Name: measuring with accuracy in the lab
Uploaded: 2020-03-26T07:22:24.000Z
Duration: 4 min 14 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Specialised Topic in Science

112.51.c.1.

112.51.c.1.D.

Lab Techniques

En la investigación científica, la reproducibilidad de un experimento es extremadamente importante. Por lo tanto, mantener un cuaderno de laboratorio con registros detallados de los procedimientos junto con las técnicas de laboratorio adecuadas ayuda a reproducir los hallazgos experimentales.
Por ejemplo, las mediciones simples, como el volumen de líquido, deben realizarse utilizando la cristalería adecuada para garantizar la precisión. Mida siempre el volumen con cristalería volumétrica, como un matraz aforado, un cilindro graduado o una pipeta volumétrica. El volumen se mide en la parte inferior del menisco.
Las marcas de volumen en los vasos de precipitados y los matraces Erlenmeyer no son precisas y solo sirven como guías. Al seleccionar la cristalería volumétrica, seleccione el recipiente más pequeño posible para el volumen necesario. La cristalería volumétrica se calibra para entregar o para contener.
Los recipientes calibrados para la entrega están diseñados para proporcionar el volumen indicado con el entendimiento de que una pequeña cantidad de líquido permanecerá en la cristalería después de vaciarla. En este caso, no es necesario eliminar el líquido restante o se vaciará más del volumen deseado. La cristalería calibrada para contener mantendrá y entregará el volumen indicado, pero requiere que el líquido restante se vierta para que se reciba el volumen completo.
Medimos para obtener un valor verdadero. Sin embargo, siempre habrá algún nivel de incertidumbre y error. El valor medido es nuestra mejor estimación del valor real, que a menudo es desconocido para nosotros. El error es la diferencia entre los valores medidos y los reales. La incertidumbre de medición describe el rango en el que creemos que es probable que se encuentre el valor real. Al registrar las mediciones, es importante mantener el número adecuado de cifras significativas.
Las cifras significativas son los dígitos de una medida que tienen significado. El último dígito registrado define el nivel de incertidumbre. Todos los números, excepto los ceros iniciales y finales, son significativos. Y los ceros finales son significativos cuando hay un punto decimal que los precede.
Por ejemplo, en una medición de longitud usando una regla, vemos que la longitud es de al menos una pulgada, pero ciertamente no de 2 pulgadas. Entonces, el primer dígito significativo es uno. La siguiente marca de verificación representa 0,1 pulgadas y también es significativa.
Una grabación de 1,1 pulgadas tiene dos dígitos significativos e implica que la incertidumbre se encuentra en las décimas. Sin embargo, el ancho real se encuentra entre dos marcas de graduación. Por lo tanto, la incertidumbre se encuentra aquí en el lugar de las centésimas, ya que la longitud se informa como 1,15 pulgadas.
Al realizar cálculos utilizando valores medidos, recuerde no realizar cálculos a una resolución superior a la medición original. Esos dígitos adicionales no son significativos y no deben incluirse. Por ejemplo, al calcular el área del cuadrado con una longitud de lado de 1,15, vemos que la longitud tiene tres cifras significativas. Por lo tanto, la respuesta también debe tener tres cifras significativas.
El área calculada de 1,3225 pulgadas cuadradas tiene cinco cifras e introduce certeza en el cálculo que no estaba en la medición original. Por lo tanto, el área correcta es de 1,32 pulgadas cuadradas.
En este laboratorio, practicará las habilidades de laboratorio adecuadas midiendo la densidad de un huevo y utilizando cifras significativas en sus cálculos y registros. Además, practicará el mantenimiento de registros en su cuaderno de laboratorio y examinará la precisión de las mediciones con cristalería volumétrica.

técnicas de laboratorio

<h4>Precisión de la medición</h4>
La capacidad de repetir un experimento y obtener los mismos resultados, o reproducibilidad, es esencial en la investigación científica. Sin embargo, es imposible repetir un experimento si no sabes cómo lo hiciste. Por lo tanto, los científicos mantienen registros detallados de sus experimentos en cuadernos de laboratorio. Estos registros incluyen información importante como la cantidad de cada material utilizado y descripciones de cada paso del procedimiento.
Los equipos, como las balanzas y la cristalería volumétrica o graduada, se utilizan para medir compuestos sólidos y líquidos para experimentos. La precisión de cada medición está limitada por el equipo utilizado. Las balanzas y otros equipos con lecturas digitales muestran automáticamente los valores con la máxima precisión que pueden soportar. La precisión de la cristalería graduada y las reglas depende del incremento, que es la cantidad representada por la distancia más pequeña entre dos marcas.
La superficie de la mayoría de los líquidos se curva hacia arriba en los bordes de los recipientes de vidrio. Este tipo de superficie líquida curvada se denomina menisco cóncavo. Cuando los bordes del líquido se curvan hacia abajo, se denomina menisco convexo. Al medir volúmenes de líquido, el punto de referencia para la parte superior del líquido es la parte inferior de un menisco cóncavo o la parte superior de un menisco convexo. Para identificar con precisión la posición del menisco, los volúmenes de líquido se leen mirando la superficie del líquido desde un lado a la altura de los ojos. Mirar el líquido desde arriba o desde abajo hará que el nivel de la superficie del líquido parezca más alto o más bajo de lo que realmente es.
Cuando el punto de referencia del menisco toca una marca en la cristalería graduada o volumétrica, la cristalería mantiene el volumen definido para esa marca. La cristalería volumétrica está diseñada para medir un volumen específico, por lo que solo tiene una marca. La cristalería graduada está diseñada para medir un rango de volúmenes, por lo que tiene muchas marcas o graduaciones, que se etiquetan a intervalos regulares. Tanto la cristalería graduada como la volumétrica pueden tener información adicional proporcionada por el fabricante sobre su precisión.
<h4>Calibración e incertidumbre</h4>
Los fabricantes calibran el equipo para garantizar que las mediciones sean precisas dentro de un rango específico de incertidumbre. Por ejemplo, una balanza analítica de alta precisión puede leer hasta cuatro decimales (0,0000 g) con una incertidumbre de ± 0,0001 g. Una lectura de 0,0345 g indica que el valor real está entre 0,0344 g y 0,0346 g.
Por lo general, la cristalería tiene impresa la incertidumbre de medición. Para registrar con precisión las mediciones de volumen, debe tener en cuenta tanto el incremento como la incertidumbre. Por ejemplo, una medición precisa del volumen en un cilindro graduado de 100 mL con incrementos de 1 mL y una incertidumbre de ± 0,5 mL se registraría hasta las décimas (000,0 mL ± 0,5 mL). Si el punto de referencia del menisco se encuentra entre dos marcas, debe estimar un valor para el décimo lugar; de lo contrario, déjelo como 0. Dado que la temperatura afecta al volumen, la temperatura para la que está calibrada la cristalería se imprimirá en ella.
La cristalería se calibra para contener (TC) o para entregar (TD). La cristalería TC contiene el volumen especificado de líquido cuando se llena hasta la marca, pero quedará una pequeña cantidad de líquido cuando se vierta en otro recipiente. Las soluciones a menudo se preparan en cristalería TC porque la precisión del volumen de la solución afecta la precisión de la concentración. Los matraces volumétricos suelen calibrarse para contener.
La cristalería TD contiene un poco más de la cantidad de líquido especificada cuando se llena hasta la marca, pero dispensa solo el volumen especificado. Por lo tanto, la cristalería TD no debe vaciarse por completo al dispensar líquidos. Este tipo de cristalería es útil para transferir un volumen preciso de líquido a otro recipiente. Tanto las pipetas graduadas como las volumétricas suelen calibrarse para funcionar.
A menos que se especifique lo contrario, se supone que la incertidumbre de un número está en la escala del último dígito. Por lo tanto, es particularmente importante informar la incertidumbre de una medición si la incertidumbre es a mayor escala. En el caso de las reglas y la cristalería graduada sin una incertidumbre conocida, la incertidumbre se estima como la mitad del incremento más pequeño. El dígito incierto se basa en dónde cae el menisco entre dos marcas de graduación.
<h4>Cifras significativas</h4>
Las cifras significativas son los números de un valor que son significativos o esenciales para expresar ese valor con la precisión adecuada. Todos los números que no sean ceros a la izquierda (0,001), ceros a la izquierda (1.000) o multiplicadores de notación científica (10x) son siempre significativos.
Los ceros a la izquierda nunca son significativos porque se pueden eliminar reescribiendo el número, ya sea en notación científica o, si se trata de una unidad SI, en unidades más pequeñas. Por ejemplo, solo hay tres cifras significativas en el valor 0,00123 m porque se puede reescribir como 1,23 × 10-3 m o 1,23 mm sin perder ninguna información. Tenga en cuenta que el factor de escala entre diferentes múltiplos de la misma unidad SI no afecta al número de cifras significativas.
Los ceros finales son significativos cuando están antes o después de un separador decimal. Por ejemplo, la medición 100,110 mL tiene seis cifras significativas y 100,0 mL tiene cuatro. Se supone que los ceros finales en un número escrito sin punto decimal son insignificantes a menos que estén marcados de otra manera. Existen varias convenciones diferentes para marcar ceros significativos en esos casos, pero para evitar ambigüedades es aconsejable reescribir esos valores en notación científica. Si la medición se realiza en unidades SI, utilice unidades más grandes.
La precisión de una medición limita la precisión de los valores calculados a partir de ella. Un valor calculado tiene el mismo número de cifras significativas que la medición o el valor menos preciso utilizado en su cálculo. Si el resultado tiene demasiadas cifras significativas, se redondea con la precisión adecuada. Si el resultado tiene muy pocas cifras significativas, se amplía con ceros finales significativos para preservar la precisión. Las cifras significativas de los valores derivados experimentalmente, como la masa molar, la densidad o algunos factores de conversión unitarios, también deben tenerse en cuenta cuando se utilizan en los cálculos.
El número de cifras significativas en un valor calculado no está limitado por constantes físicas o matemáticas en fórmulas o números exactos, como el número de puntos de datos adquiridos en un experimento. Por ejemplo, la fórmula para calcular el volumen de una esfera es V = 4/3πr3. El número de cifras significativas en el volumen calculado solo se ve afectado por el número de cifras significativas en la medición del radio r. 4/3 y π son constantes, y 3 es simplemente la notación de una operación matemática.
Los factores de conversión definidos también se tratan como constantes. Por ejemplo, la pulgada se define exactamente como 25,4 milímetros; Por lo tanto, el número de cifras significativas del valor convertido depende solo de la medición original. Compruebe siempre si el factor de conversión está definido exactamente o es un valor experimental al convertir entre unidades.
Para la suma y resta simple de mediciones, usamos la aritmética de significación para determinar el número de cifras significativas. En este caso, la respuesta tiene tantos decimales como la medición con el menor número de decimales, independientemente del número de cifras significativas. Así, 15.643,7 mL + 0,613 mL = 15.644,3 mL.
En el caso de cálculos de varios pasos o ecuaciones complejas que se dividen en partes más pequeñas para resolverlas, debe mantener al menos una o dos cifras insignificantes en los valores intermedios. Por ejemplo, si calcula los valores del numerador y el denominador de una fracción por separado antes de dividirlos, debe conservar las cifras insignificantes del numerador y el denominador al dividirlos. Esto minimiza el error de redondeo, que es la diferencia con el número que obtendría si ingresara toda la fórmula en su calculadora de una vez. Si anota estos valores intermedios, debe anotar qué dígitos son insignificantes.
<h4>Referencias</h4>
Harris, D.C. (2015). Análisis químico cuantitativo. Nueva York, NY: W.H. Freeman and Company.

Técnicas de laboratorio

Precisión de la medición
La capacidad de repetir un experimento y obtener los mismos resultados, o reproducibilidad, es esencial en la investigación científica. Sin embargo...

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Educación

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measuring with accuracy in the lab

Measuring with accuracy in the lab starts with careful records, the right tools, and correct reading of data. Scientists use lab notebooks to record how an experiment was done. These notes include the amounts of materials used and each step of the procedure. Without that record, an experiment cannot be repeated in the same way.

Balances and glassware are used to measure solids and liquids for experiments. The precision of a measurement depends on the equipment used. Digital balances show the maximum precision they can support. For rulers and graduated glassware, precision depends on the increment, which is the smallest distance between two marks.

Liquid levels must also be read correctly. Most liquids curve upward at the edges in glass containers. This curved surface is called a concave meniscus. When the edges curve downward, it is called a convex meniscus. To measure volume, the reference point is the bottom of a concave meniscus or the top of a convex meniscus. The reading should be taken from the side at eye level. Looking from above or below can make the level seem higher or lower than it really is.

Graduated and volumetric glassware are designed for different jobs. Volumetric glassware measures one specific volume and has only one mark. Graduated glassware measures a range of volumes and has many marks, or graduations, at regular intervals. When the meniscus reference point touches the mark, the glassware is holding the volume defined by that mark. Both types may also include manufacturer information about precision.

Manufacturers calibrate equipment so measurements stay within a stated range of uncertainty. A high-precision analytical balance may read to four decimal places, such as 0.0000 g, with an uncertainty of ±0.0001 g. That means a reading of 0.0345 g could represent a true value between 0.0344 g and 0.0346 g. Glassware usually has its uncertainty printed on it. For a 100 mL graduated cylinder with 1 mL increments and an uncertainty of ±0.5 mL, the volume should be recorded to the tenths place.

Temperature also affects volume, so calibrated glassware lists the temperature it is made for. Glassware can be calibrated to contain, or TC, or to deliver, or TD. TC glassware holds the stated volume when filled to the mark, but some liquid remains after pouring. This is why volumetric flasks are usually TC and are often used to prepare solutions, since solution concentration depends on volume accuracy. TD glassware dispenses the stated volume and should not be emptied completely. Graduated and volumetric pipettes are typically TD and are useful for transferring an exact volume to another container.

Recording numbers correctly also requires understanding uncertainty and significant figures. Unless stated otherwise, uncertainty is assumed to be on the scale of the last digit. If a ruler or glassware has no known uncertainty, estimate it as half of the smallest increment. Significant figures are the digits that are meaningful for the precision of a value. Leading zeroes are not significant, while trailing zeroes are significant when they appear before or after a decimal point. A number written without a decimal point may need scientific notation to avoid ambiguity.

Calculated values must match the precision of the measurements used to find them. The result should have the same number of significant figures as the least precise value in the calculation. If the answer has too many significant figures, round it. If it has too few, add significant trailing zeroes to show the correct precision. Constants such as 4/3 and π in the sphere volume formula do not limit the number of significant figures, and exact conversion factors like 1 inch = 25.4 millimeters also do not. For addition and subtraction, the answer keeps the same number of decimal places as the least precise value. For multi-step problems, keep one or two extra digits during intermediate steps to reduce rounding error.

Measuring with Accuracy in the Lab

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Measuring with accuracy in the lab starts with careful records, the right tools, and correct reading of data. Scientists use lab notebooks to record how an experiment was done. These notes include the amounts of materials used and each step of the procedure. Without that record, an experiment cannot be repeated in the same way. Balances and glassware are used to measure solids and liquids for experiments. The precision of a measurement depends on the equipment used. Digital balances show the...

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identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

Videos completos y enfocados en el plan de estudios, tanto para los instructores como para los estudiantes de los cursos de laboratorio de introducción a la química.

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Learn how students measure liquids and solids, read meniscus levels, use TC and TD glassware, and apply uncertainty and significant figures.

Measuring with Accuracy in the Lab | JoVE K12

CHEM.1

CHEM.1.D

Biology

BIO.1

BIO.1.E

Prepárese para enseñar Técnicas de laboratorio con esta guía para instructores de 1 minutos. Cubre los pasos clave de preparación para el laboratorio Chemistry en JoVE Lab Manual.

Preparación del instructor para Técnicas de laboratorio en Chemistry - Más de 9.8K visualizaciones | JoVE Lab Manual

Siga los procedimientos paso a paso de Técnicas de laboratorio en este video de protocolo estudiantil de 9 minutos. Parte de la colección Chemistry en JoVE Lab Manual.

Protocolo estudiantil de Técnicas de laboratorio en Chemistry - Más de 30K visualizaciones | JoVE Lab Manual

The Scientific Method

The scientific method is a detailed, empirical problem-solving process used by biologists and other scientists. This iterative approach involves ...

Inductive Reasoning

Inductive reasoning is a form of logical thinking that uses related observations to arrive at a general conclusion. It is uncertain and operates in ...

Deductive Reasoning

Deductive reasoning, or deduction, is the type of logic used in hypothesis-based science. In deductive reasoning, the pattern of thinking moves in the ...

Correlation and Causation

Correlation and CausationStatistical tests can calculate whether there is a relationship, or correlation, between independent and dependent variables. A ...

Ask questions and define problems related to specialized topics of study based on observations or information from text, phenomena, models, or investigations;

112.51.c.1.A.

Scientific Method

The scientific method is used to solve problems and explain phenomena. The development of the scientific method coincided with changes in philosophy ...

Demonstrating the Scientific Method with Tree Branches
ExpandObtain branches containing a few dozen typical leaves from five different local tree species. ...

Observation and Background Research
ExpandNOTE: The first two steps in the scientific process involve studying and observing natural phenomena of ...

Problem-Solving

A problem-solving strategy is a plan of action used to find a solution. It involves trial and error, algorithms, heuristics, and insights. Effective ...

Problem Definition

Problem definition is the initial and the most essential step in research.
The researcher identifies and explains the specific issue or gap to be ...

Apply science practices related to specialized topics of study to plan and conduct investigations or use engineering practices to design solutions to problems;

112.51.c.1.B.

Proper Personal Protective Equipment

Robert M. Rioux &#38; William A Elliott, Pennsylvania State University, University Park, PA
Hazards are many and varied in the laboratory, but the right ...

Emergency Eyewash and Shower Stations

Robert M. Rioux, Pennsylvania State University, University Park, PA
The Occupational Safety and Health Administration (OSHA) mandates that an emergency ...

Electrical Safety

Robert M. Rioux &amp; Suprita Jharimune, Pennsylvania State University, University Park, PA
Among the many hazards present in the laboratory, electrical ...

Working with Centrifuges

Source: Robert M. Rioux &amp; Suprita Jharimune, Pennsylvania State University, University Park, PA
Earth's gravitational force is capable of separating ...

Working with Hot and Cold Sources

Source: Robert M. Rioux &amp; Suprita Jharimune, Pennsylvania State University, University Park, PA
Working with extreme temperatures, both high and low, ...

guidelines-in-case-of-a-laboratory-emergency

Guidelines in Case of a Laboratory Emergency

Robert M. Rioux &amp; Zhifeng Chen, Pennsylvania State University, University Park, PA
The most common laboratory emergencies include chemical spills, ...

chemical-storage-categories-hazards-and-compatibilities

Chemical Storage: Categories, Hazards And Compatibilities

Source: Robert M. Rioux &amp; Taslima A. Zaman, Pennsylvania State University, University Park, PA
While the use of various chemicals in experimental ...

Safe Handling of Mineral Acids

Source: Robert M. Rioux &#38; Taslima A. Zaman, Pennsylvania State University, University Park, PA
A mineral acid (or inorganic acid) is defined as a ...

Handling Chemical Spills

Source: Robert M. Rioux &#38; Taslima Zaman, Pennsylvania State University, University Park, PA
Since chemicals are commonly used for laboratory research ...

Proper Use of Autoclaves

Robert M. Rioux &amp; Zhifeng Chen, Pennsylvania State University, University Park, PA
Autoclaving is one of the most commonly used methods in the ...

Fume Hoods and Laminar Flow Cabinets

Robert M. Rioux &amp; William A. Elliott, Pennsylvania State University, University Park, PA
Fume hoods and laminar flow cabinets are engineering controls ...

Decontamination for Laboratory Biosafety

Robert M. Rioux and Zhifeng Chen, Pennsylvania State University, University Park, PA
Decontamination is essential for laboratory biosafety, as the ...

Proper Waste Disposal

Robert M. Rioux and Taslima A. Zaman, Pennsylvania State University, Pennsylvania, PA

Users are responsible for the proper disposal of the waste ...

Use appropriate safety equipment and practices during laboratory, classroom, and field investigations as outlined in Texas Education Agency-approved safety standards;

112.51.c.1.C.

Common Lab Glassware and Uses

Source: Laboratory of Dr. Neal Abrams &#8212; SUNY College of Environmental Science and Forestry
Glassware is a regular appearance in the professional ...

Measurement Accuracy
The ability to repeat an experiment and get the same results, or reproducibility, is essential in scientific research. However, ...

Source: Smaa Koraym at Johns Hopkins University, MD, USA

	Preparation of LaboratoryExpand
	Here we show the laboratory preparation for 10 students ...

Source: Smaa Koraym at Johns Hopkins University, MD, USA

	Determining the Density of an EggExpand
	Density is a measure of how compact a substance or ...

Proper Lab Notebook Keeping

Organic Lab Notebook Keeping
Record keeping is a vital skill in science and organic chemistry labs. Without proper notes, descriptions, quantities, or ...

Source: Lara Al Hariri at the University of Massachusetts Amherst, MA, USA

	Preparation of the LaboratoryExpand
	Here, we show the laboratory preparation ...

Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA

	Nitration of Methyl BenzoateExpand
	From this lab onward, ...

Use tools appropriate to the specialized topic of study;

SI Units: 2019 Redefinition

Measurement is integral to analytical chemistry. Each record comprises a number &#8211; denoting the magnitude&#160; &#8211; and the unit &#8211; a ...

International System of Units

The International System of Units, known as the SI system, is a universally accepted measurement system recognized and used worldwide. The SI system is ...

Measurement: Standard Units

Every measurement provides three kinds of information: the size or magnitude of the measurement (a number), a standard of comparison for the measurement ...

Measurement: Derived Units

The International System of Units or SI system, by international agreement, has fixed measurement units for seven fundamental properties: length, mass, ...

Collect quantitative data using the International System of Units (SI) or qualitative data as evidence as appropriate to the specialized topic of study;

112.51.c.1.E.

Scientific and engineering practices. The student, for at least 40% of instructional time, asks questions, identifies problems, and plans and safely conducts classroom, laboratory, and field investigations to explain phenomena or design solutions using appropriate tools and models. The student is expected to:

Measuring with Accuracy in the Lab

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