Brooklyn College
Department of Chemistry
Chem 3420 / 7420G
Instrumental Analysis
Laboratory Reports
Reports will generally consist of the following components:
Title:
Abstract:
Name of the Experiment
Your Name
Your Lab Partner’s Name
Date of Experiment
A brief summary (200 words maximum) of the experiment performed.
Give code number and results for any unknowns analyzed.
Introduction: State the objectives of the experiment in a concise, but complete, manner.
A brief statement reviewing the status of that particular field is appropriate
here.
Experimental: Equipment, chemicals and procedures used are described briefly.
Results and
Discussion:
References:
Tables and Figures are presented here. The must be numbered with
appropriate titles. All axes/columns/rows should be clearly labeled and the
units of all values given. Present a representative example to show how
you obtained the data and concluded the results, but do not show all the
data or all the calculations. The Discussion should refer to the Tables and
Figures. The results should be analyzed for their precision and accuracy,
and, when possible, compared to literature data.
List full references cited in your report. Refer to an ACS journal for
examples.
Each ACS journal publishes a Guide to Authors, typically in the first issue of the year but
also on their website. Review the Guide to Authors to see how published reports are
typically presented.
Reports are due two weeks after completion of the experiment – late reports will not be
accepted. Reports will be grades on the clarity of your presentation of the results in
addition to their accuracy. Neatness, punctuation, grammar and spelling are also
important.
Attached below is an excellent example from a former student.
Determination of Weak Acids in Soft Drinks by Potentiometric Titration and Computer Data Analysis Michelle Leuenberger Phosphoric acid is found in Coca-­‐Cola drinks in variable amounts and compositions in various drinks. In this experiment the concentration of H3PO4 and H2PO4-­‐ in a sample of Coca-­‐Cola drink are determined using potentiometric titration. It is concluded that almost no H2PO4-­‐ is present in the initial sample because the average ratio of Volume of titrant necessary to reach the second equivalence point to Volume of titrant necessary to reach the first equivalence point is 1.99. The average calculated concentration of H3PO4 in this sample is 6.7 x 10-­‐3M with a standard deviation of +/-­‐ 0.2M. This standardized Sodium hydroxide solution is then used to titrate the unknown sample of Coca-­‐Cola drink. This potentiometric titration requires the use of a glass electrode which has both an indicator electrode (which responds to analyte) and reference electrode (which maintains a fixed reference potential) in one. Experimental Prior to this experiment, carbon dioxide is boiled off of the Coca-­‐Cola drink sample so that carbonic acid, which forms as carbon dioxide reacts with water in solution, will not interfere with detection of phosphoric acid. A Sodium hydroxide solution of approximately 0.10 molarity is prepared and then standardized by titration against KHP (Potassium Hydrogen Phthalate) standard solution. From the mass of KHP in solution and the volume of Sodium hydroxide solution necessary to reach the end point, the concentration of the sodium hydroxide solution is calculated to be 0.0947 +/-­‐ 0.0002 moles per liter. Figure 1 shows a plot of pH versus volume for this titration. Figure 1 pH meter reading 12.00 10.00 8.00 6.00 4.00 2.00 0.00 0.00 5.00 10.00 15.00 20.00 25.00 30.00 Volume NaOH sol'n (+/-­‐0.02mL) The Average volume of titrant needed to reach the first equivalence point is 7.09mL. The average volume needed to reach the second equivalence point is 14.12mL. The ratio of the two is 1.99 to 1 indicating that only H3PO4 is present in solution initially. The concentration of H2PO4-­‐ is negligible. It is predicted that the Volume at the third equivalence point would be three times that of the first. The average pH at the first observed equivalence point is 4.99 and the average pH at the second observed equivalence point is 8.08. The average concentration of H3PO4 in this sample is calculated to be 6.7 +/-­‐ 0.2 x 10-­‐3 equation pH = pKa + log [H2PO4-­‐]/[ H3PO4] reduces to pH = pKa. Relevant points are labeled. moles per liter. The standard deviation is 2.0 x 10-­‐4. Figure 2 shows the change in pH of the Coca-­‐
Cola drink as sodium hydroxide is added during titration. Change in Volume (mL)/Change in pH Figure 4: T2-­‐ Vol. vs pH First DerivaXve Figure 2: T2-­‐pH vs. Volume 12.00 10.00 pH 8.00 6.00 4.00 2.00 50.00 45.00 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 2.42; 43.00 9.72; 16.75 6.66, 4.42 0.00 2.00 4.00 6.00 8.00 10.00 12.00 Average pH 0.00 0.00 5.00 10.00 15.00 20.00 25.00 Volume NaOH Added (mL) Figure 3 shows the first derivative of the plot in figure 2. The first derivative plot shows distinct peaks at the volumes of each equivalence point. The relevant points are labeled. RelaXve Change in pH Figure 3: T2-­‐ pH vs. Vol. First DerivaXve 2.000 1.800 1.600 1.400 1.200 1.000 0.800 0.600 0.400 0.200 0.000 0.00 7.00; 1.810 The average calculated pKa1 value is 2.40 and the average pKa2 value is 6.81. These values exhibit 16% and 5.4% percent errors respectively when compared to known values. The pKa1 value has a much higher error percentage because the equilibrium has already begun to establish itself even before any base is added. This is why the Volume required to reach the end point of the second equivalence may be lower than the volume required to meet the end point of the first equivalence. A table of the data used to generate the plots shown in figures 1-­‐4 is shown in figure 5: 14.42; 0.831 Figure 5: Data for Titration #2 5.00 Titration #2 VNaOH pH added 10.00 15.00 20.00 25.00 Average Volume (mL) Figure 4 shows the first derivative of the plot in figure 1 if the axes are switched and shows peaks at the pH that corresponds to each pKa value. This is the point at which half of the H3PO4 has been converted to H2PO4-­‐ so that the V2-­‐
V1 PH2-­‐
pH1 (V1+V2)/2 (pH2-­‐
pH1)/(V2-­‐V1) 0.25 2.40 0.25 0.01 0.25 0.040 0.46 2.41 0.21 0.01 0.36 0.048 1.32 2.43 0.86 0.02 0.89 0.023 2.19 2.48 0.87 0.05 1.76 0.057 2.93 2.54 0.74 0.06 2.56 0.081 3.55 2.61 0.62 0.07 3.24 0.113 4.34 2.74 0.79 0.13 3.95 0.165 5.01 2.90 0.67 0.16 4.68 0.239 5.59 3.13 0.58 0.23 5.30 0.397 6.21 3.53 0.62 0.40 5.90 0.645 6.89 4.44 0.68 0.91 6.55 1.338 7.10 4.82 0.21 0.38 7.00 1.810 7.36 5.19 0.26 0.37 7.23 1.423 7.51 5.36 0.15 0.17 7.44 1.133 7.88 5.66 0.37 0.30 7.70 0.811 8.29 5.88 0.41 0.22 8.09 0.537 8.79 6.08 0.50 0.20 8.54 0.400 9.34 6.27 0.55 0.19 9.07 0.345 10.05 6.47 0.71 0.20 9.70 0.282 10.57 6.60 0.52 0.13 10.31 0.250 11.10 6.72 0.53 0.12 10.84 0.226 11.71 6.89 0.61 0.17 11.41 0.279 12.32 7.05 0.61 0.16 12.02 0.262 12.97 7.28 0.65 0.23 12.65 0.354 13.58 7.55 0.61 0.27 13.28 0.443 14.09 7.89 0.51 0.34 13.84 0.667 14.74 8.43 0.65 0.54 14.42 0.831 15.41 8.81 0.67 0.38 15.08 0.567 16.04 9.02 0.63 0.21 15.73 0.333 16.60 9.16 0.56 0.14 16.32 0.250 17.06 9.25 0.46 0.09 16.83 0.196 17.51 9.33 0.45 0.08 17.29 0.178 18.10 9.41 0.59 0.08 17.81 0.136 18.70 9.48 0.60 0.07 18.40 0.117 19.20 9.54 0.50 0.06 18.95 0.120 19.72 9.59 0.52 0.05 19.46 0.096 20.35 9.64 0.63 0.05 20.04 0.079 21.04 9.70 0.69 0.06 20.70 0.087 21.71 9.74 0.67 0.04 21.38 0.060 22.40 9.79 0.69 0.05 22.06 0.072 23.28 9.83 0.88 0.04 22.84 0.045 because equilibrium has begun to establish itself before any Sodium hydroxide is added. Errors may occur in this titration because strong base solutions are unstable and tend to absorb atmospheric carbon dioxide which reacts with water in solution forming carbonic acid which, in turn reacts with the base in solution. Also, end point detection is difficult because the steep parts of the curve are relatively short. Sample Calculations: Concentration of Standardized NaOH solution: Mass KHP=0.5112 +/-­‐ 0.0001g MW KHP= 204.23 g/mol 0.5112g x 1mol/204.23g = 2.5031 x 10-­‐3mol KHP 1 mol NaOH is consumed for every mol KHP. Therefore moles KHP=moles NaOH. Concentration NaOH =moles NaOH/Volume NaOH needed to reach end point. =2.5031x10-­‐3mol/26.36x10-­‐3L=0.0950M NaOH pKa1: The pKa values were found from the first derivative graphs of pH vs. volume. Percent error: {(Accepted value-­‐calculated value)/accepted value}x100% Concentration H3PO4: Average concentration NaOH solution = 0.0947M Volume to reach end point = 6.82mL Volume Coca-­‐Cola = 100mL Conclusions 0.0947mol NaOH/L x (6.82x10-­‐3L)=6.64x10-­‐4mol The calculated value for pKa2 is within a reasonable range of the actual value. The calculated value for pKa2 is much less accurate 6.64x10-­‐4mol H3PO4/0.100L sol’n= 6.63x10-­‐3M