VARIOUS USEFUL TECHNIQUES

In this exercise we wi ll learn severa l things. Fi rst , how to weigh a solid. Place a pieceof paper, about 15 em (6 in .) square on the left pa n of a trip balan ce and cou nterbalancethis pape r either with ano ther piece of paper of approxima tely equal size or by ad just ingthe ' ....ei gh ts.

\ Veigh out 8 grams (g) of potassium chromate as follows. From the bott le of K2 Cr0 4on the side shelf carefully po ur out, 011 a sepa rate sheet of paper, wha t you estim ate to beabout 6 g of the ma terial. (T his 'NiH be about 2 tablespoons in volume.) Adj ust the balanceweights to weigh 8 g of substance . T ra nsfer the K2 CrO" carefully and slowly to the pap eron the left pan of the balance, keeping one eye on the poi nter needle of the balance.

If more is needed, estimate the amount and pour it on to the separa te sheet of pa per,then add it to the pile on the left pan of the balance u ntil enough has been added to bri ngthe balance to equ ili brium, wi th the poi nter needl e at the center of the scale .

A n)' excess rem aining sho uld no t, repeat: should not , be returned to the original bottle.T oo many economically minded individuals have, in the past, ret urned m ate rial to thewrong bottle, or have somehow contaminated the excess material prior to ret urning it tothe right bottle. T he results in both cases were uniortunate for the IJerson who next wedthat contam inated bottle. I t is less wastef ul in the long run to adhere to this ru le: Excessmaterial is discarded. T o be economical, just Ire sure you take as little as possible i ll excess0/ )'our estimated need.

Also, note the use of the paper between the ba lance pan and the potassium chromate.You were asked to put the pa per there to prot ect the balance pan as 'well as to facil itate yourhandling of the solid. Always, when weighing solids, take care to protect the balance pallfrom con tact with the solid itself. Use a shee t of JJaJJer, a watch glass, beaker, or oth er su itabledevice. Liquids, of course, must be weighed in containers, thus affording protect ion to thebalance pa n without attention to th is detaiL

To resume , clean u p an y spill ed ma teria l and take the 8 g of reagent back to your bench .Be sure to clean up the area around the balance. Alway s leave the balance as clean, orcleaner, than it was before '}'ou used it.

Pour the weighed sal t in to a 250-1111 beaker . Knowing that , when full , this beake rcontains approximately 250 ml, esti mate how high 100 ml of water will fill the beaker . Ad dapproximately 100 1lI1 of water to the beaker. Sti r unti l all of the salt has d issolved. If youwish , the dissolu t ion can be hasten ed by heating. If you do dec ide to heat the water, use awire gauze to pro tect the beaker from direc t con tact with the flame of the burner .

Now you need to use your graduate (your graduated cylinder). Fill it about half -fullwith water and note that the surface of the 'water is curved. T his curved surface is calleda men iscus. The volume of the wa ter prese nt is read from the center, or lowest , po rtion ofthe meniscus by com paring its height with the calibration marks on the cylinder. Writedown the volume of water presen t and ask your instructor to read the volume.

Your reading: _

r e

Instructor 's read ing: _

Empty the grad ua te. Notice that some residual droplets of 'water ad here to the interior.If not removed, th is residual water will d ilut e the solution of bariu m chloride which isnext to be put into the graduate. These water droplets can be removed by dryin g thegrad uate or they can be re placed wit h droplets of barium chloride solu tion. Ad d about5 ml of bar ium ch loride solution to the graduate and twist and turn the graduate , thuswash ing the in teri or surface ,.... ith the barium chloride solut ion. D iscard this liq u id. Theadherent droplets now on the walls of the gradua te are approximately eq ua l in compositionto the bar ium chloride solut ion in the stock bottle . (Keep th is trick in mind. I t is qu ickeran d easier than drying the grad uate ; and of course it will also be su itable wi th a beaker,or any ccnta iner.) Now fill the graduate to the 50-ml ma rk with barium ch loride solution .

Back to the solut ion of potassium chromate in water : Pour about one-fou rth of thissolut ion into another 250-m l beaker. Save the remain ing three-fourths; it will be used later.

Slowly, wit h st irring, pour abou t 25 ml of the ba rium chloride solution from thegrad uate into the one-fourth portion of the potassium chromate solut ion. N ote that theliq u id becomes cloudy. The precipitate which has formed is bar ium chromate , BaCrO..;it is insoluble, or almost so, in wat er. Some of the particles of precipi tate arc so sma ll thatthey sett le \-cry slowly. Let the mixtu re stand withou t agita tion for about five m inutes an dnote that this is so.

Our immediate object is to separate the preCipita te from the liqui d. Set up theap para tus for filtering: Fol d a circle of filter pap er exactly in half . Fold th is half so as (O

for m an approximat e q uarter circle, with the two folded edges form ing an angle of about10° , as shown in Figure 6. Tear a small triangular piece off of the inside corner, ma kingsure that the ou ter edge is torn back further than the inner edge, as illustrated . Open thefolde d filter paper in to a cone , mak ing sure tha t the flap with the torn corn er is on theoutsi de of the cone . Place the cone loosely in a fu nnel and mo un t the funnel on a fu n nelsupport with the stem touch ing the inner wall of a beaker, near the r im. Turn the funnelso that the diagonally cut end of the fu nnel stem slants downward toward the beaker wall,as illustrated. Fi ll the filter paper cone with di stilled water and press the up per edge ofthe paper cone gen tly against the conica l sides of the funnel. If these dire ctions are correctlyfollowed , a column of water, which promotes rapid filt ration, will form in the stem of thefu nn el. Discard any water which has collec ted in the beaker and ask your instructor tocheck your setu p. His init ials go here:

Pour a few mi llil iters of the cloudy liqu id into the filter pa per cone. No te that no t allof the pan icles of precipitate ar c re tained on the filt er paper. Some are small en ough to

pass through the pores of the paper. Show your instructor that th is is so. His init ials:

So, to separate the prenpItate from the liq uid it is obvious that we must first removethese \'ery small part icles of precipitate before filtering. This is easy to do . Just le t themixtu re stand for about on e year. By then the very sma ll particles will have slowly di ssolvedand rc-prccipitared on the larger part icles. T he n we can filter without d ifficult y.

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HaC Iz solu t ion to the supe rn atant liqu id. If no preCipitate forms, precipi tat ion is indeedcomplete. If a precip itate does for m, more BaC h m ust be ad ded, followed by an otherd igestion and test for complete precipi tation . \ Vhen you believe pr ecipitat ion to be com­plete, show your instructo r that the supern atant liqui d is colorless and that no addit iona lprecipi tate forms when a few dro ps of BaC h solu tion are added. H is in iti als:

Discard the or iginal filter pap er , prepare another fi lter pa per cone, and separate theprecipitate from the liqu id as follows: First , le t the mix ture stand wit hout agitat ion, sothat the precipi tate settles to the bottom of the beaker. Most of the supernatant liquidcan be de canted from, that is, poured off, the to p of the precipi tate and passed thro ugh thefi lter paper into the beaker below. To d irect the flow of the liqui d, touch a st irring ro dto the beaker spout, holdi ng the st irring rod so that its lower end is within the conical spaceof the funnel. If the st irring rod is held in this position, all the liqui d will flow as desired ;even the last drop, which otherwise would run down the outside of the beaker, will godown the stirring rod into the funnel.

During the decantation, try to minimize any transfer of prec ip ita te to the filter pape r.If a large q uanti ty of precipitate iuadvcrtcn tlv gets on to the filter papcr, no har m will resu lt,but the t ime req u ired for filt ra tion will be unnecessar ily lengthened.

\Vhen most of the su pernatant liq uid has been decanted, t ilt the beaker until it is almostinverted , and most of the precipitate will slide out on to the filter paper.

At all tim es, exercise care to insure that the level of the liquid in the funnel remainsbelow the top edge of the filter paper cone and tha t no droplets of liq uid r un down theoutside of th e beaker.

\Vh en almost all of the supernatant liqu id has passed through the filter paper and mostof the precipitate has been transferred to the filter paper, rem ove the collecting beakerbelow the funnel and replace it with a test tu be . T he liqui d in the collec ti ng beaker, an dthe liqu id which will run in to the test tube, is called the " filtrate."

Make sure that a litt le filtra te has collected in the test tube and then remove the testtube, re placi ng it with the original beaker. Put approximately 5 ml of silver nitrate solu­tion in another test tube, est ima ting the volume for practice. Using your medic ine dropper ,add two drops of th is silver nitrate solut ion to th e filtrate in the test tube. Do not immersethe dropper in the filtra te. T hi s wou ld contaminate the dropper. Hold it well above theliq u id , j ust inside the mouth of the test tube, and force out the two drops.

A precipitat e of silver chloride will form. T he filtrate contains potassium chlor ide,KC I, wh ich reacts wi th the AgN 0 3 to form a white, inso luble substance, AgC l.

Notice tha t you were not directed to take the silver nitrate solution di rectly from th estock bou le wit h your medicine dropper . H ad you done so, the stock solu tion would havebeen contaminated if your dropper was not thoroughly clean. H ence a r ule :

NEVER insert any object into a bottle of liquid or solid unless ,'our instructor firstmakes sure that the object to be inserted is truly clean and free from contamination.

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N ow a few part icles of barium chromate st ill remain in the beaker. Also, not all ofthe supernatant liqu id has yet passed thro ugh the filter paper . A small q uantity, whichconta ins KCl in solu tion, is held u p by the HaCrOl in the fil ter papcr cone . Since we wishto obtai n pure barium chromate, thi s potassium chloride must be removed, and at the sametime. we can tra nsfer the residual precipita te from the beaker to the filter paper.

Using your wash bottle , gently squ irt a little d istill ed water in to the bea ker , wh ile ho ld­ing the beaker t ilted so that th e wash water. and residu al precipitate, run out int o th e filterpaper cone. Using a min imum amount of wa ter , wash the en tire interior surface of thebeaker with the stream from your wash bott le, thus washi ng all of the prec ip itate out. If afew particles stick to the beake r walls. set th e beaker dow n and loosen them with a pol ice.man affixed to the end of a st irring rod. \ Vash any part icles, as necessary, off the policeman.Make su re not to let the water in th e filter paper cone overflow in your zeal to get that lastlittl e ch unk of stu bborn barium chromate.

\\' hen most of the wash water has passed throu gh the filter paper. collect the last fewdrops in a d ean test tube and test th is liq u id wit h silver ni tra te for the presence of chloride.Addit ional AgCl will probably precipi tate. Repeat the washing of th e BaCr04 (since thebeaker is cleaned out. direct the wash water direc tly upon the precipitate) test ing for chlorideas before. Repeat as necessary until the precipitate is washed free of chlor ide, as determ inedby the results of th e test on the filtered wash water. Show your inst ructor that the precipi ta teis washed free of chlor ide . His ini tials:

It is reasonable to assume that if th e barium chromate is free of potassiu m ch lor ide, itis also free of all other solu ble contaminant s wh ich migh t have been present or iginally.Since there are no likely insolu ble contam inants, it may be assum ed th at the bar iumchromate is reason abl y pure, except for the wate r that is still present.

Let us now turn our atten tion to the remai n ing three-fourths of the origin al potassiumchromate solution. If the water is evaporated from th is solut ion , the original salt willrema in. In genera l, th ere are two ways to evaporate a solven t (water in this case) from asolut ion. \Ve can evapora te ra pidly or slowly. If we evaporate rapidly, some of the K2 Cr04will be lost: slow evaporation does not result in loss of the solu te (K2Cr04 in th is case).

T o demonstrate that rapid evaporat ion causes loss of solute, pour about half of th eremai ning K2 Cr0 4 solution into an evapora ting dish and heat it. You will note that eventhough care is taken to hea t it gently. as the q uan tity of water remain ing becomes less andless, the solution spu tters and bubbles,

Slow evapo rat ion. which you should no w attemp t, is more satisfactory. Pour all of therem aining potassium chro mate solution in to an evaporating dish and place this d ish on abeaker which is about half filled with water . The beaker sho uld be small enough so tha tthe evaporat ing d ish is supported on the r im of the beaker : the evapora ting dish shou ld

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no t be inside the beaker, Gently boil the water in the beaker . replenishing it from time tot ime , u nt il all the water in the K2CrO~ solut ion has. by slow evaporat ion , been driven off.Note that there is no loss of solute in th is slower evapora t ion process .

If you wish. some or all of the filtrate (\v·h ich contains KC I) may be evaporated bythe slow or fast process. or a com binat ion of the two-fast evaporation to remove most of thesolvent. then slow evaporation when it becom es likely that loss of material will occur if thefast evaporat ion is continued .

T h is has been a long experiment: what have you learned?

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Continue on the next page.

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Your instructor's init ials here' _