filtrates e residues
edited by JAMES 0. SCHRECK University of Northern Colorado Greeley,CO 80639
Using Silica Gel Beads to Dispense Indicators and Reagents Akiko Furuhashi and Kunihiko Endo Department of Chemistry, Aoyama Gakuin University, 1-1, Morinosato-Aoyama, Atsugi, Kanagawa, 243-01 Japan Akira Yamasaki Department of Applied Chemistry and Physics, University of Electro-Communications,1-5-1, Chofu, Tokyo, 182 Japan Silica gel beads that have adsorbed certain Table 1. Acid-Base lndicator chemical reagents can be prepared easily in the laboratory. These beads can be used in Color Change Indicator Original classrooms and field studies either a s teachBead Color ing tools or for students' laboratory work. In acidic solution, the beads turn red in hours. The Methyl Orange (MO) Yellow The beads can be classified into four categosolution becomes red araduallv over a lonaer " - .oeriod. ries according to the use: Phenolphthalein Colorless In basic solution, the color of the solution changes to red immediately. It is the same as using the indicator aeid-base indicators solution directly. detecting reagents ealor-producing metal ions In aclolc so Jllon of pH about 2, tne oeads lLrn pale Thyrno Bide (TB) Orange organic dyestuffs reo as ooes !he so &on. In oastc solul on. (PH>8). me Deads and me so ~t on botn become blJe. Silica gel has been used not only a s a desIn acidic solution (pH I), the beads turn red in 5 Yellow iccant but also for several chemical experi- Cresol Red (CR) minutes, then the solution turns pale red after 2 ments. Because of its chemical structure, hours. In basic solution (pH 8),the beads and the cationic species and certain neutral species solution become red within 2 hours. can be adsorbed to it. Applying these charac- Red cabbage Depending on pH, the color of the beads changes Blue teristics, the authors have prepared many gradually as when the extracted solution is used kinds of beads useful in chemical education directly. both a s experimental tools and lecture devices. Table 2. Color Change of Beads in Colored Solutions
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OH TB CR Red cabbaae Procedure Solution Colorless silica gel b e a d s from Wako orangejuice 3.7 Orange-Yellow Yellow Reddish-Violet Chemical Company were used for the experi- Worcestersauce 3,1 Orange-Yellow Yellow Violet ments, but any colorless beads from packets t h a t a r e used to dessicate manufactured "inegar 2.5 Red Orange Reddish-Lavender 2.5 Red Orange Reddish-Lavender products also may be used. The beads first Cola 3.8 Orange-Yellow Yellow Reddish-Violet are saturated with water vapor by being B~~~ placed over water in a desiccator for a few 0.1% Soap solution 10.2 Orange-Yellow Yellow Yeiiowish-Green days. This allows the beads to avoid destruc7.0 Orange-Yellow Yellow Yellowish-Green tion when they are placed in aqueous solu- Milk tions. Beads of 2-3 mm diameter are most suitable. Very small beads or gel powder adseparately in 95%ethanol solution. Red cabbage extract is sorb well but are not convenient to give to students, and obtained as follows. Sliced cabbage is placed in water of very big beads adsorb poorly. equal mass and boiled 10 min. After cooling the solution is Next, quantities of the beads are placed in flasks condecanted into another container. taining appropriate reagent solutions (usually about 5 g of Beads adsorbed with such reagents a s methyl orange beads in 40 mL of solution) and are allowed to stand for several days. Then they are spread on filter paper to dry in (MO), tbymol blue (TB), cresol red (CR), and red cabbage the open air. As stated above, the beads should not be dried extract can be used even in colored solutions, where most completely and should be stored in a n air-tight container. liquid indicators cannot be used, although reactions might The beads last for a long time if stored in a n appropriately sometimes be slower than with liquid reagents. This is a dark (AgNO3) or refrigerated (red cabbage) place. (We are major advantage of these beads. Phenolphthalein is restill using beads made more than two years ago). leased quickly into solution from beads, and it is very convenient for acid-base titration. Only two to three beads per Results student per titration are needed. The results are summaAcioLBase hdicators rized in Tables 1and 2.
l? solution concentrations are suitable. Phenolphthalare dissolvedein, Cresol Red tCR , and Thymol Blue ,TR)
832
Journal of Chemical Education
Th s paper was presented to tne Twe fln Internal ona Conference on Chem ca Eddcalon, Decemoer 18, 1992. Bangkok, Tna land.
Table 3. Detecting Reagents
Reagent AgN03 (Silver nitrate) KSCN (Potassium thiocyanate)
K4[Fe(CN)e](Potassium hexacyanoferrate) CHBC=NOH I CH3-C=NOH (Dirnethylglyoxime)
Original Color Change Bead Color Colorless In a solution containing CI-, beads turn pale violet in 2 minutes. Colorless In a solution containing ~ e ~both + ,beads and solution turn reddish brown in less than a minute. Pale green In a solution containing ~ e ~both + ,beads and solution soon turn blue. Colorless In a solution containing ~ i ~ only + , beads turn pink.
Detecting Reagents Many analytical reagents used for qualitative analysis of metal ions can be adsorbed onto silica gel beads. The heads are very convenient for detecting certain species existing i n solution. The results are summarized in Table 3. Color-Producing Metal Ions AO.l M solution of metal salt with the required amount of ligand reagent i s suitable. Because the gel beads are originally colorless, t h e complex ion colors are shown clearly. Ligand replacement reactions also can be observed from beads color changes. The results are summarized in Table 4. Colored Dyestuffs
A 1%solution of dyestuff is sufficient. With dyestuffs such as Auramine and Rhodamine B showing fluorescence, the beads shine brilliantly under ultraviolet radiation. The results are summarized i n Table 5.
Table 5. Colored Dyestuffs
Dyestuff Acridine Orange R Auramine Bismarck Brown Brilliant Green Chrysoidin G Fuchsin (Basic) Methylene Bluea Malachite Green Methvl Violet
Bead Color Orange Yellow Brown Green Reddish brown Red Blue Green Violet
Fluorescent dyestuff Calcein Orange Calcon Red Eosin Red Rhodamine B Red Safranin 0 Red Titan Yellow Yellow 'Easily decomposed by sunlight. Keep in the dah. Discussion These beads are light and unbreakable and can be carried easily to any place or used simply on the spot without spillage. If desired, the beads can be reused by soaking them in water for several days then repeating this a few times, changing the water. It is best to use the washed beads with the same reagent to avoid any possible interferences. Any reagent solution, of course, can be used many times. When the concentration becomes dilute, some reagent should be added. However, the concentration of the reagent solution is not a critical parameter.
Further Uses of the Beads By using pre-weighed dry beads that are placed in a sealed container containine water. the Drocess of water adsorption into t h l beads' can 'be observed by Table 4. Color-Producing Metal Ionsa simply weighing the beads after some t i e has passed. The reverse process can be observed as well by using moisturized beads. Reagents Bead Color Adsorbed Ion Also, pre-moistened beads i n a column can be Nickel nitrate Pale green [Ni(Hz0)eIL' used well to demonstrate chromatography of a Nickel nitrate and ammonia Blue [N~(NH~)~I~+ mixture of colored reagents, a s can be seen in Nickel nitrate andethylenediamine Deep violet [Ni(en)312+ standard textbooks. Coppernitrate Blue (cu(Hzo)~]~+ Any number of applications of the beads i s possible depending on the individual's imaginaCoppernitrate and ammonia [cu(NH~)~~' tion. Not only dyestuffs and water, but also Coppernitrate and ethylenediamine Deep blue [cu(en)z12' odors can be adsorbed usefully 'Only typical examples are shown. Many other combinations can be tried.
Volume 72 Number 9 September 1995
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