Titration of Chromate-Dichromate Mixtures A New Experiment for Quantitative Analysis L. H. Kalbus, R. H. Petrucci, and J. E. Forman California State University, San Bernardino. CA 92407 G. E. Kalbus Caiifornia State University, Long Beach, CA 90840 An important laboratory experiment often included in a quantitative analysis course is the titration of mixtures of sodium carbonate and sodium bicarbonate. This experiment illustrates the orincioles of acid-base eouilibria and oresenti an interesting p;oblem in stoicbiometry for studen&. A recentlv develo~edexoeriment involvine the back titration of mixtures o f chromate and dichromate illustrates the above principles, poses a more challenging puzzle in stoichiometry, and allows more variations than the titration of standard carbonate-bicarbonate mixtures. In this experiment the student is presented with the problem of determining whether the sample contains only chromate, only dichromate, or a mixture of chromate and dichromate, and the total quantities of each present. Aqueous solutions of chromate and dichromate contain several species in equilihrium. Above pH 8 CrOa2- predominates, whereas in the pH range 2-6 HCrOa- and C r ~ 0 7 are ~the principal species. When excess standard acid is added to potassium chromate-potassium dichromate mixtures, H+ from the acid reacts only with Cr0a2-, forming HCr04-. To simolifv the descri~tionof the titration reactions, we assume thai C; 0 - 2 - prese& in the original mixture remains exclusivelv as CnO-'- in the acidic solution. If the acidified solution i s nowback-titrated with standard base solution, two breaks in the pH titration curve are obtained. The first break results from the titration of the excess strong acid: H+ (excess) +OHHzO. Between the first and second break, two reactions occur. H+ from HCrOa- is neutralized, HCr04- OH- Hz0 CrOaz-; and Crz072-is converted to CrOa2-, Cr2072- 20H2Cr042- + H z 0 Thus, the volume of base required to reach the fust break represents the excess acid and is related to the amount of CrOa2present in the original sample, and the volume between the first and second break to the total quantity of the chromate and dichromate present (Fig. 1). The number of millimoles of NaOH required to reach the first endpoint represents the number of millimoles of HCI in excess. If these are subtracted from the total number of millimoles of HC1 added, the number of millimoles of HC1 reacting with the CrOa2- is obtained. Since the acid and C r 0 4 2react one to one, this equals the number of millimoles of CrOaz- present. The number of millimoles of NaOH used between the first and second endpoint is equal to the number of millimoles of Cr04--olus 2 times the number of mmol of Cr70? oresent. ~ i n c kt i e number of millimoles of CrOa2- has-&eady been determined, subtraction of this quantity from this number of millimoles of NaOH will equal 2 times the number of millimoles of C r ~ 0 7 present. ~The following equations summarize these calculations.
+
-+ + -
(T:z~L) (
)
-
first no *O break ' H ta) ( M ~= mmol ~ ~ ~ )
(MNaOH) - mmol Cr0:-
= 2 mmol Cr&-
second break
In general, the first break in the titration curve comes a t a volume of NaOH where mmol NaOH < mmol HC1 added, and the second break comes at a volume of NaOH where mmol NaOH > mmol HC1 added. If the first break comes at mmol NaOH = mmol HCI added, there is no chromate
IN ME PRESOUCE ff EXCESS STANDARD ACID
ORIGINAL MUCRlRE
CrO:.
$
c~,o:-
H C ~ O ~(a)
c ~ ~ o : -(b) xsHCI
FIRST BREAK
=CON)
m~~~o~t~lsaandth excess HCI (d
(c)
BREAK
mLNaonto mscf
MhHCrO;
(a)
andc;~:'
(b)
Flgure 1. Slolchlomehyof cllromatedichromate back tiballon
mL 0.1000 N NaOH
Based on a paper presented at the 198th National Meeting of the American Chemical Society. Miami, September 1989.
Figure 2. Back libation ofa midure of 0.2126 g K&04 and 0.2452 g K2Cr20, following the addition of 25.00 mL 0.1000 M HCi. Volume 68 Number 8 August 1991
677
present. If the second break comes a t mmol NaOH = mmol HC1 added, there is no dichromate present. A typical backtitration plot i q shown in Figure 2. Strrdmt Instructions i'.. .t 25.00 mL of your 100-mL unknown' into a 150-mL beaker. P:. 15.00 mL of 0.1000 M acid into the sample in the 150-mL Titrate your sample with 0.1000 M base, recording millilih, t, ase added and pH as measured with a pH meter. Use your :lot a titration curve, and determinethe milliliters of NaOH to reach the first and second endpoints. From these data vhether only potassium chromate, only potassium dichrod rn* a a mixture of the two is present. Calculate the mass of each eomp,nent present in the 100-mL unknown. Instead of using a pH meter, you can use methyl orange as an indicator for the first break and phenolphthalein for the second break.2
Since chromium compounds are considered hazardous, they should be disposed of properly according to the following r ' rections. Students may be assigned this procedure as an a ' 'itional laboratory exercise. T. dispose of the waste K~CrOdaq)generated in the tit~ations,~
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first adjust a dilute solution (less than 5%CrOd2-1 to a pH < 3 with sulfuricacid. Slowly add a 50%excess of NaHSOdaq), while gradually stirring. C r 2 0 ~ ~is-reduced to Cr3+in this reaction. Following this reduction, add NaOH to precipitate the Cr(OH)z,maintaining the pH in the range 7 to 10. Separate the precipitate by filtration or decantation,and pack the precipitate or sludge for landfill disposal. This determination possesses a number of advantages over the carbonate-bicarbonate titration. Amone these advantages are better p H breaks, a back titration I& possible in the carbonate-bicarbonate titration, and a more challeneing stoichiometric problem for students.
-
'Each 25.00-mL aliquot of unknown should contain from 0 to 2 mmol of potassium chromate and 0 to 1 mmol of potassium dichromate. Kalbus, L.H.: Petruccl, R.H.: Kalbus. G.E. J. Chem. Educ. 1976, 53, 719.
Adapted from procedures described In Prudent Practices for Disposalof Chemicals from Laboratories. National Research Councll, National Academy Press: Washfngton, DC. 1983.
