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I
Se miqua ntita tive Esti mati o n of Dithionite THOMAS P. WHALEY’ and JOSEPH A. GYANl Research laboratory, Ethyl Corp. , Detroit, Mich.
b Ammoniacal Naphthol Yellow S solution i s a specific indicator for detecting small quantities of dithionite, the color changing rapidly from yellow to red. Although the color change is nearly permanent for small amounts of dithionite, an apparent reversal of this color change occurs when larger amounts of dithionite are present. The time required for the second color change to occur is roughly proportional to the amount of dithionite and can be used for estimating it semiquantitatively.
T
HE KEED for a rapid method for estimating sodium dithionite in the presence of sodium methoxide motivated a rather exhaustive search for potential analytical procedures. A specific qualitative test (1) for dithionite in the absence of cesium, mercury, potassium, rubidium, and tin ( 2 ) has been described. A small amount of dithionite changes the color of a dilute ammoniacal solution of Naphthol Yellow S (sodium salt of 2,4-dinitroI-naphthol-7-sulfonic acid) from yellow t o red. Inasmuch as other common anions do not effect this color change, the reaction can be used to detect the presence of trace amounts of dithionite. The color change is transitory-i.e., the red coloration slowly fades and eventually changes to the original yellow. The reversal of the color change is extremely slow when the dithionite concentration is low, but it is relatively rapid when dithionite concentration is high. Subsequent experimentation
1 Present address, Ethyl Corp., Baton Rouge, La.
showed that the rate a t which the color reversal from red to yellow took place was roughly proportional to the amount of dithionite present. This led to a study of the concentration-time relationship and subsequent adaptation to a semiquantitative method for estimating dithionite content. The test was unaffected by the presence of sulfite, hydroxide, alkoxide, chloride, sulfide, sulfur dioxide, or other sodium sulfoxy compounds, so long as all of the ammonia was not neutralized. PROCEDURE
-4solution of 0.8 gram of Naphthol Yellow S (Kational Aniline Division, Allied Chemical & Dye Corp.) and 10
0 1 1 1 1 1 1 1 1 1 1 1
IO0
50
0
Na2S204, WT. Figure 1. Color reversal after reduction of Naphthol Yellow S b y sodium dithionite
ml. of concentrated ammonium hydroxide in 1000 ml. of distilled water was prepared as the indicator solution. Synthetic mixtures of anhydrous sodium sulfite and anhydrous sodium dithionite (commercial grade, approximately 90% pure) were prepared over the entire 0, 20, 30, concentration range-Le., 40, , . . 100% commercial-grade sodium dithionite, diluted with sodium sulfite. One-gram samples of the dithionitesulfite mixtures were dissolved in 25-ml. portions of distilled water, and 25 mi. of the ammoniacal Naphthol Yellow S test solution was added to each. The color change from yellow to red occurred immediately and the time required for the color reversal to take place was determined by a stop watch. The color reversal was gradual, going through several color graduations from red to yellow; consequently, the time recorded for the complete reversal was somewhat arbitrary, depending on the judgment of the observer. Kevertheless, the times were reproducible for a given observer and they were used successfully for estimating semiquantitatively the concentration of sodium dithionite in unknown samples. Unknowns were analyzed for sodium dithionite content (based on commercial purity) by noting the time required for color reversal with 1-gram samples and reading the concentration directly from the prepared curve (Figure 1). $11 operations were conducted a t room temperature. LITERATURE CITED
( 1 ) Jelley, E. E., Analyst 55, 34-5 (1930). (2) Welcher, F.,, J., “Organic Analytical Reagents, pp. 18-20, Van Nostrand, Xew York, 1948.
RECEIVEDfor review October 24, 195G. Accepted May 15, 1957. VOL. 29, NO. 10, OCTOBER 1957
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