Using a Control Chart to Detect Problem in Analysis

Suppose  you are working at your local sewage treatment facility.  One of your responsibilities is to monitor the effluent’s biological oxygen demand (BOD), which is a measure of the amount of biologically available organic material in the effluent.  Basically, a high BOD means that bacteria present in the water will require a large amount of O2 to decompose the available organic matter; thus, an effluent with a high BOD will ultimately have a lower concentration of dissolved O2.  A high BOD is undesirable as it may indicate that a water system is unable to support aquatic life.  As part of your job you use control charts to monitor the sewage treatment plant’s analysis of BOD levels.  Several scenarios are described below.  For each, discuss how you can use a control chart to detect the problem; feel free to sketch the resulting control chart if it helps to illustrate your answer.

Scenario A. The BOD levels are measured using an amperometric electrode that is positioned behind an O2-permeable membrane.  With time, the membrane begins to fail and the amount of O2 crossing the membrane decreases.

Scenario B. BOD measurements are temperature sensitive and appropriate temperature corrections must be made if results are to be accurate.  During a brief 3-day heat wave, the analyst forgot to make the appropriate temperature corrections.

Scenario C. A new standard is introduced to calibrate the BOD electrode; unfortunately, that standard has a negative determinate error.

Scenario D. The analysis is assigned to a less experience chemist whose work shows an increase in indeterminate errors.


Kinetic Determination of Ascorbic Acid

One method for determining the concentration of glucose in fruit juices is to take advantage of its enzymatic oxidation in the presence of glucose oxidase:

β-D-glucose + O2 + H2O → δ-D-gluconolactone + H2O2

To follow the kinetics of this reaction, the hydrogen peroxide is reacted with phenol and 4-aminoantipyrene to form a colored antipyrene dye:

2H2O2 + phenol + 4-aminoantipyrene → antipyrene dye + 4H2O

a reaction that is catalyzed using horseradish peroxidase. The formation of the dye is monitored by measuring the absorbance at 505 nm. Concentrations of the enzymes are chosen such that the second reaction is much faster than the first reaction; thus, any kinetic data is controlled by the first reaction only. One complication is that ascorbic acid, which is often present in fruit juices, also reacts with hydrogen peroxide

2H2O2 + ascorbic acid → dehydroascorbic acid + 2H2O

a reaction that is significantly faster than the enzymatic reaction between H2O2, phenol, and 4-aminoantipyrene.  Shown below are the results of three kinetic runs using a fixed concentration of glucose and variable concentrations of ascorbic acid (Series 1 has the most ascorbic acid and Series 3 has the least).  Clearly explain, in as much detail as you can, the similarities and differences between these three curves.

Kinetic Method for Manganese

Trace levels of manganese can be determined by measuring its catalytic effect on the oxidation of diethylamine by sodium periodate.  The rate of formation of the oxidation product is followed spectrophotometrically.  A series of external standards are prepared by adding 1.00-mL aliquots of standards containing 0.010, 0.020, 0.030, and 0.050 mg/mL of manganese to separate tubes containing 5.00 mL of diethylamine.  A 5.00-mL aliquot of a sodium periodate solution is added to the first tube and the absorbance is measured 10.0 min after mixing, yielding an absorbance of 0.050.  The remaining standards are treated in the same fashion, yielding absorbance values of 0.081, 0.109, and 0.169, respectively.  An analysis of a reagent blank gives an absorbance of 0.020.  A 5.00-mL sample of blood was dry-ashed and the manganese extracted using ion-exchange chromatography.  After diluting the sample to 3.00 mL, a 1.00-mL aliquot was treated in the same manner as the standards, yielding an absorbance of 0.098.  What is the concentration of Mn in the blood sample in ppm?

Standardization of a Kinetic Method of Analysis

Shown below is a plot of initial rate vs. substrate concentration for an enzyme-substrate reaction that follows Michaelis-Menton kinetics. An analyst developing a quantitative method for the substrate wishes to use a one-point standardization. The standard selected by the analyst, and the corresponding initial rate, are shown on the diagram. Will this one-point standardization give accurate results? Be sure to clearly justify your response.

HPLC Separation of Phenacetin, Salicylic Acid, Nicotine, Methylamphetamine, and Phenobarbatuic Acid

The graph shown below is a plot of retention time vs. pH for the reverse-phase HPLC separation of a mixture containing phenacetin, salicylic acid, nicotine, methylamphetamine, and phenobarbituic acid.

Based on the structures of these compounds and their pKa values, match each compound to one of the five curves.  Be sure to clearly justify your reasoning.

Separation of Amino Acids

In ion-exchange chromatography, a solution containing a mixture of compounds is passed through a column packed with a resin containing covalently attached anionic and/or cationic functional groups. One use of ion-exchange chromatography is the separation of compounds based on differences in charge. For example, an anionic ion-exchange resin separates cations, which bind to the resin, from anions and neutral compounds, which pass through the column without binding to the resin. Suppose you need to separate the amino acids lysine and proline using ion-exchange chromatography.  To what pH should you adjust the amino acid solution so that the separation is possible? Briefly defend you choice and indicate which amino acid will elute from the column and which will remain on the column.

Analysis of Anaerobic Sediments for Sulfides

A paper in the Journal of Chemical Education describes an experimental method for determining the concentrations of both insoluble metal sulfides, such as FeS2, and gaseous H2S in anaerobic sediments. Samples of sediment are placed in a flask along with 100 mL of water. After agitating the sediment using a magnetic stir bar, nitrogen gas is bubbled through the flask and the N2 and any gaseous H2S are swept into a second flask, which contains 50.00 mL of a basic buffer that converts the H2S to S2-. The concentration of S2- in this second flask is then determined using a 2- ion-selective electrode. Next, to release the sulfide from the insoluble metal sulfides, 50 mL of concentrated HNO3 is added to the first flask. The H2S that forms is, once again, swept into the second flask using N2 where its concentration is determined with the same ion-selective electrode.

(a) To calibrate the apparatus a standard solution of Na2S is placed in flask A and acidified with HNO3. The resulting H2S is swept into flask B with N2 and the potential of the ion-selective electrode is measured.  The author’s report that the electrode’s response is

E = –0.6535 – 0.0323log[S2-]

When a 34.88-g sample of sediment is first placed in flask A, the potential of the ion-selective electrode is –0.7193 volts. How many μmol of H2S are present in this sample of sediment as a gas?

(b) After acidifying the sample with HNO3, the potential of the ion-selective electrode changes to –0.7302 volts. How many μg of insoluble sulfide are present in this sample of sediment, assuming that it is present as FeS2?

(c) What is the total concentration of sulfide (that present as H2S and that present as insoluble sulfides) in the sediment sample expressed as μg H2S/g sediment?

(d) The purpose of the N2 is to extract the H2S (although H2S is a gas it has some solubility in water) from the solution in flask A.  It is unlikely that the extraction is 100% efficient. Would the failure to extract all H2S lead to a determinate error? Explain.

(e) The procedure indicates that 100 mL of H2O is placed in flask A, while 50.00 mL of buffer is placed in flask B. Note the difference in significant figures. Why does the volume in flask B need to be measured more precisely?

(f) Hydrogen sulfide is a diprotic weak acid with Ka1 = 9.5 x 10-8 and Ka2 = 1.3 x 10‑14. The solution in flask B is prepared by adding 20 g of NaOH to 1 liter of water. What is the pH of this solution and why was it chosen?

(g) The theoretical response of a sulfide ion-selective electrode is

E = K – 0.02958log[S2-]

while the author’s report the response as

E = –0.6533 – 0.0323log[S2-]

Does the disagreement between the theoretical and experimental slope lead to a determinate error? Explain.

Neocuproine Method: Analysis of Procedure

One method for determining copper is the “Neocuproine Method”. In this method, copper in a +1 oxidation state reacts with neocuproine (2,9-dimethyl-1,10-phenanthroline) to form a complex of Cu(neocuproine)2.  The complex is extracted into a chloroform-methanol mixture, giving a yellow solution with a molar absorptivity of 8000 M-1 cm-1 at 457 nm. Beer’s law is obeyed up to a concentration of 0.2 mg Cu/25 mL of extraction solvent. Full color development occurs when the sample’s pH is between 3 and 9.  A typical procedure is provided here:

A 100.0-mL sample is placed in a 250-mL beaker, acidified with 1 mL of H2SO4 and 5 mL of HNO3, and boiled to destroy any traces of cyanide, sulfide, or organic material that may be present. The remaining sample is transferred to a 100-mL volumetric flask and diluted to volume. A 50-mL portion of this sample is transferred to  a 250-mL separatory funnel and 5 mL of with hydroxylamine hydrochloride is added to reduce Cu2+ to Cu+. A 10-mL portion of a sodium citrate solution is added to complex any metal ions in the sample that might precipitate when the sample’s pH is adjusted.  A solution of 5 M NH3 is added in 1-mL increments until the pH is between 4 and 6. A 10-mL portion of neocuproine is added along with 10 mL of CHCl3. The contents of the separatory funnel are shaken and the layers allowed to separate.  The CHCl3 layer is drained into a 25-mL volumetric flask and diluted to volume with methanol. The absorbance of the CHCl3-CH3OH solution is measured at 457 nm in a 1.00-cm cell.

(a) Explain how you would prepare an appropriate blank for this analysis.

(b) The minimum absorbance that can be measured with confidence is 0.010. To what mass of Cu in the original sample does this correspond?

Standards and Samples

The spectrophotometric methods for analyzing Mn and for analyzing glucose use a chemical reaction to produce a useful signal. In the analysis for Mn in steel, colorless Mn2+ is oxidized to give the purple MnO4 ion, while in the analysis for glucose, colorless glucose is used to reduce the yellow Fe(CN)63- ion to the colorless Fe(CN)64- ion. In the analysis for manganese no attempt is made to precisely control either the time or temperature at which the reaction occurs, and samples and standards are treated separately.  The glucose analysis, however, requires that all samples and standards be treated simultaneously at exactly the same temperature and for the same length of time. Briefly explain why these experimental procedures are so different.

Standards for the Spectrophotometric Analysis of Acetone

Suppose you need to prepare a set of calibration standards for the spectrophotometric analysis of acetone at a wavelength of 211 nm using a 1.00 cm cell. The molar absorptivity for acetone at this wavelength is 562. To maintain an acceptable precision for the analysis you wish to keep the %T for your standards between 15% and 85%. What is the most concentrated and the least concentrated standard that you would want to prepare for this analysis?