/i:/ is the vowel that we find in the word FLEECE. I put that word in capitals because that is how that vowel is often referred to by linguists: the fleece vowel - or FLEECE for short. (This is not random, the word was specifically chosen for a number of specific reasons.) It is the vowel sound at the end of the word guarantee. In transcriptions of British English it has a colon [ : ] in the symbol to describe the length.
/ɪ/ is the vowel in the word KIT. It is known as the kit vowel - or KIT for short. It is the vowel we find in prefixes and suffixes, the bits we stick onto the beginnings and ends of words. So, for example it is the vowel we hear in --ing verb endings.
The vowel represented by /i/ at the ends of words in dictionaries is usually referred to as the happy vowel - HAPPY. This vowel may sound like either FLEECE or KIT, but is always short in duration.
If you say the < y > sound that we find in the word yes, and then say the < e> we find in the word end, the kit vowel is somewhere between the two sounds. This is the first vowel in the word infinitely. This word would sound very odd to a native speaker if it was said with a fleece vowel, /i:/! It would sound like a made-up word: eenfinitely.
The Original Poster asks if there is a big difference between these vowels. If we are talking about the physical difference between the sounds, the answer is: no. In fact, it is very unusual to have two vowels that are so similar in one language. They are very close together. In most languages these would count as one vowel. However, if we are talking about the meaning, or the effect on a listener, the answer is: yes! There is a big difference. There are very, very, very many words that we can be confused about if you say the wrong vowel. For example, the words peace and piss. Nobody wants to say Piss man!, when they mean Peace man!.
If you want to type IPA script, this website is very useful
Hope this helps,
Peace!
Short answer
To master the /z/ sound, make a long /s/ sound and sing loudly at the same time. It won't sound anything like a J, /dʒ/ at all.
Full answer
The Production of /z/
/z/ is a voiced alveolar fricative. To understand how we make a /z/, we need to think about some different parts of the mouth.
If you look in the mirror you will see a line running down the middle of your tongue (called the mid-saggital line).
If you feel behind your teeth with your tongue, would will feel a little shelf. That's your alveolar ridge. Behind that your mouth suddenly arches upwards to form the roof of your mouth.
To produce a /z/, you have to place the blade of your tongue (that's the bit just behind the very tip) on the alveolar ridge. You create a little furrow or ridge down the mid-saggital line. The rims (sides) of your tongue rest against the inside of your side teeth.
You then force air from your lungs down the narrow channel, this creates friction, or turbulence in the air as the air is forced through this furrow and through the narrow hole created between your tongue and the alveolar ridge. This turbulence causes a hissing noise. While this is happening, your vocal folds (sometimes called your vocal cords) vibrate. This gives the sound pitch which is heard at the same time as the turbulence
It might be helpful at this point to think about how we make the sound /s/.
The production of /s/
To produce a /s/ you have to place the blade of your tongue (that's the bit just behind the very tip) on the alveolar ridge. You create a little furrow or ridge down the mid-saggital line. The rims (sides) of your tongue rest against the inside of your side teeth.
You then force air from your lungs down the narrow channel, this creates friction, or turbulence in the air as the air is forced through this furrow and through the narrow hole created between your tongue and the alveolar ridge. This turbulence causes a hissing noise.
This probably sounds a bit familiar!
How to make a /z/ if you are accidentally making a J sound as in jump, /dʒ/
Now, you will have noticed that to make a /z/ we do exactly the same thing as we do for /s/. There is no difference between the position of your tongue, teeth, or any other part of your mouth at all. The only difference is that when we make a /z/ we have voicing, or vocal fold vibration. This gives the /z/ pitch; we can make a high pitched /z/ or a low pitched /z/. When we make an /s/ we just get a hissing sound. It does not have the same quality of pitch. This is because there is no vibration from the vocal cords.
Because of this, if you can already say /s/ with no problem, you just need to add vocal fold vibration to make a /z/. You need to add pitch. How can you do this? The answer is: you need to sing while you make an /s/. Start making an /s/ and then sing while you are making it. You need to make the /s/ for several seconds. First do it at a high pitch then a low pitch. If you can hear a high or low pitch then you are making a /z/. You can then start practising it at a normal speaking type of pitch.
It is much easier to do this than to try and follow the instructions for making a /z/. If you are making a /dʒ/ sound, the sound in the word jump, then I could give you advice like "move your tongue slightly forward in your mouth towards your front teeth - but don't make a complete closure with your tongue when you start the sound". However, this is very, very difficult to do without anyone to help you. In my experience, singing a note whilst making an /s/ sound is quite easy to do, more effective and more fun.
Hope that's helpful!
References:
You can read about /s/ and /z/ in Gimson's pronunciation of English by Alan Cruttenden, 8th Edition 2014
Best Answer
In the American standard, you should be able to guess the spelling of primary or secondary stressed "des" or "dis" at the beginning of a word. In "destiny" and "destination", "des" is stressed and has the vowel of "pet", "met" and so on. In "distant", "disproportionate", "dis" is stressed and has the vowel of "sit", "bit", and so on.
Immediately before a stressed second syllable, however, "des"/"dis" can be stressless and the vowel can vary. The reduced vowel schwa is always okay, so far as I know, but in "desire", I find also acceptable the "i" of "pit" or the "e" of "scene". The "i" pronunciation is perhaps a variant form of reduced vowel, and the "e" is probably a spelling pronunciation.
Unstressed "dis", immediately before a stressed syllable, I find varies between schwa and the "i" of "pit", at least when a consonant follows, for instance in "dismay", "disturb", "dispassionate", which I would also attribute to two variant reduced vowels. When a stressed vowel follows the "dis", as in "disable", the schwa doesn't sound as good, and in "dissect", a tense "ay" diphthong is also possible.