Titration of Monoprotic Acids with Sodium Hydroxide Contaminated by sodium Carbonate rn
Tad& Michalowski Jagiellonian University, 30-060Krakow, Poland Sodium hydroxide solutions stored in leaky flasks are usually contaminated by carbon dioxide originating both from the preparation and the air. If this solution is used as a titrant for a solution of a weak monoprotic acid HA, the resulting titration curve is distorted in comparison with the one obtained when uncontaminated hydroxide is used.
Let us then consider a titratiou of VomL of CoM HA with V mL of the solution of C1 M MOH (strong base) Ct M
+
MzC03 used as a titrant. From the charge and concentration balances, [HI + [MI = [OH] + [A]
+ [HCO,] + 2[C0,]
Figure 1. Theoreticaltitration curves obtained for weak monoprotic acids HA of different strength in titrand at different values of the fraction Din titrant: (a) D = 0.30.
(b) D = 0.10, (c)D = 0.03. (d) D = 0. The numbers at the corresponding curves denote lag Kvalues: VO= 100 mL, CO= 0.01 M. C = 0.1 M.
Volume 65 Number 2
February 1988
181
. .
[HA]
[H,CO,]
+ [A] = C,Vd(V, + V) [MI = (C, + 2C,)V/(V, + V)
:
.'
+ [HCO,] + [CO,] = C,V/(V, + V )
a n d the equilibrium constants expressed b y eqs 1-4, [HA1 = K[HI[AI [HCO,] [H,CO,]
= K,[HI[CO,I
(log KI = 10.1)
= K,[H]~[CO,]
(log Kz = 16.4)
(2) A distortion of titration curves has great significance from analyticalviewpoint. Far D > 0, the results obtained from titrations in using a visual indicator having a color change in the acidic range are loaded with a negative, systematic error. In all instances, use of more basic indicators, such as phenolphthalein, gives more accurate results. (3) A number of inflection points on a given titration curve depend on K and D values. The inflection point, that is related to a first local minimum of a buffer capacity (e.g., point A on Fig. la), is related directly to the quantity of carbonate in a sample. The abscissa of this point (V = ViDf)fulfills the equation
(pK, = 14.0)
[HI [OH] = K,
equivalent to d2pH/dV = 0. The relationship between (V, Vhd/V., and D values for K = 0 + 10, is presented in Figure 4; V, = CoVa/C. Vjntand V, are found experimentally from the abscissae corresponding toinflection points (e.g.,A and B in Fig. la) related to minimal (local) buffer capacities.
we get t h e relationship
v = v,
C, .Z, - [HI + K,I[H] C(1+ D .Z,) [HI - K,I[H]
+
where C = C,
+ C,
D = C21C Z, = lI(K[H]
+ 1)
Z,
- I)/(K~[H]~ + KJH] + 1)
= (K,[H]'
T h e plots of p H vs. V, a s expressed b y e q 5, are shown in Figure 1for different D (eq 7) a n d log K (eq 1) values. T h e corresoondine olots obtained for a strong m o n o ~ r o t i cacid (K= i n t h l t k r a n d are presented in Figure 2. An example of oH metric titration in such a svstem is given in Fiaure 3. more detailed discussion of t h e titratcon curves reads t o t h e following conclusions:
6)
k
(1) The curves corresponding to the systems with MzC03 present in the tirant (D > 0) are distorted in comparison with the curves related to the systems where D = 0 (Fig. Id). This distortion rises with increasing D value and becomes smaller for weaker acids (i.e., decreases with increasing log K).
Figure3. Curveof pH metric titration of chlaroscetic acid with a mixed (NaOH Na2COs)titrant: VO= 100mL of titrand contains a weighed amount 0.1015 g of CH2CICOOH(pa.)preparation; composition of titrant: 30 mL of Na2C03(0.1 M) + 70 mL of NaOH (0.1 M). Titration was performed in open system.
+
Figure 2. Theoretical titration curves for strong monoprotic acids in titrand solution (Vo = I00 ml, Go = 0.01 M) at different values of the fraction D: (a) 0.30,(b)0.10,(c)0.03,(d)O:C=O.l M.
182
Journal of Chemical Education
Figure 4. me 1 - L&,/V., vs. Ddependence found theoretically for monoproticacids withstability constants K = 0 + to3: (10 = 100 mL. Go = 0.01 M. C =
