34 BULLETIN 1390, U. S. DEPARTMENT OF AGRICULTURE 
For ponceau 8R, amaranth, orange I, and tartrazine.—Treat 2 grams. 
of the dye with 25 c.c. of a saturated solution of sulphur dioxide 
(p. 15) and 1 gram of zinc dust, and warm the mixture gently until it | 
becomes colorless. This should take place in from two to three 
minutes. If it does not, add more sulphur dioxide solution in small 
portions at a time until the color is destroyed. Then add 30c.c. of 
concentrated sulphuric acid and 0.7 gram of mercuric oxide or its 
equivalent of metallic mercury and digest the mixture. Finally 
make alkaline and distil as usual in the Kjeldahl method. 
TaeLeE 13.—Percentage of sulphur, nitrogen, and sodium in permitted food dyes 
Sodium 
sulphate 
corre- 
sponding 
to sodium 
content 
Dye Sulphur Nitrogen Sodium 
ff 
Per cent Per cent Per cent Per cent 
9. 30 28. 
PIGHCCAW ee te ee eee, SP Aes 12. 97 5. 66 71 
PAIMIAT AIG Mas. eee ae ee a Ee Ee ae ee ee eee 15. 91 4. 64 11. 41 35. 23 
PS TaViU EOSIN 22. he ae Le ee ee CE oe ee | eee ne 5. 12 15. 81 
MANGO a Sak CSS eR ee Ss a ee) See ee 9.15 8. 00 6. 57 20. 28 
Naphtholeyellow iS: 22 =< eae ee ee 8. 95 7. 82 12. 84 39. 64 
Tartrazine: 
‘risodium: Salt #332228 2552 St cue bee sa Be ts es 12. 00 10. 49 12. 91 39. 85 
IDISOGIOM Salt. 25 en ae ee eee 12. 52 10. 94 8. 98 27. 72 
Guinea jgrebn, By) leas 3 fst ke ee oh ee ee BS 9, 29 4. 06 3. 33 10. 28 
Light green S F yellowish: 1 
PD ISOGTUMISAl bb Sets" See Pe ee ee ee eee 12. 13 3.53 5. 80 17. 90 
Mion osodiumy Saltese sf ae ee ee ee 12. 48 3. 64 2. 98 9. 20 
AN GIFOLING =o. oo os Sao eS ee ee 13.75 6. 01 9. 86 30. 42 
MelloOWwJATB ) $35 2h ys CoS EEE ie 2 SEER eRe Ee eee ee BE Ee OO jonas: et Ale = ee 
MelloweO'B Ue c= oS ee eS oe a hy eee Re 16.09 j-24_2--2 4: 3)22 eee 
1 Recently acquired evidence indicates that the disodium salt of guinea green B and the trisodium salt 
oflight green S F yellowish form colorless solutions (72). 
COLOR BY SPECTROPHOTOMETER 
The accuracy of the spectrophotometer for the quantitative esti- 
mation of color depends upon the skill of the operator in distinguish- 
ing between slight changes in the intensity of monochromatic light. 
For this reason quantitative estimations should be undertaken only 
by those who are thoroughly familiar with this instrument. 
BuFFER SOLUTIONS 
Prepare a buffer solution by one of the following methods: 
(1) Dissolve 71.4 grams of disodium phosphate (Na,HPO,.12H,O) 
and 9.6 c.c. of glacial acetic acid in water and dilute to 1 liter. For 
use, dilute 5 c.c. with 95 c. c. of water. 
(2) Dissolve 3.540 grams of primary potassium phosphate (KH,PO,) 
and 14.568 grams of disodium phosphate (Na,HPO,.12H,O) in water 
and dilute to 1 liter. For use, dilute with an equal volume of water. — 
PREPARATION OF SAMPLE 
All water-soluble dyes.—Dissolve 10 mg. of dye in 500 c. c. of the 
diluted buffer. ‘. 
Yellow A B and yellow O B.—Dissolve 10 mg. of dye in 500 e. ¢. of 
alcohol (95 per cent). 
“" 
e 
