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Thread: pH reference solutions

  1. #16
    Join Date
    May 2015
    Location
    South Australia
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    67

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    Hi Grubs,

    Sorry no, l have ordered from another supplier Southern cross science their price is AUD9.50 for 500ml as l also needed some storage solution and Chem-supply doesn't carry. Send them an email to sales@chemsupply.com, they came back quickly today to my questions.

  2. #17
    Join Date
    Oct 2011
    Location
    Melbourne
    Posts
    1,907

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    I came across this link with recipes for preparing reference solutions outside of the 6-8pH range:

    http://delloyd.50megs.com/moreinfo/buffers2.html

  3. #18
    Join Date
    Oct 2011
    Location
    Melbourne
    Posts
    1,907

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    After 2 years I finally had time to follow GoTroppo's recipe and attempted to create a 7pH buffer solution.

    Everything I used:



    ~4 litres distilled water, 1x 1000ml + 1x 250ml measuring jugs, 2x 250ml baby bottles (for storing prepared buffer solution, any bottle will work), KH2PO4 powder, NaOH (Caustic Soda, Diggers brand bought at Bunnings), a few plastic cups, plastic spoons, 100ml syringe, 10ml syringe, pH meter, gram scales, 7pH buffer powder (to compare result against), a funnel, gloves and eye protection (NaOH is a nasty chemical). I used the 1 litre distilled water bottles to store 0.1M solutions.

    First I made my 0.1M solutions, one 0.1M KH2PO4 the other 0.1M NaOH:



    This was pretty easy, I simply added 13.60g of KH2PO4 to 1L distilled water using a 1000ml measuring jug. Stirred very well, returned to 1L bottle and I had a 0.1M KH2PO4 solution. For the 0.1M NaOH solution I added 4g of NaOH to 1 litre of distilled water, stirred very well and returned to 1L bottle. These keep for up to a year provided they're refrigerated. Having these two solutions means I can make a wide range of pH buffers using different combinations of KH2PO4 and NaOH solutions.

    Next I made the 7pH solution by mixing 100ml 0.1M KH2PO4 with 58.2ml 0.1M NaOH, then made it up to 200ml using distilled water. I now have a 7pH buffer solution.. or do I???? Better test to find out.

    To test whether the 7pH buffer I created was accurate, I made up a separate 7pH buffer solution using a 7pH buffer powder sachet I bought online.

    The results: (cup on the left is GoTroppo's recipe, cup on right is 7pH powder sachet).



    So known 7pH solution is 7.0pH on the dot

    GoTroppo's recipe is.... 7.1pH.. so damn close!!



    I'm really happy with that result. As you can see I only use a dodgy pH pen so I don't really trust it to be accurate within 0.1pH to be honest, plus my chemistry skills are pretty poor. At the end of the day it doesn't matter to me whether I have a pH of 7 or 7.1 for calibration because I'm not really testing for pH in such small ranges. For me GoTroppo's advice was great and I will continue using this recipe. I think I was a little crude with measurements perhaps and might have affected accuracy. In hindsight I should have used a 2.5ml syringe instead of 10ml to be more accurate when making up the buffer solution. Will try that next time.

    As GoTroppo mentioned you can keep the buffer solution for up to 2 weeks. I'll keep mine in the fridge but let it warm to room temp before using.

    The recipe makes up around 10x 200ml 7pH buffer solutions in total before more 0.1M solution is needed. Enough to last me the year.
    Last edited by GR1KTR; 19-01-16 at 09:43 PM.

  4. #19
    Join Date
    Oct 2011
    Location
    Melbourne
    Posts
    1,907

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    I've made 3 solutions using this method now and I've been able to achieve 7pH consistently. Turns out the 0.1M solutions were fine, it was the mixing of the two 0.1M
    solutions and distilled water to make the 7pH buffer solution I was doing incorrectly.

    I invested in a 2.5ml syringe which improved accuracy greatly. It's hard to measure in 0.1ml with anything bigger.

    I keep the 7pH buffer in the fridge and replace every 2 weeks. Holds it's value for the whole time. Interestingly it doesn't matter whether it's chilled straight out of the fridge or room temp, it still tests at 7pH. I'm going to leave the solution in the fridge for a month this time and see if it holds for that long.

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