C h a r l e s H. Henrickson Western Kentucky University Bowling Green. 42101 P a u l R. Robinson University of Illinois Urbana. 61801
Gravimetric Determination of Calcium as CaC204 H20
I
A tractable, inexpensive experiment for undergraduate quantitative analysis laboratory
T h e gravimetric determination of calcium i n calcium carbonate samples has served a s a n attractive replacement for the gravimetric determination of chloride a s silver chloride and nickel as the dimethylglyoximate. A class of 249 freshman chemistry majors a t t h e University of Illinois performed t h e analysis using twenty-five different calcium carhonate samnles ohtained from Thorn Smith. 1023 T r o v Court. Trov, Michigan 48084. T h e samples ranged in calcium content from 20-40 nercent. T h e accuracv obtained b y these students is represented in the figure in terms of frequency versus relative percent error. T h e procedure used was b a d d on a n analysis originally described by Ingols a n d Murray.' I n outline, dried, weighed calcium carhonate samples were dissolved in HCI. Solid urea ,and acidified (NHdzCz04 were then added, and the urea was hydrolyzed a t ca. 90°C to slowly and homogeneously generate oxalate ion (eqns. 1-4). T h e resulting calcium oxalate was filtered while h o t into tared sintered glass crucibles, washed with water, then acetone, a n d dried for 60 min a t 105'C.
.~-~~
~~~~
~
CaCOs
+ 2HC
-
Caz+ + Hz0 + COz
(1)
Experimental The instructions given to the students2 are summarized below. Theanalysis is to hecarriedout in duplicate with theaverage value reported as the result. The impure calcium carbonate sample should he dried for one hour at 11O0Cbefore use. Two clean, medium frit sintered glass crucibles should be clearly labeled with a lead pencil.. .then brought to constant weight by drying at llO0C [note 11. Cool to room temperature in desiccator for about 30 minand then weigh the crucibles. Store the tared (preweighed) crueihles in a covered beaker until needed. In each of two 250-ml beakers, place a 0.35-0.38 g sample of the calcium carbonate unknown and record the sample mass to the tenth of a mdligram. To eaeh sample add 100 ml of distilled or deionized water and slowly add 6.0 ml of 6 M HCI as you swirl the solution to dissolve the sample (eqn. 1). Once the sample is dissolved, check the pH of the solution by using a clean stirring rod to remove a small droplet and touch it toa piece of pH paper. If the pH is higher than pH I, lower the pH to at least I by adding a drop or two of 6 M HCI [note 21. From this point on, eaeh solution should be treated individually. Add 4-5 droos of methvl red indicator and mix the solution bv w~rl~ng.'l'hr wlutmn should be red st this puint.Thenadd ?tImlg~rfsol:durea. Swrl tlw wluticm t o d i w h t thr urea [note 41. Cover the beaker with a clean watchglassand heat thesolution to near boiling. Hold it there [note 51 until the color of the methyl red indicator changes from red to yellow [note 61, which indicates a pH of ahout 6. This should reauire from 2&35 min. The formation of the crystals of CaCzOIHzO will be complete at this time. Rinse theeondensation from the watch glass into the beaker using a wash bottle.
'R. S. Ingols and P. E. Murray, Anal. Chem., 21,525 (1949). 2C. H. Henrickson, S. Zumdahl, and G . P. Haight, Jr., "An Introduction to Chemical Systems in the Laboratory," Stipes Publishing Co., Champaign, Ill., 1977, pp. 113-120.
Hdding the hesker with a towpl, or using beskrr tongs. filter the hot sdurim [note 71 through the sinrrrrd gIss9 c~ucibk~ 8 t suctiun h L'le a rubber rmlictman and small wlumci of distilled ur deionbed water from a-wash bottle to transfer the entire precipitate to the crucible. Wash - t.h e~ervrtals ~ . with two 10-rnl oortions of cold distilled or ~, d e i m ~ e ds,atcr. S a t , rinw rhe cr).stsls with t w o 10-ml portions of ncetmr. 1)rdu air through the prtsipirat~for five rninuws u, partially dr) the ~,rystaljandremove trarciot acetone Inow 81.Then nilou the sample t
