Nov., 1916
T H E J O l i R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
trouble in using a reagent lasting five times1 as long should alone decide i n i t s favor.” I t is hoped t h a t these figures will show t h a t , as long as potassium hydroxide is obtainable even a t prices many times t h e present extraordinary one, t r u e economy dictates t h a t i t should be employed in preference t o sodium hydroxide for t h e preparation of alkaline pyrogallol. , . ..... IN D E F E N S E O F T H E H E M P E L P I P E T T E
During t h e reading of Shipley’s article for t h e preparation of t h e compariisons in t h e preceding pages, t h e author noted a paragraph of criticism of t h e Hempel pipette upon which he desires t o make brief comment. The paragraph i n question follows: “It is hard to understand why the Hempel pipette should be longer used for any but very special work. I t is difficult to fill with any reagent and especially so if the reagent is somewhat viscous. The long bent Capillary is a source of weakness in structure and of irregularity in use. The enormous friction of the liquid in the capillary requires, even with comparatively fluid reagents, a considerable excess of pressure to overcome and prohibits entirely the use of many concentrated reagents because of their viscosity. Moreover, the pipette requires a special and expensive stand while shaking has to be resorted to in order t o obtain efficient absorption. Should the pipette be broken anywhere only an experienced glass-blower can repair it.” Relative t o sentence two, i t should be mentioned t h a t a n opening2 between t h e second a n d third bulbs of t h e Hempel double pipette for t h e insertion of a funnel renders t h e filling of t h e pipette with a n y reagent a simple matter, even if t h e reagent is somewhat viscous. As regards sentence three, t h e eliminations of t h e unnecessary U-tube of t h e original Hempel pipette increases t h e strength of t h e apparatus a n d facilitates its manipulation. With t h e V-tube removed from t h e p:ipette, t h e pressure required t o force viscous liquids such as alkaline pyrogallol through capillary of I mm. bora is not unduly large (see sentence four). As t o whether shaking has t o be resorted t o (sentence five) depends upon t h e style of pipette t h a t is employed. A special form of t h e Hempel double pipette4 for solid a n d liquid reagents is especially adapted for the absorption of oxygen on I-minute contact with alkaline pyrogallol. A properly constructed frame for t:hese pipettes is extremely longlived a n d has proven a n economy on account of t h e protection which it affords t h e pipette. CORKELL L-NIVERSITY, I’CHACA, NEW YORK
RAPID VOLUMETRIC DETERMINATION OF INDIGO By SAMt-EL h%. JONESA N D WALTER SPAANS Received July 3, 1916
Of t h e various met:hods proposed for t h e determination of indigo, t h e one which depends upon t h e reduc1 Reagent S o , 9 has about 5 times the specific absorption of the potassium reagent recommeniied for use in the Hempel double pipette for liquid reagents to which Shipley refers. This reagent contains 0.136 g . of pyrogallol and 0.715 g. of potassium hydroxide per cc. and, a t the present time, costs 0.242 cent per cc. Using 20 as the specific absorption of this solution-Shipley’s value--the cost of absorbing 1 cc. of oxygen is 0.012 cent. The cost of absorbing 15.3 liters is then in this case $1.84, an excess of $1.38 over the cost with the sodium reagent. Four extra fillings would be required with this reagent, so that the saving in time might be estimated a t 28 hrs. 2 This construction is shown on a special pipette for use with cuprous chloride, designed by the United Gas Improvement Co., in the catalogs of Eimer and Amend and A. H. Thomas Co. 3 White and Campbell, J. .4m.Chem. SOC.,27 ( 1 9 0 3 , 731; Anderson, Trns JOURNAL, 6 (19141, 237. 4 Anderson, THIS JOURNAL, 8 (1916), 133.
IO01
tion of t h e sulfonated product b y means of a sodium hydrosulfite solution is t h e one i n most general use a t t h e present time. This method, which was first proposed b y A. Muller1 a n d fully described in t h e Badische Indigo Book, was a material improvement over t h e older oxidation methods. Briefly s t a t e d , i t consists i n comparing t h e sample with a n indigo of known strength b y titrating t h e sulfonated product with a solution of sodium hydrosulfite, also of known concentration, until t h e solution becomes colorless. By comparing t h e results, t h e relative value of t h e indigo in question may be obtained. Although t h e principle involved in this method is correct, there are several factors which render i t not only difficult t o perform b u t in many cases inaccurate. I n t h e first place, indigo white, t h e leuco derivative formed b y reducing indigo, is very unstable, a n d is nearly quantitatively reoxidized t o indigo b y exposure t o air. Muller tried t o overcome this difficulty b y performing t h e titration i n t h e presence of a n inert gas, such as coal gas, b u t , according t o our experience, this is not ’sufficient. I n spite of maintaining a constant atmosphere of coal gas, t h e indigo white reoxidizes very quickly if there is not a n excess of hydrosulfite present. Consequently, i t is impossible t o ascertain t h e a m o u n t of hydrosulfite necessary t o produce t h e endpoint. Moreover, t h e sodium hydrosulfite, however pure, is like t h e indigo white. very susceptible t o oxidation, as conservation trials of a n aqueous solution will readily show. These two factors render t h e method inaccurate. Realizing t h e importance of a rapid accurate method for t h e determination of indigo, especially in t h e textile industries, we have succeeded in devising one which produces t h e desired results. Like t h e method of Muller. our new method is based on t h e reduction of indigo to indigo white. I n stead, however, of working in t h e presence of coal gas, we have found t h a t b y using a current of hydrogen, t h e titration may be carried out the s a m e as any other volumetric titration, without fear of subsequent oxidation. I n other words, working with our a p paratus in a current of hydrogen, only t h e amount of hydrosulfite actually necessary t o produce .the reduction need be used. We have, in addition t o this, substituted formaldehyde sodium sulfoxylate, which is manufactured in a very pure form b y t h e Badische Company under t h e name of Rongalite C, for t h e unstable sodium hydrosulfite. A4queous solutions of formaldehyde sodium sulfoxylate may be conserved for hours or days without suffering t h e slightest decomposition, which makes it especially suitable as a “standard” t o be used for volumetric reduction methods. This very stability, however, especially a t low t e m peratures, a t first presented considerable difficulty, for on running t h e solution of sodium formaldehyde sulfoxylate into t h e sulfonated indigo, no reduction takes place a t ordinary temperatures, a n d only on 1
Amevccan Chemryt 126.
T H E J O r R A ' A L O F I S D I I S T R I d L .4,qTD ELTGIXEERIA'G CHEMISTRY-
I002
heating nearly l o the boil is t h e indigo reduced quickly enough for volumetric purposes. I n order t o obviate t h e necessity of titrating a t a boil, we found t h a t in t h e presence of sodium bisulfite the speed of the reaction m a p be accelerated, due t o the liberation of free sodium sulfoxylate by double decomposition according t o t h e following reaction:
OH
i
OH
The standard hydrosulfite-formaldehyde solution is made b y dissolving I g. of t h e solid hydrosulfiteformaldehyde (Rongalite C) in I liter of distilled water. The sample of indigo t o be tested and the standard (C. P.)indigo are sulfonated and prepared for titration in t h e following manner. PREPARATIOX
OH
'
I
S a SNo 'OH
+
/O
'OH
sas\< I O
OH Formaldehyde Sodium Sodium BisulfiteSod iab tn B i s u l j t e Suljoxylate formaldehyde Suljoxylate In malting the titration we have found t h e apparatus illustrated especially suitable (see Fig. I ) . The solu-
Burei'fe with S t a dard Hyd=os u/ph ih?forma/dehyde So/utio n
Rubber Tube tonnechon with burette
to per/n/f
S a k / n y Of
&>
f L k
Thermomefer Hydroy en
9QS
FIG.I-INDIGO TITRATION APPARATUS
tion of indigo carmine is placed in a small Erlenmeyer flask of about 300 cc. capacity closed with a rubber stopper provided with four holes. Through one of these holes a thermometer is placed, projecting into t h e liquid in t h e flask: tubes for t h e admission and exit of hydrogen gas are provided and a small, tapering t u b e connecting with t h e burette cioses the fourth hole. A small Bunsen burner and a n efficient hydrogen generator of some kind complete the apparatus.
1-01, 5. S O .J I
OF T H E I S D I G O S O L U T I O S BX SU LF 0 SA T I 0h-
If t h e sample is in pon.der form, grind up in ii small mortar and weigh out exactly I g. into a small beaker. -4dd 1 5 cc. concentrated sulfuric acid (sp. gr. 1 . 8 4 ) and rub up into an even paste with a glass stirring rod. Wash down t h e glass rod and sides of the beaker with another I j cc. concentrated sulfuric acid and thcn heat on the water b a t h or oil b a t h for 2-4 hrs. a t 55-60" C., stirring once or twice. Sulfonation should be coniplete in 2-3 hrs. Allow the sulfonated indigo t o cool down and then carefully wash out the beaker with 300 cc. distilled water. Again cool down t o I j-18' C. and make u p t o I liter. Filter and t h e solution is ready for titration. If t h e indigo is approximately a 20 per cent paste, use j g. for sulfonation, if a 50 per cent paste use 2 g.: etc. Add the sulfuric acid slowly so t h a t t h e temperature does not exceed jo-60" C.. for a loss may result by locally carbonizing a part of t h e indigo a n d darker solutions will be produced. At the same time and in the same manner sulfonate I g. of C. P.indigo for use as a standard. TITRATION
Measure accurately into t h e small Erlenmeyer flask j o cc. of t h e standard (C. P.)indigo carmine solution and add j o cc. sodium bisulfite solution (60" Tw., 33-36 per cent NaHSOs). Connect as shown in t h e figure, t u r n on t h e hydrogen current and heat to 7 j C. Then run in t h e standard sodium formaldchyde sulfoxylate solution from t h e burette until the blue color disappears. With a good strong current of hydrogen t h e titration can he carried out slowly without fear of reoxidation of t h e indigo solution before the final end-point is reached. T h e first determination will be only approximate and should be repeated until t h e results coincide. Repeat t h e titration, using t h e indigo carmine solutionfrom the sample t o be tested. An example will make clear the method of caiculation: j o cc. of t h e standard sulfonated indigo solution required 14. 50 cc. of t h e sodium formaldehyde sulfoxylate solution for reduction; 50 cc. of an indigo carmine solution made from j g. of a n approximately 20 per cent indigo paste required 1 6 . I O cc. of t h e same sodium formaldehyde sulfoxylate solution. Since t h e standard indigo contains I g. per liter, I cc. sodium formaldehyde sulfoxylate equals (o.ooI)(5o) = 0.003448 1 4 . 50 indigo and jo CC. of t h e sample equals (0.00j ) (5 0 ) = 0 . 0 2 j g. indigo paste. Then, (0.003448) ( 1 6 . I O ) ( r o o ) - z 2.206 per cent indigo in the O
~
0.025
sample. ARNOLD PKINTWORKSLABORATORIES ~ - O R T I I ADAXS. MASSACHUSETTS
