METHYLENE BLUE-REDUCTION OXIDATION
AND
Submdted by: F. B. 1)utton from various source5 Checked by: C. X, McCart,y, Mrhigan State Univels~ty,ICast 1,ansing
PREPARATION
REMARKS
Dissolve 8 g of potassiun~hydroxide in approximately 300 ml of ~vat,erin a 500-ml. flask. Allow t,o cool, m d dissolve 10 g of dext,rose. .%dd a few ml of met,hylene blue solut,ion (0.25 g in 1000 ml of water), The blue solut.ion should become colorless on standing a few minutes hut should resume &heblue color when shaken. Add more of t,he dye solut,ionif necessary.
This demonstrat,ion can be pesfmned repeat,edly in front of a class, without comment, or exp1mat.iin1, t,o st,inn~lat,eint,erest. Students may be asked t . 1 ~ mini~nu~n numlm of components present, in t,he flask t,o p r ~ ~ d u rt,he e effert,ol~served. (They should no1 he expected t,o predict the presence of the base.) Met,hylene blue is reduced hy alkaline dextrose solution, On shaking wit,h air the reduction product is reoxidized to t.he hlue dye. The solution will gradually t,urn yellow tan brown on long standing and should he freshly prepared for use.
DEMONSTRATION
Shake t,he flask, show to t,he class, allow lo st,and, and call attent,ion t , the ~ cnlorless solution,
HEAT OF CRYSTALLIZATION
PREPARATION
Ihough G-in. test tuhes are placed in a xire t,mL t,ube rack so t.hat, therc will be one test t,uhe for each t , ~ oto three studeuts. To each test tuhe add approximately 1 g of sodium thiosulfate, Apply heat, to the t.est tuhes, e n masse, until a clear liquid has heen obtained. Place t,he test tulx rack with hot tubes in a deep pnmnnat,ic trough and carry t,olecl~ure. I ~ n g t h s of fi-mm glass tuhing (ends not fine-polished) are provided, one f o ~ each test, t,uhe. Ahout 5 n ~ i nhefore t,he demonstrat,ion is delivered fill the pneumat,ic trough d h cold water from the kzp. DEMONSTRATION
Issue the cold test, tuhes of molten sodium t,l~iosulfate and lcngt,hs of glass t,ubing. Direct, t,hc students to induce cryst~allization hy rot,ating t,he glass lnbing against the hot,tom of the t,cst, tuhe. The crystds
fwm immediately and 1,he entire me11 cryslallizes in 1-3 s e ~ . 1:eeI l)ot,t,omof the test tuhe. It has hccome hot.. A mid solution has hemme warmed without, direct addition of heat,. REMARKS
It is r w ~ m ~ n e n d et.hat, d the initial heating he don? vit,h a burner. The demonst,ration has the folloving advantages : It, provides a liquid-solid transition near hody tcmperatme with a large enough heat, value to he detect,cd with the palm of t,he hand, It utili%cs a commm crystallizing t,cchnique ill scratching the test, t,uhe. It provides an excellent opening for discussion of exchange of energy forms (i.e., molecular motion to thermal energy), It varies usual demonstration t.eehnique hy allowing student participation.
