Name: reference and indicator electrodes in potentiometry
Uploaded: 2024-12-05T18:31:56.000Z
Duration: 1 min 19 s
Channel: JoVE

אלקטרודות ייחוס מספקות פוטנציאל בסיס קבוע למדידות פוטנציומטריות, בעוד שאלקטרודות אינדיקטור מגיבות לשינויים בהרכב התמיסה.
אלקטרודת המימן הסטנדרטית, או SHE, היא בעלת פוטנציאל אפס בתנאי מצב סטנדרטיים, אך דורשת זרימת גז מימן רציפה, מה שהופך אותה ללא מתאימה ליישומי מעבדה או תעשייה שגרתיים.
חלופה אחת היא אלקטרודת קלומל רוויה, או SCE, שהיא רגישה לטמפרטורה ועלולה לגרום לזיהום כספית.
אפשרות נוספת היא אלקטרודת הייחוס כסף-כסף כלוריד, המטפלת בטמפרטורות גבוהות יותר אך עלולה לחוות חיבור צומת כתוצאה מתגובה עם תמיסת האנליט.
לעומת זאת, אלקטרודות אינדיקטור מתכתיות רבות מתוכננות להגיב ישירות עם הקטיונים שלהן, עם אניונים שבעזרתם הן יוצרות מלחים מסיסים במשורה, או עם קטיונים שונים באמצעות תרכובת מתווכת.
אלקטרודות מתכתיות אינרטיות פועלות כתורמי אלקטרונים או כמקבלים בתגובות חמצון-חיזור.
בינתיים, אלקטרודות ממברנות משתמשות בקרומים סלקטיביים של יונים כדי לקבוע ריכוזי יונים ספציפיים בתמיסות.

פוטנציאומטריה: סוגי אלקטרודות

Potentiometry: Types of Electrodes

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אלקטרודות ייחוס משמשות כנקודת ייחוס יציבה למדידות פוטנציאליות, בעוד אלקטרודות אינדיקטור ואלקטרודות עבודה מגיבות לשינויים בהרכב התמיסה.

אלקטרודת מימן סטנדרטית (SHE) היא אלקטרודת ייחוס נפוצה המשמרת פוטנציאל אפס בכל הטמפרטורות. עם זאת, הצורך באספקת גז מימן רציפה הופך אותה לפחות מעשית לשימוש יומיומי.

אלטרנטיבה ל-SHE היא אלקטרודת קלומל רוויה (SCE). אלקטרודה זו כוללת גוף זכוכית בצורת H המכיל אלקטרודה מפלטינה הטבולה בכספית רוויה בכספית(I) כלוריד ובמלחי אשלגן כלוריד. למרות השימוש הנרחב, SCE רגישה לשינויים בטמפרטורה ומעלה חששות סביבתיים בשל תכולת הכספית שבה.

אלקטרודות כסף-כסף כלוריד, המבוססות על חיזור AgCl ל-Ag, מהוות אפשרות נוספת. אלקטרודות אלו כוללות אלקטרודת כסף הטבולה בתמיסת KCl רוויה בכסף כלוריד. למרות שהן עמידות יותר לטמפרטורה מ-SCE, הן עשויות לגרום למדידות לא מדויקות אם יוני הכסף מגיבים עם מרכיבי הדגימה, דבר שעלול להוביל לסתימת הצומת.

כדי להבטיח מדידות פוטנציאליות מדויקות עם אלקטרודות ייחוס, יש לשמור על רמת הנוזל הפנימי מעל רמת תמיסת הדגימה, כדי למנוע זיהום וסתימות בצומת. תחזוקה וניקוי סדירים חיוניים גם הם לתוצאות אמינות.

אלקטרודות אינדיקטוריות מתכתיות ניתנות לסיווג לשלוש קטגוריות: אלקטרודות סוג I מגיבות ישירות לתמיסות המכילות את היונים שלהן; אלקטרודות סוג II יוצרות מלחים מסיסים חלקית עם היונים הנמדדים; ואלקטרודות סוג III מגיבות עם קטיונים שונים. אלקטרודות מתכתיות אינרטיות מתפקדות כמקורות או כקולטות של אלקטרונים בתגובות חמצון-חיזור.

בנוסף, אלקטרודות ממברנה, כולל אלקטרודות סלקטיביות ליונים ואלקטרודות זכוכית רגישות ל-pH, מיועדות למדידת ריכוזים ספציפיים של יונים בתמיסות.
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אלקטרודות ייחוס משמשות כנקודת ייחוס יציבה למדידות פוטנציומטריות, בעוד אלקטרודות מחוון ואלקטרודות עבודה מגיבות לשינויים ב ...

Education

JoVE Core

Analytical Chemistry

כימיה אנליטית

Unit 1

יחידה 1

Electrochemical Analyses and Redox Titration

אנליזות אלקטרוכימיות וטיטרציית חמצון-חיזור

electrochemical-analyses-and-redox-titration

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

reference and indicator electrodes in potentiometry

Potentiometric measurements use reference electrodes and indicator electrodes to track changes in a solution. A reference electrode gives a stable baseline, while indicator and working electrodes respond to changes in the solution’s composition.

The Standard Hydrogen Electrode, or SHE, is a common reference electrode. It keeps a zero potential at all temperatures. However, it needs a continuous supply of hydrogen gas, so it is not practical for routine use.

The Saturated Calomel Electrode, or SCE, is another reference option. It has an H-shaped glass body with a platinum electrode in mercury that is saturated with mercury(I) chloride and potassium chloride salts. SCE is widely used, but it is sensitive to temperature changes and raises environmental concerns because it contains mercury.

Silver-silver chloride electrodes are also used as reference electrodes. They rely on the reduction of AgCl to Ag and contain a silver electrode in KCl solution saturated with silver chloride. They resist temperature changes better than SCE, but silver ions can react with parts of the sample and cause junction plugging, which can lead to inaccurate measurements.

Good handling is important for accurate results with reference electrodes. The internal liquid level should stay above the sample solution level to prevent contamination and junction plugging. Regular cleaning and maintenance also help keep measurements reliable.

Metallic indicator electrodes fall into three groups. Class I electrodes respond directly to solutions that contain their ions. Class II electrodes form slightly soluble salts with the ions being measured, and Class III electrodes react with different cations.

Inert metallic electrodes act as sources or sinks of electrons in redox reactions. Membrane electrodes are another major type. These include ion-selective electrodes and pH-sensitive glass electrodes, which are designed to measure specific ion concentrations in solution.

Reference and Indicator Electrodes in Potentiometry

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electrochemistry: overview

Electrochemistry and Redox Reactions

Electrochemistry connects electrical measurements with chemical reactions, especially oxidation and reduction. It focuses on how electron movement helps explain reaction behavior. Oxidation is the loss of electrons from a substance, and reduction is the gain of electrons. A species with strong electron affinity is an oxidizing agent, or oxidant. A species that donates electrons is a reducing agent, or reductant. These redox steps are often described with half-reactions. When electrons move...

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Potentiometry measures the potential difference between two electrodes in an electrochemical cell. It does this without drawing significant current, so the solution’s composition is not altered. The method uses an indicator electrode, which exchanges electrons with the analyte solution, and a reference electrode, which keeps a constant potential. Each electrode is placed in a solution that makes up a half-cell. In a common setup, the reference electrode is the anode, or left-hand electrode,...

potentiometry: types of electrodes

Potentiometric measurements use reference electrodes and indicator electrodes to track changes in a solution. A reference electrode gives a stable baseline, while indicator and working electrodes respond to changes in the solution’s composition. The Standard Hydrogen Electrode, or SHE, is a common reference electrode. It keeps a zero potential at all temperatures. However, it needs a continuous supply of hydrogen gas, so it is not practical for routine use. The Saturated Calomel Electrode, or...

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Membrane Electrodes in Ion Sensing

Membrane electrodes measure ions by using a selective membrane that creates a potential difference across it. This membrane potential is called the asymmetry potential, and it can remain nonzero even when analyte concentrations are the same on both sides of the membrane. The signal usually depends on all ions in the sample that can interact with the membrane, not just one analyte. Selectivity is described by a selectivity coefficient. This value compares the response to an interferent with the...

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Redox titration uses electron transfer in a redox, or reduction-oxidation, reaction to find the concentration of an unknown solution. The titrant has a known concentration, and it is added gradually to the analyte, which is the solution being tested. The endpoint shows when the reaction between the two substances is complete. Before a redox titration begins, the analyte should be in a single oxidation state. If needed, this is done by pre-oxidizing or pre-reducing the sample with an auxiliary...

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Choosing Oxidizing Titrants in Redox Titration

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Potentiometric titration is a quantitative analytical method used to find the concentration of an analyte by measuring the potential difference between two electrodes in a solution. The endpoint is the point where this potential changes sharply. It usually appears when the stoichiometric reaction between the analyte and the titrant is complete. To find the endpoint, the measured potential difference is plotted against the volume of titrant added. This graph is called a titration curve. A...

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Controlled-Current Coulometry in Redox Analysis

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key terms and concepts

Learn how reference, indicator, and membrane electrodes work in potentiometry, including SHE, calomel, silver-silver chloride, and pH glass electrodes.

Reference and Indicator Electrodes in Potentiometry | JoVE K12

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Reference and Indicator Electrodes in Potentiometry

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