Analytical Chemistry

Analytical Chemistry CHEM 2310 Prof. Monzir S. Abdel-Latif 1

خطة مساق كيمياء تحليلية مدرس المساق: أ.د. منذر سليم عبد اللطيف عنوان المدرس: مبنى المختبرات الجديد ، غرفة 630 الساعات المكتبية: سبت اثنين أربعاء 12-10

وصف المساق (حسب الدليل): المفاهيم الأساسية في الكيمياء التحليلية، والمفاهيم العامة للاتزان الكيميائي والذي يشمل الأحماض والقواعد، أسس التحليل الوزني والحجمي ومعايرات الأحما ض والقواعد ومعايرات الترسيب، ومعايرات المعقدات، ومعايرات الأكسدة والاختزال، وطرق التحليل المعتمدة على الخواص الكهربائية .

الأهداف العامة للمساق )أو مخرجات المساق:( • امتلاك المهارات الأساسية النظرية في التحليل الكيميائي التقليدي ، بما في ذلك تقييم • ومعالجة البيانات بطريقة حسابية وإحصائية مناسبة ، التعرف على طرق التحليل الوزني والحجمي والأسس النظرية التي يقوم عليها كل منهما ، دراسة الأنواع المختلفة من الكواشف ومجال عمل كل نوع منها ، تحضير المحاليل المختلفة بما فيها المحاليل المنظمة والحسابات الكيميائية لكل ما سبق.

أسلوب تدريس المساق: المحاضرة والنقاش في بعض الأحيان مراجع المساق: أ. المرجع الرئيس : AnalyticalChemistry, 2004, Gary D. Christian, 6th Ed. ب. المراجع الإضافية : 1. Analytical Chemistry, Principles and Techniques, Larry Hargis, Prentice Hall. 2. Arts and Science of Chemical Analysis, Christie Enke, Wiley and Sons. 3. Principles of Analytical Chemistry, Miguel Valcarcel, Springer. 4. Fundamentals of Analytical Chemistry, Skoog, Holler, and West, Brooks/Cole ج. المعلومات الواردة على صفحة المساق الالكترونية

التقييم • 3 امتحانات ساعية: 50% • الامتحان النهائي (شامل): 50% • ____________________________________________ • * ينص البند (4) من مادة (8) في النظام الأكاديمي على أن : • تشمل الأعمال الفصلية لكل مساق الامتحانات التحريرية و الشفهية و التقارير و البحوث و الأعمال المخبرية والتطبيقية على أن يخصص له 50 % كحد أقصى من العلامة النهائية للمساق وفق ما يحدده مجلس الكلية . • ** ينص البند (6) من مادة (8) في النظام الأكاديمي على أن" يكون الامتحان النهائي شاملاً للمنهج كله ".

Areas of Chemical Analysis and Questions They Answer Quantitation: How much of substance X is in the sample? Detection: Does the sample contain substance X? Identification: What is the identity of the substance in the sample? Separation: How can the species of interest be separated from the sample matrix for better quantitation and identification? 8

Chapter 1: Introduction This course is a quantitative course where you have seen some qualitative analysis in general Chemistry Lab and will also encounter the topic in other classes. In addition, Analytical Chemistry can be classified as Instrumental or Classical (wet Chemistry). 9

In this course, we will cover the classical methods of Chemical Analysis. However, it should not be implied that the term classical means something old, which is studied like history, but rather the term means understanding the basics of Chemical Analysis that were eventually laid down long time ago. Some of the classical methods still serve as the standard methods of analysis, till now. 10

The analytical process involves a sequence of logical events including: 1. Defining the problem This means that the analyst should know what information is required, the type and amount of sample, the sensitivity and accuracy of the results, the analytical method which can be used to achieve these results, etc... 11

2. Obtaining a representative sample It is very important to collect a representative sample for analysis. This could be appreciated if, for example, an ore is to be analyzed to decide whether the ore concentration in a mine or mountain can be economically produced. One should take several samples from different locations and depths, mix them well and then take a sample for analysis. 12

3. Preparing the sample for analysis Most analytical methods require a solution of the sample rather than the solid. Therefore, samples should be dissolved quantitatively and may be diluted to the concentration range of the method. 4. Chemical separations The sample may contain solutes which interfere with the determination of the analyte. If this is the case, analytes should be separated from the sample matrix by an accepted procedure. 13

5. Performing the measurement This implies conducting the analytical procedure and collecting the required data. 6. Calculations The final event in the analytical process is to perform the calculations and present the results in an acceptable manner. 14

RANGE The size of the sample can be used to describe the class of a method where: A method can be described as meso if the sample size is above 100 mg or 100 microliters. A semimicro method describes a sample size from 10 to 100 mg or 50 to 100 microliters. When the sample size is in the 1 to 10 mg or less than 50 microliters, the method is said to be a micro method A sample size less than 1 mg denotes an ultramicro method. 15

An analyte in a sample can be classified as a major constituent if it constitutes more than 1% of the sample or a minor in the range from 0.1 to 1.0 %. It is classified as trace if it constitutes less than 0.1%. Analyze versus Determine These terms are sometimes misused. Always use the term ‘analyze’ with the sample while use the term ‘determine’ with specific analyte. Therefore, a sample is “analyzed”, while an analyte is “determined”. 16

Chapter 2 Data Handling 17

Accuracy and Precision Accuracy can be defined as the degree of agreement between a measured value and the true or accepted value. As the two values become closer, the measured value is said to be more accurate. Precision is defined as the degree of agreement between replicate measurements of the same quantity. 18

Assuming the correct or accepted value is represented by the center of the circles below; If all values occurred close together within, for example, the red or blue circles, results are precise but not accurate. If all values occurred within the yellow circle, results are both accurate and precise. If results were scattered randomly in one direction of the center, results are neither precise nor accurate. 19

Example The weight of a person was measured five times using a scale. The reported weights were 84, 83, 84, 85, and 84 kg. If the weight of the person is 76 kg weighed on a standard scale , then we know that the results obtained using the first scale is definitely not accurate. However, the values of the weights for the five replicate measurements are very close and reproducible. Therefore measurements are precise. Therefore, the measurement is precise but definitely not accurate. 21

Significant Figures At the most basic level, Analytical Chemistry relies upon experimentation; experimentation in turn requires numerical measurements. And measurements are always taken from instruments made by other workers. 22

Some information about measurements • Examples we will study include the metric ruler, the graduated cylinder, and the scale. 2) Because of the involvement of human beings, NO measurement is exact; some error is always involved. This means that every answer in science has some uncertainty associated with it. We might be fairly confident we have the correct answer, but we can never be 100% certain we have the EXACT correct answer. 23

3) Measurements always have two parts - a numerical part (sometimes called a factor) and a dimension (a unit). Significant figures are concerned with measurements not exact countings. 24

Identifying significant digits The following rules are helpful in identifying significant digits 1. Digits other than zero are significant. e.g., 42.1m has 3 sig figs. 2. Zeroes are sometimes significant, and sometimes they are not. 3. Zeroes at the beginning of a number (used just to position the decimal point) are not significant. e.g., 0.025m has 2 sig figs. In scientific notation, this can be written as 2.5*10-2m 25

4. Zeroes between nonzero digits are significant. e.g., 40.1m has 3 sig figs 5. Zeroes at the end of a number that contains a decimal point are significant. e.g., 41.0m has 3 sig figs, while 441.20m has 5. In scientific notation, these can be written respectively as 4.10*101 and 4.4120*102 6. Zeroes at the end of a number that does not contain a decimal point may or may not be significant. If we wish to indicate the number of significant figures in such numbers, it is common to use the scientific notation. 26

e.g., The quantity 52800 km could be having 3, 4, or 5 sig figs—the information is insufficient for decision. If both of the zeroes are used just to position the decimal point (i.e., the number was measured with estimation ±100), the number is 5.28×104 km (3 sig figs) in scientific notation. If only one of the zeroes is used to position the decimal point (i.e., the number was measured ±10), the number is 5.280×104 km (4 sig figs). If the number is 52800±1 km , it implies 5.2800×104 km (5 sig figs). 27

Exact Numbers Exact numbers can be considered as having an unlimited number of significant figures. This applies to defined quantities too. e.g., • The rules of significant figures do not apply to (a) the count of 47 people in a hall, or (b) the equivalence: 1 inch = 2.54 centimeters. • In addition, the power of 10 used in scientific notation is an exact number, i.e. the number 103 is exact, but the number1000 has 1 sig fig. It actually makes a lot of sense to write numbers derived from measurements in scientific notation, since the notation clearly indicates the number of significant digits in the number. 28

Look at the following example 29

The length is greater than 2.6 cm but less than 2.7 cm, and so the estimated value is 2.62 cm. The measurement can be written as 2.62±0.01 cm or 26.2±0.1 mm. The number 26.2mm contains three significant figures. 30

How do you read it? How do you read it? 31

Lecture 3 Data Handling Significant Figures, continued…. Math using significant figures 36

Math With Significant Figures Addition and Subtraction In mathematical operations involving significant figures, the answer is reported in such a way that it reflects the reliability of the least precise operation. Let's state that another way: a chain is no stronger than its weakest link. An answer is no more precise that the least precise number used to get the answer. Let's do it one more time: imagine a team race where you and your team must finish together. Who dictates the speed of the team? Of course, the slowest member of the team. Your answer cannot be MORE precise than the least precise measurement. 37

For addition and subtraction, look at the decimal portion (i.e., to the right of the decimal point) of the numbers ONLY. Here is what to do: 1) Count the number of significant figures in the decimal portion of each number in the problem. (The digits to the left of the decimal place are not used to determine the number of decimal places in the final answer.) 2) Add or subtract in the normal fashion. 3) Round the answer to the LEAST number of places in the decimal portion of any number in the problem. 38

Find the formula weight for Ag2MoO4 given the following atomic weights: Ag = 107.870, Mo = 95.94, O = 15.9994. The number with the least number of digits after the decimal point is 95.94 which has two digits for expression of precision. Also, it is the number with the highest uncertainty. The atomic weights for Ag and O have 3 and 4 digits after the decimal point. Therefore if we calculate the formula weight we will get 375.6776. However, the answer should be reported as 375.68 ( i.e. to the same uncertainty of the least precise value. 39

Multiplication and Division The number having the least number of significant figures is called the KEY NUMBER. The LEAST number of significant figures in any number of the problem determines the number of significant figures in the answer. In case where two or more numbers have the same number of significant figures, the key number is determined as the number of the lowest value regardless of decimal point. 40

Note that: When the uncertainty of a number is not known, the uncertainty is assumed to be +1 of the last digit to the right

2.5 x 3.42 = ? The answer to this problem would be 8.6 (which was rounded from the calculator reading of 8.55). Why? 2.5 is the key number which has two significant figures while 3.42 has three. Two significant figures is less precise than three, so the answer has two significant figures. 42

Relative uncertainty in key number = ( +1/25) = +0.04 Now find the absolute uncertainty in answer: (8.55/25) = 0.342 Therefore, the uncertainty in the answer should be known to one decimal point. The answer can be written as 8.6 + 0.3. The relative uncertainty in answer can now be calculated: Srel = (+0.342/8.6) = +0.04 The relative uncertainty in answer is the same as that of the key number, which should be the case.

2.33 x 6.085 x 2.1= ? How many significant figures should be in the answer? Answer - two. Which is the key number? Answer - the 2.1 Why? It has the least number of significant figures in the problem. It is, therefore, the least precise measurement. Answer = 30. 45

Relative uncertainty in key number = ( +1/21) = +0.048 Now find the absolute uncertainty in answer: (29.77/21) = 1.4 Therefore, the absolute uncertainty in the answer should be known to integers. The answer can be written as 30. + 1. The relative uncertainty in answer can now be calculated: Srel = (+1/30.) = +0.033 The relative uncertainty in answer is close to that of the key number, which should be the case.

How many significant figures will the answer to 3.10 x 4.520 = (Calculator gives 14.012) have? 3.10 is the key number which has three significant figures. Three is supposed to be the correct answer. 14.0 has three significant figures. Note that the zero in the tenth's place is considered significant. All trailing zeros in the decimal portion are considered significant. 47

Another common error is for the student to think that 14 and 14.0 are the same thing. THEY ARE NOT. 14.0 is ten times more precise than 14. The two numbers have the same value, but they convey different meanings about how trustworthy they are. However, the correct answer should be reported as 14.01. Note that an additional significant figure is included in the answer. This is because the answer is less than the key number. 48

Relative uncertainty in key number = ( +1/310) = +3.2*10-3 Now find the absolute uncertainty in answer: (14.012/310) = 0.0452 Therefore, the absolute uncertainty in the answer should be known to one hundredth. The answer can be written as 14.01+ 0.05. The relative uncertainty in answer can now be calculated: Srel = (+0.05/14.01) = +3.5*10-3 The relative uncertainty in answer is very close to that of the key number, which should be the case.

Why do we add an additional significant figure in the answer when the answer is less than the key number? The answer to this question simply is to reduce the uncertainty associated with the answer. When the answer is less than the key number, the uncertainty associated with the answer is unjustifiably large. 50

Analytical Chemistry