f T e c h n o lo g y , C a m b rid g e, M a ss.

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tories h as n ecessitated a te s t for th e m easurem ent o f traces of copper in oxidized oils. T h e procedure develop ed is one w hich em p loys dith izon e (diphenylthiocarbazone) for titrim etric extraction o f th e copper.

T h e various com plex organic substances and m eth od s for an alysis for copper (and other m etals) h a v e b een discussed b y C onn, Johnson, Trebler, and K arpenko (£), Fischer (4), W ichm ann (8), etc. M an y procedures for th e analysis of copper h a v e been described. T h e spectrophotom etric m ethod o f L iebhafsky and W inslow (7) w as tried b u t discarded be­

cause of th e extrem e precautions in volved . F ischer’s titri­

m etric “ tw o-color” m ethod em ploying excess dith izon e w as also tried; how ever, in con sisten t results were obtained be­

cause of th e difficulty of m atch in g th e variou s shades of purple obtained a t th e end point.

T h e procedure described herein is one of extractive titra­

tio n w hich is som ew h at sim ilar to th a t em ployed b y W ilkins, W illou gh b y, Kraem er, and S m ith (9) in th e determ ination of traces of lead in biological m aterials. T h e m ethod consists of oxid izin g a sam ple of th e deteriorated oil w ith a m ixture of nitric, sulfuric, and perchloric acids, and evaporating th e colorless solu tion alm ost to dryness. T h e residue is dissolved in redistilled w ater, th e p H adjusted, and th e solu tion finally titrated w ith a standardized solu tion of dith izon e in carbon tetrachloride.

P re p a r a tio n o f R e a g e n ts

Concentrated sulfuric acid, concentrated nitric acid, and distilled water are all distilled from Pyrex glassware. Baker’s 72 per cent redistilled perchloric acid need not be distilled again, but is transferred to a Pyrex bottle. The dithizone reagent is prepared (7) by shaking 0.1 gram of Eastm an dithizone with about 25 ml. of c. p. carbon tetrachloride; the solution is filtered,

transferred to a separatory funnel, and shaken w ith an equal volume of 0.2 N ammonium hydroxide, the resulting brownish- yellow carbon tetrachloride layer being discarded. Four ad­

ditional extracts of the aqueous layer are made, 10 ml. of carbon tetrachloride being used each tim e and then discarded; the last portion of carbon tetrachloride should show a greenish tint.

A 25-ml. portion of carbon tetrachloride is then added, and the aqueous layer is acidified w ith 3 N perchloric acid added drop- wise.

After the funnel has been shaken to transfer the dithizone to the carbon tetrachloride layer, the aqueous layer is discarded and the dithizone solution washed twice w ith redistilled water.

The solution is filtered into a glass-stoppered Pyrex glass bottle b y m eans of a filter paper moistened w ith carbon tetrachloride to remove the water. The filtered dithizone solution is finally diluted to 400 ml. w ith c. p. carbon tetrachloride, and should be stored in the dark.

The standard copper solution is made by dissolving M allinc- krodt’s c. p. copper in nitric acid and evaporating to dryness.

The final residue is treated with 2 to 3 drops of glacial acetic acid to make it more soluble, and is then transferred quantita­

tively w ith redistilled water to a volumetric flask. The final concentration should be about 2 micrograms (2 X 10-8 gram) of copper per milliliter of solution. The pH of the copper solution is adjusted to 3.5 by adding a few drops of 0.04 per cent bromo- cresol green and the proper am ount of dilute ammonium hy­

droxide. All equipm ent is of Pyrex glass and m ust be washed w ith dithizone and flushed with c. p. carbon tetrachloride before use.

S ta n d a r d iz a tio n

T he apparatus used for the titrimetric extraction consists of a Pyrex buret graduated in 0.05 ml. and a Pyrex separatory funnel made w ith about 20 cm. of 25-mm. tubing, a pair of N o.

11 glass joints (the male being sealed at both ends, acting as a stopper), and a stopcock of 2-mm. bore.

T he walls of the funnel are indented by heating th e glass in spots and pushing inward w ith a blunt-pointed rod. Betw een the 25-mm. tubing and the stopcock is about a 1.25-cm. (0.5-inch) length of 7-mm. tubing wherein the carbon tetrachloride layer settles after separation. The stopcock is lubricated with grease a t the ends ana ethylene glycol in the center. After lubrication,

696 IN D U S T R IA L A N D E N G IN E E R IN G C H EM ISTRY droplets along the walls, but can be brought down by a swirling m otion of the funnel. The red- colored copper dithizonate is allowed to run out through the stopcock, care being taken to avoid any loss of the aqueous layer. Then 0.4 ml. more of dithizone is added, the contents are shaken again, and the carbon tetrachloride layer is drained out.

The titration is continued until the end point is reached—i. e., the green carbon tetrachloride layer does not change color. The change from red to green is through a transition of red-purple- green. A t the first sign of a purple color, the di­

thizone is added in 0.1-ml. portions because the change from purple to green is effected within 0.2 ml. of dithizone. A t this stage the shaking funnel is shaken for 3 to 5 minutes. The volume of di­

thizone equivalent to 2 micrograms of copper is then read off the buret. The funnel is drained of its entire contents and flushed with redistilled water preparatory to a second determination. E quiva­

lent milliliters of dithizone are determined for 2, 4, 6, and 8 micrograms of copper by diluting 1, 2, 3, and 4 ml. of standard copper solution to 20 ml.

and subsequently titrating w ith dithizone solu­

tion. A t the end point, a mechanical shaker is preferable. B e­

fore plotting the results for a calibration curve, a blank deter­

mination is made on 20 ml. of the distilled water. After allowing for this blank, the standardization curve should be a straight line through the zero point on the chart. glasses (concave down) resting on triangular-shaped glass sup­

ports, and set on a hot plate (under a hood) whose top is covered whereupon it becomes colorless. The solution is then allowed to evaporate alm ost to dryness a t 220° C. T he beakers are al­

lowed to cool, the contents of each beaker are quantitatively transferred to a shaking funnel w ith redistilled water, and the volum e is made up to 20 ml. One drop of a 0.04 per cent bromo- cresol green solution is added, and the pH is adjusted to approxi­

m ately 3.5 w ith ammonium hydroxide. T he sample is now ready for titration, the equivalent milliliters of dithizone being determined in precisely the same manner as th at employed in water) h ave been freshly distilled, th e reagent blank is eq u iva­

len t to 0.8 to 1.2 m icrogram s o f copper. H ow ever, after ab ou t a w eek th e reagents (principally nitric acid) apparently begin to react w ith th e b ottles, and increase th e blank valu e copper containing 4 and 8 m icrogram s of copper, respectively, were m ade up w ith p H ranging from 0.8 to 6 and titrated w ith d ith izon e. T h e sharpest end p oin ts (w ithin 0.1 to 0.2 m l. of dithizone) were obtained a t p H betw een 3 and 4, w hile

WAVELENGTH IN MILLIMICRONS

Fi g u r e 1.

A NA LY T IC A L E D IT IO N 697 solution containing 4 micrograms of copper was extracted by the procedure described above. The solution was then shaken with 1 ml. of dithizone to determine whether or not the copper extrac­

tion had been complete. The dithizone was separated and aqueous solution containing 1 microgram of copper, separated and diluted to 10 ml. w ith c . p. carbon tetrachloride. Curve 4 represents the transmission characteristics of this dithizone sample. These experiments were checked and found to give identical results.

In their procedure for th e spectrophotom etric an alysis of copper w ith dithizone, L iebhafsky and W inslow (7) considered th e absorption a t 510 m/j. From a stu d y (Figure 1) of the

range of purple shades m ade it difficult to establish a standard end p oin t in th e purple.

T h is titrim etric extraction procedure has been used suc­

cessfully in th e authors’ laboratory as a routine m easurem ent of th e copper co n ten t of oxidized m ineral insulating oils.

E v a n s and D a v en p o rt (3) describe a procedure for extracting copper and lead from m ineral insulating oils and their sub­

sequent determ ination b y th e m ethod of Fischer and Leopoldi (5). H ow ever, th e am ou n t of oil used for each determ ination

term ination are com paratively sm all. A single determ ination (in triplicate) can be m ade in ab ou t 8 hours, 4 to 5 of w hich long-term program of research on electrical insulating-oil deterioration, w hich u n til Septem ber 1, 1939, w as carried on

Localization o f Certain Chemical Constituents in