Determination of KHP

For determining purity of unknown KHP, acid – base titration was used. Molarity of titrant in this case NaOH was standardized as 0.04958(±0.00011) M. Purity of unknown KHP was calculated as 35.88(±0.16) %.

Introduction

Acid – base titration is typical method of determining unknown figures. Acid – base titration means that by using neutralization between acid and base. Abrupt changing of pH says whether procedure is done or not. pH indicators are tools for determining neutralization is reaching end point. When reaction reaches equivalent point, one of species, titrant or analyte is used up completely and there is no more neutralization. But By pH indicator, there is very few possibility to distinguish equivalent point. When, neutralization reaches at that point, changing pH is too sensitive to distinguish that. Blank titration is one of way to decrease interval between equivalent and end point. Blank titration is executed in solution with no analyte, it involves same volume of diluted water with final volume of mixture of actual titration, pH indicator and titrant. From this titrant, interval is obvious. Before doing titration, standardization of titrant is first step. If concentration of titrant is not exact, final result is not exact. And then, doing titration by standardized titrant is going to be second.

Method

All procedures were taken from Analytical Chemistry Lab – spring2013 First, estimating how much NaOH solution was needed became first step. NaOH solution was prepared in advance. For this step, dried KHP was weighed out 0.2 to 0.3 g and put into 250 mL of Erlenmeyer flask. 25-50 mL of distilled water was added to dissolve KHP. Even though some KHP remained, added NaOH solution would dissolve it. From this estimating titration, mass of KHP neutralizing 40 mL of NaOH solution was computed. Blank titration was performed with 65 mL of distilled water and 2-3 drops of phenolphthalein.

Next standardization of NaOH was performed by titration between computed mass of KHP and prepared NaOH solution. In three 250 mL of Erlenmeyer flask, 25-50 mL of distilled water, computed mass of KHP, 2-3 drops of phenolphthalein were added to each flask. In the procedure, concentration of NaOH, its average, standard deviation and 95 % confidence limits were calculated. Standard deviation had to be less than 0.00015 M. After standardization of NaOH, impure KHP was titrated with standardized NaOH solution. As the same reason of estimating how much NaOH solution was needed, 1.0 g of impure KHP was titrated in 250 mL of Erlenmeyer flask with 25-50 mL of distilled water and 2-3 drops of phenolphthalein. Mass of impure KHP to neutralize 40 mL of NaOH was calculated, actual titration was performed with calculated mass of impure KHP, 25-50 mL of distilled water and 2-3 drops of phenolphthalein in 250 mL of Erlenmeyer flask.

Discussion

At standardization of NaOH, overall standard deviation was 0.0005 M, despite of lab manual said 0.00015 or less. Standardization of NaOH was performed max 7 trails that was limited by lab manual. There was one chance to decrease interval between 0.00015 M and 0.0005 M, but the results of Grubbs-test said there was no outlier. So, there was no way to reduce standard deviation. When solution turned to pink color, titration was stopped. But titrations were not stopped at same pink color. Some trails were stopped by very dim pink color, other trails were stopped by very strong pink color. What if whole trails were stopped by blank titration’s pink color? It might decrease error coming from color of indicator. Calculated % KHP was 35.76(±0.17) % at 95 % confidence interval, actual percentage of impure KHP was 36.15 %. This actual percentage was not included in range of 95 % confidence interval 35.59 %≤ × ≤35.93 %. If standardization of NaOH was more accurate, % KHP was more exact. % KHP was included in 99 % confidence interval, 35.76(±0.38) %, but 99 % confidence interval also included more error.