-,
thwnicroscak laboratory SulfuricAcid, 0.5 M and 0.3 M Solid Potassium Iodide
Vinay ~umar: Philip Courie, and Shari Haley Northern Kentucky University Highland Heights, KY 41076
1% Starch Indicator Solution Add about 0.5 g of soluble starch to 5 mI, of water. Add a trace of HgIz as a preservative and mix the ingredients into a paste. Pour the paste into 45 mL of boiling water and stir mntinuously. Continue boiling until clear.
During the last five years many colleges and high schools have introduced microscale (or downscaled) chemistry experiments into their curriculum. The introduction of t h i s approach provides answers to some of the challenges (e.g., expenses, waste disposal, etc.) associated with teaching large numbers of students in the laboratories. In addition, the use of nontraditional, safe, and inexpensive plastic equipment that can be used for many experiments cuts down the costs considerably. Because the use of real-life samples (e.g., tea, coffee,cereal, vitamin C, ete.) as unknowns in laboratory courses helps the students to relate chemistry to everyday life, their use adds variety to the analysis and heightens their interest. This paper reports a simple microscale method for the determination of aswrbic acid (I)in vitamin C tablets, fruit drinks, and orange juice. The other methods (14 ) available for the determination of (I)require expensive instrumentation or uncommon reagents l i k i phosphotungstic acid (2)and thallic perchlorate (31. A microscale method for determining (I)has been reported previously using the dye 2,6-dichioroindophenol(5,. However, this method has the following ~hortcominas:the end point is "sluggish" and is hard to discern; the method cannot be used if the sample is highly colored (e.g., purple grape drink); and 2,6-dichlorindophenol is reduced by the Fez+salts that are often present in the multivitamin supplements (5).The microscale iodimetric method described in this paper is very precise, has a sharper end point, and works fine with the colored samples. Aknown excess of 1, is generated by the reaction of iodate with iodide and is allowed to react with ascorbic acid (6).The excess 13-is then back-titrated against sodium thiosulfate. Reactions
lo;
+
81-
GH+
-
(2)
Potassium Iodate Standard, 1.00 x lo3 M Accurately weigh 65.0 mg of solid KI03and dissolve it in 250 mL of distilled water in a volumetric flask. Experimental Procedures
p,pet Cai,b,at,on Since the volume per drop may vary with the micropipet and the angle a t whichit is held (7),all the reagent and sample solutions should be added with the same type of pipet. In this work microtip Beral pipets (Catalog no. AP1517; Flinn Scientific,Inc., Batavia, IL) were used. The optimum pipet angle during drop delivery was about 4 E . To calibrate each micropipet, deliver 30 drops of distilled water into a small previously weighed beaker. Figure out the weight per drop of water, and using the density of water at room temperature, calculate the number of drops needed to deliver 1mL of the liquid. Standardization of Sodium Thiosulfate
31- 3H20
+ ZH*
OH
I ascorbic acid
(3)
Sodium Thiosulfate, 3 x l o 4 M The concentration of NazSz03.5Hz0 solution is an important factor in the determination of (I).Aconcentrationlessthan 0.0002 M causes a sluggish end point, and a concentration of 0.0004 M or greater results in lower precision. To prevent dispnoportionation of thiosulfate ion by dissolved COz, the latter is removed hy boiling 300 mL of distilled water. Cwl to raom temperature. Weigh about 19rng of NazSz03.5Hz0into a 250mL volumetric flask; add a trace of NaZC03to solution as a preservative and dilute to the mark with distilled water. Store sodium thiosulfate solution in an amber bottle.
H0'
HO
Ij + 2 q 3 3 1
thiosulfate
0
-
SUNY-Cortland c0~land~Ny13045
Reagents
Quantitative Microscale Determination of Vitamin C
(1)
ARDENP. ZIPP
dehydroascorbic acid 31
+
S,OB"
tetrathionate
.
3-
Transfer two drops of standard KI03solution into a well of a 24-well reaction plate (catalog no. AP1447; F l i m Scientific, Inc.). Add about three granules of K l and five drops of 0.5 M HzS04to the same well. Titrate by adding thiosulfate solution. During titration keep the we11 plate on the bench top and swirl with a circular motion. Keep adding thiosulfate solution until the solution turns pale yellow. Add one drop of starch indicator. The solution a t this point should be violet or dark blue. Continue titration until the solution is 'Author to whom correspondence should be addressed.
Volume 69 Number 8 August 1992
A213
the microscale laboratory
Determination of Ascorbic Acid in Fruit Drhks and Orange Juice
Amounts of Ascorbic Acid in Different Types of Samples
Sample Type 100 mg tablet (Nature
Made brand) 250 mg tablet (K-Marl brand) Boku drink (white grapelraspberry) Hawaiian Punch drink Kool-Aid Mix (a)pink lemonade (b) lemon-lime Veryfine orange juice Trauth orange juice Adams orange juice '~er6.00 or. s8rvin.a.
No. of Trials
Average (mg) aswrtic acid
Label I RDAvalue
SLC
(mg)
10
100.4M.4
100
9
251.8i9.5
250
9
11 11 9 4 4 3
~6.8~m.92
60
Determination of Ascorbic Acid in Kool-Aid Powder Drink Mix
61.5+1.4
60
Transfer quantitatively about 1.0 g of the contents from the package into a 500-mL volumetric flask and dilute to mark with distilled water. Transfer 40 drops of Kool-Aid drink solution into a well of a 24well plate. Cany out the rest of the titration by following the above procedure for the determination of ascorbic acid in vitamin C tablets. The end point of this titratiou is not colorless, but it will be similar to the original color of the drink or the juice.
8.IaM.41 8.9'M.15 156Sb?d.7
6 6 60
193.4~i2.9
60
H0.1a*3.5
60
'per 8.00 or. sewing. 95% Confidence limits
colorless. End point color may be confirmed by viewing solution through the side of the well. Repeat titration at least three times. Determination of Ascorbic Acid in 100 mg and 250 mg Vitamin C Tablets
Weigh a commercial vitamin C tablet to the nearest 0.1 mg. Pulverize tablet thoroughly and transfer the powder to a dry weighing bottle. Weigh about 75 mg of the powder to the nearest 0.1 mg and add to 30 mL of 0.3 M H$04 in a small beaker. Stir to dissolve the powder; some binding material will remain undissolved. Filter the solution through filter paper into a 100-mLvolumetric flask. Rinse the b'aker well with three small aliquots of distilled water; transfer to the flask and dilute to mark with 0.3 M H2S04. Shake well. If 100 m g tablet is being analyzed, transfer five drops of the vitamin C solution into a well (or use three drops for the 250 mg tablet). Add five drops of standard KIOa to the same well. To the well add about three granules of solid KI. Dissolve all the granules before titrating with thiosulfate to a colorless end point as described above under the standardization procedure. The color at the start of the titration should be light amber. If the solution is colorless or pale yellow, discontinue the titration and repeat the procedure in a different well using one less drop of vitamin C solution. In order to obtain a light amber color, you may have to repeat this procedure with even fewer drops of vitamin C. Once the correct number of drops of vitamin C is determined, use the same amount in subsequent titrations.
A214
No sample preparation is needed for this analysis. Shake sample well before titration. Transfer four drops of fruit drink (or two drops orange juice) into a well of a 24-well plate. Carry out the rest of the titration by following the above procedure for the determination of ascorhic acid in vitamin C tablet. The end point in this titration is not wlorless but will be similar to the color of the drink or the orange juice before the titration.
Journal of Chemical Education
Results and Discussion
Ascorbic acid (vitamin C) is a mild reducing agent and is quantitatively oxidized by iodine (8).In solution, ascorbic acid (1)also can be oxidized due to atmospheric oxygen. Factors that can accelerate the decomposition of (1) are heat, light, alkalies, oxidative enzymes, and trace amounts of iron or copper. The oxidation can be slowed by the use of an acidic medium and cold temperatures (9). For the Hawaiian Punch drink,we obtained a figure of 61.5 mg ascorhic acid per 8.45 oz. serving. In our calculations, the conversion factor, 1 oz. (liquid) = 29.6 mL was used. Our result compares very well with the reported label value of 100% of the recommended daily allowance (RDA) figure of vitamin C for adults (60 mg) (10). The results for the analysis of vitamin C in the various brands of pure orangejuice samples were found to be in the range of 157-193 mg per 8 oz. serving. Selinger (11)reported similar results for (1)in several Australian brands of orange juice samples with a range of 36k719 ppm (or 86.4-170.3 mg ascorbic acid) per 8 oz. serving. These values are considerably higher than the RDA figure for vitamin C reported above. This may be due partly to the way the product is packaged. During the processing of orange juice some vitamin C is lost, and the juices are normally fortified with ascorbic acid to meet the legal minimum requirements (11). Literature Cited
iiA~tmlinlgSl,36,4.
1. Haddad, P. R.P& &hm@ 2. Mwdikrishna,U. ; Mwtx J. A.Analystl989,114,407408.
3. Cupta, D.;Shama, P.D.; Gupta,Y.K Talonfa lS75,22,913--914. 4. Baker, W L.;Iawe,T. TheAnol~~t1985,110,ll8~1191. 5. Thompson,S. Chpmtmk Allyn and Bamn: Beeton, 1990;pZll. 6. Harris, D. C. Q~ontitoliueChpmixl Andy&; 3rd 4.; W H. h e m a n : New York, 1991; p 746.
7. Ealy, J.;Pickerhg, M.J. C h . Edw. 1881.68, A12O. 8. Bailey, D. N. J 9. Ref. 5; p 196. 10. Ref 5; p 195.
Chpm. Edve
1974.51.488.
11. Selinger, B.;Chomisuyiit h e M a ~ k e t p I ~ ; I4,; th H H H ~ ~ ~ B B B B J J O ~ ~ E ~ , I ~ I ~ . : San Diego, 1989:p 533.
