团队Ethanoic Acid

The characteristic smell of vinegar comes from the inclusion of ethanoic acid.

Introduction

The characteristic smell of vinegar comes from the inclusion of ethanoic acid (CH3COOH), also known as acetic acid.  However, unless pure distilled vinegar is purchased, with a guaranteed and stated composition, there are other acids present in vinegar.  These include:

• citric acid - HOOCCh1C(OH)(COOH)Ch1COOH),
• malic acid (2-hydroxybutanedioic acid)  - HOOCCH(OH)Ch1COOH
• lactic acid (2-hydroxypropanoic acid) - CH3CH(OH)COOH
• tartaric acid – (CHOH)2(COOH)2

For the purposes of this investigation the most volatile acid – ethanoic acid, is taken to be a measure of the acidity.  An attempt will be made to purify the vinegar so that it is mainly the ethanoic acid that is contributing the acidic content, but this cannot be guaranteed.  
Vinegars chosen will be malt vinegar (A), red wine vinegar (B) and cider vinegar (C).

Stage 1 – removing the colour from the vinegar

Materials required

100cm3 of Vinegar samples A to C.
3 volumetric flasks, labelled A to C (250 cm3)
3 conical flasks, labelled A to C (250 cm3)
Activated charcoal
Metal spatula
Glass stirring rod
Filtration funnel
Filter paper

Method

Place vinegar samples A in volumetric flask A and add 1 spatula measure of activated charcoal.  Stir well using clean glass rod.  Filter resulting mixture through filter paper and funnel into conical flask labelled A. 
Repeat using vinegar sample B with flasks labelled B; Likewise sample C.
Note that the filtration process may need to be repeated if colour remains in the vinegar.  If charcoal is not fully removed from mixture then suction filtration may be required.

Stage 2 – distilling the vinegar

Due to the presence of other acids and additive to the vinegar the mixture needs to be distilled to purify the mixture to ethanoic acid and water.

Materials required

Distillation column and separating funnel
Pear shaped flask
Conical Flasks A to C from stage 1
Anti bumping granules
Bunsen burner
Heatproof mat
Clamp and stand
Goggles
Ice bath
Universal indicator paper
Pippette

Method

Add filtered vinegar sample A to a pear shaped flask and set up distillation column above Bunsen, using clamp and stand.  Ensure all gaps are well sealed. 
Wash conical flask A with distilled water and use for collection of distillate
Arrange column so that distillate drips into conical flask A, placed in ice bath.
Gently heat mixture in pear shaped flask until distillate begins to collect in conical flask.  Test to see when all volatile acid has been distilled by placing drops of distillate onto universal indicator and checking pH colour. When pH registers over 7 cease heating.
Repeat using vinegar samples B and C using clean apparatus.

Stage 3 – determining the concentration of ethanoic acid

Option A - chromatography

Materials

Filter paper with pencil line drawn 2cm from bottom edge and pencil spots marked and labelled at 2 cm intervals along it
Samples of ethanoic acid of known concentration eg 0.01 mol/dm3, 0.1 mol/dm3 and 0.05 mol/dm3 
Glass beaker
Vinegar samples A to C
Glass pipette
Water
tweezers
Bromocresol green indicator solution
Household plant spray bottle
goggles
Method
Place spots of known concentration ethanoic acid on each of the labelled pencil spots on the filter paper.  Place spots of vinegar samples A to C on remaining spots.  
Place chromatogram in beaker with water up to 1 cm, ie below pencil line.
Leave until water has spread ¾ up filter paper.  Remove with tweezers and leave to dry.
When dry spray with bromocresol green solution from household plant spray bottle until paper is damp but not sopping.  Dry chromatogram.
Compare colour of indicator of samples A to C to known concentrations of CH3COOH, the one closest in colour would represent the actual concentration.
This method would not be chosen in the final experiment, as there is a lot of subjectivity to comparing the colours.  Also the technique of spraying the chromatogram is not easy to master.

Option B – titration using sodium hydroxide (NaOH).

Equations and scientific calculation methodology

Ethanoic acid (CH3COOH) is a weak acid , which only dissociates partially in solution with water.  As an acid CH3COOH donates protons, which are accepted by water in solution to form the hydroxyonium ion: 
CH3COOH (aq) + h10(l) ↔ CH3COO-(aq) = H30+(aq)
Sodium hydroxide is a strong base, so it dissociates fully in water:
NaOH(aq) {+ h10 }↔ Na+(aq) + OH-(aq) {+ h10}
Note, that the water is effectively unchanged by the NaOH as the strong base dissociating causes an excess of OH-, which would shift the normal water dissociation equilibrium h10 ↔ H+ + OH- to the left.
When titrated 1 mol of CH3COOH would react with 1 mol of NaOH in a neutralisation reaction to form the salt and water:
NaOH (aq) + CH3COOH(aq) ↔ CH3COONa (aq) + h1O(l)
Therefore if a known amount of a known concentration of NaOH is used to titrate a known volume of vinegar, the concentration of ethanoic acid in the vinegar could be calculated as shown in the example below:
Molarity of solution x volume of solution = number of mols used.
If 23ml of NaOH is used to titrate 20ml of vinegar and 
Molarity x volume / 1000 = no of mols used
1 x 23/1000 = 0.023mols of NaOH used to neutralise 20ml vinegar
As 1 mol of NaOH is needed to neutralise 1 mol of CH3COOH then 20ml of the vinegar solution must also contain 0.023mols of CH3COOH.
Using molarity x volume /1000 = no of mols used with the data that is known:
Molarity x 20 / 1000 = 0.023
0.023 x 1000 = molarity x 20
0.023 x 1000 / 20 = molarity
= 1.15M
Therefore the vinegar is of concentration 1.15M.
However, it is known however that vinegar contains around 5% ethanoic acid in water.  Therefore it can be assumed that the concentration of ethanoic acid would be closer to 0.1 mol/dm3 than 1mol/dm3.  As 1mol of CH3COOH neutralises 1 mol of NaOH, it would therefore be sensible to use NaOH of 0.1 mol/dm3.

• Part i – standardisation of alkali

NaOH solution left for any length of time includes gases that have diffused in from the air.  Therefore, in order to be certain that the concentration is precise the alkali should be newly mixed immediately prior to use.

Materials

Sodium hydroxide (solid)
Distilled water
Measuring cylinder
Glass stirring rod
Metal spatula
volumetric flask (1000 cm3)

Method

As NaOH has a molar mass of 40g/mol to make a 0.1 mol/dm3 solution 2.0g of NaOH(s) should be dissolved in 500ml distilled water in a volumetric flask and stirred until dissolved. 

• Part ii - titration of ethanoic acid with NaOH

Materials

Burette
White tile
Clamp and stand
Funnel
Conical flask (250cm3)
Volumetric flask (250cm3)
Phenolphthalein indicator solution
Phenol red indicator
0.1M NaOH solution
Vinegar samples A to C

Method

Fill clean burette with 0.1M NaOH solution.
Place 10cm3 of vinegar sample A in conical flask with a few drops of phenolphthalein indicator.
Add NaOH to vinegar sample slowly, swirling flask to ensure mixing.  Phenolphthalein will turn from colourless to pink at the equivalence point (the point at which the amount of moles of one reactant is just enough to react exactly with the measured amount of the other reactant) as there would be the same amount of NaOH and CH3COOH present in the flask, leading to a neutralisation reaction, as explained above.  It will be assumed for this experiment that the end point and equivalence point are the same.
The experiment would be repeated with vinegar samples B and C.  
The experiment would also be repeated using phenol red as an indicator.  This would be expected to change from yellow to red at the equivalence point .
The amount of 0.1M NaOH used could be used to calculate the concentration of ethanoic acid in each of the samples, via the equation:
Molarity x volume / 1000 = no of mols used

Risk assessment and safety – chemicals

NaOH is an irritant and can cause burns.  Ethanoic acid is a corrosive agent.  Phenolphthalein is an irritant and can cause burns.
Goggles and protective clothing should be worn throughout.  
If chemical comes into contact with the skin the area should be washed with plenty of clean running water.  If a large area of skin is affected or skin shows signs of burning (eg redness / swelling) medical advice should be sought immediately.
If chemical comes into contact with the eyes wash out immediately with sterile eye wash.  
If chemical is ingested wash out mouth with clean water and seek medical advice immediately. Do not induce vomiting as this may cause further damage.
Do not breathe fumes of fresh NaOH solution.  Ideally experiment should take place in a fume cupboard.  If fumes are inhaled move immediately to an area of fresh air.  In case of difficulty in breathing medical advice should be sought.

Risk assessment and safety - equipment
Burette should be filled with a funnel and should not be above shoulder height to avoid spillage.
Goggles and protective clothing should be worn when using flames.
Long hair should be tied back and loose clothing fastened securely when using flames.
Care should be taken when assembling distillation equipment so that breakage does not occur.  Particular attention should be paid when inserting one tube into another and this should not be done in palm of hand in case of slippage and possible damage to hand.

References and Bibliography
Daintith, J. (ed) 1990, A concise dictionary of chemistry, 2nd edn, Oxford University Press, Oxford.
Harwood, P. & Hughes, M. 2001, "Acids and bases" in Chemistry A2, eds. P. Harwood & M. Hughes, 2nd edn, Harper Collins, London, pp. 28-53.
Hunt, A. 2001, "Physical Chemistry" in A2 Chemistry, ed. A. Hunt, 1st edn, Hodder & Stoughton, London, pp. 6-86.
McNeil, K. 2004, COSHH Standard Assessment for Work in Chemistry Research Laboratories.University of Bristol Online. Available at: , Accessed 8th February 2006 .
The Nuffield Foundation 2003, 13th November 2003-last update, Investigating Vinegars tutorial, The Nuffield Foundation Online. Available at:   Accessed, 7th February 2006.

醋的特有的气味来自乙酸列入。 介绍 来自列入乙酸（CH 3 COOH），也称为乙酸醋的特有的气味。然而，除非购买纯蒸馏醋，保证并表示组成，有其他目前在醋的酸。这些措施包括： 柠檬酸 - HOOCCh1C（OH）（COOH）Ch1COOH） 苹果酸（2 - 羟基丁二酸酸） - HOOCCH（OH）Ch1COOH在 乳酸（2 - 羟基丙酸） - CH3CH（OH）COOH 酒石酸 - （CHOH）2（COOH）2 对于本次调查的目的，最挥发酸 - 乙酸，是一个衡量的酸度。试图将被净化醋，以便它是造成的酸性的内容主要是乙酸，但是这不能保证。 选择将醋麦芽醋（A），红酒醋和醋（B）（C）。 第1阶段 - 从醋去除颜色 所需材料 100立方厘米醋样品A至C 3容量瓶中，标记为A到C（250立方厘米） 3个锥形瓶中，标记为A至C（250立方厘米） 活性炭 金属锅铲 玻璃搅拌棒 过滤漏斗 滤纸 方法 放置醋样品在容量瓶和1锅铲措施活性炭。搅拌均匀后，用干净的玻璃棒。过滤所得混合物通过滤纸和漏斗成锥形烧瓶中，标记为A。 重复使用醋样品B与烧瓶标记为B;同样的样品C. 请注意，在过滤过程中可能需要重复，如果颜色仍然在醋。如果木炭还没有完全从混合物中除去，然后抽滤可能需要。 第2阶段 - 蒸馏醋 由于其它酸和添加剂，醋的混合物需要将蒸馏净化乙酸和水的混合物的存在下。 所需材料 蒸馏塔和分液漏斗 梨形瓶 锥形瓶从第一阶段的A至C 防碰撞颗粒 本生灯 耐热垫 钳和站立 风镜 冰浴 通用试纸 Pippette 方法 添加过滤醋样品A的梨形瓶中，并成立本生以上，蒸馏塔采用钳和立场。确保所有缝隙都密封良好。 锥形瓶中洗净甲收集馏出液，用蒸馏水和使用 排列列，以便馏分油滴入锥形瓶中，放置在冰浴。 轻轻将混合物加热梨形瓶中，直到馏分开始收集在锥形瓶中。测试一下，看看滴馏出到通用的指标，并检查pH值的颜色时，所有已挥发酸蒸馏。当pH值寄存器7停火加热。 重复使用清洁设备用醋样品B和C。 第3阶段 - 确定乙酸的浓度 选择A - 色谱 物料 滤纸铅笔线绘制2厘米2厘米的间隔沿着它从底部边缘和铅笔点标记和标记 乙酸已知浓度的样品，例如0.01为mol/dm3，0.1为mol/dm3 0.05为mol/dm3 玻璃烧杯 醋样品A至C 玻璃吸管 水 镊子 溴甲酚绿指示剂溶液 家庭植物喷雾瓶 风镜 方法 放点的已知浓度的乙酸在各点在滤纸上的标记的铅笔。放点醋样品A至C剩余的斑点。 将烧杯中的色谱与水可达1厘米，即铅笔线以下。 离开，直到水已经蔓延¾高达滤纸。取出镊子和离开干。 当溴甲酚绿溶液喷雾干燥，从家庭工厂喷雾瓶，直到纸是潮湿的，但不吸水。干色谱图。 颜色样品A至C已知浓度的CH3COOH，一个最接近的颜色，代表的实际浓度指标进行比较。 在最后的实验中，这种方法不会被选择，因为这里的特定的主体比较颜色。喷涂色谱技术不容易掌握。 选择B - 滴定使用氢氧化钠（NaOH）。 方程和科学的计算方法 乙酸（CH 3 COOH）是一种弱酸，只部分在溶液中与水的离解。作为酸CH3COOH捐赠质子，溶液中的水以形成hydroxyonium离子接受： 的CH3COOH（aq）+ H10（L）↔醋酸（AQ）= H30 +（aq）的 氢氧化钠是一种强碱，所以它完全在水中分解： 氢氧化钠（AQ）{↔娜+ H10} +（aq）的+ OH-（aq）的{+ H10} 请注意，水是有效不变的NaOH作为强碱解离导致过量的OH-，这将正常的水的解离平衡H10↔H + + OH - 的左转向。 当1摩尔的CH3COOH在中和反应中形成的盐和水的反应，用1摩尔的NaOH滴定： 氢氧化钠溶液+ CH3COOH（aq）的↔乙酸钠（AQ）+ H1O（L） 因此，如果已知量的已知浓度的NaOH滴定已知体积的醋，在醋中乙酸的浓度可以计算在下面的例子所示： 解x的摩尔浓度溶液的体积=使用的摩尔数。 如果的NaOH的23毫升是用于滴定20ml的醋 摩尔浓度X宗卷/ 1000 =没有使用的摩尔 使用1×23/1000 = 0.023mols的NaOH以中和20毫升醋 由于需要1摩尔的氢氧化钠中和1摩尔的CH3COOH然后20ml的醋液中还必须包含的CH3COOH 0.023mols。 使用摩尔浓度X宗卷/ 1000 =没有使用，是已知的数据的摩尔： 摩尔浓度x 20/1000 = 0.023 0.023×1000 =摩尔浓度×20 0.023×1000/20 =摩尔浓度 = 1.15M 因此，醋的浓度1.15M。 然而，它是已知的，然而，的醋含有约5％乙酸在水中。因此，可以假定，乙酸的浓度将接近于0.1为mol/dm3比1mol/dm3。由于1mol的NaOH中和1摩尔的CH3COOH，因此，这将是明智的使用氢氧化钠0.1为mol/dm3。 第I部分 - 标准化碱 NaOH溶液放置任何长度的时间包括在从空气中的气体扩散。因此，在以一定的浓度精确的碱应在使用前立即重新混合。 物料 氢氧化钠（固体） 蒸馏水 量筒 玻璃搅拌棒 金属锅铲 容量瓶（千立方厘米） 方法 以NaOH的摩尔质量为40g/mol，使0.1为mol/dm3溶液2.0克氢氧化钠（次）应溶解在500ml蒸馏水中，在容量瓶中，并搅拌直至溶解。 第二部分 - 用NaOH滴定乙酸 物料 滴定管 白瓷砖 钳和站立 漏斗 锥形瓶（250立方厘米） 容量瓶（250立方厘米） 酚酞指示液 酚红指示剂 0.1M的NaOH溶液 醋样品A至C 方法 填写干净的滴定管用0.1M的NaOH溶液。 将10立方厘米醋样品A的锥形瓶中，用几滴酚酞指示剂。 醋样品添加氢氧化钠缓慢​​，旋转烧瓶，以确保混合。当量点时（刚够正好与其他反应物的量反应的一种反应物的摩尔量是在该点），酚酞由无色变为粉红色，因为将相同量的NaOH和CH 3 COOH本在该烧瓶中，导致中和反应，如上面所述。本实验中，将假定的终点和等当点是相同的。 将重复实验用醋样品B和C 使用酚红作为指示剂，也可以重复该实验。这将有望改变由黄变红的等价点。 0.1M的NaOH的量，使用的可用于计算在各样品中的乙酸的浓度，通过下式： 摩尔浓度X宗卷/ 1000 =没有使用的摩尔 风险评估和安全 - 化学 氢氧化钠是一种刺激性，可致灼伤。乙酸是腐蚀性代理。酚酞是一种刺激性，可致灼伤。 应佩戴护目镜和防护服各地。 如果化学品与皮肤接触面积应用大量清洁的流动的水洗涤。如果大面积的皮肤受到或皮肤有迹象显示燃烧（如发红/肿胀）建议应立即寻求医疗。 如果化学接触到眼睛立即用无菌洗眼洗出。 如果摄入化学用干净的水漱口，并立即就医。不要催吐，因为这可能会造成进一步的损害。 新鲜的NaOH溶液，不要吸入烟雾。理想的实验应在通风橱内。如果被吸入烟雾立即移至新鲜空气的区域。应寻求在难以呼吸医疗咨询的情况下。 风险评估和安全 - 设备 滴定管应填用漏斗，不应该是肩膀以上的高度，以避免溢出。 使用火焰时，应佩戴护目镜和防护服。 应将长发绑背部和宽松的衣服扣紧时使用的火焰。 组装时，应采取蒸馏设备，这样，不会发生破损。应特别注意支付一管插入到另一个时，这不应该在手掌手手延误和可能造成的损害的情况下完成。 参考文献和参考书目 Daintith，J.（ED）1990年，一个简洁的化学，第二版，牛津大学出版社，牛津字典。 哈伍德，P.休斯，M. 2001年，“酸和碱”的化学A2，主编。 P.哈伍德＆M休斯，第二版，哈珀·柯林斯，伦敦，第28-53页。 2001年亨特，A.，A2化学，主编的“物理化学”。 A.亨特，霍德和斯托顿，伦敦，第1版，第6-86页。 麦克尼尔2004年，K.，COSHH标准评估工作在化学研究实验室。大学布里斯托尔在线。 ：2006年2月8日访问。 纳菲尔德基金会2003年，2003年11月13日最后一次更新，调查，纳菲尔德基金会在线醋教程。可访问：2006年2月7日